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UNIVERSITÉ DU MAINE - UFR SCIENCES ET TECHNIQUES
École Doctorale de l’Université du Maine
THÈSE
Pour obtenir le grade de Docteur de l’Université du Maine
Spécialité: Physique de la Matière Condensé
Présentée et soutenue publiquement le 18 novembre 2005 par:
Kateryna Fatyeyeva
ELABORATION AND INVESTIGATION OF
CONDUCTING POLYMER COMPOSITES BASED ON
POLYANILINE AND POLYAMIDE
Composition du Jury:
M. A. Bernès Professeur, Université Paul Sabatier de Toulouse Rapporteur
M. D. Graebling Professeur, Université de Pau et des Pays de l’Adour Rapporteur
M. A. Korzhenko Docteur, CERDATO, Arkema, SERQUIGNY Examinateur
M. J.-F. Pilard Professeur, Université du Maine, LE MANS Examinateur
M. A. Pud Docteur (HDR), ICBP, Kiev, UKRAINE Codirecteur de thèse
M. M. Tabellout Maître de Conférence, Université du Maine, LE MANS Directeur de thèse
This thesis was prepared in:
Laboratoire de Physique de l’Etat Condensé Université du Maine, CNRS-UMR 6087
Av. O.Messiaen, 72085 Le Mans cedex 9, France
and
Laboratory of electrochemistry of organic compounds Institute of Bioorganic and Petrochemistry, National Academy of Science of Ukraine
50, Kharkovskoe shosse, 02160 Kiev, Ukraine
email: [email protected]
Acknowledgement
Taking the opportunity I would like to express my gratitude to the people who
have helped me in my work over a few past years.
Special thanks should be given to my supervisors Dr. Alexander Pud (Institute
of Bioorganic chemistry and Petrochemistry (IBCP), Ukraine) and Dr. Mohamed
Tabellout (Université du Maine, France) for their invaluable assistance in performing
this work and discussing the results.
I would also like to thank the committee members including Prof. Alain
Bernès, Prof. Didier Graebling, Dr. Alexander Korzhenko and Prof. Jean-François
Pilard for finding time in their busy schedule for serving in the committee, reviewing
my thesis and offering some valuable suggestions and comments on the work.
I would also like to thank Prof. Galina Shapoval (IBCP, Ukraine) for giving
me an opportunity to begin my research work in her laboratory.
I would like to thank the French government for providing me with the
financial support as a grant during my stay in France.
I am very grateful to the colleagues from Laboratoire de Physique de l’Etat
Condensé (LPEC) (France) as well as from the IBCP (Ukraine) for their help in making
experiments, for their encouragement and kindness. I greatly acknowledge the
assistance of Dr. Pascal Ruello (LPEC) in performing the four-probe resistivity
measurements. I would like to thank Dr. Jean-François Bardeau (LPEC) for assistance
in taking Raman spectrometry measurements, Yann Bulois (LPEC) for the electron
micrographs of the composite films, Gérard Guevelou (Laboratoire Polymères,
Colloides, Interfaces) for performing DSC measurements and Dr. Nikolay Ogurtsov
(IBCP) for his help in performing thermal analysis measurements, Olivier Schneegans
and Frédéric Houzé (Laboratoire de Génie Electrique de Paris, UMR CNRS 8507,
Supélec, Université Paris VI and Paris XI) for performing AFM (electrical and
topographical images) of the composite films. I’m grateful to Pierre-Yves Baillf for his
constant help and kindness. Also, I am appreciating the help of Jeannette Le Moine
(LPEC) very much.
In particular, I would like to thank Arkema company (France) for giving the
polyamide samples.
Special acknowledgement should be directed to Dr. Sergey Rogalsky (IBCP)
and to Dr. Jean-François Bardeau (LPEC) for their helpful discussions and comments
on the work and for their readiness to help.
3
Acknowledgement
I owe my thanks to Karolina Galicka for her assistance and friendship and to
all those who have directly or indirectly helped me in the research work and during my
stay in France.
A giant thank you goes to my parents for their love and patience they showed
in the course of taking this thesis and especially during my study in France. This work
would not be possible without their support. Finally, a big thank you to Andrey for his
support, love and belief in me.
4
Table of contents
Abbreviations…………………………..………………...…………………….... 9
Introduction……………...……………………….….……….………………..…. 11
Chapter 1. Literature review
1.1. Structure and properties of polyaniline…………………......................... 13
1.1.1. Chemical properties………………………………..................... 13
1.1.2. Electrochemical properties……………………………………... 16
1.1.3. Thermal, dielectric and mechanical properties of polyaniline…. 24
1.1.4. Influence of acid-dopants on the polyaniline preparation and
properties…………………………………………………....................
26
1.2. Synthetic methods of the polyaniline preparation………………............ 28
1.2.1. Chemical way of obtaining polyaniline………………………... 28
1.2.2. Electrochemical way and the mechanism of the polyaniline
synthesis...……………………………………………………………..
29
1.2.3. Comparison of chemical and electrochemical methods of the
polyaniline synthesis…………….…………..………………………...
35
1.3. Mechanism of electrical conductivity in polyaniline………………….....36
1.4. Practical application of polyaniline…………………………....................41
1.5. Composite materials based on polyaniline and polyamide………………43
1.6. Research motivation…………………………………………………….. 47
Chapter 2. Experimental part
Introduction…………………………………………………………………...49
2.1. Materials and reagents………………………………………….……….. 49
2.2. Preparation of the composite materials…………………………..............52
2.2.1. Synthesis of polyaniline………..………………………………...52
2.2.2. The formation and pre-treatment of the polymer matrix............... 52
2.2.3. Formation of the surface conductive composites…..…………….54
2.2.4. Formation of the bulk conductive composites……….….............. 56
2.3. The main investigation methods and techniques………...…………...…. 57
2.3.1. Method of determination of the real polyaniline content in the
composites………………….……………………..…….………..……..
58
2.3.2. Electrochemical investigations………….…………………..……60
5
Table of contents
2.3.3. Electrical conductivity measurements..………….…….................63
2.3.4. Spectroscopy investigations…………..…………………………. 65
2.3.5. Dielectric relaxation spectroscopy…………..………….……….. 67
2.3.6. Thermal analysis and differential scanning calorimetric
measurements………..………………………………………..………... 71
2.3.7. Mechanical analysis……………………..………………………. 72
2.3.8. Optical and atomic force microscopies……………..…………… 72
2.3.9. pH-potential-temperature measurements……………................... 72
Chapter 3. Electrochemical synthesis of polyaniline
Introduction………………...………………………………………………… 73
3.1. The peculiarities of the electrochemical formation and stability of
polyaniline on the surface of bare electrodes..……………………...………... 73
3.1.1. Polyaniline formation by cyclic voltammetry……..…….............. 73
3.1.2. Polyaniline formation in the potentiostatic mode……………….. 79
3.1.3. Polyaniline formation under galvanostatic conditions…………... 79
3.1.4. Electrochemical properties of the polyaniline film in the
background solution…….………..……………….................................. 82
3.2. The formation of the polymer composite materials……………………... 86
3.3. Conclusions………………………………..…………………….............. 94
Chapter 4. Chemical aniline polymerization in solid and water dispersed
polyamide media
Introduction……………………………..……………………………………. 96
4.1. The swelling kinetics of the polyamide films in the reaction media.…….96
4.2. The influence of the oxidative media on the polymerisation process…… 99
4.3. Evaluation of the structure of the surface composite films…..………….. 105
4.4. Kinetic peculiarities of the polyaniline formation in the polyamide
matrix…………………..………………………………….…………………. 110
4.4.1. Polymerization process in the PA-12 film………………………. 110
4.4.2. Influence of the polymer matrix structure………………………. 119
4.5. The aniline polymerization in the dispersed polyamide media…............. 125
4.6. Conclusions…………………………..………………………….............. 131
6
Table of contents
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman
properties
Introduction…………………………………………..…………………….... 134
5.1. Spectroelectrochemical investigation of polyaniline and its composites...134
5.1.1. Spectroelectrochemical properties of the polyaniline films.……..134
5.1.2. Spectroelectrochemical properties of the conducting composite
films……………………………………………………………….….…
136
5.2. Thermal behaviour of polyaniline and its composite materials………….140
5.2.1. Thermal behaviour of polyaniline…….………..………….……..140
5.2.2. Thermal behaviour of the conducting composite powders….…... 142
5.3. Mechanical investigation of the conducting composite films……………147
5.4. Structural studies of the surface conducting composite films……………150
5.5. Raman spectrometry measurements………………………...……………156
5.5.1. Raman spectrometry investigation of polyaniline……..…………156
5.5.2. Raman spectrometry study of the surface composite films……... 159
5.5.3. Raman spectrometry study of the bulk composite powders…….. 162
5.6. Conclusions………………..……………………………………..............166
Chapter 6. Dielectric and electrical properties
Introduction………………………………..………………………………….168
6.1. Chemically synthesized polyaniline………………………..…………….168
6.1.1. Dedoped polyaniline……………………………………..………168
6.1.2. Doped polyaniline…………..……………………………............173
6.2. Surface conducting composite materials………………………………....183
6.2.1. Dielectric properties of the polyamide matrices...……….…........ 183
6.2.2. Dielectric and electrical properties of the surface conducting
composites based on the PA-12 matrix..………………………………..
187
6.2.3. The influence of the polymer matrix structure...………………....192
6.3. Bulk conductive composite materials based on polyaniline and
polyamide……………………………………………………………………..
197
6.3.1. Effect of the dopant nature on properties of the composite PA-
12/PANI powders……………………………………………….............
198
7
Table of contents
6.3.2. Influence of the polyaniline content……………………………...203
6.3.3. Influence of the polyamide matrix structure.……………………. 215
6.3.4. Properties of the composite films…………..…………………….216
6.4. Conclusions…………………………………………..………….............. 226
General conclusions and suggestions for future work…………………................. 229
References……………………………………………..………………………….. 232
8
Abbreviations
13C-NMR – nuclear magnetic resonance
ac-conductivity – alternating-current conductivity
AFM – atomic force microscopy
AMPSA – 2-acrylamido-2-methyl-1-propanesulphonic acid
APS – ammonium persulphate
BeOH – 2-hyroxybutanol
BN – acid eutectic mixture NH4F+2.35 HF
CELT – charge energy limited tunnelling
CSA – camphor sulphonic acid
DBSA – dodecyl benzene sulphonic acid
dc-conductivity – direct-current conductivity
DiBHP – diester [bis-(2-methylpropyl) hydrogen phosphate]
DiOHP – diester [bis-(2-ethylexyl) hydrogen phosphate]
DRS – dielectric relaxation spectroscopy
DSC – differential scanning calorimetry
E – emeraldine
EB – emeraldine base
EPR – electron paramagnetic resonance
EQCM – electrochemical quartz crystal microbalance
ES – emeraldine salt
FIT – fluctuation-induced tunnelling
FTIR-spectroscopy – Fourier transform infrared spectroscopy
HN function – Havriliak-Negami function
ICP – intrinsically conducting polymers
IR – infra red
LE – leucoemeraldine
MSA – methane sulphonic acid
MWS effect – Maxwell-Wagner-Sillars effect
NMA – N-methylaniline
NMP – N-methyl pyrrolidone
NSA – β-naphtalene sulphonic acid
PA – polyamide
PANI – polyaniline
9
Abbreviations
PET – poly(ethylene terephtalate)
PM – polymer matrix
PMMA – poly(methyl methacrylate)
PNA – pernigraniline
POMA – poly(o-methoxyaniline)
PPN – polyphenylnitrenium
PVA – poly(vinyl alcohol)
PVDF – poly(vinylidene fluoride)
SANS – small-angle neutron scattering
SCE – saturated calomel electrode
SHE – standard hydrogen electrode
TEM – transmission electron microscopy
TGA – thermogravimetric analysis
TSA – p-toluene sulphonic acid
UV-Vis spectroscopy – ultraviolet-visible spectroscopy
VRH – variable range hopping
XPS – X-ray Photoelectron Spectroscopy
-N=Q=N- – quinoid diimine unit
-NH-B-NH- – benzenoid diamine unit
10
Introduction
Introduction
Conducting polymer materials are widely used in different fields of science and
industry – as electrodes in rechargeable batteries, as electrochromic displays, anti-
corrosion coatings, electromagnetic interference shielding, sensors, membranes, etc.
Nowadays, two groups of conducting polymer materials exist. The first group
consists of composite materials, which receive their conductivity after introducing
conductive dispersed materials with electronic conductivity (carbon, metallic powder or
fibres) into a polymer. Such materials are called “materials with external conductivity”
– in order to underline that conducting particles were introduced externally. The other
group of conducting polymer materials is a class of materials, conductivity of which is
reached by the motion of charge carriers along the polymer backbone. These materials
were given the name of “intrinsically conducting polymers” (ICP).
Before these materials were discovered (more than 25 years ago), conducting
polymers were obtained only by the cardinal change of the polymer insulators or
semiconductors structure, for example as a result of their heating till the temperature at
which pyrolysis or partial grafitization of the polymer took place.
Conducting polymers are a rather new class of materials which attracts
attention due to the combination of electrical, magnetic properties of metals with optical
property of usual polymers. They receive a high conductivity value during both the
redox and the doping processes, i.e. after the oxidation (reduction) and the introduction
of a necessary quantity of acid (dopant) anions in the polymer matrix (PM) of
conjugated polymers with a conductivity value about 10-15-10-10 S/cm. As a result
materials with semiconducting and metallic conductivity (from 10-9 to 103 S/cm) can be
obtained depending on the method of synthesis, the nature of polymer and dopant. The
typical representatives of this kind of materials are polyacetylene, polyaniline (PANI),
polypyrrole, polythiophene and their derivatives.
Major works dealing with conducting polymers are devoted to the problems of
their synthesis and possible applications [1-5]. In 2000 Hideki Shirakawa, Alan G.
MacDiarmid and Alan J. Heeger, who discovered the existence of conducting polymers,
were awarded by the Nobel Prize in Chemistry.
PANI is the oldest polymers among the known ICP. It was synthesized for the
first time as early as 1862 [1]. PANI takes a special place among other polymers due to
the unique combination of easy synthesis, low monomer (aniline) cost, high value of
conductivity, high stability of both doped and dedoped states in the air with sensor,
11
Introduction
optical, catalytical and other properties. PANI is a conjugated polymer with an electrical
conductivity value similar to semiconductors.
But there are some limitations in the PANI application, which are connected
with its poor solubility in common organic solvents, its infusibility and fragility. These
disadvantages often hinder PANI practical applications. Some ways to overcome these
problems such as doping PANI by functionalized acids (such as camphor sulphonic acid
(CSA), dodecyl benzene sulphonic acid (DBSA)), using substituted PANI and forming
composites with conventional polymers have been found. From our point of view, the
last way, i.e. the preparation of conducting polymer composites, is the most promising,
as in this case we obtain a material, which combines mechanical properties of common
polymers with the electrical conductivity of PANI.
So, the purpose of this work is to obtain new composite materials by
combining PANI and traditional thermoplastic polymers - polyamides in order to take
advantage of the properties of both kinds of materials. Two kinds of such composites –
surface layered and bulk – are synthesized and investigated by standard techniques.
The first chapter of this thesis is devoted to the literature review of the
conducting polymers, especially of pure PANI and composite materials based on
polyamide (PA) and PANI. The used materials as well as the composite preparation
methods are expounded in the second chapter. The description of the methods used in
the course of the investigation are also given. In the third chapter the peculiarities and
the mechanism of the aniline electrochemical polymerization on the bare electrodes and
on the electrodes covered with polymer matrices are stated. The fourth chapter of this
thesis is devoted to the chemical aniline polymerization process inside the PA films and
in the presence of the water PA dispersions. The kinetic features of the process are
described. The fifth chapter deals with the spectroelectrochemical, thermal, mechanical,
structural and Raman properties of the obtained surface and bulk conducting composite
materials. It was confirmed that in the course of forming the composite materials the
interaction between PANI and the PM takes place. The sixth chapter describes the
dielectric and electrical properties of the composite materials. Large frequency range
(10-1–109 Hz) allows revealing different relaxation processes in PANI as well as in the
composite conducting materials. Percolation theory is used to analyze the obtained
experimental results.
12
Chapter 1
LITERATURE REVIEW
Chapter 1. Literature review
1.1. Structure and properties of polyaniline
1.1.1. Chemical properties
In a broad sense the name of PANI combines a family of polymers (Fig. 1.1),
which have the same length of the polymer chain, but differ in the oxidation state, redox
properties and the value of conductivity. It is accepted that PANI exists in three well
defined oxidation states: leucoemeraldine (LE), emeraldine (E) and pernigraniline
(PNA) (Fig. 1.1). LE and PNA are the fully reduced (all the nitrogen atoms are amine)
and the fully oxidized (all the nitrogen atoms are imine) forms, respectively, and in E
the ratio amine/imine is ∼0.5. Starting from the electrically insulating LE, electrically
conducting E can be obtained by standard electrochemical or chemical oxidation, as for
all other conducting polymers [6]. But upon further oxidation a second redox process
occurs, which yields a new insulating material - PNA. In addition to this unusual
behaviour, a decrease of conductivity by ten orders of magnitude is obtained just by
treatment of the conducting E in neutral or alkaline media. Protonation induces an
insulator-to-conductor transition, while the number of π-electrons in the chain remains
constant (Fig. 1.2).
Figure 1.1. Structures of the non-conducting PANI forms
N N
x
N
H
N
H H H
NN
x
N
H
N
H
y
NN
x
N N
1-y
Leucoemeraldine (y=1, completely reduced form)
Emeraldine base (y=0.5)
Pernigraniline (y=0, completely oxidized form)
13
Chapter 1. Literature review
Figure 1.2. Illustration of the transition between the PANI base form and the protonated
emeraldine form (one of the possible chemical representations). A- represents an
arbitrary anion
For a long time the PANI structure was arguable and only recently for its
emeraldine base form it was established as poly(p-phenyleneamine quinonediimine)
(Fig. 1.3) [7]. For this purpose the synthesis of the compound of ascribed structure was
made and the following comparison of its properties with the properties of PANI
received by usual methods (by the chemical and electrochemical aniline
polycondensation) was carried out [7, 8].
Figure 1.3. Structure of the poly(p-phenyleneamine quinonediimine)
The PANI structure (Fig. 1.3) was also confirmed by the 13С-NMR [9], IR and
Raman spectroscopy experimental data [10, 11]. In accordance with these results, bands
which are typical for the vibrations of benzenoid and quinoid rings are observed.
Moreover, their ratio depends on the oxidation state, C=N vibrational mode, C-N
bending mode. The type of the C-H out-of plane bending vibration of benzenoid groups
is characteristic of a para-substitution of the aromatic ring. However, Genies et al [8]
paid attention to the fact that in IR-spectra of the specially synthesized poly-о-, p-
phenylenediamine essential differences from the PANI spectrum are present.
NN
x
N
H
N
H
y
1-y
NN
x
N
H
N
H
x
N N N N
H H H H
-
Emeraldine base (EB) (blue)
2xHA
+. +.
A- A-
Emeraldine salt (ES) (green)
+2xHA
14
Chapter 1. Literature review
The theoretical calculations (extended Hückel theory) of the planar sandwich
geometry of the aniline dimer cation (С6Н5NH2)+2 [12] showed that the optimal
geometry of this cation is head-to-tail coupling. Moreover, in such conformation the
effective overlap comes mainly from benzene rings and carbon atom interacts with the
corresponding atom from the other fragment (Fig. 1.4).
Figure 1.4. Schematic diagram showing the configuration considered in (С6Н5NH2)+2
[12]
The chemical or electrochemical PANI oxidation leads to the augmentation of
the part of quinoid diimine units and, accordingly, to the decreasing of the part of
benzenoid diamine units. Analytical results obtained earlier [13] and results of the
FTIR-spectroscopy [14, 15] testify to this fact. The deep oxidation of PANI leads to
almost quantitative formation of p-benzoquinone [16]. PANI can be reduced
electrochemically or chemically, for example, by hydrazine, to colourless
polyphenylamine, which does not contain quinoid diimine chromophore groups [16].
Due to the presence of basic amine and imine nitrogen atoms, the polymer
reacts with protonic acids, forming the PANI salts. PANI in the state of the salt is
usually formed during the synthesis in an acidic solution. Interaction of these salts with
alkaline or ammonium solutions converts PANI into the base form (Fig. 1.2). It was
established that only 50% of nitrogen atoms could be protonated [17] that was explained
by the existence of strong effective pushing away between protons near to neighbouring
nitrogen atoms [18].
PANI-base is partly soluble in some polar organic solvents, such as aromatic
amines, phenols, cold pyridine, N,N-dimethylformamide [13], and also in aqueous
solutions of acids: in cold 80% acetic acid, 60-88% formic acid [13, 16]. The molecular
weight of the soluble in tetrahydrofuran fraction of PANI-base, which was
electrochemically obtained in aqueous solution, changes in wide ranges – minimum
established value is 4300 [13], and maximum - ~50000-55000 [19]. These
NH2
H2N
.+
15
Chapter 1. Literature review
disagreements can be explained by differences of synthesizing methods and methods of
treatments resulted in the molecular-weight distribution shifts to the low molecular
weight side [20]. Also, it was found [21] that the temperature of the synthesis
influenced the molecular weight: as the polymerization temperature was decreased, the
molecular weight of PANI increased. However, the PANI molecular weight was not
affected by the acidity of the reaction medium. The soluble in N,N-dimethylformamide
the PANI fraction, which was synthesized in the acid eutectic mixture NH4F + 2.35 HF
(the medium abbreviation is BN), has molecular weight about 80000-90000 [22]. The
PANI salts with inorganic acids are soluble only in concentrated sulphuric acid [10].
However, using organic proton acids of large molecular size makes the PANI salts
partly soluble also in common organic solvents [23].
It is known that PANI can exist in several oxidation states. The protonation-
deprotonation and oxidation-reduction processes do not change the PANI chain and are
practically reversible. That’s why it was proposed [24, 25] to depict the relation
between these states as a two-dimensional potential-pH diagram. However, a point
location in this “external” diagram does not give directly the PANI structure in this
point. For theoretical analysis of transformation processes with the participation of the
different PANI forms the system with the coordinates of the electronic band filling (fe),
0<fe<1, and the location of the available sites which are protonated (fp), 0<fp<2, is more
convenient [26]:
fp = ne / nN; fе = (nN + np - ne) / 2nN
where ne, np, nN are amount of electrons, protons, and nitrogen’s, respectively.
The full diagram of the PANI states in “internal” coordinates fp, fe, which
includes all of the possible PANI forms in head-to-tail coupling is depicted in Fig. 1.5
[16], where -N=Q=N- - quinoid diimine units, and -NН-В-NН- - benzenoid diamine
units.
1.1.2. Electrochemical properties
PANI displays electroactivity only in acidic media [27]. The PANI
electroactivity is associated with the polymer transition between salt and base forms
(Fig. 1.2). In organic solvents PANI also reveals electroactivity, but only in the presence
of protonic acid as well as organic salt as electrolyte [28].
The PANI tendency to redox transitions was found with the help of cyclic
voltammograms [29]. If the PANI form displays electroactivity, one can see current
(1.1)
16
Chapter 1. Literature review
Figure 1.5. State diagram of the theoretically possible PANI forms:
1 – -В-NH-B-NH-B-NH2-, LE-base;
2 – -B-N+Н2-B-N+Н2-B-N+Н2-, protonated LE;
3 – -B-N=Q=N-B-NH-B-NH-, Е-base;
4 – -B-N+Н=Q=N+Н-B-NН-B-NH-, protonated Е, salt form;
5 – -B-N=Q=N-B-N=Q=N-, PNA-base;
6 – -B-N+Н=Q=N+Н-B-N+Н=Q=N+Н-, protonated PNA;
7 – -B-:N+-B-:N+-B-:N+-В-:N+-, or =Q=N+=Q=N+=Q=N+=Q=N+-, polyphenylnitrenium (PPN)
peaks on the cyclic voltammograms. The height and potential of these peaks will
depend on many factors. Cyclic voltammograms of PANI, which were obtained on the
platinum electrode in the acidic aniline solution depicted in Fig. 1.6, have three anodic
and three cathodic peaks. During the analysis of the electrochemical PANI behaviour
only couples 1-1’ and 3-3’ were usually considered, since the couple 2-2’ is not
specifically associated to PANI [29]. Even in the case of presence of this redox couple
on the polymer cyclic voltammograms, its behaviour does not depend on the other two
redox peaks, which are connected with each other as we can see from the results of
chronocoulometric study during the synthesis in the BN media [30]. There aren’t any
peculiarities on EPR spectra which may correspond to the couple 2-2’ in contrast to the
couples 1-1’ and 3-3’ [31].
There isn’t a generally accepted explanation of the reason of the emergence of
this middle peak couple 2-2’ [32-36]. For example, Park S.-M. et al [33] have supposed
that in the aqueous acidic solution this peak is connected with the redox reactions of the
17
Chapter 1. Literature review
Figure 1.6. Multisweep cyclic voltammograms during
deposition of PANI in 0.5 M H2SO4 containing 0.5 M
aniline [34]. Potential vs saturated calomel electrode
(SCE). Electrode – Pt.
PANI degradation which are accompanied with formation of quinoneimines and p-
benzoquinones. Genies E.M. et al [34] have demonstrated that the middle peak which
often appears during potential cycling between -0.2 and 1.2 V (vs Cu/CuF2) in BN, is
attributed to the presence of polymer containing phenazine rings. This peak appears
during the polymerization of aniline at high potential and also can be a result of
oxidation of the formed polymer at a higher potential. Yoneyama H. et al [32] have
demonstrated the conformity of couple 2-2’ to the redox reactions of p-benzoquinone
which they found with the help of spectroelectrochemical measurements. The
appearance of this couple means the beginning of the loss of the PANI electroactivity
that is observed at the PANI overoxidation. In aqueous solution such degradation, as
studied by rotating ring-disk electrode [33], leads to the formation of soluble products.
They may be a quinone/hydroquinone couple [37]. On the other hand, in the BN media
the couple 2-2’ is connected with the PANI cross-linking [34]. On the basis of
theoretical discussions, the authors [38] assumed that in the range of potential of the
couple 2-2’ the electroactive PANI oligomers with the conjugation length less than 10
aniline units can be formed.
The models of the redox PANI transitions (corresponding to the couples of
peaks 1-1’ and 3-3’) proposed in literature can be presented in the form of the schemes
(1.2) and (1.3).
On first sight, it seems that we can determine the PANI oxidation state at each
potential with the help of coulometric method on the basis of LE, colourless of which
has been already indicated by the absence of chromophore quinone diimine groups [16].
Existing results show [38] that for the complete reduction of emeraldine to LE we need
18
Chapter 1. Literature review
about 0.5 ē per aniline unit, and the comparison of results [31] and [39] allows m
the conclusion that for the LE-PNA transition we need one electron per aniline
accordance with the scheme (1.2). However, the results of coulometric study [30]
give reason for such a simplified interpretation. The interpretation of coulo
measurements in the case of conducting polymers becomes more complica
significant effect of capacitive currents [40]. The study of the impedance-fre
dependencies indicates that the storage up to 40% of the PANI charge in pro
carbonate electrolyte takes place due to the capacitive current [41]. Such ef
significant also in the aqueous media [42, 43]. So, to explain redox transitions in
it is necessary to have additional information.
The most substantiated is the following variant of scheme (1.2) of the
redox transitions (scheme (1.4)). At рН > 4 PANI is non electroactive.
The redox potential-pH dependency is determined in accordance with
equation (1.5).
The potential of peak 1 (Fig. 1.6) in weak acidic solution does not dep
pH, but in strong acidic solution its value shifts to the anodic region by 59 mV at
decreasing by a unit [34]. Consequently, in weak acidic media protons do not ta
in the reaction and as a result the potential for the first redox process is indepen
pH. In contrary, in strong acidic media the relation of one Н+ to electron occu
analogy with this, in weak acidic solution, in which the study of the potential
)
1:- 2e
1':+ 2e 3':+ 2e
3:- 2e
-
--
NH+
NH NH.. ..
NH+
-
3':+ 2e
3:- 2e-
-++NN
LE PNA
PPN
NH NH.. ..
- NH NH.. +
. 3:- e
3':+ e1':+ e
1:- e
-
-3:- e
3':+ eNH+
NH+
LE E
PNA
19
(1.3)
q
k
d
(1.2
aking
ring in
do not
metric
ted by
uency
pylene
fect is
PANI
PANI
Nernst
end on
the pH
e part
ent of
rs. By
of the
Chapter 1. Literature review
NH B NH B
.. +NH B NH B
- e, + A- -
A-
- e, - 2H , - A- -+
BN.. ..
Q N
NH B B
+
+
..
NH2
A-
--e, - H
+NH B
-
NHNH
NH.
BA
A ABQ
- e, + A--
+
- -
couple 1 - 1'
couple 3 - 3'
Ox + mH+ + nē → Red,
Е = Е0 + (0.059/n) lg(Ox/Red) – 0.059(m/n)pH (at Т = 298 К)
peak 3 of the pH dependency was successfully performed (the polymer degradation
makes it difficult), the shift of the potential by 118 mV by pH unit makes the
participation of two Н+ per electron possible [27, 44]. The possibility of the using PANI
as pH-sensitive electrode-sensor is based on its potential-pH dependency [45, 46].
The anion migrations during redox transitions of PANI were studied in
sulphuric acid solutions on the basis of changes of the electrode weight by the
electrochemical quartz crystal microbalance (EQCM) technique [47, 48] and with using
the sulphuric acid with marked 35S by a radiotracer method [49], and also in the solution
of perchloric acid [50]. The obtained results are in good agreement with the scheme
(1.4). The anion desorption at the potentials of peak 3 which was noted in these works is
particularly essential for proving the PANI oxidation mechanism.
The above presented data exclude the possibility of the PANI oxidation more
than PNA, at least in aqueous media at pH > 0. The investigation of the second stage of
the PANI oxidation in strong acidic media is difficult because of the fast polymer
degradation.
The scheme (1.4) lets us to suppose that the redox properties of PANI-base in
the intermediate oxidation emeraldine state are stronger than in more reduced and more
oxidized states. This fact is also confirmed in [27].
The phenomenon of electrochromism, i.e. the change of electronic spectrum
and the colour depending on electrode potential and, correspondingly, on the oxidation
state (Fig. 1.1 and 1.2), is typical for PANI, like for other conducting polymers [14, 48,
(1.5)
(1.4)
pH = 1 - 4 pH < 0
20
Chapter 1. Literature review
51-53]. In reduced LE state, as it was pointed out above, PANI is colourless as the
electron transition at approximately 300 nm, i.e. in UV-region, is observed. This
transition is preserved in all oxidation states of PANI and interpreted as excitation from
valence band to conduction one. This excitation is also responsible for the π-π*
transition in aniline [54]. In the intermediate emeraldine salt oxidation state two
additional peaks appear in UV-Vis region, at approximately 400 and 800 nm, but in the
fully oxidized state instead of these peaks a broad band at approximately 600 nm is
observed. The colour of PANI during this transition changes from green to blue. The
analogous change of UV-Vis spectra is observed during the transition from substituted
phenylenediamines to their cation-radicals and to appropriate quinone diimines:
The authors [55-57] paid attention to this fact and used it for the interpre
of the PANI redox transitions. Besides, the two transitions in the spectru
emeraldine salt are in good agreement with the predicted transitions for pol
structure of PANI [58], and the transition in fully oxidized state corresponds
predicted formation of the molecular exiton [59, 60] for which the presence
quinone diimine units is necessary.
The PANI structure changes were investigated in situ by FTIR-spectrosco
HCl aqueous solution with different рН values [11]. The oxidation from LE with
first peak on the cyclic voltammogram leads to the decreasing of N-H stre
vibrations intensity and to the transition from benzenoid structure to partially qu
one. Essential changes in the intensity which would be caused by the vibratio
anions do not take place. These facts allow interpreting the first oxidative proces
oxidation connected with the deprotonation, but without the exit of anions.
interpretation agrees with “strong acid” variant of the scheme. Further PANI oxi
(peak 3) leads to the increase of the intensity of N-Н vibration bands [11]. This a
adding to the scheme (1.4) the oxidation reaction of the emeraldine salt with the en
anions to the PNA salt. It is necessary to do this as the deeper oxidation will requi
process of deprotonation in accordance with the scheme (1.3).
However, in organic media the redox mechanism of PANI does not inclu
proton exchange with solution on which the possibility of multi-sweep cycling o
polymer in the system with lithium counter electrode indicates [1]. Althoug
conclusion contradicts to some of the given above results. The facts of monot
NH B NH B BBB NH B NHB +.B N Q N
21
(1.6)
tation
m of
aronic
to the
of the
py in
in the
tching
inoid
ns of
s like
This
dation
llows
try of
re the
de the
f this
h this
onous
Chapter 1. Literature review
growth and decreasing of the electrode mass during oxidation and reduction,
respectively, in the LiClО4/propylene carbonate electrolyte presuppose the participation
of only anions in the PANI redox transitions [61]. These data explain the mechanism
which combines the left top and right bottom reactions of the scheme (1.4) for the aprotonic solution [57]. It should be noted that in such media non protonated emeraldine
also exhibits electroactivity which can be described by the following reaction [55]:
The electrochemical PANI transitions are accompanied by changes
conductivity. As it is shown by in situ carried resistance measurements on direct c
in aqueous [56, 62] and nonaqueous (propylene carbonate) [61] electrolytes a
impedance measurements on alternating current in aqueous solution [63] the cond
PANI form is only the intermediate oxidative state of protonated PANI, i
emeraldine salt (Fig. 1.2). Beginning at certain potential, both reduction and oxi
of this form of PANI lead to the sharp growth of electric resistance. Cathod
anodic limits of the existence of the conducting PANI form are potentials of peak
and 3-3’, respectively: these limits are being shifted at changes of pH solution to
with the peaks on the cyclic voltammograms (Fig. 1.7).
Figure 1.7. Influence of pH on the PANI resist
(Е = 0.35 V vs SCE, рН 1) [44].
рН: 1 – 1.0; 2 – 2.6; 3 – 3.6; 4 – 4.4; 5 – 5.0
The PANI conductivity in reduced and oxidized states can be determine
electrochemical data: oxidation and reduction of compounds from the solution
electrode surface covered with PANI occur only in the cases when these reaction
NH B B
NH
NH BN Q N
..+ +
- 2e+ 2A
--
--BNH BN Q N
AAQ
..
22
(1.7)
of its
urrent
nd by
ucting
.e. the
dation
ic and
s 1-1’
gether
ivity
.
d from
on the
s take
Chapter 1. Literature review
place in the potential range of polymer electroactivity [30]. The charge transfer through
PANI in a wider potential range (at potentials less than -0.2 V and more than 0.8 V vs
SCE) [64] were not confirmed in the mentioned work [30]. Due to the presence of conductivity in limited range of potentials PANI was called a “chemical diode” [65] and
this property was used for creating the light-emitting device [66].
Besides conductivity, magnetic properties of PANI also change during the
redox PANI transitions: the maximums of intensity of EPR signal are in agreement with
the current peaks on cyclic voltammograms [67-69] (Fig. 1.8). In addition, the character
of magnetic sensitivity changes. These phenomena find satisfactory explanation within
the limits of the scheme (1.4).
Arguments in favour of the scheme (1.3) of the redox PANI transitions have
been reported in the literature [6, 22]. However, the scheme (1.3) cannot be accepted as
basic for the PANI redox transitions, since in some cases the schemes (1.2) and (1.4) are
realized too.
Also, it should be taken into account that the PANI electrochemical behaviour
(i.e. the form of cyclic voltammograms) in aqueous media significantly depends on the
acid anion which is present in the solution [64, 70-72].
Figure 1.8. Cyclic voltammogram (1);
EPR absorption and injection current
(2) for the PANI film on a platinum
electrode in 0.5 M H2SO4. Potential
was scanned between -0.2V and 0.8 V
vs SCE at 10 mV/s [67]
The results considered above show that the process of the aniline
polymerization is rather a complicated one, and runs through many stages. The rate of
each stage depends on many factors (pH of the solution, potential, temperature, solvent,
etc.). It should be also noted that all given above schemes do not take into account the
nature of the acid anion.
23
Chapter 1. Literature review
1.1.3. Thermal, dielectric and mechanical properties of polyaniline
Thermal properties. Determination of optimal processing conditions, under
which proper conductivity and physical properties are allowed, is the most important for
the PANI applications. To study the PANI processing behaviour and to select the
suitable materials for the application of a given polymer it is necessary to take into
account its dimensional stability and thermal history [73]. These properties - thermal
stability and degradation behaviours – are useful in modifying polymers for new
applications. Investigations of the thermal stability of conducting PANI are, hence, of
great importance. But the question of the PANI thermal stability hasn’t been given
enough consideration up to now.
Several researchers [73-76] have studied thermal stability of PANI in both
conducting and insulating forms by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The typical thermogram of PANI shows a three-step
weight loss process. The first step occurring in the range of 65-125 0C is attributed to
the evaporation of water or solvent molecules from the polymer [73]. The second-step
weight loss occurs between 125-350 0C and is due to the loss of low molecular weight
polymer and unbounded dopant ions from a PANI chain [74]. The third-step weight loss
occurs between 350 and 520 0C and is due to the degradation of the main PANI chain
after the elimination of bounded dopant [75]. But it should be mentioned that the
thermal stability of doped PANI is dependent on the counter anion. For example,
methane sulfonic acid (MSA) doped PANI was found to be stable up to 250 0C [70].
Dielectric properties. Only few articles deal with the dielectric properties of
the conducting polymers although the dielectric function ε*(ω) can provide information
about transport mechanism in the system. But a few studies which can be found in the
literature show that the response of PANI to the electric field depends on many factors
(temperature, electric field frequency, acid doping, water content) [77-80].
The dielectric response is generally described by the complex permittivity
(1.8), where real ε’(ω) and imaginary ε”(ω) components are the storage and loss of
energy in each cycle of applied electric field.
ε*(ω) = ε’(ω) - iε”(ω)
It was found that the values of ε’(ω) were very high at low frequency an
temperature but they are relatively constant at high frequency [79]. Such high
)
24
(1.8
d highvalues
Chapter 1. Literature review
may be due to the interfacial effects.
The increase of the dielectric permittivity with doping is a result of
contributions from the backbone emeraldine base and the formed polaron and
bipolarons to the polarization [79]. During the study of ac-conductivity and dielectric
permittivity of emeraldine base and doped PANI it was found that at frequencies below 100 Hz for y = 0.07 (Fig. 1.1) the dielectric permittivity is seen to increase much more
rapidly as compared with y = 0 and y = 0.03 [80]. This fact can suggest stronger
coupling between polarons. It was revealed the presence of the conductivity relaxation
with the help of the dielectric modulus representation M*(ω). Thus, it was shown that
the increase of the dopant concentration leads to an increase in the distribution of
relaxation times [79] and the impedance strength decreases while the relaxation times
become shorter. So, it is concluded that this relaxation times behaviour suggests
multiple paths for the system to relax due to the diffusion of polaron and bipolarons
[80].
Also, it was discovered that dielectric properties of PANI give a good
agreement with the physicochemical parameters which are correlated to statistical
distribution of insulating and conductive segments along the chain [78].
Mechanical properties. As it is known, the main disadvantages of PANI are
its poor processability and mechanical properties caused by its backbone stiffness. It
was found that the mechanical property of PANI can be substantially enhanced when it
was prepared from the gel and after the crosslinking [81, 82]. The influence of the PANI
oxidation state on the mechanical property was also established [81]. It was found that
the tensile strength of EB film was improved by about 25% upon heating to 150 0C.
This improvement was attributed to the interchain crosslinking of PANI at high
temperature and the formation of the PANI aggregates through interchain hydrogen
bonding. The tensile strength of LE improved only upon its exposure to air at 200 0C,
but, at the same time, the colour of the film changed from yellow to blue [81]. The XPS
results revealed the conversion of amine units to imine ones at this temperature [83].
Also, with the help of XPS analysis it was found that the mechanical property of the
PANI film is closely related to the oxidation state of the polymer and the degree of
crosslinking [83].
The film orientation also was found to influence the mechanical properties.
25
Chapter 1. Literature review
Monkman et al [83] showed that the tensile strength increases with elongation which
indicates the alignment of polymer chains along the stretch direction. X-ray diffraction
analysis suggested [82] a possible link of the mechanical behaviour and changes in
crystallinity with elongation.
1.1.4. Influence of acid-dopants on the polyaniline preparation and
properties
The rate of polymerization as well as conductivity and others properties of
PANI strongly depend on the used acid-dopant [84-86]. It is known that the electrical
conductivity is influenced by the size and shape of the counter ion [70].
On one hand, higher the penetration of counter ions in the formed film is, the
higher its conductivity is. But, on the other hand, the small molecular weight anions are
more quickly removed from the polymer. That’s why the choice of dopant might be a
very important factor – if we need thermally stable conducting PANI we should use
larger molecular weight acids, if the high value of conductivity is more important – it is
better to use small protonic anions. The size of counter ion can also affect the interchain
distance and, correspondingly, intermolecular interaction that can change the disorder in
the polymer system [87]. Therefore, the choice of the dopant depends on the PANI
application.
It should be mentioned that the rate of the aniline polymerization also changes
depending on the acid in such sequence: HCl < HClO4 < HBF4 < HF < H3PO4 < H2SO4
[86, 88]. The acid nature at the same doping level determines also the PANI
conductivity as one can see from the results in Table 1.1.
Therefore, it is shown the same anion differently influences the rate of
polymerization and the conductivity value of formed PANI.
In recent studies of PANI doped with various protonic acids [70, 89, 90], it was
found that hydrochloric and sulphuric acids are the best dopants in terms of stability of
conductivity. Kiattibutr et al [91] producing sensors for SO2-N2 mixtures found that at
the same doping level specific conductivity of PANI-HCl is greater as compared with
PANI-CSA because of two factors: a more closely packed crystalline mobility, and the
ability to absorb more water molecules which induced ionic conductivity.
But, in the other work [92] the opposite results can be found – the authors
showed that poly(o-methoxyaniline) (POMA) doped with HCl is easily
26
Chapter 1. Literature review
Table 1.1. – Influence of the anion nature on the PANI conductivity [86]
Anion Doping level Conductivity, Ohm-1⋅cm-1
CF3COO- 0.33 0.7
BF4- 0.34 0.8
ClO4- 0.31 2.0
Cl- 0.36 1.4
NO3- 0.33 1.9
SO42- 0.23 0.5
deprotonated in comparison to polymer doped by p-toluene sulphonic acid (TSA). The
Cl- anion is a weaker base than p-toluene sulphonate, thus, Cl- ions are more easily
removed [92].
Also, amic acids were found to be dopants for PANI [93]. PANI reacted with
the selected amic acid by mixing the N-methyl pyrrolidone (NMP) solutions of the two
materials in appropriate ratios. It was found that the conductivity in all the polymer
matrices was limited by geometric limitations between the two polymers.
Phosphoric acid diesters with long alkyl substituents are also very good
candidates for protonating agents which induce solution and melt processability of
conducting PANI [94]. Due to their lower pK value, compared to the parent acid, they
are sufficiently acidic to protonate emeraldine base. Their hydrophobic alkyl chains also
induce solubility of PANI. Phosphoric acid esters are known as plasticizers for a variety
of polymers. Thus, the esters can serve as plasticizers and protonating agents for PANI.
Protonation of PANI was achieved by treatment with diester [bis-(2-ethylhexyl)
hydrogen phosphate] (DiOHP) and diester [bis-(2-methylpropyl) hydrogen phosphate]
(DiBHP) dissolved in an appropriate solvent (toluene, decaline, chlorinated
hydrocarbons, m-cresol) or by mechanical mixing of PANI with neat diester.
Phosphoric acid esters are also known to form strong intramolecular H-bonds [94].
The PANI-dopant interaction can be related to an increase in the polymer
molecular weight, the polymer crystallinity and/or the molecular conformation of doped
PANI that changes from a compact coil to an open coil-like structure.
Not only conductivity, but also electrochromic properties depend strongly on
the acid-dopant. Gazotti et al [92] established that POMA doped with TSA presents a
higher optical contrast in the visible region than the same polymer doped by HCl.
27
Chapter 1. Literature review
Many researchers also reported that the dopant mixtures induced high
conductivity and processability [90, 95]. Dopant mixtures gave higher conductivity to
PANI than a single dopant, because they could provide an effective conjugation length
and a high protonated level.
It should also be pointed out that not only dopant can influence the PANI
properties, but also the used solvent. As it was shown, the conductivity of the PANI
samples varies with dopants (TSA, CSA, benzene sulphonic and naphtalene sulphonic
(NSA) acids) in the same solvent [95]. However, using different solvents (m-cresol,
chloroform, 2-hyroxybutanol (BeOH)) conductivity with the same dopant dramatically
changes [95], which indicates that transport properties can be controlled by both
dopants and solvents.
Taking into account a specificity of used systems, the observed divergences in
obtained experimental results may be explained by using different polymerization
methods and conditions. Significant fact may have been here also attributed to the
different PANI-dopant interactions.
1.2. Synthetic methods of the polyaniline preparation
PANI is synthesized by the chemical or electrochemical oxidative
polymerization of aniline [6]. There exists also a method of plasma polymerization [96],
but this method is not very convenient and not easy in application. So, we will speak
only about two ways of the PANI obtaining – chemical and electrochemical
polymerization.
It is possible to select such conditions under which PANI is the main reaction
product. In this case the chemical and electrochemical ways of the PANI preparation
produce polymers of the same composition since their electrochemical behaviour is
practically identical [27, 97-99].
1.2.1. Chemical way of obtaining polyaniline
The chemical methods of the aniline polymerization are based on the aniline
oxidation using suitable oxidants such as ammonium persulphate (APS) (NH4)2S2O8,
sodium chlorate NаСlO3, potassium dichromate К2Сг207, Fenton reagent, hydrogen
peroxide H2O2, etc. [99-103] in solutions containing mineral or organic acids. As a
28
Chapter 1. Literature review
result, dark green product is obtained, which because of its colour received the name
“emeraldine”. The treatment of this product by alkaline or ammonium solution converts
it into dark blue emeraldine base (Fig. 1.2). Under these conditions emeraldine can be
received quantitatively: if oxidant is taken on the basis of 1.25 mol per one aniline
molecule, so the yield reaches 97% [16]. Rather similar values have been found recently
[84]: 1.15 ± 0.04 mol of APS per mol of aniline. These results are in good agreement
with the emeraldine formula proposed on the basis of elemental analysis and
quantitative ratio [22] including quinoneimine units 1/4, brutto formula С6Н4.5N:
С6Н5NН2 + 1.25 [О] → С6Н4.5N + 1.25 Н2О.
This formula is, certainly, approximate, but is close enough to reality.
The aniline oxidation reaction is an exothermal reaction with an induction
period [104]. It was established that the presence of PANI leads to the decrease of
induction period time, i.e. the chemical oxidation as well as electrochemical one (look
part 1.2.2) is an autocatalytic process.
The properties of formed PANI are practically independent of the chosen
oxidant, but only under conditions when the oxidant is not taken in excess. In the case
of oxidant excess the decrease of the PANI yield is observed which is due to the PANI
decomposition [84]. Moreover, the polymer formed is a little more oxidized and as a
result the value of its conductivity is lower [84].
1.2.2. Electrochemical way and the mechanism of the polyaniline synthesis
Like many other conducting polymers, PANI can be easily obtained in the film
form on the electrode surface during anodic oxidation of the monomer solution at the
appropriate potentials. Formation of the film with high adhesion to the electrode is
essential for the PANI use in electronic devices and electrochemistry [105].
More often PANI is obtained from the aniline solution in aqueous 0.1-2.0 М
acid solutions as in this media polymer is the main reaction product. The beginning of
oxidation is observed at 0.45-0.65 V (vs SCE) on platinum electrode [64, 89, 106]. A lot
of articles have also been devoted to the PANI electrochemical synthesis in nonaqueous
electrolytes [22, 107-109]. The mechanisms of the initial oxidation have been proposed
[6, 60]. As it has been found from the EPR measurements in the BN media, the
intermediate nature depends on the potentials used [22]. Thus, at 0.7 V (vs Сu/СuF2) the
intermediate is paramagnetic, it is considered to be С6Н5NH2+., whereas at + 1.0 V it is
(1.9)
29
Chapter 1. Literature review
diamagnetic and on the basis of experiments authors [108] have supposed that it must
be the nitrenium cation (С6Н5NH+). The properties of the obtained polymer also depend
on the maximum value of anode potential (Fig. 1.9).
Figure 1.9. Cyclic voltammograms of PANI synthesized at high (а) [29] and
low (b) [47] potentials (vs SCE). Media – aqueous solution of sulphuric acid 0.24 М (а),
рН 1.75 (b). Sweep potential rate 50 mV/s
The process of the PANI formation is characterized by three couples of peaks
which were displayed on cyclic voltammograms in the case of the high anode potential
limit, which is striving for augmentation of the polymer growth rate (Fig. 1.9а). Such
product will be obtained in aqueous as well as in the BN media. However, at lower
potential couple of peaks 2-2’ is absent (Fig. 1.9b) in both aqueous [27, 44] and BN
[108] electrolytes. The limitation of the current densities leads to the same effect [28,
32]. Only in solutions of aqueous hydrofluoric and trifluoro acetic acids it is impossible
to avoid this couple [34]. The couple of peaks 2-2’ can be connected with intermediate
processes, including the cation radical formation in polymer volume as well as the
degradation of the formed polymer. An absence of this couple on the cyclic
voltammogram of previously chemically synthesized PANI [27] can be associated with
careful polymer washing from soluble impurities before the electrochemical study. The
aniline polymerization can take place with noticeable rate at much lower potentials
beginning at 0.55 V (vs SCE) after short polarization at 1 V (vs SCE) which leads to the
formation of the primary PANI film layer [110]. The peak of the aniline oxidation
completely disappears already after 1-2 cycles during the synthesis in cyclic
voltammetry mode, although the polymer growth on the electrode surface continues as
30
Chapter 1. Literature review
the area of cyclic voltammograms grows up (Fig. 1.10). The method of redox catalysis
[111] can be used for lowering the potential of the PANI formation; however, such
complication is, perhaps, unnecessary because of autocatalytic reaction behaviour.
Figure 1.10. Cyclic voltammograms of the PANI electrochemical synthesis. Media –
aqueous solution containing aniline (0.1 М), Nа2SO4 (0.5 М) and Н2SO4, рН 1.0 [114].
Potential sweep rate 50 mV/s. Potential – vs SCE:
1 — first; 2 — second; 3 — tenth cycle
The rate of the electrochemical polymerization reaction is directly proportional
to the aniline concentration and to the concentration of the acid [112]. It has been also
determined that the limitation stage of the process is the aniline oxidation resulted in the
formation of cation radical. The measurement performed in galvanostatic mode also
confirms the existence of induction period [109]. The obtained results again confirm the
above assumption that acid anions also take part in the aniline polymerization process
and they can considerably influence the rate of polymerization.
However, the data of the redox PANI transitions mentioned above do not give
a simple explanation of the mechanism of the aniline oxidation process. Among
different proposed mechanisms of polymerization the most real one seems to be a
mechanism of the chain radical polymerization (scheme 1.10). According to this scheme
after the cation radical formation at an electrode surface, the reaction of cation radical
with monomer molecule and the loss of proton take place. Further oxidation of aniline
monomer and the growth of the polymer chain take place after repeated oxidation of the
dimer and the loss of proton. The confirmation of the fact that the aniline
polymerization process is accompanied with the process of proton exchange was found
31
Chapter 1. Literature review
Scheme 1.10.
а) monomer oxidation:
b) loss of proton:
c) growth of radical:
d) repeated oxidation and proton loss:
e) further chain growth:
in the article [113].
The stability of the cation-radicals is a very important factor for the
electrochemical polymerization process. Cation radicals of a middle stability display a
chemical selectivity toward the chain polymerization [12]. More stable cation radicals
diffuse from the electrode surface that leads to the oligomers and to the PANI
formationin the solution volume. Less stable cation radicals are more reactive.
Therefore, they react with the solvent and with other nucleophilics near the electrode
surface before they had time to diffuse out from the electrode. Thus, the electrochemical
aniline polymerization in the presence of high nucleophilic agents (halogens,
NH
H-e +-
NH
H. . .
+
H
H+
- HN
H
N+N
H
H.. NH2
H +. . ..
++
NH
H - H;
.. NH2
+H
. H
HN
H
N. ..- 2 e
. H
HN
H
N.+ +
-
. H
HN
H
N.+ +NH2+ ..
.N
H
N.
H
NH2. . ..NH2+ ..
Polymer
-2H+
-2e-
32
Chapter 1. Literature review
hydroxides) can lead to a decrease of the polymer yield and molecular weight and, also,
to the formation of soluble products.
As for subsequent stages of the polymerization process, the following
mechanism has been widely spread in the literature [6, 86, 111, 113]. Firstly, PNA
formation takes place. Secondly, pernigraniline further transforms to the final product –
emeraldine. The transition of PNA to emeraldine is a reduction process, in which PNA
plays a role of the electron transfer mediator as it is rather strong oxidizer. According to
this mechanism an anilinium cation formed in the course of the aniline polymerization
(scheme 1.11) [115]. This cation is very reactive and takes part in the process of the
PANI synthesis too. Such mechanism of polymerization received the name of a
“catalytical process” [86].
This mechanism seems to be more probable than that proposed by Sasaki et al
[37] according to which all aniline molecules are oxidized to cation radicals. This cation
radical is formed during the electrochemical oxidation process on the electrode surface
according to the scheme 1.12. Such high reactivity of rather delocalized polymer cation
radical gives rise to doubts.
Proceeding from catalytic mechanism of the aniline polymerization, it was
(1.12)
(1.11)
NH NH+.
n+1
N y + NH3
+NH+.
y + NH2
+.
-e, -H+-PNA E
NH n NH
+.+ H2N
- H+
NH n NH +
NH
H+- H- e-
NH n+1 NH - e
33
Chapter 1. Literature review
established that kinetics of the growth of subsequent polymer layers is influenced by
structure of the formed PANI layer [29, 106].
The obtained results [9] confirm the catalytical mechanism that includes the
stage of the PNA formation. According to these results the induction period during the
polymerization process is observed at the aniline oxidation in 0.5 М aqueous solution of
sulphuric acid on a gold electrode surface in the potentiostatic mode (Fig. 1.11). It can
be supposed that the whole quantity of electricity in this initial period is consulted for
the formation of double electric layer, for the adsorption process and for the formation
of oligomer layer on the electrode surface. Only after the formation of this double
electric layer, i.e. after the induction period, the current increase begins.
Fig. 1.11. Dependency of current density і and charge q on time during the
potentiostatic PANI synthesis [9]. Gold electrode, potential 1 V (vs standard hydrogen
electrode (SHE))
1, 2 — 0.5 М Н2SO4 + 0.01 М aniline; 3 — 0.5 М Н2SO4 without aniline,
I — double layer; II — adsorption; III — oligomer formation;
IV — nucleation, polymer growth
All these mechanisms, as it was already said, do not take into account the
anion, though the morphology of formed PANI depends on it [48, 52, 61, 64]. Granular,
porous structure is characteristic of PANI formed in aqueous solutions of perchloric and
acetic acids, and PANI obtained in sulphuric, hydrochloric or nitric acids has dense
unporous structure
34
Chapter 1. Literature review
1.2.3. Comparison of chemical and electrochemical methods of the
polyaniline synthesis
The chemical way of synthesis is rather simple and cheap in comparison with
electrochemical one. However, the electrochemical method has several advantages over
the chemical polymerization.
Firstly, it is more preferable when the polymer product is intended to be used
as a polymer film electrode, thin layer sensor, or in microtechnology because the
potential or current control is a precondition for the production of high-quality material
and the polymer film that serves as an anode during synthesis is formed at the desirable
spot.
Secondly, the electrochemical way of the PANI synthesis makes it possible to
receive films of different thickness and morphology by a simple change of electrolysis
conditions, electrolyte composition and temperature.
Thirdly, the current yield is near 100% and this permits receiving films of
necessary thickness.
At last, fourthly, the electrochemical polymerization enables us to avoid by-
products of the process implying that PANI obtained in this way is pure, without any
impurities.
Potentio- and galvanostatic modes of the PANI synthesis can be considered to
be close to their final results as the current and potential reach steady value after some
transition period. These methods are rather simple in realization. However, it was noted
that films formed in the cyclic voltammetry mode have a better adhesion to electrode
[48], better optical characteristics, they are more homogeneous and dense [85]. The film
homogeneity decreases with increasing the potential sweep rate [42]. The most
qualitative and homogeneous PANI films are formed with decreasing the aniline
concentration [85].
At the same time, electrochemical method has rather essential disadvantages –
the size of the conducting films formed by this method depends on the electrode
surface, which is not very convenient for obtaining such films on a large scale. Besides,
it is rather difficult to obtain thick, more than 150-300 nm, homogeneous and dense
PANI films.
35
Chapter 1. Literature review
1.3. Mechanism of electrical conductivity in polyaniline
The conductivity of a material depends on its electronic energy level structure.
In a crystalline solid, the degenerate atomic energy levels of atoms or molecules are
combined to form non-degenerate energy band. The width of these bands depends upon
the strength of interaction between atoms or molecules, and the wave functions
describing electrons in these band states extend over the solid. The energy difference
between the highest occupied band (valence band) and the lowest unoccupied band
(conduction band) is called the band gap. The electrical properties of conventional
materials depend on how the bands are filled. Conduction occurs when an electron is
promoted from the valence band to the conduction band, but this can not occur when the
bands are empty or full [116]. If the band gap is small, then thermal excitation can be
enough to give rise to conductivity. That is why it happens in conventional
semiconductors.
The electronic ground state of the conducting polymers is that of an insulator,
with a forbidden energy gap between the valence and conducting bands. The
conductivity of these polymers is transformed through the process of doping [10]. The
term doping is derived by analogy with semiconductor systems. However, in contrast to
semiconductor systems, doping does not refer to the replacement of atoms in the
material’s framework. Doping in the case of a conjugated polymer refers to the
oxidation or reduction of the π-electronic system, p-doping and n-doping, respectively,
and can be effected chemically or electrochemically. To maintain electroneutrality,
doping requires incorporation of a counter-ion. Moreover, the conductivity of a polymer
can be increased by several orders by doping it with oxidative/reductive substituents or
by donor/aceptor radicals. Doping is accomplished by chemical methods of direct
exposure of the conjugated polymer to a charge transfer agent (dopant) in the gas or
solution phase, or by the electrochemical oxidation or reduction.
PANI holds a special position among conducting polymers in that its most
highly conducting form can be reached by two completely different processes – protonic
acid doping and oxidative doping. Protonic acid doping of EB units with, for example,
1M aqueous HCl results in a complete protonation of the imine nitrogen atoms to give
the fully protonated emeraldine hydrochloride salt [17]. Upon protonation of EB to ES,
36
Chapter 1. Literature review
the proton induced spin unpairing mechanism leads to a structural change with one
unpaired spin per repeat unit, but with no change in the number of electrons [94]. The
same doped polymer can be obtained by chemical oxidation (p-doping) of LE base
[117]. This actually involves the oxidation of the σ/π system rather than just the π
system of the polymer as it is usually the case in p-doping [118].
Neutral (non-doped) PANI – leucoemeraldine (Fig. 1.1) – is a dielectric with
the width of band gap 3.9 eV [119]. During oxidation (doping) π-electrons are removed
from the top levels of valence band and the shift of the boundary π-levels (higher than
occupied and lower than valence bands) occurs towards less energies. The gap between
boundary π-levels (forbidden band gap) becomes less (its value is ≤ 2.7 eV [120]) and
the polymer becomes a semiconductor.
The electrical conductivity and other physical properties of doped PANI are
usually explained in the frame of polaron-bipolaron theory [121]. H. Reiss [18]
proposed for the highly conducting state the structure of polaronic lattice which is in
good agreement with theoretical calculations and optical properties of the polymer.
Figure 1.12 provides a schematic representation of the effect of dopants on the
conductivity of materials with reference to the band gap theory.
Polaron energy band
Bipolaron energy band
d
Undoped polymer
Slightly doped polymer
Heavily dopedpolymer
Figure 1.12. The effect of dopants on the conductivity
The charge carriers’ nature (polaron and bipolarons, wh
doping process) is of special interest in the conduction mechanis
point of view, a radical cation (caused by a dopant acting as an ele
partially delocalized over some polymer segment is called a pola
medium around it. If another electron is removed from the pol
happen. If it is removed from a distant section of the chain it f
Incr
easi
ng e
nerg
y
37
Energy level in
conduction ban
Energy level in
valence band
of PANI
ich are formed during
m. From the chemical
ctron acceptor) that is
ron, as it polarises the
ymer, two things can
orms another polaron,
Chapter 1. Literature review
which is independent of the first. If the electron goes out (removing an unpaired
electron) it forms a dication, which is called a bipolaron. The charged fragment can
travel along the polymer chain by rearrangement of double and single bonds. Low
doping levels give rise to a polaron, while high doping levels tend to give rise to a
bipolaron [116].
The presence in the PANI structure of the basic nitrogen atoms, which are
capable to exchange protons with the media, leads to the possibility of injection of large
quantity of charges, i.e. to acid doping (Fig. 1.13).
Figure 1.13. Conductivity-pH dependency
during emeraldine doping [17]
The intensity of an EPR signal is directly proportional to the concentration of
paramagnetic centres and is also connected with the potential according to the Nernst
equation (1.5): signal intensity increases by one order at potential shift byapproximately
59 mV toward anode region at room temperature [122].
During the further PANI oxidation the EPR signal intensity (look Fig. 1.8,
curve 2) passes through the maximum and sharply decreases [17, 39, 122] — according
to the current on cyclic voltammogram (Fig. 1.8, curve 1) [9, 67], the conductivity value
increases as the potential approaches to the emeraldine salt form. With the growth of
polaron concentration on the polymer chain the polaron lattice is formed.
The conductivity mechanism has been widely investigated for solving both
fundamental and practical problems. To explain the mechanism of conductivity on the
basis of carried out measurements the following models have been proposed:
- fluctuation-induced tunneling (FIT) model;
- charge energy limited tunneling (CELT) model;
- variable range hopping (VRH) model.
In FIT model, regions of metallic conductivity separated by insulating barriers
are assumed, the voltage across which shows large thermal fluctuations. This model was
applied successfully to highly conducting polyacetylene and polypyrrole [123]. Also,
38
Chapter 1. Literature review
FIT model was used to the description of the thermal degradation of the electrical
conductivity of the PANI and polypyrrole composite materials containing polymers
with sulphonic and phosphoryl groups [124].
The CELT model, proposed by Sheng et al [125], describes the charge
transport in the system of metallic particles embedded in a dielectric matrix. It is
considered that the doped (protonated) PANI particles are conducting, and dedoped are
nonconducting. Kivelson [126] supposed that a variation of the conductivity of lightly
protonated emeraldine samples with temperature follows a power law.
The mechanism which is often proposed to explain the dc-conductivity in
disordered and amorphous materials is Mott’s VRH model [127]. The mechanism is
based upon the idea that carriers tend to hop more remote distances to sites which are
more advantageous energetically rather than to the neighbouring, but not advantageous
energetically sites. Besides, the VRH model was used to explain the conductive
property of PANI [128]. In the VRH model [128-130] the temperature dependence T of
conductivity σ follows the relation:
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠⎞
⎜⎝⎛−=
γ
σσ/1
00 exp
TT ,
where T0 is the Mott characteristic temperature and σ0 is the conductivity at T=∝
σ0 = e2⋅υ⋅R2⋅N(E)
and
)(30 ENkT
B ⋅⋅=
αλ ,
where e = 1.602⋅10-19 C is the electronic charge, υ = 1013 Hz is a phonon fre
[126], λ = 18.1 is a dimension constant [128], α is a localization length of the lo
states (cm), kB = 8.616⋅10-5 eV/K is Boltzmann’s constant, N(E) is the de
localized states at the Fermi level (cm-3⋅eV-1), and R given by
R = γ
απ
/1
)(89
⎥⎦
⎤⎢⎣
⎡⋅⋅⋅⋅⋅ ENTkB
is the average hopping distance (cm). γ is determined by the dimension of the
system. For the one-dimensional, two-dimensional and three-dimensional syste
equal to 2, 3 and 4, respectively.
The average hopping activation energy W (eV) can be estimated by the
)
39
(1.13
:
)
(1.14(1.15)
quency
calized
nsity of
)
(1.16studied
ms, γ is
known
Chapter 1. Literature review
hopping distance R and the density of states at the Fermi level N(E) by the following
relation:
W = )(4
33 ENR ⋅⋅⋅π
.
Recently, Li et al [129] have investigated the conductive mechanisms
doped and DBSA-doped PANI, which consist of microparticles and micr
respectively. It has been concluded that the conductive mechanism can be consid
be three-dimensional variable range hopping (3D VRH) for PANI-HCl an
dimensional (1D VRH) for DBSA-doped PANI depending on the formed p
structure.
Acid-dopant was also found to determine the hopping barriers. Thus
established from conductivity-temperature measurements that 2-acrylamido-2-m
propanesulphonic acid (AMPSA) produces lower hopping barriers than CSA
This can be due to comparative geometrical structures of AMPSA and CSA. The
is essentially a branched but flexible structure, while the latter is more spherical
likely to be a more rigid molecule. It is assumed [131] that the PANI-AMPSA
can be packed together more closely, giving rise to smaller interchain distanc
hence, lower barriers for carriers to surmount in hopping between chain
explanation was confirmed by X-ray studies [74].
It should be noted that the PANI conductivity may vary not only depen
the acid-dopant used, but also depending on quantities of residual water retaine
polymer. It is known that PANI in the salt form is very hydrophilic (scheme 1.1
In this case it has been assumed that hydrogen bonds are formed between the p
backbone and water molecules and affect the conductivity. The placing of the dr
salt into the environment containing water steam leads to an increase of its cond
by one-two orders [113]. The electronic conduction mechanism of proton exch
based on this phenomenon [132] described below.
In accordance with this mechanism the role of water consists in providi
)
NH2+ ..
+- e- -+ e
+
+
.
+ H2O NH H3O
NHNH
+
or
40
(1.17
of HCl-
ofibers,
ered to
d one-
olymer
, it was
ethyl-1-
[131].
former
, and is
chains
es, and,
s. This
ding on
d in the
8) [77].
olymer
y PANI
uctivity
ange is
(1.18)ng such
Chapter 1. Literature review
protonation of nitrogen atoms in polymer, at which the electron transfer between amine
and imine units of different chains would not demand the simultaneous proton transfer
and, thus, be easier [77].
1.4. Practical application of polyaniline
Ability to transfer a charge due to the presence of charge carriers in the PANI
structure causes its very important and various applications. Due to the redox process
combined with the intercalation of anions, the PANI chemical, optical and ionic
properties can be attached.
Materials for energy technologies. The electronic conductivity of PANI makes
possible to use it as a cathode in rechargeable batteries. There are many advantages of
these materials, such as the ease of fabrication, processability, low cost and light weight.
Batteries with the PANI electrode have long life, are rechargeable and can produce
current density of up to 50 mA/cm2 and an energy density of 10 Watt·h/kg [133].
Among the batteries, lithium ones are especially important since they show a high
discharging voltage (∼3.0 V).
The redox properties of PANI have conditioned its utilising as a cathode
material in rechargeable Li battery. But the poor charge-discharge capacity of PANI has
hampered its successful application in this respect. This limitation could be overcome to
some extent by incorporating active cathode materials into the PM [134].
Thin film deposition and microstructuring of conducting materials. In
microelectronics PANI may be applied as charge dissipaters for electron beam
lithography. Electron beam lithography is a direct writing method with a very high
resolution in the sub-micron range. The charging of the insulating electron beam resistor
can lead to a deflection of the electron beam and so it results in an image distortion. To
avoid the problem, conducting resistors or layers must be applied. Water-soluble PANI
was introduced by IBM as a discharge solution [135].
Electrooptical and electrochromic devices. The absorbance changes in PANI
make it quite attractive for fabricating electrooptical display devices [88, 94]. PANI
shows all colours as a result of its many protonation and oxidation forms (Fig. 1.1 and
1.2). The electrochromic properties [105, 120, 136] of this polymer can be easily
employed to produce a number of different electrochromic devices, such as display and
41
Chapter 1. Literature review
thermal “smart windows”. The smart windows absorb some of the sunlight, which
changes colour in response to sunlight or temperature changes, thus saving air
conditioning costs.
The use as a membrane. Due to its porosity PANI can be regarded as a
membrane. It could be used for separating gas or liquids. The free standing (on
supporting substrates) chemically prepared PANI films are permeated selectively by
gases [56]. In general, the larger the gas molecule is the lower the permeability through
the film.
Corrosion protection. PANI can be deposited as corrosion protection layer.
Favourite subject of investigations is mild steel, but dental materials are also discussed
[137]. The efficiency and mechanism of corrosion protection are not yet classified. On
iron an anodic protection is discussed [138]. Due to occurring redox processes, thicker
layers of iron oxide are formed and are then stabilized against dissolution and reduction.
An inhibition is also reasonable due to a geometric blocking and a reduction of the
active surface.
Sensors. In order to control air pollution and to detect combustible, toxic or
noxious gases at low levels, efforts are being made towards the development of simple
and inexpensive semiconducting oxide gas sensors. However, such sensors generally
operate efficiently only at 300 oC. In order to overcome this limitation, new materials
are being developed. It is observed that the vacuum deposited thin PANI films exhibit
excellent gas sensing properties [45]. The electrical conductivity, optical absorption and
electrical capacitance of the metal-polymer interface are strongly influenced by the
presence of certain gas molecules. These results have led to the development of gas
sensing elements for different substances, for example for gases like CO, NH3, HCl,
SO2, Cl2 and NO2, for glucose, urea, haemoglobin [45, 46, 139-141]. The thin-film
PANI-based gas sensing elements are inexpensive and operate at room temperatures
with satisfactory selectivity for these gases.
To realise the advantages of PANI having a rare combination of electrical,
electrochemical and physical properties, it is very essential to increase the PANI
processability, environmental and thermal stabilities. From recent studies on the PANI
family of polymers it has been demonstrated that these polymers could be highly
promising for many technological uses because of their chemical versatility, stability
and low cost. But in order to get a material suitable for application in various
technological fields one has to overcome certain limitations such as poor mechanical
42
Chapter 1. Literature review
properties and processability. Several approaches have been made by a lot of researches
to improve these properties. Among them: doping PANI by functionalized acids (such
as CSA, DBSA), using substituted PANI, and preparing composites with conventional
polymers. From our point of view, the last way, i.e. the preparation of conducting
polymer composites is the most promising, as in this case we obtain a material, which
combines mechanical properties of common polymers with the electrical conductivity of
PANI.
Main advantages of such composite systems are their rather low cost and
preserving main physico-mechanical properties of a dielectric matrix as in most cases it
acquires the necessary electrophysical properties already after addition of a small
quantity of the conducting polymer (1-10 wt.%).
1.5. Composite materials based on polyaniline and polyamide
Up to now it has been reported on a lot of composite systems based on
different common polymers such as poly(methyl methacrylate) (PMMA), poly(vinyl
alcohol) (PVA), poly(vinylidene fluoride) (PVDF), etc. [4, 5]. Nowadays, good
conductivity results are received in these systems. However, the composite materials
with the polymers of basic nature (specifically, polyamides) are not sufficiently studied.
But a few studies which can be found in the literature on this subject show quite
promising results [142-147].
It is known that the processing method may significantly determine the
properties of the manufactured composite materials. Well known methods to produce
the PANI containing composites may be essentially reduced to such groups [5]:
- chemical in situ polymerization of aniline in the subsurface layer of a
matrix or in the solution with a matrix polymer;
- dispersion polymerization of aniline in the presence of a matrix polymer in
the disperse or continuous phase of a dispersion;
- electrochemical polymerization of aniline in the matrix covering the anode;
- solution blending soluble matrix polymers and substituted polyanilines;
- dry blending followed by melt processing (mechanical mixing of doped
PANI with thermoplastic polymer, then melded in a hot press or extruder).
Each of these methods has its own advantages and limitations. The choice of
43
Chapter 1. Literature review
the used method in each case will depend on the necessary properties of obtained
composite materials.
Electrochemical polymerization of aniline in the polymer matrix. It is
known that the electrochemical polymerization in the PM will be realized only in
conditions of film swelling in proper conducting media for obtaining ionic conductivity
[148, 149]. The swelling process is realized by two ways:
- by the diffusion in pores in the PM film;
- by the swelling of the polymer film in proper conducting media
which contained aniline.
TGA analyses of potentiostatically synthesized composite film based on PANI
and aliphatic PA, coated platinum electrode [146] have revealed that the so-called
composite has a single phase as it is denoted by a single weight loss pattern. Although
the hydrogen bonding (due to C=O group of PA and N-H group of PANI) was clearly
seen in the solid state FTIR spectra, thermal properties could not explain such a strong
interaction. The DSC curves of the film showed one transition above 340 0С without the
PA transition. While the mechanical mixture of PANI and PA had the glass transition
temperature (Tg) of PA only. Obtained in this way polymer films also had very
reversible behaviour towards NH3 and HCl gases [146].
Chemical aniline polymerization in the subsurface layer of a polymer
matrix. This method allows obtaining such composite materials in which PANI is
located in a very thin subsurface layer of the polymer film. It was established that the
thickness of conducting layer and the conductivity of the composite film depend on the
polymerization conditions (method of synthesis, time of polymerization) [5, 143]. The
method of the chemical aniline polymerization allows obtaining conducting composite
materials with wide conductivity range – from semiconductors to conductivity of pure
PANI [150].
The method is based on the polymer film swelling in aniline [142] or in the
oxidant solution [35]. The swelled film is then put into the solution of oxidant or
monomer, correspondingly. The polymerization process takes place inside the film
surface layer. The PANI formation does not occur in the solution volume. The
properties of the formed composite materials depend on physico-chemical interaction
both of PANI with PA [144-147] and of the PM with aniline [151, 152].
44
Chapter 1. Literature review
Byun et al [143] established that the composite film based on PANI and
polyamide-6 (PA-6) prepared by this method in the presence of different acids-dopants
(HCl, DBSA, TSA, sulphosalicylic acid) consists of three layers. The outer two layers
are conducting composite layers and the inner layer is pristine PA-6. It was found that
despite the low percolation threshold (about 4 wt.%) such films have the value of
conductivity about 3.5⋅10-2 S/cm in the case of HCl. In addition, the existence of
hydrogen bonds between PANI and PA was also established [143]. These bonds can
decrease a little the PANI ability for doping [143]. The physico-chemical interaction
between PANI and the dielectric PA matrix was confirmed by the results of dynamical
mechanical thermal analysis results. Specifically, the deformation of the PA-6 crystal
structure because of the formation of PANI was revealed. The decreasing of the degree
of crystallinity (18.7% in comparison with 21.0% for pure PA) and of the heat of fusion
(49.4 J/g and 55.28 J/g for the composite and PA, respectively) [143] also testify to the existence of the interaction between PANI and the PA matrix.
The size of the acid-dopant anion influenced the behaviour of such surface
composite materials. Neoh et al established [142] that the Cl- counter-ions are easily
removed when the film is immersed in water causing PANI to revert to the base form.
At the same time, the film doped with larger anions (for example, sulphosalicylic acid
anions or polystyrenesulphonic acid anions) can remain in the salt conducting form after
being in water for extended time and under simulated weathering.
It should be noted, that produced by this method composite films owing to
small thickness (∼1-2 µm) [152] of the PANI containing layers can quickly react to
environmental influence, which allows applying them in sensor or optical devices [145].
The resistance of the PA-6/PANI composite films increased in the ammonium
containing environment, but after the exposition to ambient air they returned to the
conducting state. It was found [145] that the films doped by formic acid have better
sensor properties to ammonium gas in combination with high selectivity in comparison
with the films doped by other acids (sulphuric acid, maleinic acid, DBSA).
The level of interaction of the PA-6 and PA-12 films with doped and dedoped
PANI was established with the help of DSC measurements [147]. Electron microscopy
results demonstrated that even though the blends are phase-separated, the degree of the
PANI dispersion in the PA matrix is dependent upon the nature of the counterion of the
doping acid. The confirmation of this fact is the changes of the enthalpy of fusion of
45
Chapter 1. Literature review
obtained films (Table 1.2).
Table 1.2. Enthalpy of fusion (∆Hf ) of the composite PA-6/PANI and
PA-12/PANI films [147]
PANI (vol.%) ∆Hf (W/g), PA-6 ∆Hf (W/g), PA -12
0 75 80
10% PANI-EB 60 101
25% PANI-EB 41 130
10% PANI-MSA 67 88
25% PANI-MSA 42 77
50% PANI-MSA 18 73
10% PANI-CSA 71
25% PANI-CSA 62
50% PANI-CSA 63
10% PANI-DBSA 73 87
25% PANI-DBSA 66 99
50% PANI-DBSA 72 111
It was shown [147] that the conductivity of the composite films PA-6/PANI-
CSA does not depend on the content of the conducting PANI-CSA for contents higher
than 35% (wt/wt). Also, it was established that the rate of increasing conductivity
changes depending on the composition of the film. The value of conductivity for the all
PA/PANI films is increased at increasing PANI concentration up to a definite value. It
was found that the conductivity value reaches ≈10-2 S/cm even at 2% (wt/wt) of PANI-
CSA in the film of PA-6.
It was found that the value of conductivity is very sensitive to the morphology
of composite films. The results of optical microscopy showed [147] that films of PANI-
EB with PA-6 or PA-12 appear phase-separated while the blends containing PANI-CSA
appear homogeneous (and transparent for low PANI-CSA concentration) with no
obvious sign of phase separation. However, the phase-separated morphology of either
PA/PANI-CSA blends is evident in their transmission electron microscopy (TEM)
46
Chapter 1. Literature review
micrographs in which dense network-like structure of PANI-CSA in the host matrix is
clearly seen [147]. A study using small-angle neutron scattering (SANS) [153]
established that the blends with the smaller more polar dopants CSA and MSA behave
similarly and are unlike from either the PANI-DBSA blends or those with PANI-EB.
There is evidence that the simple picture of two pure phases is inadequate for these
materials. With the exception of the PANI-DBSA blend which has a relatively low
scattering contrast, the results indicate that the lower limit of volume fraction for the
application of SANS is a few percent PANI-emeraldine salt in PA-6. X-ray scattering
was used also to demonstrate the presence of the polyamide lamellae and residual peaks
attributable to the pure components [153].
Thus, although such distinctly different sulfonic acids (CSA, DBSA, MSA) do
not induce any pronounced interaction between the rigid PANI and the coil-like PA
chains, they do influence the degree of dispersion of PANI in the host polymer [147].
These factors are very important for the properties and the structure of the PANI
percolation network formed in the composite materials.
Analyzing the above mentioned methods of the PANI-containing composites
formation from structural, physical and chemical positions, one can say that the aniline
matrix polymerization (chemical or electrochemical) allows obtaining the composites
with the PANI percolation network distribution which is to a considerable extent
predetermined by the structure of the initial PM. The aniline matrix polymerization is,
obviously, a suitable way to produce materials with adjusted conductivity properties.
On the other hand, the fact that the structure of the initial PM influences the
properties of the final composite material is, probably, a factor limiting the conductivity
of the composite materials. Besides, the matrix polymerization is mainly suitable for
producing small-sized devices. If it is necessary to avoid these limitations, the aniline
polymerization in dispersion of the common polymer (the so-called dispersion
polymerization) [5] may be considered as an alternative way. Good homogeneity and a
low percolation threshold are characteristic of the composite materials obtained by this
method.
1.6. Research motivation
The results and data discussed above allow us making a conclusion that the
47
Chapter 1. Literature review
formation of composite materials based on PANI and common polymers opens new
possibilities for production of the materials with desired conductivity, mechanical,
thermal and other properties. Although a lot of such composite systems have been well
known, their industrial applications are still very limited, and in the case of the
important PA/PANI composites are unknown. This fact can be explained by a lack of
data on effects of the chemistry and physico-chemistry of the aniline polymerization
process in the presence of the matrix polymer on conductivity, thermostability,
mechanical, structural and other properties of the composite materials.
That’s why the investigation of the aniline polymerization conditions, the
investigation of kinetics and mechanism of this process in the different PA systems
(matrix and dispersion polymerization), physical properties of the formed materials are
necessary for a development of the production technology of the composite materials.
The main goal of this work is to obtain new composite materials of different
types by combining PANI and PA in order to take advantage of unique properties of
both materials. This goal will be reached through:
1. Investigation of the kinetic peculiarities of the chemical and electrochemical
aniline polymerization in the PM;
2. Establishment of the mechanism of the aniline polymerization process in the
PM films;
3. Development of methods of determining the real PANI content in the
composite materials;
4. Carrying out a systematic investigation of the influence of the composition
of the PM/PANI composites on the aniline polymerization process;
5. Studying a synthesis-structure-property relationship for the PA/PANI
composite system.
This study will also give a better understanding of the composite material
structure and provide an insight in the electrical properties of such materials.
48
Chapter 2
EXPERIMENTAL PART
Chapter 2. Experimental part
Introduction
In this chapter a brief description of the materials used throughout this work
will be provided. Also general details of the experimental procedures employed for the
electrochemical and chemical preparation of the composite materials described in this
thesis are included, as well as the electrochemical, spectroscopic, dielectric and other
methods of characterization used.
2.1. Materials and reagents
PANI as a conducting filler and PA as the PM have been chosen to be the main
system for studying the formation process and the properties of conducting polymer
materials. Specifically, the influence of the PM structure on the properties of the
obtained composites has been established for the most widely used in industry
polyamides of different structure (PA-6, PA-11, PA-12) and also for the matrix of PVA
and poly(ethylene terephtalate) (PET) (Fig. 2.1).
a) b)
c) d)
O C C
O O
O (CH2)2... ...
e)
Figure 2.1. The elementary chain of the PM:
a) PA-6; b) PA-11; c) PA-12; d) PVA; e) PET
C
O
NH... C
O
...(CH2)5 C
O
NH... C
O
...(CH2)10
C
O
NH... (CH2)11 C
O
...OH
CHCH2 CH2 CH...
OH
...
49
Chapter 2. Experimental part
The characterization of the polymers which were used as the PM is given in
Table 2.1. The reagents which were used in the course of the work are listed in Table
2.2. All solutions were prepared on distilled water.
Table 2.1. Characterizations of the used polymer materials
Thickness of the film or
particle size of the powder
Kind of polymer
Polymer form Thickness,
µm
Particle size,
µm
Producer
PA-6 film 25 Good Fellow
PA-11 BESHVO film 50 Arkema
PA-11 Rilsan B powder 14 - 42 Arkema
PA-12 film 50 Arkema
PA-12 Orgasol powder 5 Arkema
PA-12 powder 14 - 243 Arkema
PVA powder 15 - 30 Ukraine
PET film 50 Ukraine
Aniline purification procedure. Prior to use, aniline was distilled under
reduced pressure in the presence of zinc powder being a reducing agent. Such additional
purification permitted to remove and to prevent the formation of aniliniquinones and
azophenines [154], which coloured aniline solution in bright red colour. Purified aniline
and all aniline-contained solutions were stored at +3 - +5 0С in the darkness and in the
argon atmosphere for further prevention of the coloured substances formation.
Preparation of chlorine water solution. The water solution of chlorine in
accordance with the technique of [152] was used as one of oxidation solutions for the
chemical aniline polymerization in the PM. The process of saturation of pre-cooled
distilled water by chlorine gas was carried out by a dry Cl2 blowing through distilled
water for 2 hours at +20 0С till complete saturation of the solution with chlorine. The
solubility of chlorine in water is 7.5 g/l under these conditions [155].
It is known that Cl2, when dissolved in water, is hydrolyzed and the formation
of weak acid HClO and strong acid HCl takes place. The rate of this reaction is so high
50
Chapter 2. Experimental part
Table 2.2. Characterizations of the used reagents
Name of reagent
Formula
Grade of purification,
producer
Comments
Aniline
C6H5NH2
p.a., Aldrich
After additional
purification, look
above
Hydrochloric acid HCl analytical grade, Ukraine
Sulphuric acid H2SO4 analytical grade, Ukraine
TSA C7H10O4 98% monohydrate,
Aldrich
DBSA C18H31O3S mixture of isomers
С10 - С13, Acros
CSA C10H16O4S p.a., Acros
Acetone CH3COCH3 p.a., Ukraine
n-Hexane С6Н14 p.a., Ukraine
Formic acid HСОOH analytical grade, 99.9%,
Acros
APS (NH4)2S2O8 p.a., Ukraine
NMP C5H9NO Aldrich
Potassium chloride KCl p.a., Ukraine Crystallized twice
from distilled water
Water solution of
chlorine
Cl2 + H2O Preparation look
above
that the equilibrium is established immediately [155]. That’s why concentrations of Cl2
were determined by pH of the obtained solutions [156]. After the saturating process had
been finished, the obtained chlorine water solution was put into glass ampoules, which
were previously carefully washed by warm alkaline solution (60 0С) and distilled water
and then dried. These ampoules after filling by water chlorine solution were soldered
and stored in the dark.
51
Chapter 2. Experimental part
2.2. Preparation of the composite materials
2.2.1. Synthesis of polyaniline
The synthesis of pure PANI was performed in order to compare its spectral,
electrical, dielectric and thermal properties with the properties of the composite
materials.
Chemical polymerization. The emeraldine salt form of PANI was
synthesized chemically by the oxidative polymerization. According to [157], aniline (8
ml) was dissolved in 80 ml of 1.5 M HCl aqueous solution at 20 0C and then stirred
during 15 minutes to form an anilinium salt. Another solution containing 24.05 g of
APS in 50 ml of distilled water was prepared. It was then added to the aniline solution.
After the addition of the APS solution, the reaction mixture was stirred for further 15
hours. The precipitated PANI-HCl powder was collected under vacuum filtration and
washed with the distilled water to remove any impurities. The obtained powder was
dried under dynamic vacuum to a stable weight.
The emeraldine base powder was prepared by the alkaline dedoping of the
PANI-HCl emeraldine salt. This was accomplished by stirring the emeraldine salt in a
5% NH4OH aqueous solution for 10 hours. The powder was then washed and dried
under vacuum for 20 hours and stored in a refrigerator for later use.
Thus obtained the dedoped PANI powder was redoped by stirring for 24 hours
in 1M water solution of appropriate acid, producing PANI-acid complex. Finally, the
doped powders were dried under dynamic vacuum at 70 0C for 10 hours till a constant
weight.
Electrochemical polymerization. The electrochemical PANI formation on
the working electrode surface was carried out in 1M HCl water solution, containing
0.5M aniline. This solution was prepared by the dissolution of calculated aniline weight
in the necessary volume of the acid solution right before experiments.
2.2.2. The formation and pre-treatment of the polymer matrix
The PM in both film and powder forms were used for determining the
influence of the PM structure on composite materials properties.
The film samples were of two types:
52
Chapter 2. Experimental part
- commercial and industrial PA films (kindly donated by Arkema,
Serquigny, France and purchased from Good Fellow (PA-6));
- films which were made in laboratory:
- by casting method from the solution of the PM;
- by the compression-molding method from the powder.
Formation of films from the PM solution by casting. This method was
used for studying the process of the electrochemical aniline polymerization in the PM.
The solutions of PA and PVA were prepared. The solution of PVA was prepared by
dissolving 1 g of the PVA powder in 80 ml of hot water (65-70 0С) at continuous
stirring. Similarly, the PA solution was prepared by dissolving 1 g of PA-12 in 100 ml
of concentrated and warmed-up (40-50 0С) formic acid. Then, a certain volume of the
solution (0.015 ml) was deposited by syringe on the electrode surface for the film
formation followed by drying for 4 hours at room temperature.
It should be noted that PVA is known to be a water soluble polymer, so it was
necessary to convert it into a water-insoluble state in order to prevent its dissolution in
water solutions during the investigation of the electrochemical aniline polymerization in
the PM. It is known [158] that PVA loses its solubility during heating at temperatures
≈100-120 0С due to the cross-linking of the PVA macromolecules by the reaction of
intermolecular etherification (2.1).
The preliminary investigation of the character of weight mass changes du
heating at these temperatures showed that the complete water removal from the f
takes place in 2 hours. So, this time period was used for the PVA film formatio
further investigation.
Film formation by the compression-molding technique. The formatio
films by this method was performed by means of Specac device, which consists
hydraulic press and a digital temperature controller. The polymer powder of comp
material (∼0.2 g) was put in a rectangular form (S = 20.25 cm2) and then compress
(OHOH
CHCH2 CH2 CH ......- H2O
OH
CHCH2 CH2 CH ......
CHCH2 CH2 CH .....
OH
O
53
2.1)
ring
ilms
n in
n of
of a
osite
ed at
.
Chapter 2. Experimental part
controlled temperatures (195, 220 and 240 0С) under the load of 3 t during 1 min. The
thickness of the obtained film was ≈100 µm. The measurements of thickness were
performed with a digital micrometer.
2.2.3. Formation of the surface conductive composites
In order to obtain the surface layered conducting polymer composite, the PM
film was immersed into freshly distilled aniline at 20±2 0С in argon atmosphere. The
gravimetric measurements of weight changes of the PM during swelling process were
carried out by analytical balance ExplorerТМ (“OHAUS”, Switzerland). The previously
weighted film samples were placed in the aniline solution to be swelled during definite
time period. Then the films were taken from the solution, washed by n-hexane to
remove the surplus of aniline from the film surface and dried between two sheets of a
filter paper. After one minute exposition at ambient air the film weight was measured.
After this procedure the film was again immersed in the aniline solution, hold there
during a definite period of time depending on the desired aniline content. Then, all
operations described above were repeated.
The swelling degree α [159] was calculated taking into account the changes in
film weight:
%,100)(
0
0 ⋅−
=m
mmα (2.1)
where m0 and m are the weight of the primary and the swelled films, respectively.
The swelling process in the PM was performed till the achievement of the
definite wt.% of aniline in the film depending on research goals.
Swelled polymer films were then subjected to the chemical aniline
polymerization in different oxidant solutions – water chlorine solutions and solutions of
different concentrations of (NH4)2S2O8 in 1М water solutions of HCl or H2SO4. The
aniline polymerization process was controlled by optical UV-Vis measurements.
In order to obtain one-side surface composites a home-built cell was used (Fig.
2.2). The cell construction permits to block the other side of the PM from the oxidant
solution. The swelled polymer film was placed in the cell and the cell was filled with
the solution of oxidant. After the completion of the process the green transparent film
was taken from the oxidant solution, washed by distilled water and placed in Soxlet
apparatus for 6 h to extract with n-hexane the aniline livings and by-products. This
54
Chapter 2. Experimental part
Figure 2.2. Home-built cell used for the obtaining of the one-side surface composite
films
55
Chapter 2. Experimental part
procedure was followed by drying the film under dynamic vacuum up to a stable
weight.
In order to convert PANI formed in the PM into the non-conducting EB state
the film was treated with the aqueous solution of 5% NH4OH for 24 hours.
2.2.4. Formation of the bulk conductive composites
The bulk conductive composite materials were prepared on the basis of the PA
powders of different dispersion and structure (PA-11, PA-12). Inorganic (HCl and
H2SO4) and organic (TSA, DBSA and CSA) acids were used as acid-dopants. As TSA
was in a solid state, the concentration of the obtained acid solution was estimated by the
reverse titration by 0.1N solution of sodium hydroxide in the presence of methylorange
indicator.
In order to prepare a serie of the composite samples, different volumes of
aniline monomer are reacted with a constant weight of the PA powder.
For the preparation of anilinium salt the estimated amount of aniline was
dissolved in the necessary volume of proper acid solution. Thus formed solution of the
anilinium salt was vigorously stirred by magnetic stirrer for 0.5-2 hours. Then a definite
amount of the PA powder was added to this solution and thus obtained dispersion was
continuously stirred during one hour till the formation of homogeneous dispersion
mixture. After this, the solution of oxidant (water solution of APS) was added to the
dispersion.
The preliminary investigation has shown that the optimal condition of the
aniline polymerization in water dispersion of the PA powder for obtaining conducting
composite materials is the maintaining of the following molar ratios:
5.11
=acid
aniline ; 25.11
=oxidantaniline
Depending on the acid nature, in 0.1-1 hour the colouring of solution into dark
green colour took place, which is in the agreement with literature [104] and is
responsible for the formation of PANI in the emeraldine salt state. The obtained
dispersion was left for further 15 hours to be sure to complete the aniline oxidation
process. For that time the mixture was continuously stirred.
After the completion of the process the obtained dispersion of dark green
colour was flooded with the 10-fold surplus of distilled water and stirred by magnetic
stirrer for 15 minutes. This was made for the removal reagent livings and soluble by-
56
Chapter 2. Experimental part
products. Then the precipitated PA/PANI powder was filtered through a Buchner filter
funnel. After the filtration the product slurry of the composite material was placed into
the flask and dried under vacuum at 70-80 0С to a constant weight.
In the case of TSA using, the slurry of the composite material after filtration
was divided into two parts. One part was placed into the flask with 15 ml of 0.2 М
solution of TSA for 24 hours. Such additional treatment in TSA solution was necessary
because of TSA good solubility in water. Due to this, partial removal of TSA from the
composite powder can occur during the water washing. After the additional treatment
by acid solution the powder was filtered and was washed on filter by 30 ml of distilled
water to remove TSA surplus. The other part of composite powder was left without
additional treatment. Both parts of each sample were placed into round-bottomed flasks
and dried under vacuum at 70-80 0С to a constant weight.
A part of resulting powder was being neutralized with 5% NH4OH aqueous
solution for 24 hours. The precipitated composite was washed repeatedly with distilled
water till the filtrate became free of alkali (pH 7) and then was dried under vacuum for
48 hours. Thus obtained dedoped composite material was redoped by stirring for 24
hours in 1M water solution of appropriate acid, producing doped PANI complex.
Finally, the polymer doped powders were dried under dynamic vacuum at 70 0C for 10
hours till a constant weight.
Thus obtained composite powders were used for further investigation both in
the powder and film forms made by the compression-molding technique (look section
2.2.2).
2.3. The main investigation methods and techniques
The complex of physico-chemical methods of investigation (such as the cyclic
voltammetry, UV-Vis spectroscopy, dielectric relaxation spectroscopy, Raman
spectrometry, thermogravimetrical analysis, conductivity measurements, pH-potential-
temperature and swelling degree measurements, microscopy studies) were carried out
for studying the influence of the synthesis conditions, the structure of the PM, the nature
of electrode material and acid-dopant on the process of the formation of the composites
based on PANI, for investigating the effect of a composition of the materials on their
physicochemical properties.
57
Chapter 2. Experimental part
2.3.1. Method of determination of the real polyaniline content in the
composites
The possibility to determine the real content of formed PANI plays a very
important role for the kinetic investigation of the aniline polymerization. The most close
to this task is the method of determination of the PANI oxidation state in NMP by
measuring the consecutive UV-Vis absorption spectra [160]. It should be noted that
authors examined only pure PANI, but not PANI obtained in the composite materials.
And, besides, it needs some modifications allowing the use of unknown concentrations
of PANI in the presence of the PM. So, the necessity of a new method of the PANI
content determination arose in the course of the study.
Our method is based on determining the relationship between the optical
absorption density D and the PANI concentration in the polymer composites. It was
found that it is possible to use PANI in both EB and LE oxidation states. For this
purpose it is necessary to create the calibration plots for PANI in these oxidation states.
These states were chosen due to the independence of UV-Vis spectra profile of
these PANI states of the used acid-dopant unlike from the emeraldine salt form [6].
And, besides, it is known that UV-Vis spectrum of LE is characterized by one well-
defined absorption peak [6]. However, it is rather complicated to convert PANI formed
in thin subsurface layer of the PM to the LE oxidation state. Of this point the EB state
was chosen for the surface composite films and LE – for the bulk composite materials.
The synthesized EB (look section 2.2.1) was dissolved in the solution of NMP
at continuous stirring for 5 hours. This process took place in a flask with NMP seal to
prevent the evaporation of the solvent.
A series of solutions with different PANI concentration were prepared by the
subsequent dilution of the primary PANI solution. The received UV-Vis absorption
spectra of these solutions are characterized by two bands – one at λ = 331 nm (assigned
to π-π* transition of benzene rings) and the other one at λ = 647 nm (absorption of
quinonediimine groups). Two calibration plots of optical absorption density and the
PANI concentration in solution were depicted on the basis of the received results (Fig.
2.3a). The reproducibility of obtained results was checked by conducting of 5 parallel
measurements. The possibility of using the obtained calibration curve for the PANI
content determination in the PA matrix was preliminary postulated on the basis that
both the PM (aliphatic polyamide) and solvent (cyclic amide) had a similar nature. And,
58
Chapter 2. Experimental part
Figure 2.3. Dependencies of absorption density on the PANI concentrations in NMP
solution at fixed wavelengths:
а – PANI in emeraldine base state;
b – PANI in leucoemeraldine state
PANI concentration in NMP, %0,000 0,005 0,010 0,015 0,020 0,025 0,030
Abs
orba
nce
0,0
0,5
1,0
1,5
2,0
2,5
331 nm647 nm
PANI concentration in NMP, %0,000 0,002 0,004 0,006 0,008 0,010
Abs
orba
nce
0,0
0,2
0,4
0,6
0,8
1,0
phenylhydrazine, 343 nmascorbic acid, 343 nm
(b)
(a)
59
Chapter 2. Experimental part
besides, PANI-base in the solid PA matrix and in the liquid NMP has similar UV-Vis
spectra, i.e. the presence of PA does not influence on the PANI bands position. This
supposes an approximate equality of the PANI extinction coefficients in the both media.
For the determination of the PANI content in the surface composite films the
obtained doped composite material was converted into the dedoped state by the
interaction with the 5% aqueous solution of NH4OH during 20 hours. The comparison
of the optical density of this film at 331 and 647 nm with the calibration curve (Fig.
2.3a) give us the real PANI content in composite.
However, during the dissolution of the bulk PA/PANI composite powder in
NMP we were confronted with difficulties. As it was discovered, although pure PANI
and pure matrix polymer are soluble in NMP, the composite with PANI in the EB form
is insoluble in NMP. That’s why it was necessary to convert PANI into the form which
would be rather well soluble in NMP. It is appeared that PANI in the oxidation state of
LE has a good solubility in NMP and, what is very convenient for the analysis, - PANI
in this oxidation state has only one absorption band at λ = 343 nm (assigned to π-π*
transition of benzene rings) (Fig. 2.3b). The PANI conversion into LE oxidation state
can be made either by ascorbic acid [160] or by phenyl hydrazine [161]. After adding
the solution of the reducing agent the obtained solution was stirred for 8 hours, filtered
from insoluble PA and the UV-Vis spectrum was measured. It was found that both
reducing agents (ascorbic acid and phenyl hydrazine) lead to similar results (Fig. 2.3b).
2.3.2. Electrochemical investigations
The study of the electrochemical aniline polymerization was carried out with
the help of a PI-50-1 model potentiostat/galvanostat, arranged with PR-8 model
programmer (Belorussia) in a three-electrode electrochemical cell with unseparate
compartments (Fig. 2.4). The four-cable circuit board connection of the electrochemical
cell was used for increasing the accuracy of measurements. In this circuit board the
working electrode is connected to the potentiostat by a separate wire.
A variety of electrode materials was used as working electrodes. The
investigation of the aniline electrochemical polymerization was carried out on platinum
electrodes and on transparent glass electrode with conducting SnO2 layer deposited on
one side (transparent SnO2-glass electrode).
The platinum electrodes were chosen due to their stability at anode potentials.
The choice of the SnO2-glass electrode is connected not only with its stability,
60
Chapter 2. Experimental part
Figure 2.4. Three-electrode electrochemical cell used for the electrochemical
polymerization
Platinumcounter electrode
Saturated calomel reference electrode
Working electrode
61
Chapter 2. Experimental part
but also with the possibility of measuring UV-Vis spectra of the PANI layer formed
directly on the electrode surface.
Two types of platinum electrodes were used: a platinum wire (∅ 0.5 mm,
Svisible = 1.96⋅10-3 cm2) and a platinum plate (S = 3.38 cm2). Also, transparent SnO2-
glass electrode (S = 1.04 cm2) was used. The surface of this glass electrode was limited
by a transparent inert sticky film.
To obtain reproducible results the careful electrode surface treatment was
realized before the experiments. The surface of platinum electrodes was treated in
accordance with the most common method [29, 30, 37]: it was polished on fine-grained
emery paper (Carbimet, Grid 600, USA), then degreased by sodium bicarbonate,
washed carefully by distilled water and dried by the filter paper. The surface of SnO2-
glass electrode was washed by acetone and distilled water and dried by the filter paper.
Counter electrode (cathode) was also a platinum plate, a surface of which is by
two orders higher than the surface of working electrode. Calomel electrode which is
filled by the saturated aqueous solution of potassium chloride KCl (SCE) was used as
the reference electrode. Its equilibrium potential is equal to Е = 0.244 V versus SHE
[162]. All potentials reported here are versus SCE.
The PANI electrochemical synthesis was carried out in galvanostatic,
potentiostatic and cyclic voltammetry modes.
A cyclic voltammetry was used to characterize electrochemical properties of
PANI. The cyclic voltammograms were registered at the potential scan rates νscan = 5,
10, 20, 50, 100 mV/s in the potential range from -0.2 V to 0.8 V. The potential range
was chosen so that it contained the potential of the aniline oxidation (+0.6 V) [163] and,
respectively, the potential of the polymerization process initiation, as well as all charge-
discharge processes of formed PANI [163]. The limitation of the working potential
range by 0.2 V is caused by the fact that the potentials of obtaining PANI are reached.
The upper potential limit (0.8 V) is caused by the fact that during the potential cycling
to more positive values the irreversible degradation of formed PANI takes place because
of the PANI overoxidation [35].
The cyclic voltamogramms were registered by a two-dimensional X-Y recorder
LKD4-003 (Belorussia).
62
Chapter 2. Experimental part
The galvanostatic polymerization of aniline was accomplished by applying
current density in the range of 0.001-10 mA/сm2 (for the different electrodes) to the
working electrode for a pre-defined period of time.
In the potentiostatic mode the PANI films were prepared by applying potentials
of 0.6; 0.7; 0.75 and 0.8 V to the working electrode for a certain period of time.
Prior to measurements the electrochemical cell was washed carefully by
distilled water. The reference electrode was connected with the electrochemical cell by
two intermediate electrochemical keys (salt bridge). One of the keys was filled with the
saturated solution of KCl and was placed into the other one, which was filled with the
solution of the appropriate acid. In its turn, the second key was placed into the
electrochemical cell filled with the working solution. After filling the electrochemical
cell and both keys the prepared working electrode was placed into the cell. To increase
the accuracy of measurements and to support the working electrode potential the IR
compensation scheme was used. With the help of this scheme the potentiostat provided
the compensation of the voltage drop at the ohmic resistance of the solution layer in the
electrochemical cell between the end of the electrochemical key and the working
electrode surface.
The voltammetry curves began to be measured after the exposure of the
working electrode in the solution till the approximately constant stationary potential of
the working electrode (Еst ≈0.4 V). After the polymerization was completed, the
resulting films were washed with the distilled water before further use.
All measurements were carried out at ambient temperature (∼20 0С).
2.3.3. Electrical conductivity measurements
The surface conductivity (σ) of the obtained polymer PA/PANI composites
was measured by standard two- or four-electrode methods at ambient temperature (∼20 0С). The thickness of pellets and films was measured with a digital micrometer. Also, it
should be noticed that for the surface conducting composite film the thickness of the
conducting subsurface layer estimated by Raman spectrometry (see section 2.3.4) was
used.
Measurements of sample conductivity less than 10-5 S/cm were carried out with
a digital combined device Щ402-М1 (Ukraine) by measuring resistance by a two-
electrode method. The conductivity value is then calculated using the following
equation:
63
Chapter 2. Experimental part
hbRl⋅⋅
=σ , (2.3)
where l is the distance between electrodes (1 cm), cm; R is the measured sample
resistance, Оhm; b is the width of the sample, cm; h is the sample thickness, cm.
The electrical conductivity higher than 10-5 S/cm was measured using a four-
electrode method. Under this method, the film was laid across four parallel electrodes as
shown in Fig. 2.5. A constant current was passed between the two outer electrodes
while the potential drop across the inner electrodes was measured. Measurements were
performed with the help of the universal voltmeter В7-21А (Russia), universal
voltmeter-electrometer В7-30 (Russia) and external current source – accumulator
battery. The conductivity value is then calculated using the following equation:
hbUlI⋅⋅
⋅=σ , (2.4)
where I is the applied current, А; l is the distance between the two central electrodes (1
cm), cm; U – is the measured potential drop, V; b is the width of the sample, cm; h is
the thickness of the sample, cm.
The conductivity measurements were carried out on direct current, and all
electrodes were placed on one side of the sample surface.
Figure 2.5. Configuration used for the measurement of electrical conductivity of
conducting samples using a four-electrode method
I applied
U measured
Sample
Electrodes
vvv
64
Chapter 2. Experimental part
The resistance measurements were averaged out of the results of (i) three times
for each film or pellet, (ii) two films or pellets for each sample.
The four-electrode technique was also used to investigate the surface resistance
R of the conducting composite films as a function of the temperature. Four cooper wires
were attached in parallel on the sample surface by conducting silver paint for better
electrical contact. In this case the resistance values were measured using a Yokogawa
7563 electrometer. The temperature dependence of resistance was measured by raising
the temperature at the rate of 1 0C/min in the cryostat equipped with temperature-
controlled heater. The temperature control was realized with the help of Novocontrol
broadband dielectric spectrometer (description of the device look in section 2.3.5). Also
other kind of equipment (Agilent E3634A) was used in order to measure the
temperature dependence of resistance. In this case the temperature was increased with
the rate of 0.2 0C/min. The performed measurements with both type of equipment gave
good reproducibility.
2.3.4. Spectroscopy investigation
UV-visible spectroscopy. The choice of this method is motivated by two
reasons. Firstly, this method enables us to estimate the structural and quantitative
composition of the obtained PANI films. And, secondly, this method also permits to
watch in situ the kinetics of the PANI formation in the PM.
The spectral investigation was performed by a spectrometer Specord М40
(DDR) in the wavelength range of 200 and 900 nm in quartz cuvettes with a path length
of 0.1 and 1 cm.
The kinetic control of the system changes during the chemical aniline
polymerization in the PM was carried out directly in a spectrometric quartz cuvette
(volume: 3 ml, path length: 1 cm). Prior to measurements the PM swelled in aniline was
placed in the cuvette, which was filled with the oxidant solution. The time at which the
swelled PM is put in an oxidant aqueous solution was noted as a starting time of the
polymerization. The electronic absorption spectra were recorded in definite time periods
after the polymerization process began. In some cases, to simplify and to specify the
kinetics control of the aniline polymerization process in the PM, the optical density D of
the system was measured periodically at fixed wavelengths corresponding to the PANI
bands.
65
Chapter 2. Experimental part
The spectra of electrochemically obtained PANI on the transparent SnO2-glass
electrode were measured also using this spectometer. PANI covered SnO2-glass
electrode was installed in the spectrometer in the place of quartz cuvette.
The spectra of both chemically and electrochemically formed PANI were
recorded by means of a special software (Spectra). In order to obtain the spectra of pure
PANI, the background spectra of a pure SnO2-glass electrode or a virgin polymer film
were subtracted from the recorded spectra.
Resonance Raman spectrometry. The choice of this method is motivated by
the possibility to characterize and to determine the PANI structure [164-167]. It is well
known that this method enables to investigate objects very small in size (down to a few
µm2) without any damage of the investigated material. This method also allows
obtaining the information about quantity of PANI and its oxidation state in the
composites by analyzing the evolution of the Raman bands intensity.
The experiments were carried out by a multichannel Jobin-Yvon T64000
spectrometer connected to a liquid N2 cooled ССD detector and equipped with a
confocal microscope. The Raman spectra of the powder and polymer films were
recorded with the green excitation laser line (λ = 514.5 nm) of an Argon-Krypton laser
by a special software Labspec3t2. This line was chosen since it allows obtaining
simultaneously both Raman bands of the phenyl units of PANI which are enhanced in
the blue range, and Raman bands of the quinone units which are enhanced in the red
range [167]. Thus, this excitation line allows getting general view of different polymer
forms of PANI. In order to investigate the vibrational properties of different polymer
forms of PANI, the Raman measurements were focused mainly within 1100-1700 cm-1
region.
In order to avoid any local degradation and perturbation of the polymer
samples, the laser beam power was limited to 0.04 mW on a sample surface and the
spectra were collected with an integration time of 300 s. The laser beam was focused on
the sample in a ∼1 µm spot size by an ×100 objective and the spectral resolution was
approximately 1.5 µm.
Taking into account the advantage of the confocal system, Raman spectrometry
was used also to determine the thickness of the conducting layer of the polymer
composite film. For this purpose the Raman spectra were recorded through the thickness
of the composite film using a PI motor stage allowing precise step displacement along
66
Chapter 2. Experimental part
the Z axis of the sample below the objective. The first spectrum was obtained by means
of focusing the laser beam on the surface of the film. The following spectra were
collected by focusing the beam inside the composite film with the submicrometric step
between two spectra. The process continued until the end of the conducting layer. Then
the bands of the obtained spectra were compared with the characteristic bands of the
doped PANI spectrum.
The thickness of the conducting layer was found by analyzing the evolution of
the Raman band intensity along the Z displacement. The obtained value of the
conducting layer thickness was used in further calculations of the PANI yield and in
determining kinetic peculiarities of the PANI formation in the polymer films.
2.3.5. Dielectric relaxation spectroscopy (DRS)
It is known that the dielectric behaviour of the composite materials and their
conductivity vary with the structure of polymer chain, doping level, the nature of acid-
dopant, etc. [168-173]. They are also greatly affected by the film-formation method and
the selection of the dopant and solvent [171, 172]. The method of DRS allows
investigating the conductivity mechanism on the basis of measurements of dielectric
parameters of the conducting polymers [168, 169].
The application of an electric field provokes the fluctuations of the electrical
polarization and this leads to the electrical displacement. Such displacement depends on
the material polarization. Dielectric polarization arises due to the existence of atomic
and molecular forces, and appears whenever charges in a material are somewhat
displaced with respect to one another under the influence of an electric field. One can
distinguish several types of polarization [174], which will depend on the applied
frequency and temperature:
- electronic polarization – this effect involves distortion of the center of charge
symmetry of the basic atom. Under the influence of applied field, the nucleus of an
atom and the negative charge center of the electron shift, creating a dipole. This
polarization happens at very high frequencies or at very low temperatures;
- atomic polarization happens at IR frequencies and corresponds to the atoms
displacement in the case of the molecules deformations.
The real part of the dielectric complex permittivity ε’ after this two kinds of the
polarization is noticed as ε∞ - it is minimal value of ε’ observed for the studied
frequency region.
67
Chapter 2. Experimental part
- dipoles orientation polarization - this is a phenomenon involving rotation of
permanent dipoles under an applied field;
- the electrode polarization lies in the charge accumulation near the electrodes
and resulted from the ionic conductivity. These charges create an electric field with an
opposite direction to the applied electric field. Such polarization appeared at very low
frequencies;
- Maxwell-Wagner-Sillars (MWS) effect – if the material consists of different
phases, which possess different values of permittivity and conductivity, so in this case
the charges can accumulate at the limits of these phases. Hence, the relaxation time of
the MWS relaxation will depend on the conductivity and permittivity values of the
different components.
Dielectric permittivity measurements were performed using Novocontrol
broadband dielectric spectrometer (Novocontrol GmbH, Germany) (Fig. 2.6) in wide
frequency (0.1Hz to 1GHz) and temperature (173 to 423K) ranges. The spectrometer
consists in the following parts:
- impedance analyzer SOLARTRON SI 12690;
- impedance analyzer HP 4291A;
- dielectric converter NOVOCONTROL;
- temperature controller NOVOCONTROL Quatro Cryosystem;
- cryostat;
- sample cell.
To cover the frequency domain from 0.1 Hz to 1 GHz, two experimental
configurations are employed. From 0.1 Hz up to 10 MHz a SOLARTON SI 1260
analyzer combined with a broadband dielectric converter allows impedance and
dielectric measurement. The high frequency impedance analyzer is used with a
precision coaxial line (Fig. 2.7) [175].
For both configurations, the sample cell consists of two round golden parallel
electrodes filled with the material to form a capacitor. In the case of the high frequency
range the sample capacitor is used as a termination of a golden coaxial line and the
impedance is then calculated from the complex reflection factor at the end of the line.
The polymer film or pellet was placed between two electrodes as sketched in Fig. 2.8.
In the case of the bulk composite materials (Fig. 2.8a) the polymer powders were
compressed into discs of 10 mm in diameter and with thickness of about 1 mm under a
68
Chapter 2. Experimental part
Figure 2.6. Schematic representation of the dielectric spectrometer
69
Chapter 2. Experimental part
Figure 2.7. Schematic presentation of a high-frequency sample cell [175]
Sample
Electrodes
(a) (b)
Figure 2.8. Sample cell configuration for dielectric measurements in the case of (a) bulk
and (b) surface layered composites
pressure of 3 t at room temperature. The surface containing PANI composite films were
facing each other as indicated in Fig. 2.8b.
The sample temperature was varied by using a gas stream of nitrogen and the
measured temperature close to the sample is controlled within the accuracy of ±0.2 K.
The measurements determined the impedance of the sample capacitor Z
s
s
IU
iZZZ =+= "' , (2.5)
which is connected with the dielectric function of the sample by the equation:
02"'
CfZiisπ
εεε −=−= , (2.6)
where f denotes frequency and C0 is the vacuum capacity of the sample capacitor.
The method of dielectric analysis was chosen to calculate dielectric parameters
of the process (dielectric constant, imaginary and real permittivity, conductivity,
relaxation time, etc.). The values of imaginary and real parts of dielectric permittivity
70
Chapter 2. Experimental part
ε*(ω) = ε′ - іε″ were analyzed according to the empirical Havriliak-Negami (HN)
function [174]:
( )( )( )βαωτ
εωεσ
εωεi
i n+
∆+−= ∞
∗
1)(
0
0 , (2.7)
where ω = 2πf; ε0 denotes the vacuum permittivity; σ0 is the dc-conductivity; n is the
exponent factor, in most cases equal to 1; ∆ε is the difference between low and high
frequency limits of ε′ over the relaxation to which the HN function is applied and it is
also proportional to the area below the curve of the ε′′ relaxation peak; ε∞ denotes the
high frequency limit of the permittivity and is the unrelaxed value of permittivity; α and
β are shape parameters; τ is the relaxation time. Processing the experimental dielectric
spectra ε∗(ω) was made by means of WinFit 2.4 (1996) software of Novocontrol GmbH
(Germany).
The characteristic relaxation times τ were taken at the position of the maximum
of dielectric loss for each relaxation process and they were fitted according to the
equation (2.7). The temperature dependences of these relaxation times were analyzed
using an Arrhenius equation (2.8) and activation energies of relaxation processes were
obtained.
)exp(0 TkE
B
aττ = , (2.8)
where τ0 is the relaxation time at very high temperature; Еa is the activation energy; kB
is the Boltzmann’s constant ( 8.616⋅10-5 eV/K).
2.3.6. Thermal analysis and differential scanning calorimetric
measurements
TGA was carried out with a thermogravimetric analyzer MOM (Hungary). The
thermogram of the polymer sample was recorded in the range of temperatures: from
room temperature up to 1000 0C with a heating rate of 10 0C/min in air media. The
weight of the sample was ∼120 mg.
DSC measurements are used in order to obtain a qualitative comparison of the
crystallinity extent of the PM. DSC thermograms of the polymer films were obtained
from heating the samples from -50 0C to 300 0C under N2 purging at 20 0C/min by a
TA-Instruments differential scanning calorimeter. The weight of the sample was ∼2 mg.
71
Chapter 2. Experimental part
The ratio of the fusion enthalpy of the semicrystalline samples ∆Hc over the
fusion enthalpy of the pure (100%) crystalline polymer ∆H° gives the degree of the
crystallinity, Xc, of each polymer [147]:
%100⋅∆∆
=°H
HX c
c (2.9)
2.3.7. Mechanical analysis
The mechanical properties of the polymer composite films were measured on
the film strips of 10×44 mm in dimension. The measurement of their tensile strength
was performed by using a tensile-testing machine at room temperature (∼20 0C) using a
cross-head speed of 50 mm/min. For each tensile strength reported, at least three sample
measurements were averaged.
2.3.8. Optical and atomic force microscopies
The method of the optical microscopy was used in order to obtain structural
information about the composite PM/PANI samples. The electron micrographs of the
composite films were performed using a Leica DMRX polarizing microscope.
The atomic force microscopy (AFM) (electrical and topographical images) of
the composite films were realized in the Laboratoire de Génie Electrique de Paris, UMR
CNRS 8507, Supélec, Université Paris VI and Paris XI using a home-built extension of
a Digital Instruments Nanoscope III in contact mode (called “Resiscope”) [176]. The
tapping mode was used for making topographic images.
2.3.9. pH-potential-temperature measurements
This kind of measurement was used to better understand the aniline
polymerization process in the presence of the PA dispersion.
The changes of pH, potential as well as of the temperature during the
polymerization of aniline in the reaction mixture were recorded using Greisenger
electronic GMH 3530 (Digital pH/mV/Thermometer).
72
Chapter 3
ELECTROCHEMICAL
SYNTHESIS OF POLYANILINE
Chapter 3. Electrochemical synthesis
Introduction
It is known that the method of the electrochemical polymerization permits
materials with desired properties to be obtained under simply controlling
polymerization conditions (the potential and current values, time of polymerization,
material of electrode, etc.) [148, 177]. The other advantage of this method is the
possibility to avoid by-products of the process. Besides, this method is very useful when
we need to study the mechanism of the process. Up to now certain studies of the aniline
electrochemical polymerization process have been performed [1, 29, 30, 178]. But they
don’t give a complete picture of the mechanism and stages of the aniline polymerization
process especially in the polymer matrices.
In order to better understand the process, which takes place during the
electrochemical aniline polymerization in the PM we have preliminary investigated the
specificity of the electrochemical aniline polymerization on some bare electrodes
(platinum and SnO2-glass electrodes). The obtained results are discussed. Such an
approach makes it possible to understand and explain the aniline polymerization process
on the electrodes covered with the PM.
3.1. The peculiarities of the electrochemical formation and stability
of polyaniline on the surface of bare electrodes
Electrochemical synthesis of PANI on platinum and SnO2-glass electrodes was
carried out in potentiostatic, galvanostatic and cyclic voltammetry modes. All
measurements were performed in the aqueous solution of 1M HCl, containing 0.5M
aniline.
3.1.1. Polyaniline formation by cyclic voltammetry
Platinum electrode. The cyclic voltammograms of the aniline polymerization
on platinum electrode are demonstrated in Fig. 3.1a. The current increase at 0.6 V has
been observed even on the anodic branch of the first cycle. This increase is attributed to
the monomer (aniline) oxidation [163] and, respectively, to the formation of its cation-
radicals С6Н5NH.+. A continuous current increase of this peak and a shift of its potential
to the lower values are observed during further cycles. This can be connected with the
73
Chapter 3. Electrochemical synthesis
Figure 3.1. Cyclic voltammograms of the PANI formation on (a) platinum and (b)
SnO2-glass electrodes in the aqueous solution of 1М HCl, containing 0.5М aniline
(sweeping rate υscan = 100 mV/s)
(b) (a)
74
Chapter 3. Electrochemical synthesis
catalytical activity of PANI formed on the electrode surface [29] as well as with the
appearance of aniline oligomers (di-, three-, tetramers, etc.) which are easily oxidized
[163]. Thus, with the PANI accumulation the two redox couples appear on cyclic
voltammograms at potentials 0-0.2 V (the maximum of anodic current at 0.14 V and
maximum of cathodic current at 0-0.10 V) and at ∼0.7 V. These two redox couples are
attributed to the two-stage electrochemical process of the PANI oxidation-reduction
[22]. Visually, the appearance of the green film on the electrode surface corresponds to
this.
The first current peak increased continuously with successive potential scans,
indicating that the electroactive PANI layer is being deposited on the electrode surface
[179]. During the potential cycling, the cathodic current growth begins to decrease
gradually as compared to the anodic one by approximately 0.019 mA/cycle. In
accordance with the literature [33, 35, 180] this can be explained by the fact that during
the potential cycling till 0.8 V the overoxidation of formed PANI takes place because of
the subsequent chemical reactions of the PANI degradation. Such the degradation can
lead to the irreversibility of the anodic process and, respectively, to the decreasing of the
cathodic current values. This is, probably, connected with the fact that overoxidized
PANI is less electroactive and has lower conductivity value. The overoxidation makes
the possibility of the reduction of formed PANI difficult. The last fact leads to the
increase of the separation of the cathodic peak compared to the anodic one
approximately by 0.01 V/cycle due to an ohmic component. The decreasing of the
potential limit to 0.75 V permits to avoid the PANI overoxidation. Significant
separation of the cathodic and anodic peaks (∆Ер ∼0.19 V) testifies, probably, that the
aniline polymerization process is determined by the kinetics of the slow electrochemical
process [149].
The second redox couple (Е ∼0.7 V) (Fig. 3.1a) appears gradually on cyclic
voltammograms of the PANI formation. The development of this redox couple is
analogous to the first one: the current values increase with the potential sweeping; the
cathodic current growth decreases gradually in comparison with the anodic one
approximately by 0.14 mA/cycle; the cathodic peak potential is shifted during cycling in
relation to the anodic peak approximately by 0.06 V/cycle.
From the dependence of the first anodic peak current on the t1/2 for the different
sweep rates (Fig. 3.2) we can see that the current of this peak increases continuously
75
Chapter 3. Electrochemical synthesis
Figure 3.2. The dependence of the anodic peak current of the first redox process on the
t1/2 (a) and the dependence of the anodic peak potential on the cycling time (b).
Conditions of the measurements: aqueous solution of 1М HCl, aniline concentration
0.5М, on the platinum electrode at different sweeping rates νscan:
1 - νscan = 100 mV/s; 2 - νscan = 50 mV/s; 3 - νscan = 20 mV/s; 4 - νscan = 10 mV/s;
5 – νscan = 5 mV/s
t1/2, min1/20 2 4 6 8 10 12 14 16
Cur
rent
of t
he a
nodi
c pe
ak, m
A
0
5
10
15
20
25
30
35
Cycling time, min0 50 100 150 200
Pote
ntia
l of t
he a
nodi
c pe
ak, V
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
(b)
1
2
3
4
5
1
2
4
5
2
t1/2, min1/21 2 3 4 5 6 7 8
Cur
rent
of t
he a
nodi
c pe
ak, m
A
0.0
0.2
0.4
0.6
0.8
1.0 3
(a)
76
Chapter 3. Electrochemical synthesis
with √t which is indicative of an increase of the amount of formed electroactive PANI.
On the other hand, this fact testifies to the diffusion limitations of the process [148]. The
obtained diagrams (Fig. 3.2a) are characterized by the two well-defined modes with
different slopes. These regimes, probably, testified to the presence of two different
stages of the aniline polymerization process. On the first stage the current increase is not
very noticeable that is, probably, caused by the induction period of the polymerization
process. Hence, we can suppose that the first stage is connected with the formation of
the oligoaniline cation radicals [104]. The subsequent aniline oxidative polymerization
causes the change of the slope of the dependence I = f(t1/2) (Fig. 3.2a). The greater is the
quantity of the polymer deposited on the electrode, the higher is the rate of the polymer
formation. In this case the process is limited, mainly, by the counter ions diffusion
through the formed electroactive PANI film. Similar behaviour is observed in the case
of the polymerization of the aniline derivative (o-methoxyaniline) [88].
During the aniline polymerization process the first anodic peak potential is
continuously shifted to more positive values (0.35-0.4 V) (Fig. 3.2b). This can be
explained by the fact that the aniline oxidation takes place on the interface the PANI
film – solution and electrons transfer from aniline to the electrode through the PANI
layer. The growth of the PANI thickness and, respectively, of its resistance, leads to the
ohmic component increase. Obviously, the overoxidation of PANI makes a certain
contribution to this process too as it decreases the PANI conductivity.
The shape and behaviour of the obtained cyclic voltammograms are in good
agreement with the results obtained previously for the aniline electrochemical
polymerization in different solutions [37, 44, 178, 179]. Especially with the fact that the
rate of the PANI overoxidation and, respectively, conductivity and electroactivity losses
become higher during potential cycling to more than 0.8 V [35, 44, 178, 181]. From this
point the PANI oxidation becomes less reversible. Thus, during the potential cycling till
1V the middle redox couple at 0.45V appears on the cyclic voltammograms. This
couple is attributed to the PANI degradation process and to the formation of the
benzoquinone [33, 34].
The dependence of the formed PANI weight on the cycle number (reaction
time) was found (Fig. 3.3). The polymerization was finished at -0.2 V. The formed
PANI film was washed by the distilled water for 5 min, and then dried at the ambient
temperature (∼20 0C) for 3-4 hours up to the stable weight. The weight of the obtained
77
Chapter 3. Electrochemical synthesis
PANI film increases gradually as it is seen from Fig. 3.3. The dependence has the linear
character with the slope equal to 0.00125. The results make it possible to determine the
average PANI quantity which can be obtained on the platinum electrode during one
electrochemical cycle in the cyclic voltammetry regime. This value is 0.36
mg/(cycle⋅cm2) at the potential scan rate equal to 50 mV/s in the potential range from -
0.2 V to 0.8 V.
Cycles0 10 20 30 40 50 60 70
go
ee
,ig
htr w
lym
P
Figure 3.3. Dependency of the weight of the deposited polymer on the platinum
electrode on the cycle number (scan rate νscan = 50 mV/s) in the aqueous solution of 1М
HCl, aniline concentration 0.5М
SnO2-glass electrode. The investigation of the aniline polymerization process
on the SnO2-glass electrode was performed in order to obtain additional information
about this process. The cyclic voltammograms of the polymerization of aniline on this
electrode are demonstrated in Fig. 3.1b. These voltammograms are significantly
different from those obtained on the platinum electrode at the same potential scan rate
(Fig. 3.1a). Thus, on voltammograms obtained on the SnO2-glass electrode the second
redox couple is not observed. Instead, one broad redox peak couple can be seen which is
attributed to the electroactive PANI accumulation on the electrode surface. However,
the anodic potential peak shifts to more positive potentials when repeating potential
0.00
0.02
0.04
0.06
0.08
78
Chapter 3. Electrochemical synthesis
cycling - ∼0.62 V for 160 cycles (compare with ∼0.35 V for 160 cycles on the platinum
electrode). Probably, this fact can be connected with a rather high resistivity of the SnO2
layer. Therefore, higher potentials are necessary for redox processes to happen.
The higher current densities which are observed on the platinum electrode (Fig.
3.4a) in comparison with those observed on the SnO2-glass electrode (Fig. 3.4b) are also
in good agreement with this explanation. During the potential sweeping the smooth
PANI film is formed on the SnO2-glass electrode. The colour of this film changes
depending on the applied potential from yellow to dark green at potentials -0.2 V and
0.8 V, respectively.
3.1.2. Polyaniline formation in the potentiostatic mode
Dependencies of the current density on the polymerization time on platinum
and SnO2-glass electrodes at different applied potentials (0.6; 0.7; 0.75 and 0.8 V)
obtained in the potentiostatic mode are demonstrated in Fig. 3.5. These diagrams testify
to the delay of the current growth with the augmentation of the PANI amount on the
electrode surface. This effect is strongly revealed at higher potentials and on the SnO2-
glass electrode, which has high resistance. The shape of dependencies testifies to the
presence of the induction time. This period on the platinum electrode decreases at
potential increasing and is 25 min for Е = 0.6 V, 9 min for Е = 0.7 V, 1.5 min for Е =
0.75 V and less than 1 min for Е = 0.8 V. For the SnO2-glass electrode the picture is less
clear because of its high resistance, but the deceleration of the process is observed on
this electrode too because of the increasing of the system resistance.
It should be noted that at potentials higher than 0.7 V the inhomogeneous film
of dark green colour is formed (at 0.8 V this film is practically black – PANI is formed
in the PNA oxidation state) and at 0.6 V rather homogeneous green film is formed. This
is charged with the formation of PANI in the ES oxidation state.
3.1.3. Polyaniline formation under galvanostatic conditions
The peculiarities of the PANI electrochemical synthesis in the galvanostatic
mode on platinum and SnO2-glass electrodes are well illustrated by the Е = f(t)
dependencies (Fig. 3.6). From the represented data one can see that the electrode
potential initially shifted to the positive side and reached the maximum value (0.8 V and
higher) and then gradually decreased to a steady value (approximately 0.6-0.7 V). This
value, in turn, increased with the augmentation of current density and corresponded to
79
Chapter 3. Electrochemical synthesis
Figure 3.4. The dependence of the first anodic peak current density on the cycle number
on (a) platinum and (b) SnO2-glass electrodes in the aqueous solution of 1М HCl,
aniline concentration 0.5М, scan rate 100 mV/s
Cycles0 20 40 60 80 100 120 140 160 180
Cur
rent
den
sity,
mA
/cm
2
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Cycles0 20 40 60 80 100 120 140 160 180
Cur
rent
den
sity,
mA
/cm
2
0
1000
2000
3000
4000
(b)
(a)
80
Chapter 3. Electrochemical synthesis
Figure 3.5. The dependence of the current density on the polymerization time at different
applied potentials on (a) platinum and (b) SnO2-glass electrodes in the aqueous 1M HCl
solution, aniline concentration 0.5М:
1 - Е = 0.6 V; 2 – Е = 0.7 V; 3 – Е = 0.75 V; 4 – Е = 0.8 V
Polymerization time, min0 10 20 30 40 50 60
Cur
rent
den
sity,
mA
/cm
2
0.00
0.02
0.04
0.06
0.08
0.10
1
Polymerization time, min0 10 20 30 40 50 60
Cur
rent
den
sity,
mA
/cm
2
0
200
400
600
800
1000
1200
1400
1600
(a) 4
3
2
1
(b) 3
4
2
81
Chapter 3. Electrochemical synthesis
the formation of a green conductive PANI film on the electrode surface. The initial
overpotential which is characteristic of the electrosynthesis under such conditions is
associated with the formation of the PANI particles on the electrode surface [182]. The
tendency of the maximum augmentation is well observed at the current density
increasing. At very high current density values (more than 5 mA/cm2 on the platinum
electrode) there is no potential maximum and the film grows rather quickly. But,
because the potential under such conditions is higher than 1V, the formed film is very
inhomogeneous, overoxidized, very dark, almost black colour. The significant
increasing of the potential in the first minutes can be explained by the fact that the
aniline oxidation and then formation of PANI in the PNA oxidation state take place. For
the PNA formation the potential value which is much higher than 1V is needed. As it is
known [9, 183], PANI in the PNA oxidation state is the catalyst of the PANI formation
process. After the formation of this initial layer, the aniline polymerization takes place
at lower potentials (the main synthesis takes place at potentials approximately 0.6-0.7
V). At higher current densities (Fig. 3.6, curves 3 and 4) the electrode potential, because
of the very quick consumption of the aniline (depolarizer), reaches higher values. Thus,
at such high potentials the additional secondary reactions (overoxidation of formed
PANI, decomposition of the water and electrolyte anions) can occur in the system [148,
184].
The potential jump which is observed at 0.1 mA/cm2 in the case of the SnO2-
glass electrode and at 5 and 10 mА/cm2 on the platinum electrode can be explained by
the fact that potential exceeds 1V and the formed PANI layer is overoxidized and
becomes less electroactive and less conductive. Further electrochemical reaction and the
aniline polymerization are caused by the potential decreasing.
But, in spite of such secondary processes, a linear relationship between the
weight of obtained PANI and the current density under galvanostatic conditions (Fig.
3.7), as well as in the case of the PANI formation in the cyclic voltammetry mode (Fig.
3.3), was discovered. Thus, by varying the current density we obtained the PANI yield
of 0.028 g/C on the platinum electrode.
3.1.4. Electrochemical properties of the polyaniline film in the background
solution
The investigation of the PANI film in the background solution (solution of 1M
HCl) was performed in order to establish the behaviour of pure PANI under conditions
82
Chapter 3. Electrochemical synthesis
Figure 3.6. The dependence of the potential on the polymerization time at different
applied current densities on (a) platinum and (b) SnO2-glass electrodes in the aqueous
1М HCl solution, aniline concentration 0.5М:
a – 1 – і = 0.1 mA/cm2; 2 – і = 1 mA/сm2; 3 – і = 5 mА/сm2; 4 – і = 10 mА/сm2
b - 1 – і = 0.001 mA/cm2; 2 – і = 0.005 mA/сm2; 3 – і = 0.01 mА/сm2;
4 – і = 0.1 mА/сm2
Polymerization time, min0 10 20 30 40 50 60
Pote
ntia
l, V
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Polymerization time, min0 10 20 30 40 50 60
Pote
nt V
ial,
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
(a)
4
3 2 1
(b)
4
3 2
1
83
Chapter 3. Electrochemical synthesis
Figure 3.7. Dependency of the weight of the deposited polymer on the platinum
electrode on the anodic current density (depositing time 50 min) in the aqueous solution
of 1М HCl, aniline concentration 0.5М
of charge-discharge. To that end, after the polymerization, i.e. after the accumulation of
a certain PANI amount, the platinum electrode with deposited PANI (discharged at -0.2
V) was washed by distilled water and transferred to a 1M HCl aqueous solution for
studying its stability.
With the help of cyclic voltammetry we observed the well-defined peak at
approximately 0.3 V (Fig. 3.8), which corresponds to the reversible inclusion of the
protons [180]. It should be noted that the redox activity of the grown polymer decreases
during the second cycle and then becomes constant. According to [178, 185] such the
behaviour can be explained by the electrochemically induced ion-transfer processes
involving dopant ions in the grown polymer. Thus, during the potential sweeping from
-0.2 to 0.8 V, the PANI oxidation takes place. In order to neutralize the appeared
positive charge on the oxidized units of polymer chain the induction of anions to the
polymer chain and the pushing out protons take place. Such process results in the
increase of the polymer weight [30, 36]. This process is reversible: during the next
potential sweeping to the cathodic side the polymer loses its positive charge and that’s
Current density, mA/cm20 1 2 3 4 5 6
Poly
mer
wei
ght,
g
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
84
Chapter 3. Electrochemical synthesis
Figure 3.8. Cyclic voltammograms of the PANI-modified platinum electrode in the
background electrolyte (aqueous 1М HCl solution), νscan = 100 mV/s
85
Chapter 3. Electrochemical synthesis
why the anions transfer back to the solution and the protons again enter the polymer.
However, the complete removal of anions doesn’t take place because of transport
limitations and a certain portion of them thus remains in the polymer film. As a result,
during further sweeping to the anodic side less quantity of anions can introduce to the
film. As a consequence, the peak’s current value decreases as compared with the first
cycle. After 5 cycles the response is stabilised due to stable quantity of anions
introduced in the film.
The process of charge compensation by moving the anions is unexpectedly
displayed when changing the sweeping rate in different directions on the same sample
(Fig. 3.9). Specifically, from Fig. 3.9 it was found that for the same PANI film at
different sweeping rates the current of the first anodic peak was higher during the
gradual increasing of the scan rate from 20 mV/s to 100 mV/s (Fig. 3.9a and 3.10,
curves 3 and 4) than during the decreasing of the scan rate in the opposite direction –
from 100 mV/s to 20 mV/s (Fig. 3.9b and 3.10, curves 1 and 2). It means that during the
slow potential scan rates the electrochemical processes (charge transfer) take place at a
greater depth, i.e. all PANI layers take part in it, resulting in the appearance of the
additional peak at approximately 0.5 V (Fig. 3.9a). This peak is attributed to the
formation of overoxidized PANI [35]. On the contrary, this peak is not observed during
the potential cycling at scan rate decreasing (Fig.3.9b). Also, it should be noted that the
cathodic and anodic peaks potential and the potential difference between them are not
dependent on the scan rate in the studied range of rates.
A very good linear relationship between the first anodic peak and the scan rate
(Fig. 3.10) is found, indicating a surface-controlled redox process [148]. With
increasing the scan rate the magnitude of the peak current increased. Thus, the process
of the counter ions transfer to the polymer film and back – to the solution during the
oxidation-reduction of PANI takes place practically inside the PANI layer.
3.2. The formation of the polymer composite materials
Preliminary investigation of the electrochemical aniline polymerization was
necessary for determining the conditions and the peculiarities of such a process in the
PM.
It is obvious that the electrochemical polymerization in the PM can be realized
only under the condition of the matrix film swelling in the proper conducting media to
86
Chapter 3. Electrochemical synthesis
Figure 3.9. Cyclic voltammograms of the PANI-modified platinum electrode in the
background electrolyte (aqueous 1М HCl solution) in the stationary mode:
a – from 20 mV/s to 100 mV/s; b – from 100 mV/s to 20 mV/s
1 - νscan = 20 mV/s; 2 - νscan = 50 mV/s; 3 - νscan = 100 mV/s
87
Chapter 3. Electrochemical synthesis
Figure 3.10. The variation of the anodic peak current with scan rate (a) and with √υscan
(b) for the PANI film in the aqueous solution of 1M HCl:
1, 3 – first cycle;
2, 4 – second and next cycles
υscan1/2, (V/s)1/2
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Cur
rent
of t
he a
nodi
c pe
ak, m
A
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Scan rate υscan, V/s0.00 0.02 0.04 0.06 0.08 0.10 0.12
Cur
rent
of t
he a
nodi
c pe
ak, m
A
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
3
2 4
1
3
4 2
1
(a)
(b)
88
Chapter 3. Electrochemical synthesis
insure ionic conductivity. This means that the peculiarities of such the polymerization
depend not only on electrochemical characteristics of the system, but also on the
swelling degree of the matrix. So, we investigated the swelling kinetics of the PA-12
film in the reaction media and determined the time necessary to get constant value of
swelling. The same investigation was performed in the PVA matrix in order to
determine the influence of the PM structure.
It was found that the swelling process proceeds slower in the PA matrix as
compared with the PVA matrix (the saturation in which reaches in 150 min) (Fig. 3.11).
For all further researches we used films which were in the reaction media within this
period of time.
Figure 3.11. Swelling
kinetic of the PM in the
aqueous solution of 1М
HCl, containing 0.5М
aniline:
1 – the PA-12 film;
2 – the PVA film
The voltammograms which represent the PANI formation process on
electrodes, covered by PA-12 and PVA in the cyclic voltammetry mode are illustrated
in Fig. 3.12. As one can see, this process is similar to that on the bare electrode (Fig.
3.1): at potentials 0-0.2 V and 0.65-0.75 V two couples of redox peaks are formed, the
height (current) of which also increases with each successive cycle. However, the
presence of the PM film on the electrode surface leads to the decrease of the current
values (approximately by 1.13 mA/cycle for PA-12 and by 1.47 mA/cycle for PVA)
(Fig.3.13) and also to the peak potential shift to the anodic side (on the average by 0.013
V/cycle and 0.03 V/cycle in the case of PA-12 and PVA, respectively) (Fig.3.14). The
difficulty of the aniline polymerization process on the electrodes covered with the PM
films may be explained by increasing of the system resistance, as in the case of the PM
Time, min0 50 100 150 200
Swel
ling
degr
ee, %
0
1
2
3
4
5
6
1
2
89
Chapter 3. Electrochemical synthesis
Figure 3.12. Cyclic voltammogrames of the aniline polymerization on the surface of
covered with the PM platinum electrode from 0.5M aniline + 1M HCl solution
at υscan = 100 mV/s:
a – the PVA film;
b – the PA-12 film
(a) (b)
90
Chapter 3. Electrochemical synthesis
presence the monomer molecule should diffuse not only through the formed PANI
layer, but also through the film of the PM. Really, the specific resistance of the working
electrode with the film of PA-12 was found to be increased by 107 Ohm⋅cm and in the
case of the PVA film - by 44 Ohm⋅cm.
It was found that the rate of the polymerization process depends on the PM
structure. However, the character of the PM structure influence changes depending on
the cycle number and, respectively, on the polymerization time. Thus, during the first
cycling stage the currents of redox peaks, which characterize the rate of the PANI
formation, are less in the PA-12 matrix. However, with time the ratio of the rate
changes - the peak currents in the PA-12 matrix exceed current values for the PVA film.
The obtained dependences (Fig. 3.13) testify that the PANI formation goes easier on the
bare electrode, than on the electrode covered with the polymer films. However, on the
electrode covered with the PVA film this process takes place with more difficulties than
on the electrode covered with the PA-12 film. The obtained dependence of the peak
potential on the polymerization time (Fig. 3.14) also confirms these results.
One can suppose that the reason of such behaviour can be a different degree
and rate of swelling of the PM in the working solution, which, in its turn, influences the
charge transfer process in the polymer. From the analysis of the kinetics of swelling and
the PANI peak current values it follows that in the case of the PA-12 film this factor
(swelling degree and rate of swelling) is not decisive at the swelling degree of 0.5%
beginning since 11 min of the electrode being in the working solution (compare Fig.
3.11 and 3.13). As a result, the rate of the electrochemical aniline polymerization and,
respectively, the quantity of formed PANI in the PA-12 matrix exceed the rate of the
process in the PVA matrix (even in spite of the fact that the PVA film can be swelled to
much more degree (Fig. 3.11)). Such differences testify to more effective aniline
electrochemical polymerization process in the PA-12 polymer film.
The last observation is a very important evidence of the influence of the PM
structure and can be connected with the physicochemical interaction of aniline and
PANI with PA. Such supposition is in agreement with the literature results on the
formation of hydrogen bonds between the imine PANI groups and oxygen of amide
groups in PA-6 [142, 143] and also with the obtained dependence of the first anodic
PANI oxidation potential on the cycling time (Fig. 3.14).
91
Chapter 3. Electrochemical synthesis
Figure 3.13. The dependence of the first anodic peak current density on the
polymerization time on (a) platinum and (b) SnO2-glass electrodes at 100 mV/s in 1М
HCl aqueous solution, aniline concentration 0.5М:
1 – bare electrode;
2 – electrode, covered with the PVA film;
3 – electrode, covered with the PA-12 film
Polymerization time, min0 20 40 60 80 100 120 140 160
Cur
rent
den
sity
of th
e an
odic
pea
k, m
A/c
m2
0.0
0.1
0.2
0.3
0.4
Polymerization time, min2 4 6 8 10 12 14
Cur
rent
den
sity
of th
e an
odic
pea
k, m
A/c
m2
0
2
4
6
8
10
12
14
(b)
1 2
3
2
3
1 (a)
92
Chapter 3. Electrochemical synthesis
Figure 3.14. The dependence of the first anodic peak potential on the polymerization
time on (a) platinum and (b) SnO2-glass electrodes at 100 mV/s in 1М HCl aqueous
solution, aniline concentration 0.5М:
1 – bare electrode;
2 – electrode, covered with the PVA film;
3 – electrode, covered with the PA-12 film
Polymerization time, min0 20 40 60 80 100 120 140 160
Pote
ntia
l of t
he a
nodi
c pe
ak, V
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Polymerization time, min0 10 20 30 40 50 60 70
Pote
ntia
l of t
he fi
rst a
nodi
c pe
ak, V
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
(a)
(b)
1
2
3
1 2
3
93
Chapter 3. Electrochemical synthesis
3.3. Conclusions
On the whole, the study of the peculiarities of the aniline electrochemical
polymerization on the surface of both bare and covered with the PM electrodes allows
making the following conclusions.
1. Electrochemical behaviour of the PANI formation on the bare electrodes in
potential sweeping range from -0.2 V to 0.8 V is described by two redox transitions:
leucoemeraldine-emeraldine salt and emeraldine salt-pernigraniline. Continuous cycling
of the potential results in the growth of the PANI layer on the electrode surface, which
is displayed in the increase of current peaks corresponding to the transitions. Kinetics of
the process becomes slower because of the increase of the system resistance due to the
growth of the PANI layer thickness.
2. The PANI formation occurs in two stages: the first one is connected with the
polymer nucleation on the bare electrode and is limited by the monomer diffusion. The
second one is limited mainly by the counter ions transfer through the formed PANI
layer.
3. On the SnO2-glass electrode the aniline polymerization process is strongly
limited by the resistance of SnO2 layer. On this electrode only one redox couple is
observed, which is attributed to the leucoemeraldine-emeraldine transition. However,
the anodic potential peak shifts to more positive potentials when repeating potential
cycling and becomes broader.
4. Polymerization processes studied under the galvanostatic conditions allows
easy controlling of the PANI quantity on the electrode surface. But at high current
density there is a risk to produce overoxidized PANI. Unlike this, potentiostatic mode
insures using the potential window of stable PANI. Its formation under the constant
potential is described by three distinct regions on the current-time dependencies. In the
first region an initial drop in the current was observed, which might be attributed to the
slow adsorption of the monomer. This was followed by an increase in current (region
II), which was attributed to the nucleation and growth of a new electroactive PANI
phase on the surface of the electrode. As the reaction time increased, the current became
steady (region III).
5. The voltammetry response of formed PANI is directly proportional to the
weight of the polymer deposited. Therefore, the rate of the polymer formation is directly
94
Chapter 3. Electrochemical synthesis
proportional to the increase of current, which displays the quantity of PANI. Linear
relationships between the obtained PANI amount and the current density or cycle
number (i.e. reaction time) are discovered.
6. It was found that the mechanism of the aniline polymerization process on the
electrodes covered with the PM is similar to that on the bare electrodes, but hindered by
the system resistance increase. At the same time, the efficiency of the process of the
aniline polymerization depends on the matrix structure and is higher in the PA-12
matrix that can be associated with the physicochemical interaction of aniline and PANI
with the PA matrix, specifically, with the formation of hydrogen bonds.
The obtained results allowed concluding that the composite PM/PANI films
retain electrochemical properties of pure PANI. This gives some hopes to use these
composite materials in the fields, where conductivity and electroactivity of PANI are
needed (electrochromic and optical devices, sensors, etc).
On the other hand, it should be noticed that the method of electrochemical
polymerization does not allow obtaining the conductive composite materials in a large
scale. Such limitation is connected with the necessity to consume a lot of electrical
energy and to use large equipment. This moved us to investigate the chemical aniline
polymerization inside or in the presence of the PA matrix. This method opens good
production perspectives of the PA/PANI composite in a large scale.
95
Chapter 4
CHEMICAL ANILINE
POLYMERIZATION IN SOLID
AND WATER DISPERSED
POLYAMIDE MEDIA
Chapter 4. Chemical polymerization
Introduction
The formation of the PANI composite films by the electrochemical aniline
polymerization (see chapter 3) is a rather promising method to produce such composites
for use in small size devices (sensors, etc.) [186]. However, this method does not allow
obtaining the conductive composite materials in a large scale because of the complexity
of the technological process. Hence, the chemical polymerization method can be
alternatively considered as being more suitable for practise. So, in this chapter we have
discussed the process of the chemical aniline polymerization in the PA films as well as
in the PA dispersion.
To understand characteristics of the polymerization, the kinetic peculiarities of
this process were studied. This approach allows not only judging about the mechanism
of the processes in the systems but also permits realization of the effective controlling
of such processes under real conditions. The kinetic parameters of the process are
studied depending on the oxidant nature and on the type of the PM.
4.1. The swelling kinetics of the polyamide films in the reaction
media
As in the case of the electrochemical polymerization, the chemical one may be
realized in the PM only under the condition of the matrix film swelling in the monomer
solution and in the reaction media. This means that the structure and the thickness of the
conducting layer will depend on the swelling degree of the matrix. Thus, we
investigated the swelling kinetics of the PA-12 film in aniline as well as in the reaction
media to determine the time necessary to get certain value of swelling and to understand
the influence of the PM structure on the mechanism and kinetics of this process.
It was established that the swelling process of the PA-12 film in aniline went
on faster than in the PA-6 film. It is well seen from the fact that the swelling saturation
is reached in ∼3.5 hours for the PA-12 film and only after more then 15 hours for the
PA-6 film (Fig. 4.1a and 4.1b, respectively). But, on the other hand, the maximum
swelling degree is higher in the case of the PA-6 film - 16.5 wt.% in comparison with
13.8 wt.% for the PA-12 film. On one hand, this fact may be explained, probably, by the
different hydrophilicity of the PM due to a different ratio of polar/nonpolar groups.
96
Chapter 4. Chemical polymerization
Figure 4.1. Swelling kinetic curves of the PA films in aniline:
a – the PA-12 film;
b – the PA-6 film
Time, hours0 20 40 60 80 100
Swel
ling
degr
ee, w
t.%
0
2
4
6
8
10
12
14
16
18
20
(b)
Time, hours0 2 4 6
Swel
ling
degr
ee, w
t.%
0
2
4
6
8
10
12
14
16
(a)
97
Chapter 4. Chemical polymerization
Indeed, the elementary chain of PA-12 consists of 11 aliphatic (CH2) groups,
whereas in a molecule of PA-6 there are only 5 such groups (Fig. 2.1). On the other
hand, different degree of crystallinity and, correspondingly, different free volume
amount of the used PA films can be also considered as important factor. This difference
was confirmed with the help of DSC measurements (Fig. 4.2). According to the
equation (2.9), the degree of crystallinity was calculated by the fusion enthalpy of the
polymer films (Table 4.1). It was found that the degree of crystallinity is higher in the
case of the PA-6 film when compared with that of the PA-12 film. So, as the PA-6
matrix has higher crystallinity degree, it contains less volume of the amorphous phase
than the PA-12 one. Additionally, the higher maximum swelling degree in the PA-6
film may be also explained by the fact that aniline molecules can form hydrogen bonds
with amide groups of PA, i.e. some physicochemical interaction of aniline with the PM
takes place. And as the quantity of the polar amide groups are higher in the case of the
PA-6 (Fig. 2.1), so the quantity of hydrogen bonds is higher too. Thus, the film of PA-6
can hold more aniline molecules compared with the film of PA-12. It should be
mentioned that for further kinetic investigation we used films which were set in aniline
within different periods of time in order to receive films with different degree of
swelling.
Figure 4.2. DSC
thermographs of the PA
virgin films
Also, we investigated the swelling kinetics of the PA films in the reaction
media we used, i.e. in distilled water, in aqueous HCl solution (1M) and in
Temperature, 0C-100 -50 0 50 100 150 200 250 300
Hea
t Flo
w
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
PA-6 PA-12
Exo
98
Chapter 4. Chemical polymerization
Table 4.1. Degree of crystallinity of the PA films
Type of PA
Parameter
PA-6
PA-12
∆Hc, J/g 60.7 48.6
∆H°, J/g 160 [187] 233.5 [188]
Xc, % 37.9 20.8
aqueous acidic (1 M HCl) solution of 0.1 M (NH4)2S2O8 (Fig. 4.3). As one can see in
the case of these solutions the situation is the same – the film of PA-6 (Fig. 4.3a)
reaches much higher values of the saturation in comparison with the film of PA-12 (Fig.
4.3b). And this fact is in good agreement with the explanation given above – that the
swelling kinetics goes faster in the case of the PA-6 film because of its structure (it
contains more polar groups than PA-12). This resulted in more hydrophilic nature of the
former.
4.2. The influence of the oxidative media on the polymerization
process
The determination of kinetic peculiarities of a process in combination with the
simultaneous identification of the forming products is one of the most reliable
approaches to study the reaction mechanism. This approach we applied to study the
chemical aniline polymerization process, which is accompanied by the optical density
system changes [37, 44, 108]. We used this fact to monitor the aniline polymerization
process by UV-Vis spectroscopy, since the intermediates as well as the resulting
polymer product are known to be coloured [108, 119]. As changing parameter here we
used the oxidizer nature – we used aqueous acidic solutions of APS and chlorine water
solution.
The UV-Vis spectrum of aniline is characterized by the presence of bands only
in the ultraviolet region (180-230 nm). These bands are caused by the electron
transitions of the benzene ring and of the amine group [54]. It was found that due to the
interaction of aniline with an oxidant in the solution the substantial spectral changes
took place owing to the PANI formation (bands at 315, 450 and 875 nm) [119]. In the
99
Chapter 4. Chemical polymerization
Figure 4.3. Swelling kinetic curves of the PA-6 (a) and PA-12 (b) films in the different
media:
1 – distilled water;
2 – aqueous HCl solution (1M);
3 – 0.1M (NH4)2S2O8 + 1M HCl solution
Time, hours0 5 10 15 20 25
Swel
ling
degr
ee, w
t.%
0
2
4
6
8
10
12
14
16
18
Time, hours0 5 10 15 20 25
Swel
ling
degr
ee, w
t.%
0.0
0.5
1.0
1.5
2.0
2.5
(a)
(b)
1
2
3
1
2 3
100
Chapter 4. Chemical polymerization
PA matrices similar changes are observed. Indeed, as we can see from Fig. 4.4 – 4.6, in
a few minutes after the beginning of the polymerization process in the PA-12 matrix the
PANI bands appear and continuously grow.
Visually this process in the PM swelled in aniline and immersed in the oxidant
aqueous solution begins from the formation of the blue regions at the matrix surface.
These regions rapidly change their colour to green one, merge gradually in the course of
polymerization and colour in green all the surface of the film sample. Such colour
changes testify that the PANI formation in the PM is realized through the typical stages
of the pernigraniline (Fig. 1.1) and emeraldine salt (Fig. 1.2) formation. However, the
UV-Vis spectra measured during the polymerization process do not show the stage of
the pernigraniline formation probably because of the big gap (∼3-10 min interval)
between the spectra (Fig. 4.4 – 4.6).
Figure 4.4. UV-Vis spectra of
the aqueous solution of
1 – 2.0 min; 2 – 5.0 min; 3 –
28.1 min; 8 – 32.7 min; 9 – 4
400
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
2.5
3.0
9
01 2
the aniline polymerization process in the PA-1
water chlorine solution ([Cl-] = [HOCl] = 0.003
7. 7 min; 4 1. in; 5 – 17.4 min; 6 – 20.5
5.5 10 – 76.3 min; 11 – 128.6 min; 12 – 1
Wavelength, nm500 600 700 800 900
1
23 4
5 6
7
8
min;
8 m – 1
101
1
11
2 matrix in
N):
min; 7 –
52.1 min
Chapter 4. Chemical polymerization
Figure 4.5. UV-Vis spectra of the aniline polymerization process in the PA
the aqueous solution of 1M HCl + 0.1M (NH4)2S2O8:
1 – 1.5 min; 2 – 4.8 min; 3 – 7.5 min; 4 in; 5 – 12.7 min; 6 – 1
7 – 18.3 min; 8 – 22.8 min; 9 – 26.1 min; 10 – 29.4 m – 32.8 min; 12
13 – 46.4 min; 14 – 50.7 min; 15 – 80.3 m – 119.1 min
The UV-Vis spectra of the PANI formation in the PM measured
oxidation systems (chlorine water solution, solutions of APS in hydr
sulphuric acids) reveal the presence of two bands (at 320-340 nm and at
and the shoulder at 410-440 nm. Such features of the spectra testify to the
PANI in the emeraldine salt oxidation state as in the case of the aniline po
in the solution [189]. The band at 320-340 nm can be assigned to π-π∗
benzene rings, the shoulder at 410-440 nm and the broad band at 700
obviously referred to the polaron transition [6]. Intensity of these ban
gradually in the course of the polymerization (Fig. 4.4 – 4.6). However, w
intensity and the position of the maximum change depending on the oxidati
Specifically, in case of the water chlorine solution with a small c
of Cl2 (0.003 N), the increase of the band at ∼750 nm, which is attributed t
absorbance [181], is observed (Fig. 4.7, curve 1). The gradual increase of th
– 10.1 m
in; 11
in; 16
Wavelength, nm400 500 600 700 800 900
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
2.5
1
12 13
14 15
16
102
91
011-12 matrix in
5.3 min;
– 36.2 min;
in all studied
ochloric and
700-750 nm)
formation of
lymerization
transition of
-750 nm are
ds increases
ith time the
on system.
oncentration
o the polaron
is maximum
3 4 5 6 7 8
2
Chapter 4. Chemical polymerization
Figure 4.6. UV-Vis spectra of the aniline polymerization process in the PA
the aqueous solution of 1M H2SO4 .1M H4)2S2O8:
1 – 1 min; 2 – 4.5 min; 3 – 7.8 min; 4 – 11.1 min; 5 – 14.4 m ; 6 – 17
7 – 20.6 min; 8 – 23.7 min; 9 – 27.1 min; 10 – 30.5 min; 11 – 3 mi 2 –
13 – 68.8 min; 14 – 100.5 min; 15 – 138 min; 16 – 157 min; 17 – 16
is accompanied by its widening to the blue side of the spectrum. Such beha
explained by an augmentation of the formed polymer quantity as we
bipolaron appearance [181] and the overoxidation processes. In approxim
fter the beginning of the polymerization process some deceleration of th
observed followed by the stopping of this peak growth (Fig. 4.4 and 4.7, c
can be explained by deep oxidation (and even overoxidation) of the
ontaining layers, which contact with the oxidant solution.
In the case of APS water solution in hydrochloric acid as an
increase of optical density at 750 nm is also observed during the first m
reaction (Fig. 4.5 and 4.7, curve 2). However, in this case the decele
polymerization process is observed later (at the 70-th min)
+ 0 (N
in
n; 1
a
c
Wavelength, nm400 500 600 700 800 900
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
4
9
0
23
45
67
1
103
1 2 3 56 7 8
1
11
11
11
1-12 matrix in
.5 min;
37.5 min;
8 min
viour can be
ll as by the
ately 30 min
e reaction is
urve 1). This
upper PANI
oxidant, the
inutes of the
ration of the
Chapter 4. Chemical polymerization
spectra of the polymerization process in APS solution in different acids (Fig. 4.7, curves
accompanied by the reaction of chlorine hydro
than the charged S2O82- anions. As a result, the aniline polymerization in the chlorine
containing media, i.e. in the presence of HOCl, penetrates to the deeper layers of the
PM film. In the sulphuric acid medium with only one oxidant (ASP) this process is
localized in a very thin subsurface layer of the dielectric matrix due to the limitation of
Figure 4.7. Kinetic curves of the aniline polymerization process in the PA-12 matrix in
different oxidation media:
0 18020 40 60 80 100 120 140 160
Polymerization time, min
1 – chlorine water solution, λmax = 750 nm;
2 – 0.1M (NH4)2S2O8 + 1M HCl, λmax = 750 nm;
3 – 0.1M (NH4)2S2O8 + 1M H2SO4, λmax = 670 nm
And, in the oxidation system of the aqueous solution of APS in sulphuric acid
not only stopping of the increase of the optical density is observed, but even its sharp
dropping (Fig. 4.6 and 4.7, curve 3). The observed difference between the UV-Vis
2 and 3) may be explained by the influence of the background electrolyte (HCl or
H2SO4) on the PANI formation process. It is known that in the aqueous solution of
hydrochloric acid in the presence of APS the formation of chlorine proceeds. This is
lysis resulted in the formation of HOCl
being an effective oxidant in the aniline polymerization in the PM [152]. The
nondissociated molecules of HOCl can penetrate to the dielectric matrix more easily
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
2.5
2
3
1
104
Chapter 4. Chemical polymerization
S2O82- penetration and is accompanied, obviously, by the overoxidation and degradation
of PANI formed inside the upper matrix layers. These results in higher decrease of
optical density of the formed composite (Fig. 4.6 and 4.7, curve 3) than in the case of
HCl background electrolyte (Fig. 4.4 and 4.7, curve 1).
It should be noticed that in the sulphuric acid solution the maximum of the
absorbance is shifted to the shorter wavelength region unlike other oxidation systems
(670 nm in comparison with 750 nm, respectively). The shift to 670 nm, i.e. the blue
shift, can be the evidence of the polymer formation with the shorter conjugation length.
4.3. Evaluation of the structure of the surface composite films
In order to study the kinetic parameters (e.g. reaction orders) of the chemical
aniline polymerization process and to accomplish the calculations of the PANI
formation process in the PM, it is necessary to determine the structure of the obtained
omposite film and the thickness of the conducting layer.
The structure and thickness of the composite surface films were successfully
determined by using Resonance Raman spectrometry (see section 2.3.4).
Preliminary we measured the virgin PA-12 film, pure doped PANI powder and
omposite film under the same experimental conditions using the confocal micro-
Raman spectrometry (Fig. 4.8). Spectra presented in the figure clearly demonstrate the
enhancement of the bands o A the bands of PA-12 are very weak when
recorded under the same conditions. As one can see the spectrum of the composite film
(Fig. 4.8, curve 3) do not contain any well-defined band c acteristic of the PA-12
. Due to the enhancement of the band intensity originating from PANI, the signals
om PA will be considered negligible for the complete analysis. On the other hand, one
can notice that the spectra of the composite film and the pure doped PANI powder are
similar (Fig. 4.8, curve 2 and 3). This observation is in good agreement with the results
f the electrochemical aniline polymerization inside the PM (chapter 3). It was shown
that the composite material displays the features of PANI obtained under “free”
onditions.
Hence, taking into consideration the advantage of the confocal system and the
enhancement of the PANI vibrational bands under the resonant conditions we were able
determine the thickness of the PANI containing layer (Fig. 4.9). So, the dependence
of the integrated Raman intensity in the wavenumber region 1150-1670 cm-1 on a
c
c
f P NI while
har
film
fr
o
c
to
105
Chapter 4. Chemical polymerization
sample p
also plotted versus the Z
displacem nt (Fig. 4.10b).
Fig. 4.10 clearly shows that the distribution of PANI in the PM is similar for
the PA-6 and the PA-12 films. The obtained profile of the integrated Raman intensity is
symmetric and, therefore, proves that the PANI containing layer can be considered as a
homogeneous layer of a definite thickness. The experimental points were fitted by
applying a Gaussian law. The distance d (Fig. 4.10) was obtained as a half of the width
Figure 4.8. Raman spectra collected under the same experimental conditions
(λ = 514.5 nm, t = 300 s, p = 0.04 mW):
1 – the PA-12 film;
2 – the PANI powder in the form of ES;
3 – the composite PA-12/PANI doped film
Wavenumber, cm-1
Ram
an in
tens
ity, a
.u.
osition is shown in Fig. 4.10.
The same approach and measurements were used to analyze the thickness of
the PANI containing layer formed in the PA-6 film. The evolution of the integrated
Raman intensity of the PANI characteristic bands was
e
500 1000 1500 2000 25000
500
1000
1500
2000
2500
3
1
2
106
Chapter 4. Chemical polymerization
Figure 4.9. Schematic presentation of the Raman depth analysis (a) and Raman
spectra collected on the doped composite film based on PA-12 (b):
1 – 5 - different sample positions below the objective
Wavenumber, cm-1500 1000 1500 2000 2500
Ram
an in
tens
ity, a
.u.
0
500
1000
1500
2000
2500
5
3
4
(a)
(b)
2
1
PANI containing layer
Insulating layer
12345
Z
objectiveobjective
107
Chapter 4. Chemical polymerization
1.4e+5
1.6e+5
1.8e+5
Figure 4
.10. Evolution of the integrated Raman intensity of the PANI vibrational bands
in the wavenumber region 1150-1670 cm-1:
a – the PA-12 matrix; b – the PA-6 matrix
Sample position Z, µm-2 0 2 4
3 (a)
Inte
grat
ed R
aman
inte
nsity
, a.u
.
6.0e+4
8.0e+4
1.0e+5
1.2e+5
d 4 2
4.0e+4
2.0e+4
5 0.0
1
Sample position Z, µm-4 -2 0 2 4 6 8
Inte
grat
ed R
aman
inte
nsity
, a.u
.
-2.0e+4
0.0
2.0e+4
4.0e+4
6.0e+4
8.0e+4
1.0e+5
1.2e+5
(b) 3
2 4
5 1
d
108
Chapter 4. Chemical polymerization
of the profile pea
merization we used
. Correspondingly, we can
posite film. This layer
retards diffusion of the oxidant accordingly, slows
down the form . This fact is in a good agreement
with the resu merization in the
PA-6 (Nylon) film
ANI
) of the
) is
found m
ion
of the oxidant solution od agreement with the
DSC measurements (Fig. 4. of the PM swelling kinetics in the
aqueous reaction media (Fig.
The obtained values of the
lations of the PANI yield in the composite films
and to de
(n) thickness δ, µm
k. Obtained results show that the method of the aniline poly
resulted in a double-layered conducting composite film
suppose that the process runs in an external layer of the com
solution into the interior one and,
ation of PANI inside the polymer film
lts obtained by Byun and Im [143] for the aniline poly
.
It should be mentioned that such a layered architecture is similar in the PA-6
and PA-12 films since in both cases we obtained the same depth profile (Fig. 4.10).
On the contrary, the thickness of the conducting layer strongly depends on the
structure of the PM as one can see from Table 4.2. The thickness δ of the P
containing layer was determined by taking into account the refractive index (n
PM and the full-width at half-maximum of the Raman profile (d):
δ = d ⋅ n (4.1)
The thickness of the conducting layer in the case of the PA-6 film (4.7 µm
ore than twice as much as for the PA-12 film (2 µm). This result can be
explained by the more hydrophobicity of PA-12 (Fig. 4.3), which reduces the diffus
into the PM. This explanation is in a go
2) and with results
4.3).
thickness of the conducting PANI containing layer
(Table 4.2) was used in further calcu
termine the kinetic parameters of the polymerization process.
Table 4.2. Results of the Raman depth analysis
Matrix
Distance d, obtained from
Raman depth analysis, µm
Refractive index Conducting layer
PA-6 3.1 1.53 [190] 4.7
PA-12 1.3 1.54 [190] 2
109
Chapter 4. Chemical polymerization
s solutions is rather
ecause of the
precip e rate of aniline
consum erization rate are
used 115]. Unlike this, the slow rate of the aniline polymerization process in the
transparent PM allows controlling the process in situ with the help of the UV-Vis
spectrometry as it was shown in the previous section.
As it follows from the known kinetic studies of the aniline polymerisation
[115, 191-194], under all other equal conditions the quantitative characteristics of the
t concentration
poly e thin surface layer of the PM.
.4.1. Polymerization process in the PA-12 film
4.4. Kinetic peculiarities of the polyaniline formation in the
polyamide matrix
The kinetic investigation of the PANI formation in aqueou
difficult because of the fast aniline polymerization process and also b
itation of PANI [115, 191]. Therefore, in many cases either th
ption is registered [191] or aniline derivatives with less polym
[
process depend on the aniline concentration in the matrix and the oxidan
in the solution. So, we investigated an influence of both factors on the kinetics of the
merization process inside th
4
As it is shown above, the final composite polymer film has a layered structure.
The external two layers are conducting PANI containing layers and the internal layer is
the pristine PA. The thickness of the PANI containing layer was determined by Raman
spectrometry (see section 4.3) and was found to be dependent on the structure of the
PM.
We postulate that aniline is evenly distributed only inside the PM and does not
go to the oxidant solution during the polymerization process. This is confirmed by the
fact that neither aniline nor products of its oxidation were registered in the solution. The
range of the used aniline concentrations in the polymer film is between 0.56-1.36 M and
of the used oxidant (APS) concentrations in the solution are 0.1-0.2 M.
The obtained kinetic profiles were measured for the exciton band at 750 nm.
As one
defined S- erization
in the solution [183, the existence
of three intermediate stages, which can be named as the induction period, the
can see from Fig. 4.11, these absorbance-time dependencies have the clearly
shape character, which is typical for the autocatalytic aniline polym
195-197]. Such character of the curves clearly reveals
110
Chapter 4. Chemical polymerization
intermediate propagation stage and the deceleration stage [193, 198]. The same
behaviou of the po erization process wa observed for the electrochemical method
of the PANI formation as it was noticed previously (chapter 3).
e difference between the investigated systems may be clearly seen in the
dependence of the induction period, the slope and the character of the kinetic curves on
the reagent concentrations. Specifically, in the case of the aniline polymerization in the
water solution of DBSA Madathil et al [ displa d data that allowed us to conclude
that the induction period decreased approximately by a factor of eight when doubling
the amount of APS. Slope of the kinetic curve part, which corresponded to the
propagation stage, kept practically unchanged. This observation confirmed the
independence of the rate of the propagation stage on the oxidant concentration in this
solution system. In contrast to this, for the aniline polymerization in the PM much less
change of the induction period is observed, but a rise of the slopes both of the induction
period and the propagation stage parts of the kinetic curve are noticed when increasing
the oxidant concentration (Fig. 4.11a). But the rate of the propagation step is strongly
affected by the oxidant concentration that can be seen from the change of the slope of
the kinetic curves (Fig. 4.11a).
At the same time the increase of the aniline concentration in the PM from 0.56
M to 0.81 M also did not noticeably influence the induction period duration (Fig.4.11b,
curves 1-3). But such a concentration increase resulted in more than two-fold change
(growth) of the slope of the kinetic curve part corresponding to the propagation stage
(i.e. its rate). The further increase of e aniline concentration to 1.12 M resulted in the
change of induction period time as well as curve shape (Fig. 4.11b, curve 4). Such
behaviour displays probably the change in a condition of the PA-12 film swelling.
Finally, at 1.36 M of the aniline concentration in the PM the shape of the kinetic curve
dramatically changed testifying to a significant decrease of the induction period and
also to the increase of the polymerization rate without the termination step (Fig. 4.11b,
curve 5), which is observed at lower aniline concentrations (Fig. 4.11b, curve 1-4). On
the contrary, the decreasing of the duration of the induction period was obtained with
the increase of aniline concentration (Fig. 4.11b).
period duration on n and also to the
independence of the p trations in the PA-12
r lym s
Th
193] ye
th
The kinetic curves obtained testify to the actual independence of the induction
the oxidant concentration in the solutio
ropagation stage rate on the aniline concen
111
Chapter 4. Chemical polymerization
1.0
1.5
2.0
4.11. Kinetic cu of the PANI form PA-12 film at 750 nm at
rent (a) aniline ( 4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 0.5582 M)
concentrations:
a - 1 - 0.1 M; 2 - 0.15 M; 3 - 0.17 M; 4 - 0.2 M
b - 1 - 0.56 M; 2 - 0.66 M; 3 - 0.81 M; 4 - 1.12 M; 5 - 1.36 M
4.11. Kinetic cu of the PANI form PA-12 film at 750 nm at
rent (a) aniline ( 4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 0.5582 M)
concentrations:
a - 1 - 0.1 M; 2 - 0.15 M; 3 - 0.17 M; 4 - 0.2 M
b - 1 - 0.56 M; 2 - 0.66 M; 3 - 0.81 M; 4 - 1.12 M; 5 - 1.36 M
Figuregure rvesrves ation in the ation in the
diffediffe С(NHС(NH
Polymerization time, min0 100 200 300 400
1
2
4 3 (a)
Abs
orba
nce
0.5
0.0
1
2
3 4
Abs
orba
nce
Polymerization tim ine, m0 100 200 300 400
0.0
0.5
1.0
1.5
2.0
2.5
4
(b) 5
3 2
1
112
Chapter 4. Chemical polymerization
film not higher than 0.81 M.
e typical
an the solution systems. For the polymerization in the solution
authors [ 15, 191, 192] proposed a semi-empirical kinetic model, in which aniline is
consume
These observations strongly differ from known kinetic results of th
iline polymerization in
1
d in a slow, homogeneous reaction with the oxidant (in fact this is an induction
period), followed by much faster heterogeneous reaction with the PANI precipitated
phase (an autoacceleration step). This enabled them to propose the following empirical
rate equation:
[ ] [ ] [ ] [ ] [ ]PANkAPSANkdtANd
⋅+⋅=− , (4.2)
where [AN], [APS] and [P] are the initial concentra
21
tions of aniline, APS and PANI,
correspo
ends on the aniline
concentrations and the PANI quantity formed in dispersed form in the solution phase
[115, in the case of the aniline
polymer
laniline) [115]. For the last case Sivakumar et al [115] reduced a kinetic equation
for the p
ndingly. k1 and k2 are constants of the homogeneous and heterogeneous stages,
respectively. In turn, k2 is an observed complex constant including a contribution from
the specific surface area of the precipitated PANI.
As one can see, the homogeneous stage is determined by both aniline and
oxidant concentrations, while the heterogeneous one dep
191, 192]. However, as we considered above,
ization in the PM all the formed products are located in the matrix. So, we can
say that the behaviour of the aniline chemical polymerization in the PM is more close to
the chemical polymerization of the aniline derivative, e.g. N-methylaniline (NMA),
which results in the formation of the soluble in the polymerization medium poly(N-
methy
olymerization rate Rp to the simplified one constant equation:
[ ] [ ]PDSNMAkR p ⋅= 2 , (4.3)
where [NMA] and [PDS] are concentrations of the NMA and potassium peroxodisulfate.
So, taking into account the resemblance of the polymerization of aniline
derivatives and the aniline polymerization in the PM and the fact that the kinetics was
measured by changes of the absorption of the polymer at 750 nm, we found it also
possible to use the similar simplified equation to describe the process of the PANI
formation in the PA matrix:
[ ] [ ]yxp APSANk
dtdAk
dtdPR ⋅=== 750 , (4.4)
113
Chapter 4. Chemical polymerization
114
where dtdP is the rate of the PANI formation, 750k is a coefficient of proportionality
between the content of PANI formed in the external subsurface layer [P] and its
absorptio
concentration was
k
2 and 3). A straight line is obtained
with the
n A at 750 nm. k is the observed constant; x and y are reaction orders with
respect to the aniline and APS, respectively.
In order to get suitable conditions for obtaining both reaction orders of the
polymerization process, a kinetic study was carried out with different concentrations of
oxidant in the solution at the constant aniline concentration in the PM (Fig. 4.11a) and
vice versa – with different concentrations of the aniline in the PM at a constant
concentration of the oxidant in the solution (Fig. 4.11b)
To evaluate the reaction orders we took into account the layered structure of
the composite film and the fact that during kinetic measurements the conducting layer
was formed at both sides of the pristine PM as the film was placed into the oxidant
solution. Correspondingly, the reaction zone in the case of the PA-12 film is the layer
with the thickness of 4 µm (2 µm at each side of the PM).
Thus, in order to determine the reaction order x the growth rate is calculated
from the slope of the kinetic curves on Fig. 4.11b. Since the oxidant
ept constant and only the concentration of aniline varied, the double logarithmic plot
of the growth rate V0 versus the aniline concentration [AN] is shown in Fig. 4.12a. A
straight line was obtained with the slope equal to 2.8. This indicates that the PANI
formation is 2.8-order with respect to the aniline, i.e. higher than in the case of the
aniline polymerization in the solution (equal to 2 in the case of using potassium
peroxomonosulphate [115]). Such a high reaction order testifies, obviously, to some
complication of the interaction between the localized monomer (aniline) in the PM and
APS in the solution.
To determine the reaction order with respect to APS the slope of kinetic curves
of the PANI formation in the PA-12 matrix at λ = 750 nm obtained at the fixed aniline
concentration in the film and with different oxidant concentrations in the solution (Fig.
4.11a) is counted. Again, the double logarithmic plot of the polymerization rate versus
concentration of APS is plotted (Fig. 4.12b, curve
slope equal to 1.6, which is again higher than for the aniline polymerization in
the solution (equal to 1 [115, 194]). It should be noted that we carried out measurements
for different aniline concentrations in the PM and the fixed concentration of APS in the
Chapter 4. Chemical polymerization
e
concentration of the aniline (a) and uring the aniline polymerization in
2, 3 - (NH4)2S2O8 + 1M HCl (Сaniline = 0.56 M (2); Сaniline = 1.12 M (3))
(b)
lg[AN·104]3.7 3.8 3.9 4.0 4.1 4.2
lg[V
0·104 ]
Figure 4.12. Double logarithmic plot of the polymerization rate V0 on th
the oxidant (b) d
the PA-12 film:
1 – 0.1M (NH4)2S2O8 + 1M HCl
0.4
.6
.8
1.0
1.2
.4
.6
.81
(a) 1
1
0
0
1
lg[APS·104]2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35
lg[
0·10
V4 ]
0.4
6
8
0
1.2
1.4
1.6
1.8
2.0
1.
0.
0.
(b)
3
2
115
Chapter 4. Chemical polymerization
solution (Fig. 4.12b, curves 2 and 3). This was made in order to check that the reaction
order do
cid is performed (Fig. 4.13). It is established that the dependency of the
polymerization rate on the conversion degree has parabolic character, which is the
characteristic of the autocatalytic reactions [29, 196]. In an autocatalytic reaction the
yield in the
PA-12 film
Oxidant
es not depend on the aniline concentration in the PM. It was found that in both
cases the reaction order is 1.6. Probably, the fractional reaction order with respect to the
oxidant (APS) may be explained by the formation of the other additional oxidant HOCl,
which is weakly dissociated and easily penetrates into the PM [152]. This fact can lead,
in turn, to the changes in the mechanism of the polymerization [189].
The differentiation of the kinetic curves of the PANI formation in the
hydrochloric a
rate in the beginning of the process is low because a little product is present. It increases
to a maximum as a product (PANI) is formed and then again drops to a low value as a
reactant (aniline) is consumed. The obtained curves show the same trends as for the
aniline polymerization in the solution [198].
The obtained results confirmed that the concentration of the oxidant as well as
the concentration of aniline influences the polymerization rate of the reaction. Thus, the
aniline concentration in the PM influences the initiation and propagation rates – when
increasing the aniline concentration the time of the induction period decreases and the
polymerization rate increases (Fig. 4.13a). On the other hand, when increasing the APS
concentration the rate of polymerization increases too (Fig. 4.13b).
Taking into account the thickness of the conducting layer we also evaluated
the yield of PANI in the PM (Table 4.3 and 4.4).
Table 4.3. Effect of the oxidant (APS) concentration on the PANI
General General
concentration,
M
Aniline
concentration,
M
quantity of APS
in the solution,
mol
quantity of
aniline in the
PM, mol
Yield of the
PANI
formation, %
0.1 0.56 3·10-4 6.03·10-7 49.3
0.15 0.56 -4 -74.5·10 6.03·10 63.6
0.17 0.56 5.1·10-4 6.03·10-7 90.0
0.2 0.56 6·10-4 6.03·10-7 91.9
116
Chapter 4. Chemical polymerization
117
Figure 4.13. The rate of the PANI formation versus the conversion degree of the
polymerization reaction in 1 M HCl aqueous solution:
a – at the fixed oxidant concentration (С(NH4)2S2O8 = 0.1 M);
b – at the fixed aniline concentration (Сaniline = 0.56 M)
Conversion degree, %0 20 40 60 80 100
Poly
mer
izat
ion
rate
V0·1
04 , mol
/l·m
in
0
20
40
60
80
1000.1 M APS0.15 M APS0.17 M APS0.2 M APS
(b)
(a)
Conversion degree, %0 20 40 60 80 100
Poly
me
·104 ,
rizat
ion
rate
V0
mol
/l·m
in
0
20
04
12
180
60
80
100
0
140
160
0.56 M aniline0.63 M aniline0.66 M aniline0.81 M aniline1.12 M aniline1.36 M aniline
(a)
Chapter 4. Chemical polymerization
Table 4.4. Effect of the aniline concentration on the PANI yield in the PA-12 film
Aniline
concentration,
M
Oxidant
concentration,
M
General
quantity of APS
in the solution,
mol
General
quantity of
aniline in
the PM, mol
Yield of the
PANI formation,
%
0.56 0.1 3·10-4 6.03·10-7 49.3
0.63 0.1 3·10-4 6.83·10-7 53.9
0.66 0.1 3·10-4 7.13·10-7 60.9
0.81 0.1 3·10-4 8.77·10-7 57.2
1.12 0.1 3·10-4 1.21·10-6 53.9
1.36 0.1 3·10-4 1.47·10-6 47.7
As one can see, the yield of the reaction of the PANI formation in all studied
solutions does not exceed ∼65%. Only at high oxidant concentration (0.17 and 0.2 M),
i.e. under the conditions when the more quantity of HOCl is formed, the yield of PANI
approaches ∼92
Nevertheless, the PANI amount for ed in all the cases is sufficient to provide
the value of conductivity necessary for the application of these films as antistatic
materials (Table 4.5). The conductivity of the virgin PA-12 film is given for
comparison.
Table 4.5. Influence of the polymerization solution on the conductivity of the composite
PA-12/PANI films
Polymerization solution Conductivity, S/cm
%.
m
Chlorine water solution 1-2⋅10-5
0.1M (NH4)2S2O8 + 1M HCl 5-8⋅10-5
0.1M (NH4)2S2O8 + 1M H2SO4 3-7⋅10-7
Pure PA-12 10-13-10-14
118
Chapter 4. Chemical polymerization
4.4.2. Influence of the polymer matrix structure
The investigation of the aniline polymerization process in the PA-6 polymer
film was performed in order to obtain additional information about this process and in
order to elucidate the influence of the PM structure on the polymerization process.
Kinetic curves of the aniline polymerization inside this PM are presented in
Fig. 4.14. Obtained curves are significantly different from those obtained during the
polymer
, as a result, by the deeper penetration of
the oxid
seen by changes of
the film
aximum is observed, as can be seen
in Fig. 4.14 and 4.15b. After the time corresponding to this maximum the film begins to
becom t be
explained by the continual overoxidat which resulted in the formation and
release into the solution PANI wi conjugated chains and also in the
forma e solu cts o ad as ne
[1
diminution of the optical den erve ll st s
(e.g. Fig 4). Therefo wing for th we took titative characteristic
the value of the absorbance which corresponds to the ma the kine ve for
the calc ns. Similar to the calculatio e PA-12 stimated action
ization inside the PA-12 film under the same conditions (Fig. 4.15).
First of all, it should be noticed that the bands at 320-340 nm and at 700-750
nm as well as the shoulder at 440 nm presented in the UV-Vis spectrum (Fig. 4.15a)
testify to the formation of PANI in the emeraldine salt oxidation state in the PA-6 film
as in the case of the PA-12 film (Fig. 4.5). But, on the other hand, the rate of the PANI
formation in the PA-6 film is much higher in comparison with that of the PA-12 case
(Fig. 4.15a). Such the difference may be explained by the distinction of the both PM
swelling in the reaction media (Fig. 4.3) and
ant into the PA-6 film and, correspondingly, by the thicker the PANI containing
layer in the PA-6 composite film (Fig. 4.10 and Table 4.2).
The second important observation is the fact that the kinetic curves in the case
of the PA-6 film is completely different from those obtained for the PANI formation in
the PA-12 film (Fig. 4.15). The deceleration of the process in the PA-6 film observed at
∼10 min is rapidly enhanced at 15-20 min (Fig. 4.14). This can be
absorbance. With an increase of the polymerization time the absorbance of the
film decreases and the curve with a well-defined m
e colourless. Such behaviour was also observed by Neoh et al [142] and migh
ion of PANI
of th shorter
f the PANI degr
sity is obs
tion of th
99, 200].
The
ble produ ation, such
d under a
p-benzoquino
udied condition
. 4.1 re, allo is fact as a quan
ximum at tic cur
ulatio ns for th film we e the re
119
Chapter 4. Chemical polymerization
Figure 4.14. Kinetic curves of the PANI formation in the PA-6 film at 750 nm at
different (a) aniline (С(NH4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 1.33 M)
concentrations:
a - 1 - 0.46 M; 2 – 1.10 M; 3 - 1.33 M; 4 - 1.43 M;
b - 1 - 0.05 M; 2 - 0.07 M; 3 - 0.1 M; 4 - 0.12 M
Polymerization time, min0 20 40 60 80 100 120 140
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Polymerization time, min0 50 100 150 200
Abs
orba
nce
0.0
0.5
1.0
1.5
2.0
2.5
(a) 4
3 2
Polymerization time, min
0 50 100 150 200
Abs
orba
nce
0.000
0.005
0.010
0.015
0.020
0.025
1
1
3
4 (b)
1
2
120
Chapter 4. Chemical polymerization
igure 4.15. Differences between the PA matrices, aqueous solution of
1M HCl + 0.1M (NH4)2S2O8 (Сaniline = 1.3 M):
a – UV-Vis spectra of the polymerization process after 4 min
b - Kinetic curves of the PANI formation at 750 nm
400 500 600 700 800 900
Abs
ance
orb
0.0
0.2
0.4
0.6
0.8
1.0PA-6
Wavelength, nm
PA-12
Polymerization time, min0 100 12 160
F
20 40 60 80 0 1400.0
0.5
2.5
PA-6PA-12
1.5
2.0
Abs
orba
nce
1.0
(b)
(a)
121
Chapter 4. Chemical polymerization
122
d compared them with those
obtained
n the
s with respect to aniline and oxidant (Fig. 4.16) anorder
for the PA-12 film (Table 4.6).
Table 4.6. Experimentally obtained reaction orders for the aniline polymerization i
PM in the (NH4)2S2O8 water solution in HCl
Polymer film
Reagent
PA-6
PA-12
aniline 2.8 2.8
oxidant (APS) 1.6 1.6
As one can see from the received results the reactions orders with respect to
both aniline and APS are the same irrespective of the PM. Such the result testifies that
the aniline polymerization inside the PA film proceeds according to the same
mechanism.
Differentiation of the kinetic curves of the aniline polymerization in the PA-6
films gives the same parabolic dependencies as for the PA-12 composite films (Fig.
4.17), w
ch the process allows obtaining the conductive PANI containing layer in a
thin sub
technological point of view. Besides, PANI obtained in this way can be doped only by
small-sized inorganic acids and not by organic ones because of the diffusion limitations
hich confirm the autocatalytic character of the aniline polymerization reaction
inside the PM.
The yield of PANI in the PA-6 matrix is much less than that in the PA-12 film
and increases twice at increasing the oxidant concentration – at 0.07 M APS the yield is
17.3% and at 0.12 M APS – 34.4% (Table 4.7). While, the PANI yield stays practically
constant at increasing the aniline concentration to 1.10 M, but during further
augmentation of the aniline content inside the PM to 1.33 M it rises to 28%.
On the whole, the data obtained allow both an easy control of the aniline
polymerization process and the prediction of the PANI yield in the PA matrices. At the
same time it should be noted that the polymerization of aniline inside the PM seems to
be more preferable, when using as a matrix non porous industrial polymers with a poor
solubility. Su
surface layer of the dielectric matrix. Owing to a small thickness (2-5 µm) the
PANI containing layers can quickly react to an external environment changes that
allows applying them in sensors or optical devices.
However, the method of the surface polymerization is not easy from the
Chapter 4. Chemical polymerization
Figure 4.16. Double logarithmic plot of the dependencies of the polymerization rate V0
on the concentration of the aniline (a) idant (b) during the aniline
polymerization in the PA-6 film:
1 – 0.1M (NH4)2 2O8 + 1M HCl
2, 3 – (NH4)2S2O8 + 1M HCl (Сanilin 2); Сaniline = 1.10 M (3))
and the ox
S
e = 1.33 M (
lg[AN·104]3.6 3.7 3.8 3.9 4.0 4.1 4.2
lg[V
0·104 ]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.6
1.4
lg[APS·104]2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4
lg[V
0·104 ]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
(b)
1
3
2
(a)
123
Chapter 4. Chemical polymerization
(a)
Figure 4.17. The rate of the PANI formation versus the conversion degree of
polymerization reaction in 1 M HCl aqueous solution:
a – at fixed oxidant concentration (С(NH4)2S2O8 = 0.1 M);
b – at fixed aniline concentration (Сaniline = 1.33 M)
Conversion degree, %0 20 40 60 80 100
Poly
mer
izat
ion
rate
V0·1
04 , mol
/l·m
in
0
200
400
600
800
1000
1200
14000.05 M APS0.07 M APS0.1 M APS0.12 M APS
(b)
Conversion degree, %0 10020 40 60 80
Poly
mer
izat
ion
rate
V0·1
04 , mol
/l·m
in
0.0
0.6
0.9
100.0
200.0
300.0
0.3
0.46 M aniline1.10 M aniline1.33 M aniline(a)
124
Chapter 4. Chemical polymerization
Table 4.7. Effect of the oxidant and aniline concentrations on the PANI yield in the
PA-6 film
Oxidant
concentration,
M
Aniline
concentration,
M
General
quantity of
APS in the
solution, mol
General quantity
of aniline in the
PM, mol
Yield of the
PANI
formation, %
0.05 1.33 1.5·10-4 3.39·10-6 0.5
0.07 1.33 2.1·10-4 3.39·10-6 17.3
0.1 1.33 3·10-4 3.39·10-6 28
0.12 1.33 3.6·10-4 3.39·10-6 34.4
0.1 0.46 3·10-4 1.18·10-6 14.2
0.1 1.10 3·10-4 2.82·10-6 15.7
0.1 1.33 3·10-4 3.39·10-6 28
0.1 1.43 3·10-4 3.67·10-6 27.9
of large ganic m cules penetration inside the PM. This problem can be resolved
through formation of the PANI composites when using dispersion polymerization
method [201].
4.5. The aniline polymerization in the dispersed polyamide media
mong the methods described in the literature for the preparation of the
composite materials, the dispersion polymerization allows forming core-shell particles
(where PA-core and PANI-shell). Firstly, this method has the advantage that the host
matrix is hardly changed during the preparation of the conducting composite and,
therefore preserves its desired mechanical properties. The reason for the formation of
the core-shell structure in situ polymerization is that the PANI chains are insoluble in
the medium (water) if their size exceeds a certain value. After this point, the polymer
chains can either nucleate a new particle, aggregate with other polymer chains to form
the PANI structure, or absorb onto an existing surface. If the polymerization of the
monomer is performed in the presence of a host powder matrix, a very large surface
area will be present in the medium for the aniline to absorb onto. If this happens, core-
or ole
A
,
125
Chapter 4. Chemical polymerization
shell structures can be formed. The process of the formation of conducting core-shell
composites is schematically depicted in Fig. 4.18.
Figure 4.18. Schematic presentation of the formation of the conducting core-shell
structure via an in situ polymerization
In the dispersion media it is difficult to observe the polymerization process by
UV-Vis spectroscopy. By this reason the progress of the aniline polymerization was
monitored by temperature-potential-pH changes recorded with the special equipment
(see section 2.3.9). Continuous monitoring the redox potential, the temperature and the
pH of the polymerization solution enabled us to follow each stage of the synthesis,
which involved significant changes in the oxidation state and in the protonation state of
the intermediary and final products. The acid-dopants we used are TSA and DBSA,
because they have been reported to obtain rather thermostable PANI [202]. The
obtained profiles are demonstrated in Fig. 4.19. The conditions of the polymerization
process and the concentration of the reagents see in section 2.2.4. As for all other
methods of the PANI formation (electrochemical and chemical in thin subsurface layer
of the PM) we also investigated the aniline polymerization under “free” conditions, i.e.
in the absence of the PA powder (Fig. 4.20). It was made for better understanding the
effect of the PM powder on the aniline polymerization.
rns three distinct periods can be
distinguished (Fig. 4.19a): the induction period, the propagation stage (the period of the
oxidative polymerization) and the period after polymerization [104, 193, 198].
on mixture of aniline, acid and PA
powder no colour changes are observed. During the induction period the temperature of
the reaction mixture does not change. While pH starts to decrease, the value of the
potential begins to increase (Fig. 4.19 and 4.20). The continuous decrease of pH during
the induction period is due to a cont
mixture in a course im 03]. Gradually, the
As one can see, in all obtained patte
Right after APS is added to the reacti
inuous release of TSA or DBSA and H2SO4 into the
of the aniline oxidation and d erization [2
PA particle
aniline NH2HA.
NH2HA.
NH2 HA.
NH2HA.
NH2
HA.
NH2.HA
oxidantPANI shell
PA core
acidic mediumHA
PA particle
aniline NH2HA.
NH2HA.
NH2 HA.
NH2HA.
NH2
HA.
NH2.HA
oxidantPANI shell
PA core
acidic mediumHA
126
Chapter 4. Chemical polymerization
Polymerization time, min0 20 40 60 80 100
of different acids:
a – TSA;
b – DBSA
Figure 4.19. Observed during the aniline polymerization time dependence of
temperature-pH-potential in the PA-11 dispersion at room temperature in the presence
Pote
ntia
l, V
pH
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Tem
pera
ture
, 0 C
20
21
22
23
24
25pHPotential, VTemperature, 0C
32.17 min
19.25 min
23.6 0C
Polymerization time, min0 20 40 60 80 100
Pote
ntia
l, V
p
H
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Tem
pera
ture
, 0 C
20
21
22
23
24
25pH Potential, VTemperature, 0C
62.25 min
43 min
23.6 0C
Period after polymerization Induction period
Propagation stage
(b)
(a)
127
Chapter 4. Chemical polymerization
Polymerization time, min
Figure 4.20. Time dependence of temperature-pH-pH-potential observed during the aniline
polymerization at room temperature in the presence of different acids:
a – TSA;
b – DBSA
polymerization at room temperature in the presence of different acids:
a – TSA;
b – DBSA
otential observed during the aniline
0 20 40 60 80 100
Pote
ntia
l, V
pH
0.0
0.2
40.
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Tem
pera
ture
, 0 C
pH Potential, V
22
23
25
Temperature, 0C
20
21
24
63.25 min
32.42 min
23.7 0C
(a)
Polymerization time, min0 20 40 60 80 100 120
Pote
1.6
1.8
ntia
lH
, V
p
0.0
0.2
0.4
60.
1.
0.8
1.0
1.2
4
Tem
pera
ture
, 0 C
20
21
22
23
24
25pH Potential, VTemperature, 0C
60.25 min
23.7 0C
84.5 min
(b)
128
Chapter 4. Chemical polymerization
originally colourless mixture turns blue and later gets darker. The slope of the pH curve
becomes more negative as the polymerization starts. The temperature of the reaction
m
its
m imum (∼23.6 0C (Fig. 4.19 and 4.20)). The deep blue colour observed during the
polymerization at this stage can be assigned to the protonated pernigraniline [192],
namely to its dication diradical form [205]. This fact is confirmed by Stejskal et al
[104]. They performed the aniline polymerization on the glass support and after a
certain period of time being in the reaction media removed this support from the
polymerization solution and measured UV-Vis spectra.
After some time (sufficient for the complete protonated pernigraniline
for ation) which corresponding to maximum of potential on the kinetic curves (Fig.
4.19 and 4.20), the blue reaction mixture starts turning green. During this phase, the
dication diradicals are reduced by a surplus of the aniline [189] to a stable cation radical
structure of the protonated emeraldine [191] and the potential starts to decrease:
Polymerization still proceeds as the aniline is oxidized to PANI by the
pernigraniline dication diradical, until the reduction of pernigraniline to emeraldine is
completed. Protons are released during the polymerization process (scheme (4.5)) and
pH drops even more steeply (Fig. 4.19 and 4.20). After the polymerization has been
completed, the concentration of the protons equilibrates and the pH levels stay without
further changes (Fig. 4.19 and 4.20).
Such a scheme (with the intermediate pernigraniline formation) is confirmed
by the results [206]. The experimental evidence [205] indicated that the intermediate
compound (pernigraniline) is able to act as homogeneous mediator of the electron
t
Thus, the processes involved in
sketched as (4.6).
.
ixture rises steeply to reach a sharp maximum that corresponds to the fact that the
aniline polymerization is the exothermic process [204]. The temperature reaches
ax
m
n
N
H
N. ..n
N
H
N.+ +
HH- e-
+(4.5)
ransfer in redox processes. Mediator decreases the redox potential of the reaction.
the aniline polymerization can be schematically
129
Chapter 4. Chemical polymerization
where M is the mediator and M+· is the corresponding cation radical formed in the
course of the oxidation. In this case, mediators are compounds that can easily switch
from the reduced to oxidized form, and vice versa, and that have a redox potential
between the potentials of sulphate/peroxodisulfate and aniline/aniline cation radical
pairs.
Besides, pernigraniline and emeraldine can be converted into each other
according to the scheme (4.6). It is, therefore, not surprising that PANI when formed in
the reaction mixture and promoted the aniline polymerization has an autocatalytic effect
[196
s of
all
reac
he
in
An influence of the acid-dopant nature is also significant - the aniline
polym TSA
than in the pres xplained by the
steric hindrance of the m A. Due
to this, the rate of the an
(4.6)
0.5 S2O82-
SO42-
M
M
NH2
NH2
+.
+.PANI
].
As one can see from Fig. 4.19 and 4.20 profile of the curves almost does not
change in the presence of the PA powder. In contrast to this, the characteristic point
the polymerization process (the potential and temperature peaks, beginning of pH
decrease) are shifted to much less times. Such behaviour testifies to acceleration of
polymerization stages in the dispersion medium. Table 4.8 clearly shows that the
polymerization rate increases in the presence of the PA powder compared to that in the
tion solution without the PA powders. Such effect is observed independently on the
used acid-dopant. This indicates that the polymerization of aniline on the surface of t
PA powder starts earlier than in the whole volume of the reaction media, which is
accordance with observations reported in the literature [104, 192, 198]. So, we may
point out that the polymer powder dispersion has a significant catalytic effect on the
oxidative aniline polymerization for account of the increase of the available surface on
which PANI can deposit.
erization without the PA powder is completed quicker in the presence of
ence of DBSA (Table 4.8). This difference may be e
onomer coupling in a presence of larger anions of DBS
iline polymerization is greater in the case of TSA.
130
Chapter 4. Chemical polymerization
T
polymerization pro ig. 4.19 and 4.20)
Acid
S
able 4.8. Parameters of the potential peak position (min) registered during the aniline
cess (from F
Medium TSA DB A
Aniline 63.35 4.5 8
Aniline + dispersed PA 62.232.17 5
In the presence of the dispers wder th cture is ob d – in
case of T the rate of p n i r.
, it should ntioned t the con NI in the osites
was cal ed on the b f the deve method (see section 2.3.1) it was found
that the posites pre with TSA a higher yield of PANI than that in the
presence of DBSA as listed in Table 4.9. Such observation also can be explained by the
steric hi ce in the pr e of DBSA
Estimated practical value
ed PA po e same pi serve
SA olymerizatio s greate
Also be me hat when tent of PA comp
culat asis o loped
com pared had
ndran esenc .
Table 4.9. The yield of the PA-12/PANI bulk composites prepared in the presence of
different acids
TSA DBSA
Theoretical value of
the PANI-acid
content, wt.% content, wt.% yield, % content, wt.% yield, %
1 0.48 48.53 0.61 60.72
2 1.59 79.48 1.13 56.61
4 3.19 79.72 3.61 90.32
6 5.04 83.94
8 7.53 94.14 5.64 70.47
10 9.92 99.22 7.75 77.53
4.6. Conclusions
The study of the chemical aniline polymerization inside the polymer film and
in the polymer powder dispersion enables us to draw the following conclusions:
131
Chapter 4. Chemical polymerization
1. The chemical aniline polymerization inside the PM triggers off the interface
of the oxidant solution and the swelled in aniline PM and, thereafter, occurs deep in the
PM. The polymerization process is limited by the matrix’s ability to swell in the oxidant
solution, by the oxidant and monomer diffusion into the reaction zone and, also, by the
presence of the overoxidized PANI in the upper PM layer. All these features result in
the layered structure of the composite films, which is confirmed by Raman
spectrometry. For the first time with the help of Raman spectrometry it has been
established that the thickness of the formed PANI containing layer depends on the PM
structure (2 µm
ability of the matrix to sw
of the reaction solution proves
the exist
diffusion
limitation w
for PA-12 and 4.7 µm for PA-6). This difference is determined by the
ell in the reaction medium.
2. The kinetic peculiarities of the PANI formation in the PA matrices are
connected with the solid state of the PM. Such state of the PM is determined, on one
hand, by the diffusion limitation of the process and, on the other hand, by the
distribution of the monomer (aniline) in a free volume of the amorphous phase of the
semi-crystalline polymer (PA). Also, the ratio of the hydrophilic and hydrophobic
groups in the polymer influences the polymerization process inside the PM. This was
confirmed by the higher rate of the aniline polymerization process in the PA-6 film in
comparison with that in the PA-12 film.
3. The result of the specific character of the matrix polymerization process is
noticeably higher reaction orders with respect to aniline and APS in comparison with
the same process in the solution. At the same time both in the solution and in the PM the
aniline polymerization process runs according to the autocatalytic mechanism and has
three stages – an induction period, a propagation stage and a termination stage. The
determination of the temperature-potential-pH changes
ence of three stages during the dispersed aniline polymerization process.
4. It was established that the aniline polymerization inside the subsurface layer
of the PA matrix proceeds with a rather high yield (∼92 %), in spite of the
hich were confirmed by the spectroelectrochemical measurements. High
efficiency of this process results in obtaining the composite materials with the surface
conductivity of ∼10-4 S/cm. This conductivity level gives hope on an application of the
matrix polymerization process to produce materials with antistatic or sensor properties.
5. The aniline polymerization in the water PA powder dispersion in the
presence of the organic acids (TSA and DBSA) allows obtaining the PANI layer on the
132
Chapter 4. Chemical polymerization
surface of the PA particles. As a result the particles with a core-shell structure are
formed.
6. The influence of the acid-dopant on the polymerization process has been
established. It has been found that in the presence of TSA the reaction is completed
quicker than in the presence of DBSA, which is explained by the steric hindrance of the
monomer coupling in the presence of big anions of DBSA.
7. It has been established that the polymerization rate in the presence of the
dispersed PA powder is greater than in the solution of pure aniline. The possible reasons
of this fact can be the catalytic effect of the PA particle surface and the fact that the
polymerization roceeds in the th particle surface. It
is known that the concentration of the reagents in such layers is higher than that in the
solution.
8. It has been shown that the chemical aniline polymerization both in the PM
and in the presence of the PA dispersion takes place according to the same mechanism
as the polymerization of aniline under “free” conditions.
9. Establishing parameters of the aniline polymerization process in the PM and
PA powder dispersion makes it possible to control the properties (conductivity,
thickness of the conducting layer) of such composites.
process p in absorbance layer on the
133
Chapter 5
SPECTROELECTROCHEMICAL,
THERMAL, MECHANICAL,
STRUCTURAL AND RAMAN
PROPERTIES
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Introduction
To increase the range of the PANI applications, the composite materials
including non-conducting polymer (PA) and PANI have been obtained by
electrochemical and chemical aniline polymerization (Chapter 3 and 4). In order to
obtain the composites with high technological and physical parameters and properties,
the impact of the nature of the acid-dopants and the type of PA on certain properties of
the composites has been investigated. It is believed that these parameters play an
important role in controlling the properties of the resulting composites. The use of
different methods enables us to compare characteristics of the pure components of the
composite and the composite itself and, thus, to obtain the composites with high
technological and physical properties.
5.1. Spectroelectrochemical investigation of polyaniline and its
composites
5.1.1. Spectroelectrochemical properties of the polyaniline films
The spectral behaviour of the PANI film is determined by the volume charge,
which the PANI film has obtained during electrochemical transformations [120, 207].
One can see this effect clearly from the absorption spectra of the PANI films deposited
onto the SnO2-glass electrode by the cyclic voltammetry method at different applied
potentials in a background electrolyte (Fig. 5.1).
The UV-Vis absorption spectra of the PANI films obtained at different applied
potentials show three absorption bands – at 315, 450 and 875 nm. The position and
absorbance value of these bands changed depending on the applied potentials. When the
applied potential was -0.2 V the PANI film is in completely reduced LE oxidation state
(Fig. 1.1) as expected. In this state PANI has a very low conductivity value [108]. Its
energy structure is similar to the structure of inorganic semiconductors. Absorbance
maximum at 315 nm in this oxidation state (Fig. 5.1, curve 1) is originated from the π -
π* transition of the benzenoid rings [207] and can lead to the electron transitions
between valence and conduction bands. However, a peak at ∼450 nm (cation-radicals)
[22, 108, 207] and a broad peak with the maximum at ∼875 nm [207], which are in
charge of polaron band transitions in the PANI film, in spectrum of LE seems to
134
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.1. UV-Vis spectra of the PANI film in the background electrolyte (1 М HCl) at
different applied potentials:
1 – Е = -0.2 V; 2 – Е = 0.1 V; 3 – Е = 0.3 V; 4 – Е = 0.5 V; 5 – Е = 0.6 V; 6 – Е = 0.7 V
indicate that a small fraction of oxidized units still remains in the polymer. This may be
due to incomplete reduction of the film or due to LE local reoxidation by oxygen. The
increase of the potential results in the growth of the optical density at 450 and 875 nm,
but the spectra remain similar (Fig. 5.1). With a further increase of the potential to 0.8V,
the charge delocalization on polyaromatic chain occurs, resulting in the absorption peak
shifting to the ∼600-650 nm, at which the imine form of PANI absorbs. In accordance
with [55] this absorption is associated with the bipolaron formation. The LE form is
oxidized to the emeraldine one. Such transitions are accompanied by a smooth visual
colour change of the PANI film: almost colourless at -0.2 V (absorption peak at 330
nm) with potential increasing to 0.7 V the colour changed to green (absorption peak at
875 nm). The oxidation process and the emeraldine salt formation which take place in
the PANI film are in charge of these colour changes. This transition corresponds to the
first couple of the redox peaks on the cyclic voltammogram (Fig. 3.1a), that well
confirm reversible changes, which take place in the PANI film.
1
2
7 65
4
3
Wavelength, nm300 400 500 600 700 800 900 1000
Abs
orba
nce
0.0
0.1
0.2
0.3
0.4
0.5
6 5 4 3
2
1
135
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
By analyzing the absorption spectra of the PANI films obtained under
potentiostatic conditions one can see that the films formed at 0.7 and 0.75 V (Fig. 5.2a,
curves 2 and 3) have the highest absorption at ∼650-700 nm (i.e. in the region which is
typical of the conducting PANI in the emeraldine form). As it was discussed before, this
fact testifies to the formation of bipolarons [119, 208]. The UV-Vis spectrum of PANI
formed at 0.6 V (Fig. 5.2a, curve 1) does not reveal any maximums, which testify to the
formation of PANI with a shorter conjugation length and in a non-oxidized state.
Moreover, the quantity of PANI formed at 0.6 V is very small that well agrees with
electrochemical results (see chapter 3.1.2). On the contrary, at the potential 0.8 V (Fig.
5.2a, curve 4) the deep oxidation and overoxidation take place which give rise to
decreasing the optical intensity of the peaks. Under these conditions we received the
dark blue PANI film (pernigraniline). The shape of the spectrum testifies to the polymer
degradation, which results in an irreversibility of the PANI oxidation process. Thus, in
order to obtain the conducting non-overoxidized PANI film its formation should be
performed at potentials between 0.6 V and 0.8 V.
The spectra of the PANI films formed in galvanostatic mode show similar
behaviour: at і = 0.001 mA/сm2 a yellow film of PANI in the non-oxidized state is
formed (Fig. 5.2b, curve 3); at і = 0.1 mА/сm2 a non-homogeneous overoxidized PANI
film is obtained (Fig. 5.2b, curve 1) and at the intermediate і = 0.005-0.01 mА/сm2
rather homogeneous electroactive green PANI films are formed (Fig. 5.2b, curves 2 and
3).
Obtained results are in a good agreement with the previously discussed data
(Fig. 3.1, 3.5, 3.6) and with the literature [108, 119]. The colour changes in the PANI
films are evidence of the existence of the several oxidation states which differ in the
oxidation levels. Such spectral PANI properties can be used in different colour
electrochromic displays or sensors, etc.
5.1.2. Spectroelectrochemical properties of the conducting composite films
The results of spectroelectrochemical measurements have shown that in the
case of the aniline electrochemical polymerization in the PM two absorption maxima –
at ∼440 nm and at ∼825 nm are observed (Fig. 5.3) like in the case of the pure PANI
film (Fig. 5.1). These maxima, in accordance with the literature [108, 120, 207], are
connected with the polaron (localized cation-radical) formation. It should be pointed out
136
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.2. UV-Vis spectra of the PANI films obtained under (a) potentiostatic and (b)
galvanostatic conditions in the aqueous solution of 1М HCl, aniline concentration 0.5 М:
a - 1 – Е = 0.6 V; 2 – Е = 0.7 V; 3 – Е = 0.75 V; 4 – Е = 0.8 V;
b - 1 – i = 0.001 mA/cm2; 2 – i = 0.005 mA/cm2; 3 – i = 0.01 mA/cm2;
4 – i = 0.1 mA/cm2
Wavelength, nm400 600 800 1000
Abs
orba
nce
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Wavelength, nm400 600 800 1000
Abs
orba
nce
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
(a)
3
4
2
1
(b)
1
3
2
4
137
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.3. UV-Vis spectra of the composite PM/PANI films at different applied
potentials:
1 – PVA, E= -0.2 V; 2 – PVA, Е = 0.5 V;
3 – PA-12, Е = -0.2 V; 4 – PA-12, Е = 0.5 V
that on the spectrum measured at -0.2 V the peak at 825 nm is also observed as in the
case of the PANI film (Fig. 5.1). This fact can be explained by the incomplete reduction
of the film. During the oxidation from -0.2 V to 0.5 V the peaks shifted from 825 nm to
770 nm and from 440 nm to 420 nm (Fig. 5.3). Such a hypsochromic shift resulted in
the colour change of the film from pale yellow to green which is similar to the
behaviour of pure PANI. Also, it should be noted that the absorption intensity of PANI
formed in the PA-12 matrix is higher than that in the PVA matrix (Fig. 5.3, curves 2 and
4) that can be explained by the different PANI amount obtained in the PM. This
observation is also in a good agreement with the obtained dependencies of the first
anodic peak on the bare and covered with the PM electrodes (Fig. 3.13). Such a
difference in the PANI quantity in the different PM can be explained by the
physicochemical interaction between the PANI molecules and amide groups of the PA
matrix – the formation of the hydrogen bonds.
Wavelength, nm400 600 800 1000
Abs
orba
nce
0.0
0.1
0.2
0.3
0.4
1
4
2
3
138
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
One can suppose that during the aniline polymerization in the PM the presence
of this interaction can lead to a certain orientation of aniline molecules. This orientation,
in its turn, can effect the conjugation length and the doping level of PANI obtained on
the bare and covered with the PM electrode surfaces. Indeed, the peak in the UV-Vis
spectra at 770 nm, which was attributed to polaron transition in PANI, was broader and
shifted to the shorter wavelength (Fig. 5.3, curves 2 and 4) in comparison with those for
PANI formed on a bare electrode (Fig. 5.1, curve 4). This fact allows us supposing a
shorter conjugation length of PANI formed inside the PM. However, judging by the
shifting value the PANI conjugation length in both matrices is practically the same and
doesn’t depend on the matrix structure. In spite of this, the structure of the PM is
revealed in a different doping level of the formed PANI structures. Thus, according to
the literature results, one can judge about this level by the ratio of the absorption
intensity of quinonediimine structures (400-500 nm) and polarons presented in the
polymer (800 nm). Having analyzed the received UV-Vis spectra, it is easy to see that
in the case of pure PANI the polaron absorption dominated (Fig. 5.1 and 5.2). On the
contrary, on the UV-Vis spectra of the composite films the absorption of the
quinonediimine structure is revealed (Fig. 5.3). The quantity of these structures
increases from PA-12/PANI to PVA/PANI films. In other words, the PVA matrix
makes the doping process more difficult despite of its less resistance, in comparison
with the PA-12 matrix. The reason of such difficulty may be a stronger interaction of
PANI with the matrix. Such explanation is in good agreement with the voltammetry
results of the potential shift of the PANI peaks in both PM (Fig. 3.14).
The changes of the UV-Vis spectra of PANI can be caused not only by the
different reaction conditions, but also by the arbitrary oxidation under the influence of
the external factors. During the discharge of the PANI films at -0.2 V the films become
colourless, but after being in the air during some period of time (< 3 min) their green
colour is gradually restored. At further potential applying of -0.2 V the oxidized PANI
state is rather easily reduced to the primary state, however, in the air the process of the
colour changing is repeated. This fact can be explained by the oxidation of the PANI
reduced film (leucoemeraldine state) by the oxygen which is in the air.
139
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
5.2. Thermal behaviour of polyaniline and its composite materials
For all practical purposes, the stability of the conducting composites is very
important aspect to pay attention to. The thermal stability and thermal behaviour of
PANI are the subjects of many investigations [75, 202, 209]. However, since the
synthesized composite materials are based on PANI and PA, their thermal stability can
be different from that of the neat polymers. To evaluate the possible effect of PANI in
this case, the thermal stability of the polymers and the composites was investigated by
TGA.
5.2.1. Thermal behaviour of polyaniline
The thermal gravimetric analysis results for the dedoped and doped PANI
powders are given in Fig. 5.4. A weak weight loss step between 60 and 100 0C on the
weight curve of PANI-base is attributed to the losses of small molecules such as
solvents and impurities [210]. A strong weight loss step, which commences at about 350 0C, is attributed to the decomposition of the PANI backbone involving chain scission
and fragmentation [211].
In contrast to PANI-base, the thermograms of doped PANI (Fig. 5.4) show
three-step weight loss process, which is similar to the observations done by Ding et al
[75] and Wang et al [209]. The small fractions of weight loss (∆m = 2-7 %) below 120 0C may be attributed to the loss of moisture in the PANI samples in accordance with
[75]. The amount of the weight loss due to the evaporation of moisture is greater for the
PANI-HCl and PANI-H2SO4 powders than for the PANI-TSA, PANI-DBSA, PANI-
CSA and for the dedoped PANI powders. This indicates, obviously, that the HCl- and
H2SO4-doped PANI absorbs water easier than other samples. These TGA results are in
good agreement with Matveeva et al results [212], who reported that it is very difficult
to remove completely the bound water by drying. The second weight loss, in the range
of 120-250 0C, is attributed to the dedoping process of the PANI powder [209] and the
last one is due to the decomposition of the PANI chains [211]. This occurs at 350 0C,
which is consistent with the previous results [209] and with the TGA trace for dedoped
PANI.
140
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.4. TGA traces for the chemically synthesized PANI powder, 10 0C/min
rom Fig. 5.4 and Table 5.1 we can see that the weight loss of the PANI-acid
powders in the temperature range of 180-280 0C is about 5-18 % depending on the acid-
dopant and it is due to the evolution of the free dopant [75]. The heavy weight loss at
about 370 0C is corresponding to the degradation of organic acid (Fig. 5.5). Besides, a
higher thermal stability of DBSA-doped PANI as compared to CSA-doped PANI (Table
5.1 and Fig. 5.4) agrees with Han et al results [213] and may be caused by the different
evaporation temperatures because the PANI-CSA powder contains the largest content of
moisture due to the hydroscopic nature of CSA.
rom the analysis of the obtained results it is easy to see that the PANI
powders doped by the different acids can be disposed by the thermal stability in the next
row:
HCl < H2SO4 < CSA < TSA < DBSA.
ake a conclusion that the nature of the dopant anion plays an
important ro
Temperature, 0C
0 200 400 600 800 1000
Wei
ght,
%
0
20
40
60
80
100dedoped PANIPANI-HClPANI-H2SO4
PANI-TSAPANI-DBSAPANI-CSA
F
F
So, we can m
le in the thermal stability properties of PANI.
141
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Table 5.1. Temperature values for the different weight losses for PANI
Acid-dopant T∆m=10%, 0C T∆m=20%, 0C T∆m=30%, 0C T∆m=40%, 0C T∆m=60%, 0C
HCl 144 297 405 472 601
H2SO4 132 296 350 402 498
TSA 272 321 389 443 551
DBSA 270 302 382 453 554
CSA 216 267 310 404 513
Dedoped 428 480 521 559 630
Figure 5.5. TGA traces for the acids used as dopants, 10 0C/min
5.2.2. Thermal behaviour of the conducting composite powders
Preliminarily we investigated the thermal behaviour of the pure PA-12 powder.
It was established that PA-12 degraded at temperature of about 400 0C (Fig. 5.6), which
corresponded to degradation of the PA chain [214].
The thermal patterns of the synthesized composite PA-12/PANI powders
doped with different acids are also displayed in Fig. 5.6. The composite materials show
Temperature, 0C0 200 400 600 800 1000
Wei
ght,
%
0
20
40
60
80
100TSADBSACSA
142
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
m
he
e loss is
erature up to
posite
powders sim perature
higher than ∼410 C is, probably, due to the complete degradation of the matrix polymer
and PANI backbones after the loss of the dopants [202].
CSA) at the
temperat
odriguez and Akcelrud in the
polyuret
Temperature, 0C0 200 400 600 800 1000
Wei
ght,
%
0
20
100PA-12PA-12/PANI-HClPA-12/P 2SO4ANI-HPA-12/PA TSA
40
60
80 NI-PA-12/P BSAANI-DPA-12/P SAANI-CPA-12/PA dedoped
Figure 5.6. TGA traces for the composite PA-12/PANI powders, 10 0C/min
slower polymer decomposition compared with that of the PANI powders (Fig. 5.4). The
ain feature of the TGA traces of the PA-12/PANI powders is the presence of only two
weight loss stages for all the acid-dopants with the exception of CSA (Fig. 5.6). T
TGA trace of CSA presents three decomposition stages (the additional stag
observed at the temperature from 300 0C to ∼380 0C). This may be attributed to a lower
thermal stability of CSA (Fig. 5.5) and its hydroscopic nature.
The first stage weight loss starting practically from the room temp
110 0C corresponds to a loss of water molecules/moisture presenting in the com
ilar to pure PANI samples (Fig. 5.4). The weight loss at the tem0
The minor weight loss for the composites (except PA-12/PANI-
ure up to 400 0C and the great one above 400 0C (Fig. 5.6) indicates that these
composite materials have a good thermal stability close to that of pure PA-12. The
similar thermal behaviour was observed by R
hane/PANI based composites [215].
NI-
143
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
However, it should be also pointed out the existence of a significant difference
in TGA traces of the PA-12/PANI samples doped by organic and inorganic acids (Fig.
5.6). Such behaviour can be explained by the fact that inorganic acids (HCl, H2SO
rmed more stable salts with PANI.
It was found that the PANI content in the composite is only 2.38 wt.% on the
basis of the dedoped PANI powder (∼3-5 wt.% in the doped state). Therefore,
weight loss before 400 0C, which could be ascribed to the decomposition of the PANI
backbone, is relatively small.
On the whole, the obtained thermograms of the PA-12/PANI powder
composites powders testified to the higher thermal stability of PANI in the com
aterial (compare Fig. 5.4 and 5.6 and Tables 5.1 and 5.2).
Table 5.2. Temperature values for the different weight losses for the PA-12/PANI
composites
Composition of the
4) are
fo
the
posite
m
T∆m=10%, T∆m=20%, T∆m=30%, T∆m=40%, T∆m=60%,
C sample 0C 0C 0C 0C 0
pure PA-12 438 453 460 466 474
PA-12/PANI-HCl 440 455 464 469 478
PA-12/PANI-H2SO4 448 462 469 473 481
PA-12/PANI-TSA 408 420 428 433 446
PA-12/PANI-DBSA 401 415 421 428 440
PA-12/PANI-CSA 371 430 452 463 478
PA-12/PANI-dedoped 451 463 468 474 483
An enhancement of the thermal stability of the PA-12/PANI composites is
clearly indicated by a temperature rise to about 100 0C in comparison with the pure
PANI powders (the main weight loss is observed at ∼420 0C, while for pure PANI it is
observed at ∼300 0C). Also, the increase of the thermal stability indicated some kind of
interacti
.
on between PANI and PA-12. A similar behaviour has been observed by Das et
al [216] for polyacrylamide/PANI blends and by Rodrigues et al [217] when studying
the lignin/PANI blends through TGA. They suggested the formation of the hydrogen
bonds between two polymers, which provides an increase of the thermal stability of the
composite comparing to the pure components
144
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
In order to study an influence of the PANI content on the thermal stability of
the composite materials we prepared and investigated with the help of TGA the
composites with a different PANI content and doped by different acids (Fig. 5.7, Table
5.3). As one can see from obtained thermogramms, for all studied composites two
stages of
PANI content, increasing with increasing the PANI
concentr
NI fraction.
C
sa
T∆
0C
T0C
T∆
0C
T∆
0C
T0
the weight loss are observed – a minor weight loss at 80–100 0C (inset in Fig.
5.7) and the second one – at 380-420 0C. On the whole, the thermal stability of the pure
PA-12 powder is maintained irrespective of the PANI content. The mass loss at above
380 0C is determined by the
ation, probably, for account of the dopant evolution (Table 5.3). This
observation also confirmed the above stated supposition about increasing of the thermal
stability of the composites by the presence of the dedoped PA
Table 5.3. Temperature values for the different weight loss for the PA-12/PANI
composites
omposition of the
mple m=10%, ∆m=20%,
m=30%,
m=40%,
∆m=60%,
C
pure PA-12 434 452 459 467 477
PA-12/PANI-TSA
(0.48 wt.%)
43 45 46 46 49
1
0
3
73
PA-12/PANI-
DBSA (0.61 wt.%)
41 44 45 46 49
5
6
3
74
PA-12/PANI-TSA
(7.53 wt.%)
40 41 42 42 40
5
2
7
42
PA-12/PANI-
DBSA (
7.75 wt.%) 382 406 416 423 439
As one can see from Table 5.3, the composite materials doped by TSA display
a little bit higher thermal stability irrespectively of the PANI content than those doped
by DBSA. Such behaviour may be explained by different portions of PANI in the
PANI-acid complex. On the other hand, with the rise of the weight loss the difference
becomes less significant due to the dedoping process, i.e. because of the removal of the
acid molecules from the composite powder. This means that at a temperature higher
than that characteristic of the dopant complete removal, the composite material contains
only dedoped PANI which, as was shown before (Fig. 5.4 and Table 5.1), degrades at
145
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
0 200 40
Figure 5.7. TGA traces for the composite PA-12/PANI powders doped by (a) TSA and
(b) DBSA with different PANI-acid content, 10 0C/min
0 600 800 1000
Wei
ght,
%
0
20
40
60
80
100
PA120.48 wt.% 1.59 wt.% 3.19 wt.% 5.04 wt.%7.53 wt.% 9.92 wt.%
20 30 40 50 60 70 80 90 100 110 120 130
Wei
ght,
%
97
98
99
100
101
Temperature, 0C
(a)
T eremp atur 0Ce,
(a)
(b)
Temperature, 0C0 200 400 600 800 1000
Wei
ght,
%
40
60
80
100
0
20
PA120.61 t.% w1.1 3 wt.%3.61 t.% w5.64 t.% w7.7 5 wt.%
Temperature, 0C20 30 40 50 60 70 80 90 100 110 120 130
Wei
ght,
%
97
98
99
101
100
(b)
146
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
higher temperatures in comparison with the doped samples.
Also, it is clearly seen that with the increasing of the PANI content in the
composite material the temperature, at which the beginning of the significant weight
loss occurs, is decreased. This feature is preserved independently of the used acid-
dopant (compare 438.75 0C and 399.67 0C for PANI-TSA with 419.23 0C and 381.86 0C
for PANI-DBSA for the ∼0.5 wt.% and ∼7.5 wt.%, respectively).
On the whole, one can conclude that the thermal stability of the composite
materials is close to that of pure PA and the nature of the acid-dopant was found to
influence the thermal stability of the whole composite. Also, it should be pointed out
that the temperature of the significant weight loss of the composite because of the
dopant removal and polymer degradation is significantly higher than that of the
composite processing. The obtained data testify that it is possible to produce the
conductive polymer blends with satisfactory thermal properties by the aniline
polymerization in the PA water dispersion.
5.3. Mechanical investigation of the conducting composite films
As it was already mentioned (see section 1.1.3) the main disadvantage of
PANI, which often hinders its practical applications, is its poor mechanical properties.
Up to now some efforts to improve these properties have been done. Among them is the
preparation of the PANI films of the gel state [82] or orientation of the PANI film [83].
Also, it was found that the crosslinking of the PANI films made of the concentrated (8
wt.%) NMP solutions significantly increased its tensile strength [81]. The dopant and
moisture content have a great influence on the mechanical properties of the films, as
well [82].
In spite of all these data, the preparation of conducting composite materials
based on PA and PANI seems to be more preferable from the technological point of
view for an improvement of the mechanical properties of the films. Polyamide itself can
form very good quality films of appreciable strength [190].
The mechanical properties of the PA-12/PANI films, pressed at 195 0C of the
c
and the results a
orresponding powders (detailed description see in the section 2.2.2), were measured
re shown in Table 5.4.
147
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
In the case of the pure PA-12 film the tensile strength is 52.8 MPa and for the
dedoped
sion is confirmed by
the resul
the PA-12/PANI composite films
Composition of the sampl Tensile strength, MPa
PA-12/PANI composite film this value is practically the same (51.4 MPa).
However, for the doped PA-12/PANI films the tensile strength of the films decreases.
It is known [5, 215, 216, 218, 219] that under the same conditions of
preparation of the PANI based composite materials the nature of the acid-dopant is a
factor, which determines properties of the composites. This conclu
ts of the mechanical measurements (Table 5.4). In the case of the doping by the
inorganic acids (HCl, H2SO4) the tensile strength decreases from 52.8 MPa to ∼33 MPa.
However, when organic dopants (TSA, DBSA) were used as a doping agent, composite
films with a tensile strength of ∼43 MPa could be fabricated. These data indicate that
the strength of the PANI films could be increased by blending with the common
polymers as a matrix and, besides, by using certain acid-dopants.
Table 5.4. Mechanical properties of
e
pure PA-12 52.8
PA-12/PANI dedoped 51.4
PA-12/PANI-HCl 33
PA-12/PANI-H2SO4 35
PA-12/PANI-TSA 43
PA-12/PANI-DBSA 47
The higher tensile strength in the case of organic acid-dopants used may be
explained by the known fact that organic acids perform triple role – of a dopant, a
plasticizer [220] and a compatibilizer. Besides, the composite films doped by such
function
in Fig. 5.8 as a function of
the PAN
se of the mechanical properties of composites
independently on the acid used.
alized acids when producing the film by the compression molding technique
preserve a higher conductivity value as it will be shown later (see Chapter 6).
The mechanical properties (tensile strength) of the pure PA-12 and the PA-
12/PANI-TSA and PA-12/PANI-DBSA films are presented
I-acid content. As one can see, the tensile strength of the composite films at
rather low PANI contents is not far from that of the pure PA-12 film. But the increase of
the PANI content leads to the decrea
148
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
It should be noted that in the case of DBSA, the tensile strength is not
significantly affected by the addition of PANI-DBSA up to 4 wt.%. However, the
presence of more than ∼4 wt.% of PANI-DBSA in the films leads to a decrease in the
value of
Figure 5.8. The tensile strength of the PA-12/
reasing the PANI content, the PANI component
the tensile strength. At the same time in the case of TSA-doped PANI, with
increasing the content of the conducting PANI-TSA complex the mechanical strength of
the composite film at once decreases. The tensile strength decrease testifies, probably,
that the PANI-acid complex in the composites behaves as a defect in the PM. Thus, at
low PANI-acid complex content, a good and homogeneous dispersion is obtained. A
higher PANI-acid content and the using TSA as an acid-dopant may cause great defects
in the matrix and the mechanical perturbations, which will decrease the tensile strength
(Fig. 5.8).
PANI-acid films with different conductive
complex content
The observed trend is in some agreement with previous results [219, 221, 222]
obtained for the conductive composite materials based on other common polymers.
Small amount of PANI disturbs morphology of the PA matrix itself, thereby, lowering
the tenacity value. When inc
PANI-acid content, wt.%0 2 4 6 8 10
Tens
ile s
treng
th, M
Pa
0
10
20
30
40
50
60 TSADBSA
149
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
increasin
owder. It is known that polyamides are subject to the acidic hydrolysis
[158]. So, additional amount of acid can lead to such (acidic) kind of hydrolysis
resulting in a decrease of the tensile strength of the film.
On the other hand, the rise in the pressing temperature from 195 0C to 240 0C
(Fig. 5.9b) also results in the decrease of the tensile strength. This may be, probably,
explained by the fact that with the rise in the pressing temperature the dissociation of
the PANI-acid complex increased and, as a result, the certain amount of the “free” acid
appeared in the composite. This acid acts as in the case of the additional doping, i.e.
leads to the hydrolysis of PA.
Also, the pressing of the additional doped powders at higher temperatures leads
to the same result and, correspondingly, the value of the tensile strength decreases.
Therefore, the obtained results allow confirming that the tensile strength of the
composite materials based on PA and PANI can be control by changing the nature of
the acid-dopant, the content of the conducting PANI complex and the pressing
temperature. The composite films with sufficient tensile strength value could be
obtained.
5.4. Structural studies of the surface conducting composite films
Chapter 4, the feasibility of the surface PA/PANI composites synthesis was demonstrated and the kinetic peculiarities of this process were estimated. Besides, the
influence of the PM structure on the thickness of the conducting layer was shown.
owever, we may suppose that the structure of the PM may influence not only
the thickness of the PANI containing layer, but also the distribution of PANI inside the
gly contributes to the strength of the system and tenacity somewhat decreases.
Previously Gangopadhyay et al [219] reported a similar character of the mechanical
properties of the PVA/PANI composites.
The influence of an additional doping, i.e. an additional treatment in TSA
solution (see section 2.2.4), and the pressing temperature on the mechanical properties
of the composite materials with the different PANI-TSA content has also been
investigated. The results of the measurements are depicted in Fig. 5.9. It has been
established that the additional doping by TSA leads to a decrease of the tensile strength
of the composite films (Fig. 5.9a). Such influence may be explained by the fact that
during the doping process an additional quantity of the acid is introduced inside the
composite p
In
H
150
Chapter 5. S
151
b – influence of the pressing temperature
pectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.9. Tensile strength of the PA-12/PANI-TSA composites as a function of the
PANI-TSA content:
a – influence of the additional doping;
PANI-TSA content, wt.%0 2 4 6 8 10
0
10
30
40
50
60
a M
Png
th,
stre
ileTe
ns 20
without additional dopingwith additional doping(a)
PANI-TSA content, wt.%0 2 4 6 8 10
Tens
ile st
reng
th, M
Pa
0
10
20
30
40
50
60195 0C220 0C 240 0C
(b)
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
PM layer and the final composite structure. The concernment of this fact is very
important because the conductive properties of the composites depend on molecular
distribution of the conductive clusters with respect to the PM [223, 224]. By these
reasons the investigation of the morphology of the composite films was performed. The
[147] for the PA-6/PANI
The difference between the PM is clearly seen from the optical pictures of
surface c
ANI
ob
PANI formed in the PA-6 film appears as a very homogeneous film with a continuous
network structure (Fig. 5.11b), whereas the PET/PANI film presents a granular structure
(Fig. 5.11c).
Two phases are clearly seen for the PET/PANI composite (Fig. 5.11c): the neat
PET, characterized by the fracture surface and PANI, characterized by aggregates and
agglomerates organization. It has been shown by standard AFM measurements [223]
that the doping of the composite films based on PANI and a PM such as PET drastically
modifies the film surface topography showing a re-organisation of PET and PANI
flexible chains under the influence of the acid. It has been found that the pure PET and
the dedoped form of the composite PET/PANI film exhibit comparatively flat surfaces,
whereas doping leads to the appearance of the hilly features. Also, it has been found that
not only doping (i.e. appearance of charge carriers on the PANI chains), but the acid-
dopant also influences the surface morphology of the composite films [223]. In
particular, it has been found that HClO4 molecules induce much stronger changes in
comparison with the HCl case. Specifically, it is possible to believe that this effect is
caused by a new packing of the amorphous part of the PET and PANI which is induced
composite film based on PET was also investigated in order to determine the influence
of the PM structure.
The optical microphotographs for HCl-doped PA-12/PANI composite film
show that the pure PA-12 film has uniform surface (Fig. 5.10a) in comparison to doped
composite films (Fig. 5.10b). Similar distribution of PANI was found by Zhang et al
[225] for the PA-11/PANI composites and by Basheer et al
and PA-12/PANI composites with the help of TEM.
omposites (Fig. 5.11). One can see from this figure that the structure of the PM
is important in determining the distribution and the organisation of the conducting
clusters in the synthesized composites. As it can be observed, the distribution of P
tained in the PA-11 film (Fig. 5.11a) is similar to that in the PA-12 film, probably,
due to the similarity of the molecular structures of the PM (Fig. 2.1). On the other hand,
152
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.10. Optical micrographs of the pure PA-12 film
PA-12/PANI-HCl film (b)
(a) and of the composite
PA-6/PANI-HC HCl (c) films
(a) (b)
(c)
(a) (b)
Figure 5.11. Optical micrographs of the composite PA-11/PANI-HCl (a),
l (b) and PET/PANI-
153
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
by Cl- and ClO4- anions penetration. Obviously, larger ClO4
- anions, when penetrating
into the PM, deform it stronger than Cl- ones [223].
In order to confirm the influence of the doping process on the topographical
features of the composite polymer film, we performed AFM measurements for the
composite PA-6/PANI-HCl film (Fig. 5.12a). As one can see from the obtained image,
the surface deformation is also observed. But, in contrast with the PET/PANI composite
[223, 22
ing layer reveals a distribution of
conducti
4], for the PA-6/PANI composite film a rather uniform distribution of PANI in
the surface layer of the film is demonstrated (Fig. 5.12a).
The change of the surface morphology is accompanied by the modification of
the electrical properties of the film surface. As one can see, the electrical image (Fig.
5.12b) performed on the surface of the PANI contain
ng clusters with a resistance value around 105 Ω, dispersed in the PM having a
high resistance value (over 1011 Ω). It is evident that these clusters are different by
shape and irregularly distributed in the PANI containing layer, that leads to a hilly shape
of the surface layer.
Figure 5.12. Topographical (a) and electrical (b) AFM images of the surface PA-
6/PANI-HCl composite film obtained with the “Resiscope” in contact mode
(a) (b)
154
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
In the case of the PET matrix (Fig. 5.13), the conducting clusters are rather
dispersed and no clear evidence of percolation is observed. On the contrary, for the
sample based on the PA-6 film, the conductive ilm
surface (Fig. 5.12b). It has been shown [143] th t the percolation threshold in the latter
case reaches for only 4 wt.% of PANI. As a result, the electrical macroscopic properties
are different: the film surface resistance measured at room temperature by a standard
four-electrode technique is found to be 20 kΩ and 4 MΩ for PA-6 and PET matrices,
respectively. This difference cannot be attributed to the initial concentration of aniline
in the swelled film and, correspondingly, to the PANI content. The samples being
pre t
of aniline (around 12 wt.%). I etry (chapter 4 and
[226]) that the thickness of the PANI containing layer was higher in the PA-6 matrix
(4 m) than in the PET film (∼2 µm). The structure of the PM may, probably, explain
the encountered differences. The found struct posite films have been
correlated with the observed electrical conductivity values (Chapter 6).
Figure 5.13. Topographical (a) and electrical (b) AFM images of the surface
PET/PANI-HCl com 223]
As a who estify to a phase
segregation revealed by the presence of the clusters of various conductivity values in the
thin subsurface layer of the composite film.
clusters occupy a large part of the f
a
pared in the same way, in both cases the pure films took up roughly the same amoun
t was also shown by Raman spectrom
.7 µ
ures of the com
posite film obtained with the “Resiscope” in contact mode [
le, the AFM and the optical microscopy data t
(a) (b)
155
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
5.5. Raman spectrometry measurements
1.1 and 1.2) [227, 228]. Most
of them
he fact that a lot of articles describe the Raman behaviour of pure
PANI in different oxidation states, only a few Raman studies give the characterization
of the PANI composite materials. example, Pereira da Silva et al [231] reported on
the evolution of the Raman spectra of PMMA/PANI composite films prepared by
mixing solutions of PANI-CSA and PMMA using m-cresol as a solvent. The blue shift
of the frequencies of the carbonyl band was observed and the authors correlated it with
the indication of the chemical interaction between PANI and PMMA.
In our study, we used Raman spectrometry in order to identify the presence of
PANI and its forms in the obtained composites and, also, in order to determine the
discrepancies between the composite materials with different PANI contents.
5.5.1. Raman spectrometry investigation of polyaniline
For several years, PANI was extensively studied by Raman spectrometry.
However, because of the PANI structure complexity, Raman studies remained
essentially limited to the qualitative discrimination between the different forms of
PANI. Numerous works were done in order to identify the different vibrations
characteristic of the different PANI oxidation sates (Fig.
dealt with the study of the PANI insulating forms: leucoemeraldine, emeraldine
and pernigraniline bases. More recently, several papers reported on the interpretation of
the Raman spectra of the PANI conducting form (ES) [229-231]. It was found a
correlation between the appearance of a new band around 1330 cm-1 and the PANI
protonation process. The authors [229, 230] assigned this band to the stretching of >C-
N+· bonds.
In spite of t
For
Typical Raman spectra obtained for the pure PANI powder in different
oxidation states are shown in Fig. 5.14. The positions of the main vibrations were
compared with the typical vibrations of ES and EB reported in the literature under
sim ed
(Fig. ring
stretching vibration of the benzenoid ring at 1622 cm-1 [232] shifts to 1604 cm-1 after
the dedoping process. The C=C ring stretching vibrations of the quinoid rings at 1591
ilar experimental conditions. A comparison of the spectra measured for the dedop
5.14, curve 1) and doped (Fig. 5.14, curve 2) powders reveals that the C-C
156
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.14. Raman spectra of the PANI powders in different oxidation states
(λ = 514.5 nm, t = 300 s, p = 0.04 mW):
1 – dedoped PANI (EB) powder;
2 – doped PANI-HCl powder;
3 – doped PANI-H2SO4 powder
cm-1 [230], which cannot be observed in the PANI-HCl sample, grow in intensity when
more quinoid units are formed during the dedoping process. At the same time, the C-H
in-plane bending deformation of the benzenoid rings at 1191 cm-1 [229] completely
disappeared from the Raman spectrum of dedoped PANI and a new band assigned to C-
H in-plane bending deformation of the quinoid rings at 1165 cm-1 appears for the
dedop f the
d
cm-1. They are assigned, respectively, to the C-N stretching vibrations of the benzenoid
units [44
ig. 1.2). Lindfors et al [234] have studied
2
1
3
Wavenumber, cm
ed PANI powder. Also, two new peaks appear in the Raman spectrum o
edoped PANI sample: the one at 1221 cm-1 and the broad band around 1488 - 1557
] and the C=N stretching vibrations of the quinoid units [233]. The presence of
these bands are consistent with the structure of the EB form of PANI inasmuch as it
consists of both >C=N- and >C-NH- units (F
-11000 1200 1400 1600 1800
Ram
an in
tens
ity, a
.u.
0
5000
10000
15000
20000
25000
30000
35000
1622
1191
1591
1604
1165
1221
1488
1258 13
2013
40
1557
1409
1191
1258 13
40 1622
1165
1
2
3
157
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
the ES-EB transition of the PANI films in solutions with different pH values. They
fou hen pH is increased. And that
simultaneously, the C-N+· stretching vibrations of the semiquinone radicals at 1258 cm-1
[228] dis
internal redox process associated with this type of protonation, results in
lowering
transition can be
induced
e Raman
spectrum ig. 5.14, curves 2 and 3). As one may see
in the istic bands of doped
PANI ar
nd that the band at 1488 cm-1 grows strongly w
appear almost completely (Fig. 5.14, curve 1).
In the Raman spectrum of the dedoped PANI form the C-C stretching
vibrations at 1557 cm-1, which is probably related to the quinoid units [227], as well as
the C-C stretching vibrations of the quinoid units at 1409 cm-1, are also observed.
In all cases, the ES form can be undoubtedly identified when a band around
1320-1340 cm-1, assigned to protonated nitrogen [229], dominates the spectrum (Fig.
5.14, curve 2). Indeed, it should be pointed out that none of the insulating forms of the
PANI (LE, EB and PNA) gives rise to vibrational modes in the 1300-1400 cm-1 spectral
region [228, 230]. The presence of the peaks in this region is, therefore, inherently
associated with the protonation process by means of the radical formation mechanism.
The
the order of the bond between C and N, which now becomes intermediate
between the amine (>C-NH-) and the imine (>C=N-) bonds [6].
It should be noted that the characteristic >C-N+· stretching vibrations of the
delocalized polaron charge carriers at 1320-1340 cm-1 can still be seen in the spectra of
dedoped PANI. This feature may be explained by the minor fractions of the ES form,
which are, probably, still present in the dedoped sample. On the other hand, as it was
established by Lindfors and Ivaska [234] the benzenoid to quinoid
by the incoming laser light resulting in the C-N+· stretching vibration observed
in the spectrum (Fig. 5.14, curve 1).
We also investigated the influence of the acid-dopant nature on th
of PANI in the ES oxidation state (F
Raman spectrum of H2SO4-doped PANI, all the character
e preserved as for HCl-doped PANI: the C-C ring stretching vibrations of the
benzenoid ring at 1622 cm-1, the C-H in-plane bending vibrations of the benzenoid rings
at 1191 cm-1, the >C-N+· stretching vibrations of the semiquinone radicals at 1258 cm-1
and the conduction band around 1340 cm-1. Also, it should be noticed that for H2SO4-
doped PANI two closely located Raman bands at 1191 and 1165 cm-1 are observed (Fig.
5.14, curve 3). This is in accordance with the co-existence of the benzenoid and quinoid
units in the oxidized sample.
158
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
So, taking into account all these results, we can conclude that Raman spectra
completely depend on the oxidation states of PANI, and to a less extent depend on the
acid-dopant.
5.5.2. Raman spectrometry study of the surface composite films
inary, we investigated the Raman behaviour of the pure PA films (Fig.
5.15) in order to compare the vibrational bands of PA in the composite films with those
observed in pure PA. The assignment of the Raman bands for the PM was made
according to the literature [235, 236]. For all types of PA (PA-6, PA-11, PA-12) the
Raman spectra are rather similar and all vibrational bands corresponding to PA are
present. In particular, we clearly observed the C-N-C stretching de at 930 cm-1, the
C-C stretching modes at 950-1150 cm-1, the stretching band of carbonyl group vibration
at 1642 cm-1, the stretching vibrations of the CH2-groups at 2852, 2889 and 2932 cm-1,
and the in-phase twist of the CH2-groups at 1298 cm-1. The spectra also contain the
vibrations of the N-CH2-groups at 2722 cm-1 and the N-H stretching at 3300 cm-1. It
should be noted that in the case of the PA-6 film the vibrational bands, which
correspond to the carbonyl and NH groups are more intense than those observed for the
PA-1 and
3300 cm-1). This observ he molecular structure
of the studied PM (Fig. 2.1), s is changed in the next
row: PA-6<PA-11<PA-12. In bonyl group to CH2-group
for PA-12 is less than that for
Thanks to the confocal micro-Raman Resonance spectrometry, we have shown
n
Prelim
mo
1 and PA-12 films (compare, for example, the peaks’ intensity at 930, 1642
ation is consistent with the difference in t
since the quantity of the CH2-group
other words, the ratio of the car
the PA-6 film.
(chapter 4) that it was possible to mainly observe the bands of PANI in the Raman
spectra of the composite PA/PANI films. Fig. 5.16 shows Raman spectra of the
composite PA-12/PANI films in ES and EB oxidation states. To help making a
comparison, the figure includes also the spectrum of the virgin PA-12 film (Fig. 5.16a,
curve 1) measured under the same conditions. The intensity of the PA-12 spectrum was
multiplied by a factor 10 to observe the position of the vibrational bands.
Fig. 5.16 shows clearly that the Raman spectra of the composite film are
mai ly characterized by the PANI bands. However, it can be noticed that the PANI
bands in this case are shifted in comparison with those of pure PANI (Fig. 5.14).
Specifically, in the spectrum of the doped sample (Fig.5.16a, curve 2) the semiquinone
radical >C-N+· band at 1320-1340 cm-1 is significantly shifted towards 1347-1378 cm-1,
159
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.15. Raman spectra of the PA films
in the EB oxidation state the band at
1351 cm
n
m the bands are
broader (Fig. 5.16a) than for the PA-6 film (Fig. 5.16b). The other interesting
(λ = 514.5 nm, t = 240 s, p = 1.67 mW):
1 – PA-6; 2 – PA-11; 3 – PA-12
while the dedoped sample (Fig. 5.16a, curve 3) shows a negligible shift of the band at
1165 cm-1 (C-H bending of the quinoid ring) to 1169 cm-1 and the band at 1409 cm-1 (C-
C stretching of the quinoid ring) to 1413 cm-1. Moreover, it should be noticed that on
the Raman spectrum of the composite PA-12/PANI-1 can be observed. This may be explained by the spatial difficulties of the
doping-dedoping processes, because in the case of the composite material the PM
hinders the free diffusion of the ions of the doping or dedoping agents. And this feature
can explain the uncompleted dedoping process of PANI in the composite film. All this
results in the appearance of the >C-N+· stretching vibrations on the Raman spectrum.
In the case of the PA-6 film as a PM (Fig. 5.16b) one can observe similar
behaviour as in the case of the composite film based on PA-12. Namely, on the Rama
spectra of the composite film only characteristic bands of PANI are present (Fig. 5.16b,
curves 2 and 3). For the PA-6/PANI composite film, like for the PA-12/PANI one, the
shift of the vibrational bands is observed. However, for the PA-12 fil
Wavenumber, cm-1500 1000 1500 2000 2500 3000 3500 4000
Ram
an in
ters
ity, a
u..
80000
288928
52
0
20000
40000
60000
3300
2932
2722
164214
427212
98
1064
1114
13
1127
1079
930
3
2
1
160
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure
Wavenumber, cm-11000 1200 1400 1600 1800 2000
Ram
an in
tens
ity, a
.
80000
1169
1233
161413 14
7315
00
1608
32
1351 70000
u.
5.16. Raman spectra of the composite films based on (a) PA-12 and (b) PA-6
films (λ = 514.5 nm, t = 300 s, p = 0.04 mW):
1 – the pure PM;
2 – the doped composite PM/PANI-HCl film;
3 – the dedoped composite PM/PANI film
30000
40000
50000
6000010
6411
1411
32 1298
1442
1642
1188
1248
1347
1378 15
69
x 1016
05
1152
Wavenumber, cm-11000 1200 1400 1600 1800 2000
Ram
an in
tens
ity, a
.u.
0
1000
2000
3000
4000
5000
6000
7000
× 10
1642
1442
1372
1127
1079
12 13
1
2
1625
1193
55 43
1161
1223
1449
1615
1541
1349
1592
3
3
2
1
(b)
(a)
161
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
observation is the very strong bands at 1193 and 1625 cm-1 for the doped composite PA-
6/PANI-HCl film (Fig. 5.16b, curve 2). These bands correspond to the C-H in-plane
bending vibrations of the benzenoid rings and C-C ring stretching vibration of the
benzenoid ring, respectively. For the dedoped samples (Fig. 5.16, curve 3) the band at
1161 cm , corresponding to the C-H bending of the quinoid ring is also more intense in
the case of the composite PA-6/PANI film. Such behaviour may be explained by, at
least, two reasons: by a higher quantity of the formed PANI in the PA-6 film and by a
higher doping level of PANI in the case of the PM based on PA-6. Such explanation is
well confirmed by the different thickness of the conducting PANI containing layer (Fig.
4.10). So, these results also indicate the influence of the PM molecular structure on the
properties of the obtained surface composite materials.
5.5.3. Raman spectrometry study of the bulk composite powders
-1
he Raman spectrum of the PA-12 powder is identical to the Raman spectrum
of PA-12 in the film form (Fig. 5.15, curve 3). The Raman spectra of the composite
materials with the d . 5.17. In order to
compare the Raman sp iphatic CH2-groups at
2852, 2889 and 2932 cm ity of the PA powder was
the same for all composite materials.
I)
the char
T
ifferent PANI content are presented in Fig
ectra, the stretching vibrations of the al-1 were normalized, since the quant
Fig. 5.17 shows that the intensity of the vibrational bands in the wavenumber
range 1000-1700 cm-1 increases with the PANI content. But it should be pointed out that
unlike the surface films, in the case of the PA/PANI powders the Raman bands of PA
are also seen in all spectra. Indeed, on the Raman spectrum of the composite powder
with only 0.48 wt.% of the PANI-TSA the bands at 1298 and 1445 cm-1 which
correspond to the CH2-groups vibrations of the PM are observed as well as the C-C
skeletal vibrations at 1067 and 1111 cm-1 (Fig. 5.17a, curve 1).
At the same time, even with such a small PANI quantity in the composite
powder (only 0.48 wt.% of the PANI-TSA complex, i.e. 0.25 wt.% of dedoped PAN
acteristic vibrations of PANI are already observed: at 1195 cm-1 - the C-H in-
plane bending of the benzenoid rings, the >C-N+· stretching vibration of the delocalized
polaron charge carriers – at around 1360 cm-1 and at 1638 cm-1 - the C-C stretching
vibrations of the benzenoid rings. It is established that as the PANI-TSA content
increases the intensity of the PANI bands increases too (Fig. 5.17a, curves 2-6).
162
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.17. Ram
12/PANI-DBSA powde nt PANI-acid complex content:
a – 1 - 0.49 wt.%; 2 - 1.59 wt.%; 6 - 9.92 wt.%
b - 1 – 0.61 wt.%; 2 – 1.13 wt.%; 5 – 7.75 wt.%
an spectra of the composite (a) PA-12/PANI-TSA and (b) PA-
rs with differe
3 - 3.19 wt.%; 4 - 5.04 wt.%; 5 - 7.53 wt.%;
3 – 3.61 wt.%; 4 – 5.64 wt.%;
Wavenumber, cm-11000 1500 2000 2500 3000 3500
Ram
an in
tens
ity, a
.u.
15000
20000
25000
30000
35000
40000
3300
2889
2852
2932
2727
1625
1193
1344
1064
1295
1442
1636
1440
1195
(b)
5
4
32
1
Wavenumber, cm-11000 1500 2000 2500 3000 3500
Ram
an in
tens
ity, a
.u.
0
10000
20000
30000
40000
50000
1067 1111
1195 1298
1638
2852 28
8929
32
1188 13
4512
54
16221595(a)
1
2
34
56
163
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
In contrast to the composites doped by TSA, for the composite materials
synthesized in the presence of the other doping acid – DBSA – the PANI characteristic
bands become apparent at a higher PANI-DBSA content (Fig. 5.17b). In order to make
a comparison between these two acid-dopants, the plot of the evolution of the integrated
Raman intensity as a function of the PANI content was made (Fig. 5.18). As one can
see, the evolution isn’t the same – at the same PANI-acid content in the case of TSA the
integrated Raman intensity is higher. Boyer et al [230] studied the evolution of the
Raman spectra of ES with the time of the exposure in HCl vapour or, in other words,
with the increasing of the doping level. They observed a decrease of the C=N bond and
the C-N stretching bands and the appearance of a band assigned to the C-N+· stretching
of the cation radical species at around 1332 cm-1 with the doping level increasing. On
the basis of these results, we can conclude that PANI obtained in the presence of DBSA
has a
ontent, namely, on the interaction of the cation
radical f
s of the aliphatic CH2-
groups.
re after
lower doping level than the one obtained with TSA.
But one should also noticed that the positions of the Raman bands slightly
shifts to lower wavenumbers with the increasing PANI content in the case of both acids.
The more is the PANI content, the lower are the wavenumbers of the characteristic
vibrational bands. The bands shift confirms the above stated interaction of PANI with
the matrix. On the other hand, such evolution shows that the degree of this interaction
strongly depends on the PANI-acid c
ragments of PANI with the electrodonor groups of the PA matrix (Fig. 5.18).
In order to study the influence of additional doping on the composite Raman
properties we collected Raman spectrum of the PA-12/PANI-TSA powder after an
additional treatment with TSA (see Chapter 2) (Fig. 5.19). The spectrum was recorded
under the same conditions (λ = 514.5 nm, t = 300 s, p = 0.167 mW) as the ones used
before for the initial powder, so we can compare the Raman intensity of both spectra.
The spectra were also normalized with the stretching vibration
The figure clearly shows that the additional doping leads to significant
increasing of the all characteristic vibrational bands of doped PANI, namely, the C-H
bending vibrations of the benzenoid ring at 1195 cm-1 and the band at 1345 cm-1 which
is responsible for the >C-N+· cation radical fragments. Such changes in the system may
be easily explained by the increasing of the total quantity of the acid (TSA) and, thus,
they may testify to the presence of the non doped imine sites in the PANI structu
164
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
Figure 5.18. Evolution of the integrated Raman intensity in the wavenumber range
1047-1680 cm-1 as a function of the PANI-acid content in the composite:
1 – TSA; 2 - DBSA
Figure 5 ectra of
PANI-acid content, wt.% 0 2 4 6 8 10
u.ity
, a.
nten
sg
man
i R
ara
ted
Inte
.19. The influence of the additional doping treatment on the Raman sp
0.0
2.0e+5
4.0e+5
6.0e+5
8.0e+5
1.0e+6
1.2e+6
1.4e+6
1.6e+6
1
2
1
2
Wavenumber, cm -1
the composite PA-12/PANI-TSA (9.92 wt.%):
1 – without additional doping; 2 – with additional doping
1000 1500 2000 2500 3000 3500
Ram
an in
tens
ity, a
.u.
-10000
0
10000
20000
30000
40000
50000
162415
99
1345
1195
1258
1
2
165
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
the initia
ch
composi
ed that the presence of the PM hinders the formation of PANI with long chains,
which m
l polymerization process. This is also the evidence that the doping agent (TSA)
can be partially removed during washing just after the polymerization process. Such a
result proves unambiguously that an additional doping treatment could be useful to
obtain a higher conducting composite.
On a whole, obtained results show that the composite PA-12/PANI powders
doped by different acids have similar Raman spectra. On these spectra the characteristic
vibrational bands of both PANI and PA are present. However, it can be notices that the
PANI characteristic bands in the composite materials are shifted to the low wavenumber
region in comparison with those of pure PANI. Besides, with increasing the PANI
content in the composites the intensity of the PANI bands increases simultaneously.
5.6. Conclusions
The study of the spectroelectrochemical, thermal, mechanical and structural
properties of the conducting composite materials based on PANI and PA allows us to
come to the following conclusions:
1. It was found that in spite of the preparation method PANI synthesized on the
surface of the bare electrodes or on the surface of the electrodes, covered with the PM
preserves high ability to the reversible redox processes and changes its colour
depending on the oxidation state. This fact gives us the possibility to use su
te films in sensors for different gases, as pH sensors, as electrodes in batteries,
etc.
2. The observed spectroelectrochemical differences between the composite
PM/PANI materials and pure PANI are connected with the diffusion limitations, as in
the case of the PM the counter ions should diffuse from solution to the electrode surface
not only through the formed PANI layer, but also through the polymer film. It was
establish
ay be one of the reasons of the decrease of the PANI conductivity in the
composite materials as compared with that of pure PANI.
3. It was shown that the dopant nature affects the thermal properties of pure
PANI. The thermal stability of both PANI and PA-12/PANI composites is enhanced.
Such composites showed an increase of about 100 0C of the degradation temperature in
comparison with pure PANI independently of the acid-dopant used.
166
Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties
4. It was found that the PANI introducing into the PA matrix leads to a
decrease of the tensile strength of the PA films. Also, it was confirmed that the nature of
e acid-dopant is one of the main factors, which influence the mechanical properties of
e final composite film – the tensile strength is higher when using the organic acid-
opants than when using the inorganic ones. Such behaviour is connected with the fact
at organic acids perform triple role – of a dopant, a plasticizer and compatibilizer.
prove mechanical properties of the composite films
ultaneously with their conductivity.
5. It was found that at low PANI-acid content (up to 4 wt.%) the tensile
rength values of the composite films are not far from the values of pure PA. A further
crease of the PANI content caused a decrease of the mechanical properties of the
omposite films independently of the acid used. The main reason is, probably, poor
echanical properties of PANI.
e
pressing posite
PA/PANI material. This fact is ex bility of the acidic hydrolysis of
the PA matrix since during the pressing at higher temperatures the dissociation of the
conducting complex PANI-acid takes place. Thus, in the matrix volume the molecules
of the “free” acid appear which lead to the PA chains hydrolysis and, further, to the
mechanical properties decrease.
. On the basis of the AFM and optical microscopies results, the structural
organisation of the conducting PANI clusters within the insulating PM is found to be
dependent on the PM structure. For example, the final composite material is non-
uniform in the case of the PA-11 and PA-12 films and rather homogeneous in the case
of the PA-6 film.
. The analysis of Resonance Raman spectrometry results has shown that the
composite films of both surface and bulk composites reveal the characteristic
vibrational bands of PANI even at low PANI concentrations. The influence of the PM
structure on the properties of the obtained composite materials was confirmed.
9. It was established that in the course of the formation of the composite
m
confirmed by the Ram
th
th
d
th
Therefore, such dopants im
sim
st
in
c
m
6. It was established that both additional doping and the increase of th
temperature caused the decrease of the mechanical properties of the com
plained by the possi
7
8
aterials the interaction between PANI and the PA matrix takes place. This is
an bands shift to the low wavenumber region.
167
Chapter 6
DIELECTRIC AND
ELECTRICAL PROPERTIES
Chapter 6. Dielectric and electrical properties
Introduction
In previous chapters it was stated that the conducting composite PA/PANI
materials have a phase segregation structure, revealed by the presence of clusters of
various conductance values on the composite film surface. Also, it was shown that the
interaction between the PA units and the PANI chains exists. So, we can suppose that
such a cluster organisation of the conducting layer will result in the appearance of
interfacial polarisation effects. Besides, the observed modifications occurring in the
composite materials due to the acid doping also may result in significant changes in the
dielectric behaviour of the doped films. On the other hand, it is well known that the
electrical properties of the composite material are determined by its components, their
conductance and the percolation behaviour.
To confirm the assumption of the influence of the PANI conductivity and its
organisation in the PM on the general conductivity of the composite materials, the
dielectric and electrical properties of the polymer composite materials has been studied
in this chapter.
6.1. Chemically synthesized polyaniline
In order to interpret the obtained data for the composite materials, we have
also studied the dielectric and electrical behaviour of the pure PANI samples in doped
and dedoped forms.
6.1.1. Dedoped polyaniline
Relaxation properties. The frequency dependence of the measured real part ε’
of the complex dielectric permittivity and the real part σ’ of the conductivity for the
dedoped PANI powder (pressed pellet) is presented in Fig. 6.1 at several temperatures.
The spectra in Fig. 6.1 are presented at temperatures higher than 273 K, though the
measurements were performed beginning with 173 K. This is because below 273 K the
value of ε’ is almost temperature-independent having a weak frequency dispersion.
The real part of the permittivity exhibits a relaxation behaviour which
relaxation frequency is temperature-dependent. As one can see, ε’ exhibits a low-
168
Chapter 6. Dielectric and electrical properties
Figure 6.1. Frequency dependence of the real part of dielectric permittivity (a) and
conductivity (b) for dedoped PANI at different temperatures. Solid lines are the best fit
to HN equation (2.7)
Frequency, Hz10-1 100 101 102 103 104 105 106 107
σ', S
/cm
10-11
10-10
10-9
10-8
10-7
10-6
273 K 293 K 313 K 333 K 353 K 373 K HN fit
Frequency, Hz10-1 100 101 102 103 104 105 106
ε'
100
101
102
103
273 K 293 K 313 K 333 K 353 K373 K HN fit
(b)
(a)
169
Chapter 6. Dielectric and electrical properties
frequency plateau corresponding to the static dielectric constant εs and then decreases
with increasing frequency, forming a low plateau value ε∞ in the high-frequency region
(Fig. 6.1a).
The analysis of the dielectric permittivity using HN equation (equation 2.7)
revealed a temperature dependence of the relaxation time τ (Fig. 6.2), which is well
described by a simple Arrhenius law (see equation 2.8). For the emeraldine base τ0
appeared to be 5.19⋅10-11 s and the activation energy appeared to be 0.539 eV. These
results are in agreement with the calculations performed by Zuo et al [168] for the PANI
powders with different protonation levels. For example, for the emeraldine base they
found 1.6⋅10-10 s and 0.46 eV for τ0 and the activation energy, respectively [168]. At the
same time, the temperature dependent conductivity data obtained (Fig. 6.2) can be fitted
to Arrhenius equation of conductivity:
)exp(0 TkE
B
a−=σσ , (6.1)
where σ0 is the conductivity at very high temperature; Еa is the activation energy; kB is
the Boltzmann’s constant (8.616⋅10-5 eV/K). The activation energy for the conductivity
Figure 6.2. Arrhenius plot of the characteristic relaxation time and of the conducti
was found to be 0.589 eV, which is close to that of the relaxation time (0.539 eV).
vity
for dedoped PANI. Solid line is the best fit to equations (2.8) and (6.1), respectively
1000/T, K-12.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8
rela
xatio
n tim
e τ,
s
10-4
10-3
10-2
10-1
100
σ , S
/cm
10-11
10-10
10-9
10-8
10-7
170
Chapter 6. Dielectric and electrical properties
As the value of the activation energy is rather high, one can suppose that this
ation process is a β-relaxation, as for the α-relaxation process the activation energy
alue should be much higher and its frequency relaxation ay appear at much higher
mperatures [237]. In other words, the observed relaxation could be a β-relaxation
rocess commonly observed in the traditional polymers since the activation energy is of
e same order. On the other hand, inasmuch as the value of the activation energy for
e relaxation time is close to that of the conductivity (0.539 eV and 0.589 eV,
spectively), we can surmise that the correlation between these processes exists and
at both processes originated from the same transport mechanism. Therefore, the
laxation we observed for dedoped PANI at low frequencies can be the well known
onductivity relaxation [238]. In order to support this assumption the dependence of the
d
(F d
to be nearly equal to -1 with a linearity factor of 0.9951. Besides, it was established that
the relaxation time τ and the dc-conductivity σ obey the following equation [238]:
relax
v
te
p
th
th
re
th
re
c
c-conductivity value is plotted versus the relaxation time in double logarithmic scale
ig. 6.3). The linear behaviour of the dependence was obtained, which slope was foun
)()( 0
TT s
τεε
σ⋅
= , (6.2)
Figure 6.3. Dependence of the characteristic relaxation time versus the conductivity for
dedoped PANI
relaxation time τ, s10-4 10-3 10-2 10-1 100
σ, S
/cm
10-11
10-10
10-9
10-8
10-7
171
Chapter 6. Dielectric and electrical properties
where
dc-
conductivity value although not very high
represented in Fig. 6.1a arises from hopping and/or oscillations of these charges around
xed pinn
ε0 is the permittivity of the free space (8.85⋅10-12 F/m). This relation is
characteristic to the conductivity relaxation process
It was observed some deviation between the experimental values and
calculated according to equation (6.2) which can be explained by experimental error –
for example, by the presence of the residual solvent (water) in the sample. Thus, we
measured total conductivity of the sample, not only the conductivity value of PANI.
It should be noted that in conducting polymers is a strong charge trapping [168,
239] and its localized motion when applying an alternating electric field operates as an
electric dipole. The dielectric relaxation in the presence of such an alternating electric
field is a result of charge hopping among available localized sites [174]. So, for PANI
the polarons and bipolarons are the relevant charge species involved in the relaxation
and conductivity mechanisms [168, 240, 241]. At low frequencies, such charge hopping
could extend throughout PANI in the absence of strong pinning, which can lead to a
continuous current, resulting in a dc-conductivity. It should be pointed out that the
charge carriers exist even in the dedoped form of PANI as the sample reveals a
(Fig. 6.1b). The relaxation process
fi ing centres [168].
Thermal behaviour of the conductivity. There have been some reports on the
mperature dependence of the conductivity of PANI. Zuo et al [168] studied the
mperature dependence of the conductivity in the range between 78 and 375 K for
edoped and salt forms of PANI and reported the activation energy of 0.5 eV for the
edoped form. Pingsheng et al [242] showed that the activation energy increased from
.037 to 0.493 eV when the doping solutions pH was varied between 0 and 6.3. Also,
robst and Holze [243] suggested that the method of the aniline polymerization would
e an important factor in explaining the conduction mechanism.
Fig. 6.4 shows the temperature dependence of the conductivity for dedoped
ANI at 100 Hz. As one can see, the first run from 158 K to 433 K is slightly different
rom the others but further temperature cycling of dedoped PANI does not lead to any
urther visible changes in the conductivity value. This observation may be explained by
removed from the pellet. As it was 5.4, PANI in the dedoped state is
te
te
d
d
0
P
b
P
f
f
the fact that during the first increase of temperature the residual moisture (water) is
shown in Fig.
172
Chapter 6. Dielectric and electrical properties
Figure 6.4. Temperature cycling of the PANI pellet in the dedoped state at 100 Hz
stable up to 618 K, no further change of the conductivity is observed during next
cycling till 433 K.
Temperature, K
100 150 200 250 300 350 400 450
σ, S
/cm
10-13
10-12
10-11
10-10
10-6
10-9
10-8
10-7
These results confirm the above stated explanation (see section 5.2) that during
the temperature increasing for the dedoped PANI sample only two weight loss stages
are observed – the first one is connected with the moisture evaporation and the second
one is attributed to the decomposition of the PANI backbones (see Fig. 5.4).
6.1.2. Doped polyaniline
Relaxation process. The measured real ε’ and imaginary ε” parts of the
complex permittivity as a function of frequency for the doped PANI samples (pressed
pellets) at 300 K are shown in Fig. 6.5. The doping process of PANI leads to a drastic
increase of the permittivity. We found that the dielectric constant ε’ at low frequencies
is very high, but it drops very fast with increasing frequency for all used acids (Fig.
6.5a). This may be due to the easy charge transfer through well ordered polymer chains
in disordered regions as suggested by Joo et al [244]. This is characteristic of a
conducting polymer and consistent with the literature results [168, 170, 245]. Such
start
173
Chapter 6. Dielectric and electrical properties
Figure 6.5. Frequency dependence of the real (a) and imaginary (b) parts of dielectric
10-1 100 101 102 103 104 105 106
permittivity for PANI doped by different acids at 300 K
103
4
5
6
7
8
9
10
11
12
13
1410
10
10
10
10
10
10
10
10
10
10
HClH2SO4
TSA DBSA CSA
Frequency, Hz
ε"
(b)
Frequency, Hz10-1 100 101 102 103 104 105 106
ε'
102
103
104
105
106
107
108
109
1010
1011
1012
HClH2SO4
TSA DBSACSA
(a)
174
Chapter 6. Dielectric and electrical properties
behaviou
ies, with a
slope equal to -1 (Fig. 6.5b). Such a dependence indicates that ε” is proportional to 1/f,
and, correspondingly, that the conductivity is constant at low frequencies. This testifies
to the presence of a dc-conductivity. Si
looks like a relaxation process (Fig. 6.5a). Such behaviour is characteristic of the charge
carriers systems [174]. The localized charge carriers when applying an alternating
electric field can hop and jump to neighboring localized sites which form a continuous
connected network, allowing the charges to travel through the sample and, thus, causing
the electric conduction.
In order to study in detail the relaxation and conduction mechanisms in the
PANI samples we also performed measurements in the high frequency region (from 106
Hz till 109 Hz) (Fig. 6.6). As one can notice from Fig. 6.5a and 6.6, the relaxation
process, which for dedoped PANI was observed in the low frequency region (Fig. 6.1),
shifts toward higher frequencies as a result of the doping process. Moreover, it is
established that the position of the relaxation frequency of doped PANI is dependent on
the used acid-dopant.
Therefore, we can say that a relaxation process is present in the studied doped
PANI samples and it is the interfacial polarization process, which for HCl and TSA
doped PANI is observed in the low- and in the high-frequency regions, respectively.
This supposition is in good agreement with the obtained values of conductivity – PANI
doped by TSA has a higher conductivity value than sample doped by HCl (0.031 S/cm
and 0.49 S/cm for HCl- and TSA-doped PANI, respectively). According to the literature
[171], the increase in the counter-anion size leads to an increase of the interchain
r can be also explained as one due to the increased contribution of the dc-
conductivity [174].
Furthermore, the dependence of ε” versus frequency in the double logarithmic
scale for all PANI doped samples is almost a straight line at low frequenc
milar behaviour was also observed by other
researchers, - for example, Lian et al for poly(N-alkylanilines) [170] and Chen et al for
poly(3-alkylthiophene)s [245]. So, according to Fig. 6.5b, the dc-conductivity value of
HCl doped PANI is lower than that of TSA doped PANI.
As it was mentioned above, ε’ increases with the frequency decrease for all
acid-dopants and the very high polarization may hide the relaxation process if any in the
studied low frequency range at room temperature. Nevertheless, we can say that the
feature of ε’ decreases in the high-frequency region for HCl and CSA doped PANI
175
Chapter 6. Dielectric and electrical properties
dielectric permi )
Figure 6.6. Frequency dependence of the real (a) and imaginary (b) parts of the
ttivity for PANI-TSA. Solid lines are the best fit to HN equation (2.7
(a)
Frequency, Hz106 107 108
ε'
60
80
100
120
140
160
180
200
220
240173 K193 K213 K233 K253 K273 K293 K313 KHN fit
(a)
Frequency, Hz106 107 108
ε"
100
101
102
103
104
173 K 193 K 213 K 233 K253 K273 K293 K313 KHN fit
(a)
(b)
176
Chapter 6. Dielectric and electrical properties
distance. This makes the hopping between chains more difficult and, hence, results in a
reduction of conductivity. But, on the other hand, the doping by TSA causes a compact
coil conf
6.7, respectively. As one can
see, the
e of the process and the conductivity values was found (Fig.
6.8).
ormation of the PANI chains [246]. Such conformation leads to the possibility
of the electron hopping not only along the PANI chains but also between the chains,
which may significantly increase the conductivity.
In order to study the relaxation behaviour with the temperature change and to
estimate the activation energy of the process, dielectric measurements were performed
for TSA-doped PANI in the high frequency range. The real and imaginary parts of
dielectric permittivity and real part of the conductivity for the PANI-TSA sample as a
function of the temperature are depicted in Fig. 6.6 and
real part of the dielectric permittivity exhibits a relaxation process of a rather
high dielectric strength (Fig. 6.6a), as a result of the doping process. This relaxation
process is associated with the charge carriers’ mobility. However, for dedoped PANI
form only a flat dielectric response was observed in this frequency region.
As in the case of dedoped PANI, the temperature dependencies of the
relaxation time and dc-conductivity for doped PANI may be described according to the
equations (2.8) and (6.1). It was found that the activation energy for PANI-TSA is 0.093
eV, i.e. much lower than that for the dedoped sample (0.539 eV). Also, the correlation
between the relaxation tim
Conductivity properties. As expected, the doping process is seen to lead to an
increase of the conductivity at low frequencies (Fig. 6.9). Analyzing the obtained results
we can say that at low frequencies the real part of the conductivity spectra of doped
PANI are dominated by a dc-conductivity, whereas at high frequencies the spectra
reveal an ac-conductivity increasing with frequency (Fig. 6.7).
One should notice the observed difference between conductivity values
measured at low and high frequencies. Such behaviour may be explained by the use of
two different measurement methods – low- and high-frequency devices (see section
2.3.5).
We have undertaken the measurements of the temperature dependence of the
electrical conductivity for the PANI samples doped by the different acids. Fig. 6.10
shows the obtained conductivity variation as a function of the temperature. Zuppiroli et
al [247] assumed that dopant counter ions play a very active role in the formation of the
177
Chapter 6. Dielectric and electrical properties
Figure 6.7. Real part of the conductivity for PANI-TSA as a function of temperature.
TSA doped PANI
Solid lines are the best fit to HN equation (2.7)
Figure 6.8. Dependence of the characteristic relaxation time versus the conductivity for
Frequency, Hz106 107 108
σ', S
/cm
10-5
10-4
10-3
10-2
173 K193 K 213 K 233 K253 K 273 K 293 K313 K HN fit
relaxation time τ, s10-10 10-9 10-8 10-7
σ', S
/cm
10-5
10-4
10-3
10-2
178
Chapter 6. Dielectric and electrical properties
Figure 6.9. Frequency dependence of the real part of the conductivity for PANI doped
by different acids at 300 K
conducting clusters by acting as bridges between neighbouring chains and stabilizing
the conducting regions. As can be seen from this figure, the conductivity initially
increases gradually with the rise
Frequency, Hz10-1 100 101 102 103 104 105 106
σ', S
/cm
10-10
10-9
10-8
10-710-2
10-1
100
in temperature from 173 K to ∼320 K and then
decrease
ility of organic acids in
compari
ion. The first process is likely to be responsible for the conductivity changes in
the vicin
s. It should be noted that for the PANI sample doped by organic acids (TSA,
DBSA and CSA) the maximum value is reached at ∼340 K for TSA and DBSA and at
∼380 K – for CSA-doped PANI. For the PANI samples doped by inorganic acids (HCl
and H2SO4) the decrease of the conductivity starts earlier (300 and 330 K for HCl and
H2S04, respectively). This is connected with higher thermal stab
son with the inorganic ones (see section 5.2).
There are three potential causes for the change in the conductivity: (1) the loss
of the adsorbed moisture; (2) the gradual deprotonation and (3) the oxidative
degradat
ity of 350 K. Other two processes lead to the conversion of conducting PANI
into non-conducting one and a consequent loss of conductivity is anticipated and indeed
observed. This decrease in conductivity is attributed to the separation of the dopants
from the polymer backbones, which results in the dedoping process and, thus, a large
PANI-HClPANI-H2SO4
PANI-TSAPANI-DBSA PANI-CSA dedoped PANI
179
Chapter 6. Dielectric and electrical properties
Figure 6.10. Tempe
from e
m tivity studies
the m
sam
ents of
the conductivity dependence on the frequency
sample was being held at a certain tem me time (∼20 min) until
measurements at all frequencies were done. In the second case (Fig. 6.11) we measured
rature dependence of the electrical dc-conductivity for PANI doped
with different acids
portion of imine nitrogen is being in the non-conducting state. In its turn, this leads to a
decrease in the concentration of polarons. A support for the above inference is obtained
the thermogravimetry results (see section 5.2) which indicate the presence of th
different weight loss stages for the doped PANI samples. The assumption about the
decreasing of the polarons concentration is also well confirmed by the EPR
easurements performed by Kuo and Chen [248]. They performed conduc
for PANI doped with diphenyl phosphate and found that the temperature dependence of
the spin density is consistent with that of the conductivity [248].
In order to verify the proposed assumption about the deprotonation and loss of
oisture we performed additional conductivity measurements for the PANI-HCl
ple as a function of temperature at a fixed frequency (100 Hz) (Fig. 6.11). An
observed difference between conductivity values in Fig. 6.10 and 6.11 is connected with
the use of other way of measurement. In the first case we performed measurem
and temperature simultaneously, i.e. the
perature for so
Temperature, K150 200 250 300 350 400 450
σ', S
/cm
10-4
10-3
10-2
10-1
100
101
HClH2SO4 TSA DBSA CSA
180
Chapter 6. Dielectric and electrical properties
eans that not only the moisture
was eva
Figure 6.11. Temperature dependence of the electrical conductivity for the PANI-HCl
pellet at 100 Hz
the conductivity at a fixed frequency with temperature varying continuously with a
temperature ramp of 2 0C/min. Therefore, the conductivity values obtained by the latter
method are higher than those obtained by the former method. But, in spite of the slight
difference in conductivity, the common behaviour is the same in both cases.
So, as one can see, the first scan is rather different from the next ones (Fig.
6.11). The conductivity maximum, which is observed at ∼310 K is connected with the
beginning of the moisture evaporation and is in agreement with the thermogravimetric
measurements (Fig. 5.4). The conductivity of PANI registered during the next scans is
significantly less compared with the first one, which m
porated, but also the doping acid – HCl, which is a rather volatile acid indeed.
But, nevertheless, some acid remains in the sample, since the conductivity value is still
rather high in comparison with dedoped PANI (Fig. 6.4).
The obtained conductivity measurements are also in agreement with the
investigation of Prokes et al [249], who studied the resistivity evolution of the PANI
films and powders. They found that the resistivity value increased when exposed to high
temperatures. Also, they performed resistivity measurements during the temperature
Temperature, K100 150 200 250 300 350 400 450
σ, S
/cm
10-5
1 -4
10-3
10-2
1 -1
1 00
0
0
start
181
Chapter 6. Dielectric and electrical properties
cycling [249]. The initial increase of the resistivity in the first run reflects the loss
water, while the other ones are connected with the deprotonation and a potential
degradation.
In order to calculate the activation energy value, the dependence of log
versus T-1 was plotted for the linear part of the temperature dependence of the
conductivity for doped PANI (Fig. 6.12). This plot indicates an increase in conductivity
with temperature that may be due to a hopping mechanism [127]. The linear variation of
the conductivity versus temperature for all acid-dopants suggests a thermally activated
process of an Arrhenius-type in the experimental temperature range. The values of the
activation energy calculated from the slope (equation 6.1) are listed in Table 6.1. The
obtained value for the PANI-HCl pellet is in good agreement with the activation energy
value obtained for the PANI derivative – poly(N-methylaniline) doped with chloride io
(0.11 eV) [250].
of
σ
n
significantly lower than that for dedoped PANI. Such difference can be explained by the
Figure 6.12. Arrhenius plot of the conductivity for the doped PANI samples
The obtained values of the activation energy for the doped PANI samples are
presence of localized electrons in dedoped PANI which after the doping process are
delocalized and, therefore, the conductivity process is facilitated.
1/T, K-10.0035 0.0040 0.0045 0.0050 0.0055 0.0060
1
100
σ, S
/cm
0-4
10-3
10-2
10-1
101
HClH2SO4
TSADBSACSA
182
Chapter 6. Dielectric and electrical properties
Table 6.1. Activation energy for PANI doped with different acids
Acid-dopant Activation energy Ea, eV
HCl 0.076
H2SO4 0.159
TSA 0.080
DBSA 0.187
CSA 0.118
dedoped 0.539
The obtained information of relaxation process and conductivity behaviour for
pure PANI is of great importance for a better understanding of the phenomena observed
in PA/PANI composite materials discussed in the next sections.
6.2. Surface conducting composite materials
As it was shown in section conductive composite films have a
layered film structure and a PANI clusters organisation. This can result in the
5.4, the surface
appearance of interfacial polarization effects resulting in the appearance of the
relaxation processes. Additional relaxation associated with the dipolar reorientation of
chains segments or side groups in the PM may contribute the whole spectra of the
composite film.
In order to distinguish these relaxation processes from those of the PM we have
first performed dielectric measurements for the pure PA films.
6.2.1. Dielectric properties of the polyamide matrices
As it is known [174, 237], both dielectric constant and loss factor depend upon:
- the dipole density, characterized by the total number of dipoles per volume
unit;
- the ability of the dipoles to follow the reversal of polarity when applying an
alternating electrical field. Moreover, the mobility of the dipole, in turn, depends upon
the mobility of the polymer chains to which the dipoles are attached.
183
Chapter 6. Dielectric and electrical properties
At very low temperatures, below the glass transition temperature, the dipolar
activity of the polar materials is reduced. As the temperature increases, the mobility of
the dipole and, consequently, the permittivity increase.
known to be a γ-relaxation. About 193 K a polarization process, characterised by a
lecules
the
diele tion of
β-
com
K,
trans , which
ε”
p increase with the increase in temperature as a consequence of
interfacial polarization contribution and ionic conductance (Fig. 6.13b). Therefore, the
third two
relaxation p β γ
follow Arrhenius law (equation 2.8) and the corresponding activation energies are listed
The dielectric properties of most polyamides have been examined and detailed
information can be found in McCrum et al work [237]. Dielectric spectra obtained for
PA-6, PA-11 and PA-12 in this study are shown in Fig. 6.13, 6.14 and 6.15,
respectively. The solid lines are the best fit to HN equation (2.7). For all types of PA at
least two polarization processes related to molecular motions are generally observed in
the investigated temperature range. At lower temperatures (about 173 K) a polarization
process of weak intensity can be seen at high frequencies (Fig. 6.13-6.15). This
relaxation is thought to be due to the local motion of chain segments, mainly, (CH2)
segments, located between the interchain hydrogen bonds [237, 251]. This relaxation is
undergoes a shar
symmetric distribution of relaxation time (α = 0.5, β = 1), is observed which is related
to a mechanism of motion (rotation) of the amide bonds together with water mo
that are bonded to them – β-relaxation process [237]. It should be also noticed that
ctric γ-relaxation is weak, whereas the β-relaxation, which involves the mo
the polar amide groups, is much more pronounced in the dielectric spectra (Fig. 6.13-
6.15). As one can see, the film of PA-6 (Fig. 6.13) displays an extremely broad
relaxation that may be connected with a higher content of the amide groups in
parison with PA-11 and PA-12 (Fig. 2.1).
It has been also shown in the literature that at higher temperatures (313-343
depending on the type of PA) a strong relaxation is found, which corresponds to the
glass-rubber transition of the amorphous phase (α-relaxation) [237]. Above this
ition temperature the materials show a sharp increase in dielectric constant
is due to an interfacial polarization process as a result of trapping of free charge carriers
at boundaries between crystalline and amorphous regions [174]. In our case
relaxation process (α-relaxation) is overlapped and we can distinguish only
rocesses - and ones.
The temperature dependences of these relaxations (β and γ) were found to
184
Chapter 6. Dielectric and electrical properties
Figu
several temperatures
re 6.13. The dielectric loss ε” of the PA-6 film as a function of frequency for
Frequency, Hz10-1 100 101 102 103 104 105 106
ε"173 K
β 193 K213 K
0.08
233 K
0.02
0.04
0.06
0.00γ
6
Frequency, Hz10-1 100 101 102 103 104 105 106
313 K333 K353 K 5373 K
1
2
3
4
ε"
β(b) 0
(a)
185
Chapter 6. Dielectric and electrical properties
eral temperatures
Figure 6.14. The dielectric loss ” of the PA-11 film as a function of frequency for
several temperatures
Frequency, Hz10-1 100 101 102 103 104 105 106
ε"
0.00
0.02
0.04
0.06
0.08
0.10173 K193 K213 K 233 K
γ
β
ε
Figure 6.15. The dielectric loss ε” of the PA-12 film as a function of frequency for
sev
Frequency, Hz10-1 100 101 102 103 104 105 106
ε"
0.00
0.02
0.04
0.06
0.08173 K193 K213 K 233 K
β
γ
186
Chapter 6. Dielectric and electrical properties
in Tab
atic
copoly
Table 6.2. The activation energies of polyamides
Activation energy Ea, eV
le 6.2. The calculated activation energy for the γ process is in good agreement
with previously reported activation energy for PA-12 [237] and for the aryl-aliph
amides [251].
Type of PA γ β
PA-6 0.326 0.525
PA-11 0.436 0.737
PA-12 0.379 0.688
The general trend in the dielectric data for all studied types of PA is quite
similar, but a few differences are observed. The loss peak of the β process is more
intense in PA-6 than in PA-11 and PA-12. This is in agreement with the higher
concentration of the amide groups in PA-6. A freedom of motion of these amide bonds
was assumed to be the cause of this relaxation process [237].
6.2.2. Dielectric and electrical properties of the surface conducting
composite based on the PA-12 matrix
Relaxation properties. Fig. 6.16 presents the frequency dependences of
” for the PA-12/PANI film in the doped and dedoped states. The spectrum of the virgin
PA-12 film is given for comparison. The solid lines in both ε’ and ε” are f
easured data using HN function (equation (2.7)). It should be noticed that spec
Fig. 6.16 are depicted at 193 K since at higher temperatures the α-relaxation process
ε’ and
ε
its to the
m tra in
and nic conductivity den all relaxations.
The results indicate that the aniline polymerization inside the PA film and
PANI presence in the film causes a great increase of ε’ and ε” in the whole studied
frequency region. This feature is observed for the composite films with PANI in the
dedoped state as well as in the doped one. This may be due to the additional polarization
which emerges from the presence PANI even in the dedoped state.
As it has been seen (see section 6.1.1) the value of ε’ is rather high even in dedoped
PANI. Analyzing the obtained results we can conclude that the presence of PANI (12
wt.%) even in thin subsurface layer leads to considerable changes in the dielectric
io hid
of charge carriers in
187
Chapter 6. Dielectric and electrical properties
Frequency, Hz
Fi s
a function of the frequency for ANI film (12 wt.%) at 193 K
gure 6.16. The variation of dielectric constant ε’(a) and dielectric loss factor ε” (b) a
the composite PA-12/P
10-2 10-1 100 101 102 103 104 105 106 107
ε'
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5PA-12 PA-12/PANI dedopedPA-12/PANI doped HCl PA-12/PANI doped HClO4 HN fit
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107
ε"
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
PA-12PA-12/PANI dedopedPA-12/PANI doped HCl PA-12/PANI doped HClO4
HN fit
τ1
γ
β
(b)
(a)
188
Chapter 6. Dielectric and electrical properties
spectra.
In the virgin PA film and dedoped composite film only two relaxation
processes are observed. The first one - the high frequency process - is attributed to the γ-
relaxation process associated with the local chain motion. The second process is most
likely correspondin metric distribution
of relaxation time (α ∼0.5, β 1) and by a relat y (Table 6.2).
In the doped PA-12/PANI film the great increase in ε’ (Fig. 6.16a) can be
explained by an additio larization caused high concentration of charge
carriers in conductive PANI particles dispersed within the insulating PM. As the
conductivity of the dedo -12/PANI film is closer to the virgin PA-12 film than
that of the doped PA-12/PANI, such great variation of ε’ and ε” was not observed in the
former film
s to
the
relaxation process labelled efore, the layered structure of the film
and the PANI clusters organisation in the subsurface conducting layer may result in the
appearan
rature (Fig. 6.17) showed that at high
temperat
g to a β-relaxation process characterised by a sym
= ively high activation energ
nal po by a
ped PA
, i.e. in the dedoped PA-12/PANI film. A similar behaviour was observed by
Mattoso et al [252] in the case of the PVDF/poly(o-methoxyaniline) (PVDF/POMA)
blends, which were prepared by casting the blended solution of POMA and PVDF in
dimethylacetamide.
On the contrary, the doping process (irrespective of the doping acids) lead
appearance in the studied frequency and temperature ranges of an additional
τ1. As it was stated b
ce of polarisation phenomena. Hence, this additional relaxation τ1 can be
attributed to the interaction between PA and doped PANI. It was established that the
additional relaxation process is present only when PANI is in its conducting doped
state.
The performed dielectric relaxation measurements for the composite film
doped by HCl as a function of the tempe
ures (more than 263 K) all relaxations are hidden or overlapped by the
conductivity and α-relaxation process associated with the chain motion in the polymer
matrix, which dominate the spectra. As one can see in Fig. 6.17, the relaxation process
τ1 is temperature-dependent. The characteristic relaxation time τ1 was taken at the
position of the maximum of dielectric loss and its temperature dependence was
analyzed according to equation (2.8). The obtained dependencies are shown in Fig. 6.18
and the obtained fit parameters can be found in Table 6.3. As one can see from the
obtained results, the activation energy of the relaxation process τ1 have rather small
189
Chapter 6. Dielectric and electrical properties
Figure. 6.17. The frequency dependencies of ε” for the composite PA-12/PANI
(12 wt.%) film doped by HCl.
Figure 6.18. Arrhenius plots for the relaxation processes observed in the doped
composite PA-12/PANI films
1000/T, K-1
3.5 4.0 4.5 5.0 5.5 6.0
rela
xatio
n tim
e τ,
s
10-8
10-7
10-6
10-5
rela
xatio
n tim
e τ,
s
10-4
10-3
10-2
10-1
100
101
τHCl
τHClO4τβ
Frequency, Hz10-1 100 101 102 103 104 105 106
ε"
0.00
0.05
0.10
0.15
0.20
0.25
0.35
0.30
173 K193 K213 K 233 K 253 K HN fit
190
Chapter 6. Dielectric and electrical properties
values in comparison with β-relaxation. As it was established previously, the surface
composi
Tabl
te PA-12/PANI film consists of two layers - the first one being the conducting
doped PA/PANI layer and the second one being the pure PM (see section 4.3). This
interfacial polarisation leads to a Debye type relaxation process [174]. Therefore, it may
be suggested that the charge carriers are responsible for this phenomenon.
e 6.3. The fit parameters according to Arrhenius equation (2.8) for the doped
PA-12/PANI films
HCl HClO4Acid-dopant
Parameter τ1 τβ τ1 τβ
τ0, s 2⋅10-9 7.14⋅10-14 1.03⋅10-11 7.14⋅10-14
E, eV 0.115 0.536 0.174 0.536
Conductivity. It should be noticed that not only dielectric, but also electrical
properties of the film are changed after the polymerization and doping processes since
the composite PA-12/PANI film after the doping process gains conductive properties.
Indeed, the surface conductivity measurements performed using four-electrode method
have demonstrated that the dc-conductivity of the PA-12/PANI-HCl film is higher
(namely 5⋅10-5 S/cm at 303 K) than that of pure PA-12 (10-14 S/cm). On the other hand,
it is less than the conductivity of pure PANI (3.3⋅10-2 S/cm), which is explained by the
low PANI content in the composite film (12 wt.%). The performed measurement of the
surface resistance of the composite film doped by HCl is shown in Fig. 6.19, curve 1.
The resistance increases with the decrease of temperature, clearly implying that the
obtained material has to be considered as a semiconductor in the whole range of
temperatures. The activation energy for the conduction as derived from the slope of
Arrhenius plot (Fig. 6.19, curve 2) was estimated to be 0.075 eV which is similar to the
values obtained for the pure PANI powders (Table 6.1). Specifically, for PANI-HCl the
activation energy was found to be 0.076 eV.
Also, we should conclude that the choice of the dopant is very important. The
activation energy is slightly higher in the case of using HClO4 acid as a dopant in
comparison with that for HCl-doped PANI (Table 6.3). Such a feature implies that, in
the case of this acid, the conductivity should be less than in the case of HCl. And, really,
the surface conductivity of the samples doped by HCl is 5⋅10-5 S/cm and for that doped
191
Chapter 6. Dielectric and electrical properties
200 220 240 260 280 300
Res
istan
ce R
, Ohm
10
10
106
5
4
Temperature, K
Figure 6.19. Temperature dependence of the resistance (1) and the corresponding
Arrhenius plot (2) for the composite PA-12/PANI-HCl film
by HClO4 is 6⋅10-6 S/cm (measured by four-electrode method).
6.2.3. The influence of the polymer matrix structure
As it was previously shown (chapter 3.5) the structure of the PM can
significantly vary the properties of the final composite film. In order to verify the
influence of the PM on the dielectric and electrical properties we examined the
composite films based on PA-6 and PA-11.
In Fig. 6.20 the frequency dependence of ε” for the composite PA/PANI films
are plotted. The spectrum of the virgin film is given for comparison. The figure clearly
shows that for both types of polyamide (PA-6 and PA-11) the dielectric behaviour of
the dedoped composite films is similar to that of the PA-12/PANI film (Fig. 6.16). The
presence of dedoped PANI in the composite film leads to some increase of the β- and γ-
relaxations strength.
nges significantly the dielectric spectra of the
composites. Irrespective of th
The PANI doping process cha
e PM, the doped composite films preserve the relaxation
1000/T, K-1
3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
1/R
, Ohm
-1
10-6
10-5
10-4
1
2
192
Chapter 6. Dielectric and electrical properties
Figure 6.20. The variation of dielectric loss factor ε” as a function of the frequency for
the composite PA-11/PANI (a) and PA-6/PANI (b) films at 193 K
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107
ε"
0.00
0.02
0.04
0.06
0.10
0.12
0.08
PA-6PA-6/PANI dedopedPA-6/PANI doped HClHN fit
β relaxation γ relaxation
τ1
τ2
Frequency, Hz
ε
10-2 10-1 100 101 10 103 104 105 106 1070.00
2
"
0.02
0.04
0.06
0.08PA-11PA-11/PANI dedopedPA-11/PANI doped HCl HN fit
τ1(a)
β relaxation
γ nrelaxatio
(b)
193
Chapter 6. Dielectric and electrical properties
process which is caused by β-relaxation of PA. But the doped composite films also
revealed an additional relaxation process τ1 in the investigated temperature and
frequency ranges. The additional relaxation process is, obviously, connected with the
presence of charge carriers in the PANI containing layer.
It should be pointed out that the spectra in Fig. 6.20 are presented at 193 K,
since above the glass transition temperat
respectively) these relaxations are no longer visible because of the appearance of the
ionic conductivity and the α-relaxation process, which dominate the spectra.
Moreover, it should be noted that the composite PA-6/PANI film (Fig. 6.20b)
exhibits somewhat different behaviour. The doping by HCl leads to the appearance of
two additional relaxation processes. These peaks are labelled τ1 and τ2.
The temperature dependencies of these relaxation peaks for both PA are
presented in Fig. 6.21 and the fit parameters obtained by fit to Arrhenius equation (2.8)
for the doped films are shown in Table 6.4.
Table 6.4. The fit parameters for the doped PA/PANI films
ure (323 K and 313 K for PA-6 and PA-11,
PA-6 PA-11 Type of PA
Parameter τ1 τ2 τβ τ1 τβ
τ0, s 1.27⋅10-6 1.2⋅10-8 5.79⋅10-18 4.37⋅10-10 4.48⋅10-15
E, eV 0.039 0.052 0.673 0.149 0.555
As we can see from the obtained results, the second peak (τ2) observed in the
composite film based on PA-6 has an activation energy of 0.052 eV. This value is found
to be closer to the one found in pure PANI and, therefore, the second relaxation peak
may be attributed to the interfacial polarization in PANI, as it was discussed previously
(see sect
may be explained by the higher conductivity values obtained in
the PA f
ion 6.1). On the other hand, analyzing the shape and the obtained values of the
activation energy for τ1 for PA-11 and PA-6 (Table 6.4) one can found that the
relaxation peak τ1 for the former is broader than for the latter.
Also, it should be noted that in the PA based composite films the relaxation
process are found at higher frequencies compared with the composites based on the PET
films [223]. This fact
ilms in comparison with those for the PET film. It is known that the relaxation
frequency is proportional to the conductivity. So, the increase of frequency by four
194
Chapter 6. Dielectric and electrical properties
Figure 6.21. Arrhenius plot for the relaxation processes observed in the doped
1000/T, K-14.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0
rela
xatio
n tim
e τ,
s
10-7
10-6
10-5
10-4
rela
xatio
n tim
e τ,
s
10-4
10-3
10-2
10-1
100
101
τβ
τ1
(b)
1000/T, K-1
4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0
rela
xatio
n tim
e τ,
s
10-4
10-3
10-2
10-1
100
101
rela
xatio
n tim
e τ,
s
10-8
10-7
10-6
10-5
10-4
τβ
τ1
τ2
(a)
composite PA-6/PANI (a) and PA-11/PANI (b) films
195
Chapter 6. Dielectric and electrical properties
decades signifies the increase of the conductivity value by four orders. Indeed, the
resistance of the subsurface conductive layer of the PA/PANI film is four orders lower
than that in the PET/PANI film case.
According to the variation of the activation energy depending on the t
PA (Tab
ype of
osite films
should v
ore conductive than the one based on PA-
12 – the variation of the activation energy is shown in Table 6.5. This result is contrary
to the data ob ut, as it was
mentio e relaxation peak the case of the PA-11/PANI composite is
ra rding to this observation we may believ that the re ation
process this cas ve two relaxation processes. So, the hidden
relaxatio ess could the o d disa nt in th
Table 6.
le 6.3 and 6.4) the studied composites can be placed in the following row:
PA-6/PANI-HCl > PA-12/PANI-HCl > PA-11/PANI-HCl > PA-12/PANI-HClO4
It is logical to suppose that the conductivity value of these comp
ary in the same row. In order to check this assumption we measured by the
four-electrode method the resistance value of the composite films doped by HCl (Fig.
6.22).
As one can see from this figure, the composite film based on the PA-6 film is
the most conductive one, which is in agreement with the obtained row. But it turned out
that the composite film based on PA-11 is m
tained from the relaxation processes (Table 6.3 and 6.4). B
ned above, th τ1 in
ther broad and acco e lax
τ1 in e is an o rlap of
n proc explain bserve greeme e results.
5. The activation energy for the doped PA/PANI films calculated from Fig. 6.22
Type of PA Activation energy, eV
PA-6 0.051
PA-11 0.064
PA-12 0.079
We can say that in spite of the fact that the surface conductivity value of the
composite surface conductive films can not be measured using DRS, the conductive
properties of these films can be evaluated through the study of the interfacial relaxation
processes in the films.
196
Chapter 6. Dielectric and electrical properties
Figure 6.22. Arrhenius plot for the conduction of the composite films, based on the
different types of PA:
1 – PA-6/PANI-HCl film;
2 – PA-11/PANI-HCl film;
3 – PA-12/PANI-HCl film
6.3. Bulk conductive composite materials based on polyaniline and
polyamide
In the previous chapter (section 4.5) it has been shown that it is possible to
prepare conducting composite particles with a PA core and a PANI shell (Fig. 4.18).
Such composite organization may result in the appearance of polarisation phenomena,
as it was shown for the surface conductive composite films (section 6.2).
In this subsection we have focused our attention on the bulk conducting
composite materials based on PA-11 and PA-12 powders as well as on the conducting
films obtained by compression-molding. Special attention is paid to the influence of the
doping acid and to the influence of the PANI-acid amount on the electrical and
dielectric properties of the composites.
1000/T, K-13.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
1/R
, Ohm
-1
10-6
10-5
10-4
1
2
3
197
Chapter 6. Dielectric and electrical properties
6.3.1. Effect of the dopant nature on the properties of the composite
PA-12/PANI powders
As it was widely discussed in the literature [253, 254] and in the previous
chapters, the nature of the counter ions is one of the determining factors controlling the
conductive properties of the composite materials such as the level and stability of the
conductivity. But, apart from the direct incorporation during the polymerization process,
the counter anions can be changed by ion-exchange after the formation of PANI. We
used this approach in the preparation of the samples doped by different inorganic and
organic acids. It should be noticed that the PA particles used in this study were
polydisperse (14-243 µm) and the PANI content was 2.38 wt.% counting on dedoped
PANI.
Relaxation properties. Preliminary we performed measurements for the virgin
PA-12 powder and dedoped PA-12/PANI composite. One can notice that as for the
surface conducting composite films, for the bulk composites only an increase of the β-
relaxation strength is observed in the dedoped samples in comparison with virgin PA
powder (Fig. 6.23).
ANI in the composite material leads to a great increase
in ε’ and ε” for the low-frequency range (<106 Hz), which may be associated with a
greater interfacial polarization due to a higher electrical conductivity. The results
obtained for other composite materials, for example for the composite PVDF/POMA
films [252] and for the TiO2/PANI nanocomposites [255], confirm such an explanation.
ecause of the very high values of conductivity, the dielectric permittivity
spectra of ε’ and ε” do not reveal any relaxation peaks at all studied temperatures. The
dipolar relaxation processes of PA are hidden by the large polarization of the doped
composite. Therefore, we also performed measurements in the high frequency region
(from 10 9 Hz). We found that virgin PA-12 did not reveal any relaxation in
this ran the PA-12/PANI-TSA composite a well-defined relaxation
process is observed followed by a dc-conductivity (Fig. 6.24). The peak frequency is
practically stable with increasing the temperature.
t is
shown in Fig. 6.2 f the activation
nergy is determined (0.032 eV). The value of activation energy
The presence of doped P
B
6 Hz till 10
ge, whereas in
Arrhenius plot of the relaxation time τ for the PA-12/PANI-TSA pelle
5. A straight line behaviour is obtained and the value o
e
198
Chapter 6. Dielectric and electrical properties
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106
ε"
10-2
10-1
100
101
102
PA-12PA-12/PANI dedoped
Figure 6.23. The variation of dielectric loss factor ε” as a function of the frequency for
virgin PA-12 and for the dedoped PA-12/PANI composite at 303 K
Figure 6.24. Imaginary part of dielectric permittivity as a function of frequency for the
PA-12/PANI-TSA powder
Frequency, Hz106 107 108 109
ε"
0
20
40
60173 K193 K 213 K 233 K 253 K 273 K 293 KHN fit
199
Chapter 6. Dielectric and electrical properties
Figure 6.25. Arrhenius plot of dielectric relaxation time for the
PA-12/PANI-TSA powder
for the PA-12/PANI-DBSA samples is 0.065 eV. The higher activation energy for
PANI-DBSA is in agreement with the lower conductivity in this case in comparison to
the conductivity of the PA-12/PANI-TSA composite (see below).
observed at higher fre y be d ers’ mobility. The
presence of the charge carr observed only in the doped PANI complex, as for the
composite material with dedoped PANI the high-frequency region reveals only flat
dielectric response. The ment of this rel process to a rather high
frequency in comparison with surface composite materials (Fig. 6.16 and 6.20) is
connecte
Analyzing all obtained results we can suppose that the relaxation process
quencies ma ue to the charge carri
iers is
displace axation
d with the higher conductivity value obtained for the bulk composite PA-
12/PANI-acid material.
Conductivity. The dc-conductivity value -14
-13
is found to be 1.39⋅10 S/cm and
5.23⋅10 PA-12 and for the dedoped PA-12/PANI sample, respectively. The
slightly higher conductivity value for the dedoped sample in comparison with pure PA-
S/cm for
1000/T, K-13.0 3.5 4.0 4.5 5.0 5.5 6.0
xatio
n τ
, s ti
me
rela
10-9
10-8
200
Chapter 6. Dielectric and electrical properties
12 can be explained by the presence of charge carriers. As it was shown with the help of
Raman spectrometry, even in the dedoped basic form of PANI these charge carriers
exist (the presence of the conduction band on the Raman spectrum of dedoped PANI
(Fig. 5.14)).
After the doping process the value of dc-conductivity increased greatly by
more than 10 orders of magnitude (Fig. 6.26) in spite of the fact that only 2.38 wt.% of
dedoped PANI was in the composite. As one can see from this figure, the most
conductive is the sample doped by TSA. In order to calculate the activation energy, the
measurements of conductivity as a function of temperature for all dopant-acids were
performed (Fig. 6.27). It was found that the conductivity of the composites increases
with temperature and then passes two successive maximums at 293 K and 383 K. A
linear behaviour is observed for the increase in conductivity with temperature for T <
293 K. Above this temperature, the conductivity decreases with increasing temperature
up to 343 K, then a new increase of the conductivity to 383 K is observed. A final
ecrease of the conductivity is observed at temperatures above 383 K. The first decrease
in conductivity can be related to the residual moisture loss. As it was shown by
thermogravimetric analysis (Fig. 5.6), the first weight loss started practically at room
temperature, which is synonymous with the loss of moisture hydrogen-bonded with
PANI. As one can see, in the case of DBSA-dopant this decrease is the smallest (Fig.
6.27). According to [169], on account of the hydrophobic nature of the long alkyl chain
of DBSA ess of the moisture to the PANI chain is difficult. The presence of the
moisture leads to the charge delocalization, most probably by salvation of the acid
anions and, thereby, reducing the electrostatic interaction between the positive charge
and the anions [169]. This charge distribution due to the moisture presence has been
indicated to reduce polarization effects of the anion [169]. The decrease in conductivity
value is assumed to be due to the increased
due to the loss of bounded water molecules ].
The second decrease of the conductivity after 383 K (Fig. 6.27) is associated
with the loss of the dopant ion from
The activation energy was calcu r the linear part of the obtained
dependence (Table 6.6). As one can see from
activation energy are similar, except one refore, we can
assume that the use of organic acids to replace small molecule
d
, the acc
localization of the electronic wave function
[77, 169
the PM, i.e. with the dedoping process.
lated only fo
the obtained results, the values of the
for DBSA-doped PANI. The
201
Chapter 6. Dielectric and electrical properties
Fi site
powder doped by different acids at 303 K
gure 6.26. Frequency dependence of real part of conductivity for the compo
PA-12/PANI
Figure 6.27. Arrhenius plot for the doped PA-12/PANI powder
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108
σ', S
/cm
10-3
10-2
HCl H2SO4
TSADBSA
1000/T, K-1
σ', S
/cm
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.010-3
10-2
HClH2SO4
TSADBSA
202
Chapter 6. Dielectric and electrical properties
acids as a dopant may enhance the thermal stability of protonated PANI because the
evaporation of the dopants may be restricted.
Table 6.6. The activation energy for the doped PA-12/PANI powders
Acid-dopant Activation energy Ea, eV
HCl 0.053
H2SO4 0.053
TSA 0.048
DBSA 0.061
6.3.2. Influence of the polyaniline content
One of the most remarkable effects in the conducting composite materials is
the percolation behaviour of the conductivity as a function of the PANI weight fraction
[256]. Such investigation will provide information on the influence of the PANI content
on the electrical and dielectric properties of the composite materials and will allow
estimating the percolation threshold (pc) for the conducting composite materials.
The percolation theory is based on the formation of the conducting network in
the insulating PM. For example, it was shown [221] that complete miscibility of the
conducting particles with insulating matrix is not desirable as in this case the conducting
network will not be created. In the case of the polymer composites filled with the
conducting polymers, the percolation threshold pc is defined as the minimum amount of
the conducting filler which must be added to an insulating matrix to cause the onset of
the electrical conductivity. This sudden jump in the conductivity is attributed to the
formation of the first “infinite” agglomerate pathway that allows electrons to travel
through a microscopic distance in the composite [256]. According to the theory [256],
this occurs when the filler represents 16 vol.% in the mixture. It means that below this
concentration the composites are very resistant to electrical flow, whereas above this
value the composites are conductive.
Relaxation properties. The frequency-dependent real conductivity σ’ at
various PANI-TSA contents (p) is presented in Fig. 6.28 at 303 K. It should be noticed
that the PA particles used in this study were monodisperse (5 µm).
203
Chapter 6. Dielectric and electrical properties
204
sion
]. This
he increase of the conductivity with the frequency is due to the presence of
various kinds of inhomogeneity in the composite materials [258]. The poor frequency
dependence of the conductivity at higher PANI-TSA contents indicates that more
homogeneous materials are produced. Dutta et al [172], who investigated ac-
conductivity of PANI doped with NSA at various doping concentrations, found similar
behaviour, i.e. the frequency dependence of the conductivity is much more noticeable
w
The complex permitt NI-TSA contents in the
PA-12 matrix are depicted in Fig. 6.29. It is clearly observed that the increase of the
Figure 6.28. The frequency dependence of the real part of the complex conductivity for
the different PANI-TSA weight fractions at 303 K
The usual behaviour of disordered conductors is recovered: conductivity
remains constant at low frequency up to some onset frequency ωc where it starts to
increase. This frequency is the so-called crossover frequency ωc, defined as the
frequency at which ac-conductivity differs from the dc-plateau [257]. For charge carrier
diffusion on percolation structures, ωc is related to the transition from normal diffu
(σ’ (p > pc) = σdc = const) to anomalous diffusion (σ’ (p < pc) ∼ ωs) [257
crossover frequency increases with the increasing of the PANI content (Table 6.7).
T
ith the NSA concentration decreasing.
ivity spectra for the different PA
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107
σ',S
/cm
10-8
10-7
10-6
10-5
10-4
10-3
10-2
0.48 wt.% PANI-TSA1.59 wt.% PANI-TSA3.19 wt.% PANI-TSA5.04 wt.% PANI-TSA7.53 wt.% PANI-TSA9.92 wt.% PANI-TSA
Chapter 6. Dielectric and electrical properties
Figure 6.29. Th rts of
the complex permittivity for the d at 303 K: (ο) 0.48 wt.%;
( ) 1.59 wt.%; (◊) 3.19 wt.%
Frequency, Hz
10-2 10-1 100 101 102 103 104 105 106 107 108
ε"
1
10
1
1
1
10
106
107
108
5
04
03
02
1
00
e frequency dependence of the real ε’ (a) and imaginary ε” (b) pa
ifferent PANI-TSA content
Frequency, Hz101 102 103 104 105 106 107 108
ε'
0
200
400
600 (a)
(b)
205
Chapter 6. Dielectric and electrical properties
Table 6.7. dc-Conductivity and crossover frequency for the composite
PA-12/PANI-TSA powders
PANI-TSA content, wt.% σdc, S/cm ωc, Hz
0 ∼10-14
0.48 9.89⋅10-8 3.96⋅102
1.59 1.49⋅10-6 4.51⋅103
3.19 4.57⋅10-5 1.73⋅105
5.04 2.07⋅10-4 5.85⋅105
7.53 6.43⋅10-4 1.32⋅106
9.92 1.02⋅10-3 4.44⋅106
Pure PANI-TSA 4.905⋅10-1
PANI-TSA content leads to the increase of the relaxation frequency and the static
permittivity up to a certain value. For the PANI-TSA content higher than 3.19 wt.% it
was not possible to measure reasonable values of ε*.
From Fig. 6.29a we observe that a high value of permittivity has been found.
This may be due to an easy charge transfer through well ordered polymer chains in
disordered regions as suggested by Joo et al [259]. This behaviour has been observed in
unblended PANI-HCl by Zuo et al [168].
The power law behaviour of ε”(ω) for frequency lower than some critical
frequency is directly related to the dc-conductivity. A significant difference between the
two groups is also visible in dc-conductivity (σdc = σ’ (ω→0) (Fig. 6.29), which is a
measure of the long range movements of the charge carriers.
We have studied the frequency dependence of the dielectric constant ε’ and ac-
conductivity at several temperatures for the composite material with 3.19 wt.% of
PANI-TSA as shown in Fig.6.30. It is clearly observed that the relaxation frequency
increases with the increase of temperature. The temperature dependence of the
relaxation time τ, calculated from HN function (equation (2.7)), is depicted in Fig. 6.31.
It follows an Arrhenius law (equation (2.8)), which indicates a thermally activated
behaviour of the observed relaxation process. The best fit are obtained with 3⋅10-9 s and
0.067 eV for τ0 and the activation energy, respectively.
On the other hand, the conductivity dependence of the PA-12/PANI-TSA
206
Chapter 6. Dielectric and electrical properties
Frequency, Hz102 103 104 105 106 107 108
ε'
0
50
100
150
200
25
300
0
173 K193 K(a) 213 K 233 K 253 K273 K 293 K HN fit
Figure 6.30. The real permittivity (a) and real conductivity (b) for the PA-12/PANI-
TSA (3.19 wt.%) composite as a function of frequency at different temperatures
Frequency, Hz100 101 102 103 104 105 106 107 108
σ', S
/cm
10-6
10-5
10-4
10-3
10-2
173 K193 K213 K 233 K253 K 273 K 313 K 353 K 373 K 393 K
(b)
207
Chapter 6. Dielectric and electrical properties
Figure 6.31. Temperature variation of relaxation time of the PA-12/PANI-TSA (3.19
ma erial (Fig. 6.30b) is well described by the general power law:
, (6.3)
here σdc is the dc-conductivity, A is a temperature dependent constant and s is the
requency exponent ranging from 0 to 1 [260]. The second item in equation (6.3)
epresents the frequency dependent conductivity known as ac-conductivity.
The value of s for various temperatures has been determined from the linear
lope of log σac(ω) versus log ω. The value of s was found to increase with the decrease
f temperature as exhibited in Fig.6.32 and to lie between 0.25 and 0.84. The value of s
hopping conduction in amorphous materials [260].
Conductivity.
wt.%) powder. Solid line is a fit to Arrhenius law (equation (2.8))
ts
dc A ωσσ ⋅+=
w
f
r
s
o
is in accordance with the theory of
The dependence of the dc-conductivity on the PANI content (p)
ear the percolation threshold pc can be described by the power law [256]:
, (6.4)
here σ0 is a constant, t is a critical exponent that depends on the dimension, p is the
eight fraction of the filled particles, and p is the percolation threshold.
The extrapolated values of the dc-c
composite powders with the different PANI contents are illustrated in Fig. 6.33 for two
nt
cdc pp )(0 −= σσ
w
w c
onductivity variations at 303 K of the
1000/T, K-13.5 4.0 4.5 5.0 5.5 6.0
rela
xatio
n tim
e τ,
s
10-8
10-7
10-6
208
Chapter 6. Dielectric and electrical properties
Figure 6.32. Variation of s with temperature for the PA-12/PANI-TSA (3.19 wt.%)
composite
dopants: TSA and DBSA. It is clearly seen that the electrical conductivity
systematically increases with the PANI weight fraction in PA. But the conductivity of
the bulk conducting composites that were synthesized using TSA as doping acid was
about one order of magnitude higher than that synthesized using DBSA. This can be
attributed to the influence of the counter anion. The DBSA anion has a rather long alkyl
chain and in order to be incorporated in PANI as a counter anion a sufficient place is
necessary. Therefore, the PANI chains will have to deform, resulting in a stressed
polymer backbone and leading to a decrease in conductivity [261]. Nevertheless, the
powders exhibit reasonably good conductivity even at a very low PANI content. Thus,
at the lowest content used, namely, 0.48 wt.% of PANI-TSA, the conductivity is
1.96⋅10-6 S/cm. But the value of conductivity for the PA-12/PANI-TSA composite (9.92
wt.%) is sm
shown in T
uction of the
conjugation length in the PANI chains. Similar results were obtained for other PANI
aller by 2 orders of magnitude in comparison with pure PANI-TSA as
able 6.7. On one hand, this may be because the PA particles hinder carrier
transport between different molecular chains of PANI and, on the other hand, an
interaction at the interface PA particles/PANI also leads to the red
Temperature, K150 200 250 300 350 400 450
s
0.8
0.4
0.5
0.6
0.7
0.3
0.2
0.9
209
Chapter 6. Dielectric and electrical properties
Figure 6.33. Dependence of the dc-conductivity on the PANI-acid weight fraction p at
303 K. The solid lines are fit to equation (6.4). The inset shows the values of σdc above
the perc
tion
of the P
e fitted the σdc data presented in the Fig.6.33 to
equation
olation threshold versus log (p – pc) where the solid lines correspond to the best
fitted line
composites based on common insulating polymers. For example, Chattopadhyay et al
[262] systematically studied the variation of conductivity values at 303 K for different
methyl cellulose (MC)/PANI composites in the alcohol medium. They observed that the
value of conductivity decreases with the increase of alcohol and MC concentrations.
When conductivities of the PA-12/PANI-acid powders are plotted as a func
ANI content in a semi-logarithmic scale, a sharp change in conductivity is
observed at ∼1 wt.% of PANI-acid complex as shown in Fig. 6.33. At this composition
the conductivity changes significantly (over more than 5 decades). This point, probably,
corresponds to the percolation threshold (pc). It is known that the percolation threshold
depends upon the shape of the distribution of conductive particles in the matrix polymer
[263] and, also, on the nature of the acid-dopant [264]. In order to estimate the precise
value of pc and the critical exponent t, w
(6.4) and the best-fit values giving the correlation coefficient of 0.98, are
PANI-acid content, wt.%0 2 4 6 8 10
σ dc, S
/cm
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
TSADBSA
(p-pc)10-2 10-1 100 101 102
σ dc, S
/cm
10-6
10-5
10-4
10-3
10-2
210
Chapter 6. Dielectric and electrical properties
presented in Table 6.8. The obtained low values of the percolation threshold are similar
t observed for the cellulose acetate/PANI blends [265]. We also found that the
percolation threshold pc is temperature-independent.
Table 6.8. The best-fit percolation parameters for the PA-12/PANI-TSA and PA-
12/PANI-DBSA powders
Acid-dopant Percolation threshold pc, wt.% σ0, S/cm t
to tha
TSA 0.37 5⋅10-5 1.85
DBSA 0.61 1.94⋅10-5 1.51
The fit was done by variation of pc in the interval from 0 to 1 in steps of
value of pc the value of t has been determined from the slope of the lin
relation of the dc-conductivity σdc and (p-pc) in a log-log scale (see inset for Fig. 6.
One can see in Fig. 6.33 that the agreement between the data and the theoretical la
qualitatively good on the whole range of composition while it is expected to be va
0.01.
For each ear
33).
w is
lid
only near the percolation threshold. The values of t (Table 6.8) calculated here for the
PA-12/PANI composites are in agreement with the theoretical value of t ≈2.0 for a
percolation network in three dimensions [256]. As one can see from the obtained results
(Table 6.8), in the case of PANI doped by TSA the parameter t is closer to the
theoretical value for the three dimensional network (∼2.0 [256]) than in the case of
doping by DBSA. The smaller value of t observed for the PA-12/PANI-DBSA powder
arises from induced hopping transport between disconnected (or weakly connected)
conducting parts of the network.
It should be mentioned that the obtained values for the percolation threshold
are rather low compared with those given in literature. For example, Tsotra and
Friedrich who studied the epoxy resin/PANI-DBSA blends [266] determined a
percolation value at about 2.5 wt.% of the conductive salt. Zhang et al [225] studied the
PA-11/PANI blends prepared by the dissolution of PA-11 and PANI in the sulphu
acid and established that the percolation in such a system occurs only at about 5 wt.%.
A
t
percolation threshold, which d into the composite with a
ric
t this value of the PANI content, the conductivity of the composite blend was more
han 7 orders of magnitude higher than that of pure PA-11 [225]. Lowering the
means that less PANI was adde
211
Chapter 6. Dielectric and electrical properties
good conductive property, not only reduces the cost of the materials but also improves
its proce
Fig. 6.34 shows the ith temperature for the PA-
12/PANI-TS e esti ation en qual to 0.104
eV. The observed difference between the activ ies for the composite powders
based on PA at the sam NI content (2.3 wt.% counting on dedoped PANI) can be
explained by the different PA particle size – compare activation energy of 0.104 eV for
PA particle of 5 µm an 8 eV - for 14-24 us, with increasing of the size of
the PA particles the va the activation en creased. T rvation can be
explained by the fact that the effective surface which should be covered to provide the
percolation network increases with the decrease of the PA particle size.
vation energy (Table 6.9).
ssability.
variation of conductivity w
A (3.19 wt.%) sample. Th mated activ ergy is e
ation energ
e PA
d 0.04 3 µm. Th
lue of ergy is de his obse
10
-4
Figure 6.34. Conductivity of the PA-12/PANI-TSA (3.19 wt.%) sample as a function of
temperature
Thermal variations of electrical conductivity of the PA-12/PANI-acid powders
with different weight fractions of PANI were performed for powders doped with TSA
and with DBSA. A study of the temperature dependence of the dc-conductivity in the
range of 150-450 K allowed the determination of the acti
1000/T, K-1
3.0 3.5 4.0 4.5 5.0 5.5 6.0
σ', S
/cm
-5
10-6
10
212
Chapter 6. Dielectric and electrical properties
Table 6.9. A comparison of the activation energy of the doped PA-12/PANI powders
Composition of the sample
Activation energy
Ea, eV
PA-12/PANI-TSA (0.48 wt.%) 0.230
PA-12/PANI-TSA (1.59 wt.%) 0.211
PA-12/PANI-TSA (3.19 wt.%) 0.104
PA-12/PANI-TSA (5.04 wt.%) 0.089
PA-12/PANI-TSA (7.53 wt.%) 0.086
PA-12/PANI-TSA (9.92 wt.%) 0.068
PA-12/PANI-DBSA (0.61 wt.%) 0.258
PA-12/PANI-DBSA (1.13 wt.%) 0.226
PA-12/PANI-DBSA (3.61 wt.%) 0.154
As one can see from the obtained results, the activation energy decreases with
the PAN content increasing, indicating the facilitation of the charge transport which
could be attributed to the formation of new conducting paths of PANI as observed for
PANI with different protonation levels doped by HCl [227, 267] and H2SO4 [268]. Such
feature confirms that as far as the PANI content increases, the conductivity of the
composite increases too.
he rapid increase of the activation energy with the decrease of the PANI
fraction reveals the change of geometry which occurs below the percolation threshold
with the rupture of conducting paths. Similar behaviour was observed by Jousseaume et
al [269] for the polystyrene/PANI blends doped by CSA and bis(2-ethyl-hexyl)
hydrogen phosphate.
nfluence of additional doping.
I
T
I The influence of additional doping on the
electrical properties of the composite materials with different PANI-TSA contents has
also been investigated. In Table 6.10 the results of the dc-conductivity measurements
demonstrate the change of the value of the percolation threshold and the value of the
parameter t.
a result of additional doping the value of t is closer to 2, i.e.
the percolation network occurs in three dimensions. Also, this result confirms the data
of Rama spectrometry (see section 5.5.3) about the presence of the non-doped imine
It is found that as
n
213
Chapter 6. Dielectric and electrical properties
Table 6.10. Influence of the additional doping on the percolation parame
PA-12/PANI-TSA powder
Percolation threshold pc, wt.% σ0, S/cm
ters for the
t
As-prepared 0.37 5⋅10-5 1.85
After additional doping 0.3 7⋅10-5 1.95
sites in the PANI structure after the initial polymerization process. We can make such a
conclusion as the slight increase of the conductivity of the PA-12/PANI composites,
obtained by in situ dispersion polymerization in the presence of TSA is observed due to
th
Figure 6.35. Electrical conductivity of the PA-12/PANI-TSA composites as a function
of the PANI-TSA content and doping treatment at 303 K
It is evident that the treatment with additional TSA doping enhances slightly
the conductivity of the composites and reduces a little the percolation threshold.
The temperature dependence of conductivity was also performed in the case of
the additionally doped PA-12/PANI-TSA powders and Table 6.11 shows that the values
of the activation energy are also decreased with the increase of the PANI content. The
e additional doping (Fig. 6.35).
PANI-TSA content, wt.%0 2 4 6 8 10 12
σ dc, S
/cm
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
without additional dopingwith additional doping
214
Chapter 6. Dielectric and electrical properties
Table 6.11. The activation energy for the PA-12/PANI-TSA powders after additional
doping
Composition of the sample
Activation
energy Ea, eV
PA-12/PANI-TSA (0.48 wt.%) 0.162
PA-12/PANI-TSA (1.59 wt.%) 0.156
PA-12/PANI-TSA (3.19 wt.%) 0.117
PA-12/PANI-TSA (5.04 wt.%) 0.088
PA-12/PANI-TSA (7.53 wt.%) 0.087
PA-12/PANI-TSA (9.92 wt.%) 0.074
smaller values of the activation energy for the samples after the additional doping imply
a longer decay length and a higher density of states at the Fermi level (see equations
(1
6.3.3. Influence of the polyamide matrix structure
.13) – (1.15)).
In order to reveal the influence of the PM on the conductive properties of the
bulk composite materials, the PA-11 matrix was a
rather high
conducti
0.031 eV and 0.041 eV for the composites doped with TSA and HCl, respectively.
Analyzin
, because of
the similar matrix molecular structure and close c
this case, as in
the case
lso used for the preparation of the
bulk PA-11/PANI composite materials. The PA particle size was 14-42 µm.
The increase of the complex permittivity values after the doping process for the
composites based on PA-11 was also observed, but because of a
vity value no relaxation process was observed in low-frequency region. As in
the case of the PA-12/PANI-acid composites, at high frequencies a relaxation process
was observed. The relaxation times τ at different temperatures are determined and fitted
with an Arrhenius law (equation (2.8)). The best-fit values for the activation energy are
g the obtained results we may conclude that for both PM (PA-11 and PA-12)
the parameters of the observed relaxation are practically similar, probably
onductivity values.
In Fig. 6.36 the conductivity for the PA-11/PANI composite powders doped
with different acids is plotted at 303 K. The figure clearly shows that in
of the bulk composite based on PA-12, a dc-conductivity plateau is observed
for all dopants. The value of conductivity for the PA-11/PANI-TSA composite is
215
Chapter 6. Dielectric and electrical properties
PA-11/PANI powd ith different acids
Figure 6.36. The conductivity dependence as a function of frequency for the composite
ers doped w
slightly higher than that of the PA-12/PANI-TSA composite (9⋅10-3 S/cm and 6⋅10-3
S/cm, respectively). As for the surface conducting composites (see section 6.2), a higher
conductivity is observed for the composites based on PA with a lower (CH2) groups
concentration. Therefore, we can conclude that the electrical properties of the
composites depend on the structure of the PM.
Measurements of conductivity as a function of temperature were also
performed and it was found that the composite conductivity increases with temperature
up to a certain value for all used acids. A linear relationship was observed at T < 293 K.
Two peaks of conductivity were observed on the received curves – the first one at 343
K and the second one – at 383 K, after which the final decrease of conductivity
occurred. The calculated activation energy values are presented in Table 6.12.
6.3.4. Properties of the composite films
The possibility of producing the conducting composite PA/PANI films can be
an advantage when industrial applications are considered. That’s why the composite
films corresponding to the previously studied powders were obtained by the
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108
σ', S
/cm
10-12
10-11
10-10
10-9
10-810-3
10-2
HCl H2SO4 TSADBSADedoped
216
Chapter 6. Dielectric and electrical properties
compression- molding technique from the powder samples at 195 0C (a detailed
description of the film preparation is given in section 2.2.2).
Table 6.12. The activation energy for the PA-11/PANI samples doped with different
acids
Acid-dopant Activation energy Ea, eV
HCl 0.183
H2SO4 0.161
TSA 0.072
DBSA 0.097
Influence of the dopant nature. In order to display the changes in the
conductivity value we performed measurements of the composite materials in the film
. Fig. 6.37 shows the frequency dependence of the conductivity for the Pform A-
2/PANI composite films doped by different acids measured at 303 K. The spectra of
t
c
difference as compared to pure PA-12 spectra of the doped composite films
we observed a frequency-independent conductivity region at lower frequencies. This
1
he pure and dedoped samples are given for comparison. The real part of the
onductivity of pure PA-12 increases with increasing the frequency and at 0.1 Hz gives
value of ∼10-14 S/cm. The spectrum of the dedoped composite film does not reveal any
while in the
conductivity part is identified as the dc-conductivity. Above a characteristic onset
frequency ωc, the conductivity increases with frequency (Fig. 6.37). The ωc value
increases with increasing the conductivity and it is 5.1⋅104 Hz, 7.7⋅104 Hz and 3.9⋅105
Hz for the composites doped by HCl, DBSA and TSA, respectively.
The conductivity of the composite PA-12/PANI-acid films obey Arrhenius law
and the activation energy is found to be 0.077 eV, 0.078 eV and 0.103 eV for the films
doped by TSA, HCl and DBSA, correspondingly.
It should be mentioned that for the composite materials in the powder form at
303 K only dc-conductivity is observed in the frequency range 0.1 Hz-1MHz (Fig.
6.26). dc-Conductivity value of the films is lower than that of the corresponding powder
composite. For example, for the PA-12/PANI-TSA composite in the powder form the
conductivity is 6⋅10-3 S/cm, while for the corresponding sample in the film form – only
217
Chapter 6. Dielectric and electrical properties
e film preparation we were obliged to stop at 195 0C.
During t
Figure 6.37. Frequency dependence of the conductivity for the composite
PA-12/PANI-acid films at 303 K
1⋅10-6 S/cm. But, on the other hand, the acid nature dependence remains the same in
both sample forms – the most conductive is the sample doped by TSA (Fig. 6.26 and
6.37).
The observed differences in the conductivity values for the composite samples
in powder and film forms may be explained by the thermal instability of the PANI
conducting complex as during th
his procedure the partial dedoping process takes place and a decrease of the
conductivity is afterwards observed. In addition, this high temperature may cause the
disorganization of PANI in the insulating matrix (the derangement of the percolation
net).
Influence of the PANI content. It was shown that the dopant nature has a great
influence on the electrical properties of the composite materials. In order to reveal the
influence of the conducing complex content on the composite conductivity we
performed measurements for the materials in the film form.
Frequency, Hz10-2 10-1 100 01 102 103 104 105 106 107 1081
σ', S
/cm
10-6
10-5
10-14
10-13
10-12
10-1
10-10
10-8
-7
1
10-9
10
PA-12/PANI-HCl PA-12/PANI-TSAPA-12/PANI-DBSAPA-12PA-12/PANI-dedoped
ωc
ωc
218
Chapter 6. Dielectric and electrical properties
The changes of the conductivity with the PANI contents are illustrated in Fig.
6.38. Apparently, the conductivity increased dramatically for the PA-12/PANI-TSA and
PA-12/P
Figure 6.38. The dependence of the dc-conductivity of the composite PA-12/PANI films
(pressing temperature 195 0C) at 303 K
When the PANI content approaches the percolation value, the conductivity
rapidly increases by some orders of magnitude and then increased gradually with further
increase of the PANI content in the system. The acid-dopant also affected the
percolation threshold as can be seen from Table 6.13.
as it was established (see section 5.3, Fig. 5.8), resulted in a small decrease of the tensile
strength. Therefore, introducing of PAN , on one hand, improves the electrical
conducti
both
used acids (Fig. 6.33), but for the samples in the film form they are
ANI-DBSA composite films, when the PANI content approaches the
percolation threshold.
PANI-acid content, wt.%0 2 4 6 8 10 12
σ dc, S
/
10-12
10-11
10-10
10-9
cm
The increase of the content of the conductive PANI complex in the composites,
I
vity of the composites, but, on the other hand, it has a negative influence on
mechanical properties of the composite materials.
As it was established for the samples in the powder form the value of
conductivity and its dependence on the PANI content have the same behaviour for
10-8
10-7
10-6
TSADBSA
219
Chapter 6. Dielectric and electrical properties
PA-12/PANI films
reshold pc, σ0, S/cm t
Table 6.13. Influence of the dopant on the percolation parameters for the composite
Percolation th wt.%
PA-12/PANI 4.52 2.08⋅10-9 1.40 -TSA (195 0C)
PA-12/PANI- 5.63 1.72⋅10-8 0.43 DBSA (195 0C)
dependent on hown by G t al [221] that the
completely mi k. Probably, in the
case of using takes place and, thus, the percolation
network is for small valu rameter t (0.43) also
testifies to this
It is a ent with additional doping leads to the
ecrease of the percolation threshold to 3.03 wt.% in the case of TSA-doped PANI. But
addition
r up to 2 wt.% [158]) this can lead to the acidic hydrolysis.
Thus, th
s of samples are obtained with different frequency-dependences,
which are in good agreem
twork can not be formed. As the PANI
(for example, the sample with 3.19 wt.%
of PANI-TSA) the appearance of
the acid (Fig. 6.38). It was s azotti e
scible composites do not form the percolation networ
DBSA such “ideal dissolution”
med with difficulty and later. The e of pa
fact.
lso necessary to note that the treatm
d
al doping, on the other hand, results in a slight decrease of the conductivity
value – for example, the conductivity value for 9.92 wt.% of PANI-TSA is 1.86⋅10-7
S/cm before and 4.9⋅10-8 S/cm after additional doping. This can be explained by the fact
that during such treatment the additional quantity of the dopant (TSA in this case) is
introduced inside the PM. In the presence of the scanty quantity of water (it is known
that PA can absorb wate
e percolation network can be broken. The decrease of the tensile strength of the
film after additional doping (see Fig. 5.9) also testifies to this.
The measurements of conductivity as a function of frequency were performed
for both the PA-12/PANI-TSA (Fig. 6.39a) and PA-12/PANI-DBSA (Fig. 6.39b)
composites. Two group
ent with the results of percolation threshold (Fig. 6.38). Thus,
for samples with a low PANI-acid content the conductivity is frequency-dependent in
all studied frequency region. This can be explained by the fact that below percolation
threshold the conducting PANI particles are well dispersed within the insulating non-
conducting PA phase, i.e. the percolation ne
content approaches the percolation threshold
a linear part, i.e. the frequency-independent part of
conductivity, is observed (Fig. 6.39a). Therefore, above the percolation threshold the
composite films reveal a dc-conductivity, the value of which corresponds to the flat part
220
Chapter 6. Dielectric and electrical properties
Figure 6
.39. The frequency dependence of the real part σ’ of the complex conductivity
for the PA-12/PANI-acid films with the different PANI-acid content at 303 K
Frequency, Hz10 07 108-2 10-1 100 101 102 103 104 105 106 1
σ', S
/cm
10-14
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
0.61 % PANI-DBSA1.13 % PANI-DBSA3.61 % PANI-DBSA5.64 % PANI-DBSA7.75 % PANI-DBSA
Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108
σ', S
/cm
10-14
10-13
10-8
10-5
10-4
10-3
10-7
10-6
10-12
10-11
10-10
10-9
0.48 % PANI-TSA1.59 % PANI-TSA3.19 % PANI-TSA5.04 % PANI-TSA9.92 % PANI-TSA
(a)
(b)
221
Chapter 6. Dielectric and electrical properties
of the frequency dependence.
t
measurement for the composite PA-12/ A (3.19 wt.%) film pressed at 195 0C
as a function of frequency and temperature simultaneous
real part of permittiv 40. It ved that with the
temperature incr fted to the requency region.
.
to equation (2.7)
lained by the fact
that both
In order to reveal the nature of the relaxation process we carried ou
PANI-TS
ly. The obtained results of the
is clearly obserity ε’ are depicted in Fig. 6.
ease the relaxation process is shi higher f
Figure 6.40. The frequency dependence of the real part of dielectric permittivity for the
PA-12/PANI-TSA (3.19 wt.%) film at different temperatures. Solid lines are the best fit
The relaxation process calculated according to HN function (equation (2.7)) is
characterized by an activation energy of 0.087 eV and a symmetric distribution of
relaxation time (α ~0.5, β = 1). A correlation between the relaxation time and
conductivity were found. The dc-conductivity and the relaxation frequency are both
thermally activated with the same activation energy, which may be exp
5
8
9
10
processes originated from the same transport mechanism.
Thermal variations of electrical conductivity for the PA-12/PANI films with
different weight fractions of the PANI conducting complex are shown in Fig. 6.41a - for
films doped with TSA and in Fig. 6.41b - for films doped with DBSA. In both cases the
Frequency, Hz
10-2 10-1 100 101 102 103 104 105 106 107 108
6
7
ε'
3
4
173 K193 K 213 K233 K 253 K 273KHN fit
222
Chapter 6. Dielectric and electrical properties
Fig n of temperature for the composite
10-5
10
ure 6.41. Electrical conductivity as a functio
PA-12/PANI films doped with (a) TSA and (b) DBSA
Temperature, K150 200 250 300 350 400 450
σ dc, S
/cm
10-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
-4
0.61 % PANI-DBSA 1.13 % PANI-DBSA 3.61 % PANI-DBSA 5.64 % PANI-DBSA 7.75 % PANI-DBSA
(b)
Temperature, K150 200 250 300 350 400 450
σ dc,
S/cm
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
0.49 % PANI-TSA 1.59 % PANI-TSA 3.19 % PANI-TSA 5.04 % PANI-TSA7.53 % PANI-TSA
(a)
223
Chapter 6. Dielectric and electrical properties
plots show that two different regions could be clearly distinguished. The presence of the
two regions on the temperature dependence is also observed in the cas
additionally doped by TSA and for films pressed at higher temperatures (220
24
e of films 0C and
e
conductivity value rapidly and significantly increases. One may suppose that the
presence of this transition is connected w mperature Tg of the
PA film as the value of PA-12 is 37 0C . After this temperature the
movements of the polym omolecules becom er and, as a result, the ionic
conductivity increases.
Activation energy values for the compos s with different PANI-acid
contents are estimated and listed in Table 6.14. The obtained values reveal, as in the
case of
0 0C). The transition between these regions is observed at 310 K, after this point th
ith the glass transition te
Tg for [190]
er macr e easi
ite film
the samples in the powder form, that the activation energy decreases as the
PANI content increases, or, in other words, conductivity increases suggesting the
formation of more polarons responsible for electrical conduction in PANI. However, for
the composites in the film form the values of the activation energy are much higher than
those for the composites in the powder form (compare Table 6.9 and 6.14).
Table 6.14. The activation energy as a function of the PANI-acid content in the
PA-12/PANI composite films
Composition of the sample Activation energy Ea, eV
PA-12/PANI-TSA (0.49 wt.%) 0.688
PA-12/PANI-TSA (1.59 wt.%) 0.656
PA-12/PANI-TSA (3.19 wt.%) 0.563
PA-12/PANI-TSA (5.04 wt.%) 0.410
PA-12/PANI-TSA (7.53 wt.%) 0.135
PA-12/PANI-DBSA (0.61 wt.%) 0.891
PA-12/PANI-DBSA (1.13 wt.%) 0.891
PA-12/PANI-DBSA (3.61 wt.%) 0.874
PA-12/PANI-DBSA (5.64 wt.%) 0.787
PA-12/PANI-DBSA (7.75 wt.%) 0.644
higher than that of TSA doping, which is in agreement with the value of the percolation
It should be noted that the activation energy in the case of DBSA doping is
threshold.
224
Chapter 6. Dielectric and electrical properties
In order to reveal the thermal stability of conductivity for the synthesized PA-
12/PANI composite films, the conductivity of the film with 9.92 wt.% of PANI-TSA
was recorded as a function of time at 423 K (Fig. 6.42). The obtained plot exhibits a
downward tendency as the ageing proceeds. Wolter et al [270] assumed that the
the
ent can
a
So, the obtained result shows that the conductivity after a heat treatment at 423
K durin
deprotonation of a conducting polymer, which is responsible for the loss of conductivity
value, is controlled by a diffusion-like process that starts at the surface of
conducting grains. Furthermore, it has been postulated [271] that a heat treatm
increase the concentration of structural defects in the film, which also results in
decrease of the charge mobility.
Figure 6.42. The time dependence of the conductivity for the PA-12/PANI-TSA (9.92
wt.%) composite films at 423 K
g 2.5 h decreased by less than one order of magnitude. Therefore, these
conducting films are rather promising materials used for producing, for example,
photographic films or plastic foils.
Influence of the polymer matrix structure. The influence of the PM structure
was also investigated by the example of PA-11. The observed value of the onset
Time, min
0 20 40 60 80 100 120 140 160
σ dc, S
/cm
10-5
10-4
225
Chapter 6. Dielectric and electrical properties
frequenc
composite PA-11/PANI-acid films obey Arrhenius law and the
activation energy is found to be 0.064 eV and 0.099 eV for the films doped by TSA and
HCl, correspondingly. The obtained values are of the same order of magnitude as for
the composite films based on the PA-12 film.
Fig
6.4.
rpreted. The obtained results can be
summari
y ωc increases with increasing conductivity of the composite film (Fig. 6.43) as
in the case of PA-12. So, the highest value of ωc is for the film doped with TSA (1.5⋅104
Hz) and the lowest one is for the DBSA doped film (52 Hz). It was also found that the
obtained results for the
ure 6.43. The frequency dependence of the conductivity for the composite
PA-11/PANI films at 303 K
Conclusions
The temperature and frequency dependences of the permittivity and
conductivity for pure PANI as well as for the surface and bulk conducting composite
PA/PANI materials have been measured and inte
zed as follows:
Frequency, Hz10 10 10 10
σ', S
/cm
10
10-14-2 -1 100 101 102 103 104 105 106 7 8
-13
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
HCl TSADBSADedoped
226
Chapter 6. Dielectric and electrical properties
1. High permittivity value (∼100) is found for dedoped PANI due to the
pr r
dedoped PANI and is attributed to the conductivity relaxation, since a correlation of the
conductivity with the relaxation tim
ments for the d PANI samples show a significant
i value. Nevert , a relaxation process at high
frequencies is observed. Besides, the position of this relaxation is found to be dependent
l relaxation processes are found in the doped composite related to
its cond
wt.%
esence of charge carriers. Also, a relaxation process is found at low frequencies fo
e was found.
2. Performed measure oped
ncrease of the conductivity heless
on the acid-dopant used, which is explained by the different counter-anion size and the
different PANI conformation.
3. For the surface conductive composite PA/PANI materials besides the
subglass relaxation processes (β and γ) occurring in the PM as well as in the dedoped
composite, additiona
uctive properties. Interfacial polarization relaxations in the MHz region are
attributed to the layered and clustered structure of the composite. At higher frequency,
conductivity relaxation appears to be connected with the interfacial polarization in the
PANI clusters. The structure of the PM is found to influence these relaxations.
4. Although the surface conductivity value of the composite films can not be
measured using DRS, the conductive properties of these films have been evaluated
through the study of the interfacial relaxation processes in the films.
5. The conductivity of the bulk conductive composite either in powder or in
film forms found to be dependent on the acid-dopant and on the PANI content. A low
percolation threshold is found for the PANI-TSA and PANI-DBSA complexes (0.37
and 0.61 wt.%, respectively) in the powder form whereas in the film form it
increases to 4.52 wt.% and 5.63 wt.%, correspondingly, for the PA/PANI-TSA and PA-
PANI-DBSA composites.
6. The influence of the PANI content on the frequency dependence of the
permittivity and conductivity revealed that the relaxation frequency, attributed to
interfacial polarization effect, shifted to higher frequency region and the onset
frequency of the conductivity also increased with the increase of the PANI content.
7. The activation energy was found to decrease with the increase of the PANI
content as well as after the treatment by additional doping.
8. The increase of the conducting complex content in the composite leads to the
increase of the composite conductivity value in the powder form as well as in the
pressed film form. However, it was found that the conductivity of the samples drops to a
227
Chapter 6. Dielectric and electrical properties
very low value when they are pressed to form films. This can be explained by the
influence of the high temperature value at which films are prepared.
228
General conclusions and
suggestions for future work
General conclusions and suggestions for future work
In the present study, conducting polymer composite materials based on PANI
and PA, either surface or bulk, in powder or film forms, were prepared by the in situ
electrochemical and chemical polymerization methods. The properties of the obtained
materials were investigated in detail by a set of physico-chemical methods, such as
DRS, Raman and UV-Vis spectroscopies, thermogravimetry, spectroelectrochemistry,
tensile strength and conductivity measurements, etc. For comparison, pure PANI was
prepared by the electropolymerization and by chemical polymerization methods. Its
thermal, electrical and dielectric properties in both doped and dedoped states were
investigated to allow a better understanding of the studied properties of the composites.
The peculiarity and kinetics of the chemical and electrochemical aniline
polymerization in the polymer matrix revealed that either in the polymer film or in the
presence of the polymer dispersion the aniline polymerization process runs according to
the typical autocatalytic mechanism. Three stages – an induction period, a propagation
stage and a termination stage – have been pointed out. The aniline polymerization
process in the polymer matrix has noticeably higher reaction orders with respect to
aniline and oxidant (APS) in comparison with the same process in the solution. The
efficiency of the aniline polymerization process depends on the matrix’s ability to swell
in the oxidant solution, on the oxidant and monomer diffusion into the reaction zone.
This is associated with the physicochemical interaction of aniline and PANI with the
PM (formation of hydrogen bonds).
To evaluate the real PANI content in the conducting composites a method
using UV-Vis spectroscopy has been developed.
It was shown that the dopant nature affects thermal properties of pure PANI.
The bulk conductive PA-12/PANI composites showed the increase by ∼100 0C of the
degradation temperature in comparison with pure PANI independently of the acid-
dopant used. Also, it was found that the presence of PANI in the PA matrix leads to a
decrease of its tensile strength. It was confirmed that the nature of the acid-dopant
influences the mechanical properties of the final composite film – the tensile strength is
higher when using the organic acid-dopants than when using the inorganic ones. Such
behaviour is connected with the fact that organic acids perform the triple role – of a
dopant, a plasticizer and compatibilizer.
Both surplus of the acid-dopant (which appeared after the treatment by
additional doping) in the PA/PANI composite and the increase of the pressing
temperature deteriorate the tensile strength of the final composite material. The main
229
General conclusions and suggestions for future work
reason of this phenomenon is the acidic hydrolysis of the PA matrix since the surplus of
the acid or its appearance in the material during the pressing at higher temperatures due
to dissociation of the conducting complex PANI-acid takes place.
The results of Resonance Raman spectrometry confirmed the interaction
between PANI and the PA matrix (hydrogen bonds). For the first time with the help of
Raman spectrometry the thickness of the formed PANI containing layer, which depends
on the PM structure, have been determined. In the case of the surface conductive
composite PA-6/PANI film the characteristic PANI bands were found to be more
intense than those of PA-12/PANI film due to the higher thickness of the PANI
containing layer and more quantity of formed PANI in the case of the PA-6 film. The
differences may be explained in terms of the PM molecular structure, which results in
higher swelling capacity of PA-6.
The layered and clustered structure of the surface conductive composites
revealed by AFM and optical microscopy and the core-shell organization (PA grains-
PANI) of the bulk conductive composites revealed by Raman spectrometry resulted in
the appearance of an interfacial polarization relaxation process. The relaxation
frequency of this process is found to be proportional to the dc-conductivity. These two
physical properties (relaxation process and conductivity) are found to be dependent
upon the dopant nature, the PANI content and the matrix structure.
In surface conductive PA/PANI composites, besides the subglass relaxation
processes (β and γ) occurring in the pure matrix and in the dedoped PA/PANI
composites, additional relaxation processes are found in the doped composite related to
its conductive properties. Interfacial polarization relaxations in the MHz region are
attributed to the layered and clustered structure of the composite. At higher frequency,
conductivity relaxation appears to be connected with the interfacial polarization in the
PANI clusters.
The study of the conductivity dependence on the PANI-acid complex content
for the bulk composites revealed a low percolation threshold for the composites in
powder form - 0.37 wt.% and 0.61 wt.% for the PANI-TSA and PANI-DBSA
complexes, respectively. However, in compression molded films the value of the
percolation threshold increases (4.52 wt.% and 5.63 wt.% for the PA/PANI-TSA and
PA-PANI-DBSA composites, correspondingly).
Therefore, in the obtained composites the insulating polymer (PA) provides
good mechanical, thermal properties and processability while the conducting polymer
230
General conclusions and suggestions for future work
(PANI) provides electrical conductivity. Obtained results testify to the practical and
scientific possibility of using these composite materials based on PANI and polyamide
as antistatic coatings and in sensors.
However, to improve the electrical properties of the compression molded film
and to enhance the thermal stability of the conductivity it would be useful to find (or to
synthesize) dopants, which will display higher thermal stability than those used in this
study.
Also, the study of the estimation of the PANI doping level in the PM/PANI
composites would be very effective and useful to determine the minimum necessary
amount of the doping acid leading to a high conductivity value of the final composite.
Finally, attention should be paid also to the influence of the size of the polymer
matrix particles on the composite properties. A full understanding of the behaviour of
this class of materials will be of benefit to the applications of conducting polymer
composite materials in various fields.
231
References
References
1. Scrosati B. Electrochemical properties of conducting polymers // Progr. Solid
State Chem. - 1988. - Vol. 18, l. - P. 1 – 77.
2. Bakhshi A.K. Electrically conducting polymers: from fundamental to applied
research // Bull. Mater. Sci. - 1995. - Vol. 18, 5. - P. 469 – 495.
3. Novak P., Muller K., Santhanam K.S.V., Haas O. Electrochemically active
polymers for rechargeable batteries // Chem. Rev. - 1997. - Vol. 97. - P. 207 – 281.
4. Anand J., Palaniappan S., Sathyanarayana D.N. Conducting polyaniline
blends and composites // Prog. Polym. Sci. – 1998. - Vol. 23. - P. 993 – 1018.
5. Pud A., Ogurtsov N., Korzhenko A., Shapoval S. Some aspects of preparation
methods and properties of polyaniline blends and composites with organic polymers //
Prog. Polym. Sci. – 2003. – 28. – P. 1701 - 1753.
6. Kang E.T., Neoh K.G., Tan K.L. Polyaniline: a polymer with many interesting
intrinsic redox states // Prog. Polym. Sci. – 1998. – 23. – P. 277 – 324.
7. Lu F.-L., Wudi F., Nowak M., Heeger A.J. Phenyl-capped octaaniline (COA):
An excellent model for polyaniline // J. Am. Chem. Soc. - 1986. - Vol. 108, 26. - P.
8311 - 8313.
8. Genies E.M., Lapkowski M., Noel P., Langlois S., Collomb M.N., Miquelino
F. Recent results on polyanilines. Electrochemistry, EPR substituted derivatives,
applications, displays, etc... // Synthetic Metals. - 1991. - Vol. 43, 1-2. - P. 2847 -
2852.
9. Glarum S.H., Marshall J.H. The in situ ESR and electrochemical behavior of
poly(aniline) electrode films // J. Electrochem. Soc. - 1987. - Vol. 134, 9. - P. 2160 -
2165.
10. Zeng X.-R., Ko T.-M. Structures and properties of chemically reduced
polyanilines // Polymer. – 1998. – Vol. 39, 5. – P. 1187 – 1195.
11. Ping Z., Nauer G.E., Neugebauer H., Theiner J., Neckel A. In situ Fourier
transform infrared attenuated total reflection (FTIR-ATR) spectroscopic investigations
on the base-acid transitions of leucoemeraldine // Electrochimica Acta. – 1997. - Vol.
42, 11. – P. 1693 – 1700.
12. El-Shall M.S., Kafafi S.A., Meot-Ner (Mauter) M., Kertesz M. Ionic charge-
transfer complexes. 2. Comparative ab initio and semiempirical studies on cationic
complexes of aniline // J. Am. Chem. Soc. – 1986. – 108. – P. 4391 - 4397.
232
References
13. Mohilner D.M., Adams R.N., Argersinger W.J. Investigation of the kinetics
and mechanism of the anodic oxidation of aniline in aqueous sulfuric acid solution at a
platinum electrode // J. Am. Chem. Soc. - 1962. - Vol. 84, 19. - P. 3618 - 3622.
14. Винокуров И.А., Берцев В.В., Каршина Е.А. Спектроэлектрохимическое
изучение полианилинового электрода // Электрохимия. – 1990. - т.26, 3. - С.
311 – 317.
15. Erdem E., Karakisla M., Sacak M. The chemical synthesis of conductive
polyaniline doped with dicarboxylic acids // European Polymer Journal. – 2004. - Vol.
40, 4. – P. 785 - 791.
16. Тарасевич М.Р., Орлов С.Б., Школьников Е.И. и др. Электрохимия
полимеров. – М.: Наука, 1990. – 238 с.
17. MacDiarmid A.G., Chiang J.C., Richter A.F., Epstein A.J. Polyaniline: a new
concept in conducting polymers // Synthetic Metals. – 1987. - Vol. 18, 1-3. – P. 285
– 590.
18. Reiss H. Theoretical analysis of protonic acid doping of the emeraldine form
of polyaniline // J. Phys. Chem. – 1988. – 92. – P. 3657 – 3662.
19. Davied S., Nicolau Y.F., Melis F., Revillon A. Molecular weight of
polyaniline synthesized by oxidation of aniline // Synthetic Metals. – 1995. – 69. – P.
125 - 126.
20. Watanabe A., Mori K., Iwasaki Y., Nakamura Y. Molecular weight of
electropolymerized polyaniline // J. Chem. Soc. Chem. Commun. - 1987. - 1. - P. 3 - 4.
21. Stejskal J., Riede A., Hlavata D., Prokes J., Helmstedt M., Holler P. The effect
of polymerization temperature on molecular weight, crystallinity and electrical
conductivity of polyaniline // Synthetic Metals. – 1998. – 96. – P. 55 – 61.
22. Genies E.M., Lapkowski M., Tsintavis C. La polyaniline: preparations,
properties et applications // New J. Chem. - 1988. - Vol. 12, 4. - P. 181 - 196.
23. Li S., Cao Y., Xue Z. Soluble polyaniline // Synthetic Metals. – 1987. – Vol.
20, 2. – P. 141 – 149.
24. Salaneck W.R., Lundström I., Huang W.S., MacDiarmid A.G. A two-
dimensional-surface ‘state diagram’ for polyaniline // Synthetic Metals. - 1986. - Vol.
13, 4. - P. 291 - 297.
25. Genies E.M., Vieil E. Theoretical charge and conductivity ‘state-diagrams’ for
polyaniline versus potential and pH // Synthetic Metals. – 1987. – Vol. 20, 1. – P. 97
– 108.
233
References
26. Choi H.-Y., Mele E.J. Commensurability effects and modulated structure in
polyanilines // Phys. Rev. Lett. - 1987. - Vol. 59, 19. - P. 2188 - 2191.
27. Huang W.S., Humphrey B.D., MacDiarmid A.G. Polyaniline, a novel
conducting polymer. Morphology and chemistry of its oxidation and reduction in
aqueous electrolytes // J. Chem. Soc. Faraday Trans. - 1986. - Vol. 82, 8. - P. 2385 -
2450.
28. LaCroix J.-C., Diaz A.F. Electrolyte effects on the switching reaction of
polyaniline // J. Electrochem. Soc. - 1988. - Vol. 135, 6. - P. 1457 - 1463.
29. Zotti G., Cattarin S., Comisso N. Electrodeposition of polythiophene,
polypyrrole and polyaniline by cyclic potential sweep method // J. Electroanal. Chem. -
1987. - Vol. 235, 1 - 2. - P. 259 - 273.
30. Genies E.M., Tsintavis C. Electrochemical behavior, chronocoulometric and
kinetic study of the redox mechanism of polyaniline deposits // J. Electroanal. Chem. -
1986. - Vol. 200, 1-2. - P. 127 - 145.
31. Konig U., Schultze J.W. Kinetics of polyaniline formation and redox
processes // J. Electroanal. Chem. - 1988. - Vol. 242, 1-2. - P. 243 - 254.
32. Kobayashi T., Yoneyama H., Tamura H. Electrochemical reactions concerned
with electrochromism of polyaniline film-coated electrodes // J. Electroanal. Chem. -
1984. - Vol. 177, 1-2. - P. 281 - 291.
33. Stilwell D.E., Park S.-M. Electrochemistry of conductive polymers. IV.
Electrochemical studies on polyaniline degradation – product identification and
coulometric studies // J. Electrochem. Soc. - 1988. - Vol. 135, 10. - P. 2497 - 2502.
34. Genies E.M., Lapkowski M., Penneau J.F. Cyclic voltammetry of polyaniline:
interpretation of the middle peak // J. Electroanal. Chem. - 1988. - Vol. 249, 1-2. - P.
97 - 107.
35. Pud A.A. Stability and degradation of conducting polymers in electrochemical
systems // Synthetic Metals. - 1994. - Vol. 66, 1-3. - P. 1 - 18.
36. Андреев В.Н., Майоров А.П. Исследование процесса деградации пленок
полианилина с использованием радиоизотопного и электрохимического методов
// Электрохимия. – 1993. - т.29, 2. - С. 282 – 285.
37. Sasaki K., Kaya M., Yano K., Kitani A., Kunai A. Growth mechanism in the
electropolymerization of aniline and p-aminodiphenylamine // J. Electroanal. Chem. -
1986. - Vol. 215, 1 - 2. - P. 401 - 407.
234
References
38. Glarum S.H., Marshall J.H. Electron delocalization in poly(aniline) // J. Phys.
Chem. - 1988. - Vol. 92, 14. - P. 4210 - 4217.
39. Devreux F., Genoud F., Nechtschein M., Villeret B. ESR investigation of
polarons and bipolarons in conducting polymers: the case of polypyrrole // Synthetic
Metals. - 1987. - Vol. 18, 1-3. - P. 89 - 94.
40. Feldberg S.W. Reinterpretation of polypyrrole electrochemistry.
Consideration of capacitive currents in redox switching of conducting polymers // J.
Am. Chem. Soc. - 1984. - Vol. 106, 17. - P. 4671 - 4674.
41. Mermilliod N., Tanguy J., Hoclet M., Syed A.A. Electrochemical
characterization of chemically synthesized polyanilines // Synthetic Metals. - 1987. -
Vol. 18, 1-3. - P. 359 - 364.
42. Glarum S.H., Marshall J.H. The impedance of polyaniline electrode films // J.
Electrochem. Soc. - 1987. - Vol. 134, 1. - P. 142 - 147.
43. Rubinstein I., Sabatani E., Rishpon J. Electrochemical impedance analysis of
polyaniline films on electrodes // J. Electrochem. Soc. - 1987. - Vol. 134, 12. - P.
3078 - 3083.
44. Focke W.W., Wnek G.E., Wei Y. Influence of oxidation state, pH, and
counterion on the conductivity of polyaniline // J. Phys. Chem. - 1987. - Vol. 91, 22.
- P. 5813 - 5818.
45. Agbor N.E., Petty M.C., Monkman A.P. Polyaniline thin films for gas sensing
// Sensors and Actuators B. – 1995. – 28. – P. 173 – 179.
46. Li X., Ju M., Li X. Chlorine ion sensor based on polyaniline film electrode //
Sensors and Actuators B. – 2004. – 97. – P. 144 – 147.
47. Cui S.-Y., Park S.-M. Electrochemistry of conductive polymers. XXIII.
Polyaniline growth studied by electrochemical quartz crystal microbalance
measurements // Synthetic Metals. – 1999. – 105. – P. 91 – 98.
48. Baba A., Tian S., Stefani F., Xia C., Wang Z., Advincula R.C., Johannsmann
D., Knoll W. Electropolymerization and doping/dedoping properties of polyaniline thin
films as studied by electrochemical-surface plasmon spectroscopy and by the quartz
crystal microbalance // J. Electroanal. Chem. – 2004. - Vol. 562, 1. – P. 95 - 103.
49. Horanyi G., Inzelt G. Anion-involvement in electrochemical transformations
of polyaniline. A radiotracer study // Electrochimica Acta. - 1988. - Vol. 33, 7. - P.
947 - 952.
235
References
50. Иванов В.Ф., Кучеренко Ю.А., Некрасов А.А., Ванников А.В. Влияние
рН водных растворов хлорной кислоты на процесс электроокрашивания
полианилина // Электрохимия. – 1990. - т.26, 6. – C. 732 – 737.
51. Винокуров И.А., Берцев В.В., Батова С.А. Спектроэлектрохимическое
изучение пересольватации полианилина и полипиррола // Электрохимия. – 1990. -
т.26, 10. - С. 1351 – 1353.
52. Malta M., Gonzalez E.R., Torresi R.M. Electrochemical and chromogenic
relaxation processes in polyaniline films // Polymer. – 2002. – 43. – P. 5895 - 5901.
53. Yano J., Yamasaki S. Three-color electrochromism of an aramid film
containing polyaniline and poly(o-phenylenediamine) // Synthetic Metals. – 1999. –
102. – P. 1157.
54. Свердлова О.В Электронные спектры в органической химии. – Л.:
Химия, 1985. – 248 с.
55. Watanabe A., Mori K., Iwasaki I., Nakaamura Y., Niizuma S.
Electrochromism of polyaniline film prepared by electrochemical polymerization //
Macromolecules. - 1987. - Vol. 20, 8. - P. 1793 - 1796.
56. Lindfors T., Kvarnstrom C., Ivaska A. Raman and UV-Vis spectroscopic
study of polyaniline membranes containing a bulky cationic additive // J. Electroanal.
Chem. – 2002. – 518. – P. 131 - 138.
57. Neudeck A., Petr A., Dunsch L. The redox mechanism of polyaniline studied
by simultaneous ESR-UV-vis spectroelectrochemistry // Synthetic Metals. – 1999. –
107. – P. 143 – 158.
58. Giri D., Kundu K., Majumdar D., Bhattacharyya S.P. Possibility of polaronic
structure in polyaniline lattice: A semiempirical quantum chemical approach // Journal
of Molecular Structure (Theochem). – 1997. – 417. – P. 175 - 185.
59. Harigaya K. Long-range excitons in conjugated polymers with ring trosions:
poly(para-phenylene) and polyaniline // J.Phys.: Condens. Matter. – 1998. – 10. – P.
7679 - 7690.
60. Dimitriev O.P. Origin of the exciton transition shift in thin films of
polyaniline // Synthetic Metals. – 2002. – 125. – P. 359 – 363.
61. Motheo A.J., Venancio E.C., Mattoso L.H.C. Polyaniline synthesized in
propylene carbonate medium in the presence of di- and tri-chloroacetic acids. Part I.
Polymer growth studies // Electrochimica Acta. – 1998. - Vol. 43, 7. – P. 755 - 762.
236
References
62. Boara G., Sparpaglione M. Synthesis of polyanilines with high electrical
conductivity // Synthetic Metals. – 1995. – 72. – P. 135 – 140.
63. Aoki K., Hayashi K. A frequency-dependent capacitance model and analysis
of the a.c. impedance of conducting polyaniline films // J. Electroanal. Chem. – 1995. –
384. - P. 31 - 37.
64. Tang H., Kitani A., Shiotani M. Effects of anions on electrochemical
formation and overoxidation of polyaniline // Electrochimica Acta. – 1996. - Vol. 41,
9. – P. 1561 - 1567.
65. Srivastava M.P., Mohanty S.R., Annapoorni S., Rawat R.S. Diode like
behaviour of an ion irradiated polyaniline film // Physics Letters A. – 1996. – 215. – P.
63 - 68.
66. Huang F., Wang H.L., Feldstein M., MacDiarmid A.G., Hsieh B.R., Epstein
A.J. Application of thin films of conjugated polymers in electrochemical and
conventional light-emitting devices and in related photovoltaic devices // Synthetic
Metals. – 1997. – 85. – P. 1283 - 1284.
67. Glarum S.H., Marshall J.H. In situ potential dependence of poly(aniline)
paramagnetism // J. Phys. Chem. - 1986. - Vol. 90, 23. - P. 6076 - 6077.
68. Ivanov V.F., Nekrasov A.A., Gribkova O.L., Vannikov A.A.
Spectroelectrochemical, EPR and conductivity investigations of thin films of vacuum
deposited polyaniline // Electrochimica Acta. – 1996. - Vol. 41, 11-12. – P. 1811 –
1814.
69. Langer J.J., Krzyminiewski R., Kruczynski Z., Gibinski T., Czajkowski I.,
Framski G. EPR and electrical conductivity in microporous polyaniline // Synthetic
Metals. – 2001. – 122. – P. 359 - 362.
70. Morales G.M., Miras M.C., Barbero C. Anion effects on aniline
polymerisation // Synthetic Metals. – 1996. – 101. – P. 686.
71. Tagowska M., Palys B., Jackowska K. Polyaniline nanotubules – anion effect
on conformation and oxidation state of polyaniline studied by Raman spectroscopy //
Synthetic Metals. – 2004. – 142. – P. 223 – 229.
72. Брунерс Р.У., Лицитис Г.Я., Восекалнс А.В. Исследования
электропроводящих полимерных покрытий на электродах. 1. Электрохимические
свойства пленок полианилина в растворах кислот // Изв. АН ЛатвССР. Сер. хим. -
1986. - 4. - С. 450 - 454.
237
References
73. Chandrakanthi N., Careem M.A. Thermal stability of polyaniline // Polymer
Bulletin. – 2000. – Vol. 44, 1. – P. 101 – 108.
74. Chan H.S.O., Ho P.K.H., Khor E., Tan M.M., Tan K.L., Tan B.T.G., Lim
Y.K. Preparation of polyaniline doped in mixed protonic acids: their characterization by
X-ray protoelectron spectroscopy and thermogravimetry // Synthetic Metals. – 1989. –
31. – P. 95 – 108.
75. Ding L., Wang X., Gregory R.V. Thermal properties of chemically
synthesized polyaniline (EB) powder // Synthetic Metals. - 1999. – 104. – P. 73 - 78.
76. Lu X., Tan C.Y., Xu J., He C. Thermal degradation of electrical conductivity
of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants //
Synthetic Metals. – 2003. – 138. – P. 429 – 440.
77. Matveeva E.S. Residual water as a factor influencing the electrical properties
of polyaniline. The role of hydrogen bonding of the polymer with solvent molecules in
the formation of a conductive polymeric network // Synthetic Metals. – 1996. – 79. – P.
127 - 139.
78. Diaz Calleja R., Matveeva E.S., Parkhutik V.P. Electric relaxation in
chemically synthesized polyaniline: study using electric modulus formalism // J. Non-
Cryst. Solids. - 1995. – 180. – P. 260 – 265.
79. Pinto N.J., Sinha G.P., Aliev F.M. Frequency-dependent conductivity and
dielectric permittivity of emeraldine base and weakly doped poly(o-toluidine) //
Synthetic Metals. – 1998. – 94. – P. 199 – 203.
80. Singh R., Arora V., Tandon R.P., Mansingh A., Chandra S. Dielectric
spectroscopy of doped polyaniline // Synthetic Metals. – 1999. – 104. – P. 137 – 144.
81. Li Z.F., Kang E.T., Neoh K.G., Tan K.L. Effect of thermal processing
conditions on the intrinsic oxidation states and mechanical properties of polyaniline
films // Synthetic Metals. – 1997. – 87. – P. 45 - 52.
82. Tzou K., Gregory R.V. Mechanically strong, flexible highly conducting
polyaniline structures formed from polyaniline gels // Synthetic Metals. – 1993. – 555.
– P. 983 – 988.
83. Laughlin P.J., Monkman A.P. Mechanical properties of oriented emeraldine
base polyaniline // Synthetic Metals. – 1997. – 84. – P. 765 – 766.
84. Ayad M.M., Salahuddin N., Sheneshin M.A. Optimum reaction conditions for
in situ polyaniline films // Synthetic Metals. – 2003. - Vol.132, 2. – P. 185 - 190.
238
References
85. Hussain A.M.P., Kumar A. Electrochemical synthesis and characterization of
chloride doped polyaniline // Bull. Mater. Sci. – 2003. - Vol. 26, 3. – P. 329 – 334.
86. Ковальчук Е.П., Аксиментьева Е.И., Томилов А.П. Электросинтез
полимеров на поверхности металлов. – М.: Химия, 1991. – 223 с.
87. Stafstrom S., Bredas J.L., Epstein A.J., Woo H.S., Tanner D.B., Huang W.S.,
MacDiarmid A.G. Polaron lattice in highly conducting polyaniline: theoretical and
optical studies // Phys. Rev. Lett. – 1987. – 59. – P. 1464 - 1467.
88. Norris I.D., Kane-Maguire L.A.P., Wallace G.G. Electrochemical synthesis
and chiroptical properties of optically active poly(o-methoxyaniline) //
Macromolecules. - 2000. - Vol. 33. - P. 3237 – 3243.
89. Borole D.D., Kapadi U.R., Kumbhar P.P., Hundiwale D.G. Effect of inorganic
dopants (in presence of electrolyte) on the conductivity of polyaniline, poly(o-toluidine)
and their copolymer thin films // Materials Letters. – 2002. – 57. – P. 844 – 852.
90. Kim S.H., Seong J.H., Oh K.W. Effect of dopant mixture on the conductivity
and thermal stability of polyaniline/Nomex conductive fabric // J. Appl. Pol. Sci.. –
2002. - Vol. 83. – P. 2245 – 2254.
91. Kiattibutr P., Tarachiwin L., Ruangchuay L., Sirivat A., Schwank J. Electrical
conductivity responses of polyaniline films to SO2-N2 mixtures: effect of dopant type
and doping level // Reactive and functional polymers. – 2002. – 53. – P. 29 – 37.
92. Gazotti W.A.Jr., Jannini M.J.D.M., Cordoba de Torresi S.I., Paoli M.-A. de
Influence of dopant, pH and potential on the spectral changes of poly(o-
methoxylaniline): relationship with the redox processes // J. Electroanal. Chem. – 1997.
– 440. – P. 193 - 199.
93. Angelopoulos M., Patel N., Saraf R. Amic acid doping of polyaniline:
Characterization and resulting blends // Synthetic Metals. – 1993. – Vol. 55, 2-3. –
P. 1552 – 1557.
94. Pron A., Osterholm J.-E., Smith P., Heeger A.J., Laska J., Zagorska M.
Processable conducting polyaniline // Synthetic Metals. – 1993. - Vol. 57, 1. – P.
3520 – 3525.
95. Joo J., Chung Y.C., Song H.G., Baeck J.S., Lee W.P., Epstein A.J.,
MacDiarmid A.G., Jeong S.K., Oh E.J. Charge transport studies of doped polyanilines
with various dopants and their mixtures // Synthetic Metals. – 1997. – 84. – P. 739 –
740.
239
References
96. Mathai C.J., Saravanan S., Jayalekshmi S., Venkitachalam S., Anantharaman
M.R. Conduction mechanism in plasma polymerized aniline thin films // Materials
Letters. – 2003. – 57. – P. 2253 - 2257.
97. Karakisla M., Sacak M., Erdem E., Akbulut U. Synthesis and characterization
of malonic acid-doped polyaniline // J. Appl. Electrochem. – 1997. – 27. – P. 309 –
316.
98. Delvaux M., Duchet J., Stavaux P.-Y., Legras R., Demoustier-Champagne S.
Chemical and electrochemical synthesis of polyaniline micro- and nano-tubules //
Synthetic Metals. – 2000. – 113. – P. 275 - 280.
99. Palaniappan S. Chemical and electrochemical polymerization of aniline using
tartaric acid // European Polymer Journal. – 2001. – 37. - P. 975 - 981.
100. Adams P.N., Monkman A.P. Characterization of high molecular weight
polyaniline synthesized at –40 oC using a 0.25:1 mole ratio of persulfate oxidant to
aniline // Synthetic Metals. – 1997. – 87. – P. 165 - 169.
101. Sun Z., Geng Y., Li J., Jing X., Wang F. Chemical polymerization of
aniline with hydrogen peroxide as oxidant // Synthetic Metals. – 1997. – 84. – P. 99 -
100.
102. Zhu H., Mu S. Effect of Fenton reagent on the synthesis of polyaniline //
Synthetic Metals. – 2001. – 123. – P. 293 - 297.
103. Rao P.S., Sathyanarayna D.N., Palaniappan S. Polymerization of aniline
in an organic peroxide system by the inverted emulsion process // Macromolecules. –
2002. – 35. – P. 4988 - 4996.
104. Sapurina I., Riede A., Stejskal J. In-situ polymerized polyaniline films 3.
Film formation // Synthetic Metals. – 2001. – 123. – P. 503 - 507.
105. Jelle B.P., Hagen G. Performance of an electrochromic window based on
polyaniline, prussian blue and tungsten oxide // Solar Energy Materials and Solar Cells.
– 1999. – 58. – P. 277 - 286.
106. Mazeikiene R., Malinauskas A. Electrochemical stability of polyaniline
// European Polymer Journal. – 2002. – 38. – P. 1947 - 1952.
107. Naudin E., Gouerec P., Belanger D. Electrochemical preparation and
characterization in non-aqueous electrolyte of polyaniline electrochemically prepared
from an anilinium salt // J. Electroanal. Chem. – 1998. – 459. – P. 1 - 7.
240
References
108. Genies E.M., Lapkowski M. Spectroelectrochemical evidence for an
intermediate in the electropolymerization of aniline // J. Electroanal. Chem. - 1987. -
Vol. 236, 1-2. - P. 189 - 197.
109. Fusalba F., Gouérec P., Villers D., Bélanger D. Electrochemical
characterization of polyaniline in nonaqueous electrolyte and its evaluation as electrode
material for electrochemical supercapacitors // J. Electrochem. Soc. – 2001. – Vol. 148,
1. – P. A1 – A6.
110. Mu S., Chen C., Wang J. The kinetic behavior for the electrochemical
polymerization of aniline in aqueous solution // Synthetic Metals. – 1997. – 88. – 249 -
254.
111. Zinger B. Catalytic electrosynthesis of conducting polymers // J.
Electroanal. Chem. - 1988. - Vol. 244, 1-2. - P. 115 - 121.
112. Yano J., Ohnishi T., Kitani A. Kinetic study of the constant-potential
electropolymerization of aniline in perchloric acid solution // Synthetic Metals. – 1999.
– 101. – P. 752 – 753.
113. Lapkowski M. Proton exchange processes in the electrochemical
reactions of polyaniline – spectroelectrochemical studies in aqueous and non aqueous
solutions // B. Electrochem. - 1989. - Vol. 5, 10. - P. 792 – 799.
114. Ohsaka T., Ohnuki Y., Ovama N., Katagiri K., Kamisako K. IR
absorption spectroscopic identification of electroactive and electroinactive polyaniline
films prepared by the electrochemical polymerization of aniline // J. Electroanal. Chem.
- 1984. - Vol. 161, 2. - P. 399 - 405.
115. Sivakumar C., Gopalan A., Vasudevan T., Wen T.-C. Kinetics of
polymerisation of N-methyl aniline using UV-VIS spectroscopy // Synthetic Metals. –
2002. – 126. – P. 123 - 135.
116. Kuo C.-T., Chen S.-A., Hwang G.-W., Kuo H.-H. Field-effect transistor
with the water-soluble self-acid-doped polyaniline thin films as semiconductor //
Synthetic Metals. – 1998. – 93. – P. 155 – 160.
117. Zuo F., Angelopoulos M., MacDiarmid A.G., Epstein A.J. Transport
studies of protonated emeraldine polymer: A granular polymeric metal system // Phys.
Rev. B: Condens. Matter. - 1987. - Vol. 36, 6. - P. 3475 - 3478.
118. MacDiarmid A.G. Nobel Lecture: “Synthetic metals”: A novel role for
organic polymers // Reviews of modern physics. – 2001. – 73. – P. 701 - 712.
241
References
119. Huang W.S., MacDiarmid A.G. Optical properties of polyaniline //
Polymer. - 1993. - Vol. 34, 9. - P. 1833 – 1845.
120. Foot P.J.S., Simon R. Electrochromic properties of conducting
polyanilines // J. Phys. D: Appl. Phys. – 1989. – Vol. 22. – Р. 1598 – 1603.
121. Bredas J.L., Street C.B. Polarons, bipolarons, and solitons in conducting
polymers // Accounts Chem. Res. - 1985. - Vol. 18, 10. - P. 309 - 315.
122. Иоффе Н.Т., Коган Я.Л., Майрановский В.Г. ЭПР
электрохимически генерированных радикалов в тонких пленках полианилина //
Электрохимия. - 1988. - т. 24, 1. - C. 248.
123. Schmeiber D., Bartl A., Dunsch L., Naarmann H., Gopel W. Electronic
and magnetic properties of polypyrrole films depending on their one-dimensional and
two-dimensional microstructures // Synthetic Metals. – 1998. – 93. – P. 43 – 58.
124. Dalas E., Sakkopoulos S., Vitoratos E. Thermal degradation of the
electrical conductivity in polyaniline and polypyrrole composites // Synthetic Metals. –
2000. – 114. – P. 365 - 368.
125. Sheng P., Abeles B., Arie Y. Hopping conductivity in granular metals //
Phys. Rev. Lett. – 1973. – 31. – P. 44 – 47.
126. Kivelson S. Electron hopping in a solution band: Conduction in lightly
doped (CH)x // Phys. Rev. B. – 1982. – 25. – P. 3798 – 3821.
127. Mott N.F., Davis E.A. Electronic process in non-crystalline materials. –
1979. – Oxford: Charendon.
128. Campos M., Bello B.Jr. Mechanism of conduction in doped polyaniline
// J. Phys. D: Appl. Phys. – 1997. – 30. – P. 1531 – 1535.
129. Li J., Fang K., Qiu H., Li S., Mao W. Micromorphology and electrical
property of the HCl-doped and DBSA-doped polyanilines // Synthetic Metals. – 2004. –
142. – P. 107 – 111.
130. Kaynak A. dc Conduction in electrochemically synthesized polypyrrole
films // Tr. J. of Chemistry. – 1998. – 22. – P. 81 – 85.
131. Adams P.N., Devasagayam P., Pomfret S.J., Abell L., Monkman A.P. A
new acid-processing route to polyaniline films which exhibit metallic conductivity and
electrical transport strongly dependent upon intrachain molecular dynamics // J. Phys.:
Condens. Matter. – 1998. – 10. – P. 8293 - 8303.
242
References
132. Nechtschein M., Santier C., Travers J.P., Chroboczek J., Alix A., Ripert
M. Water effects in polyaniline: NMR and transport properties // Synthetic Metals. -
1987. - Vol. 18, 1-3. - P.311 - 316.
133. Ryu K.S., Kang S.-G., Lee G.J. The charge/discharge mechanism of
polyaniline films doped with LiBF4 as a polymer electrode in a Li secondary battery //
Solid State Ionics. - 2000. - 135. - P. 229 – 234.
134. Karami H., Mousavi M.F., Shamsipur M. A new design for dry
polyaniline rechargeable batteries // Journal of powder sources. – 2003. – 117. – P. 255
- 259.
135. Cottevielle F., Le Mehaute A., Challioui C., Mirebeau P., Demay J.N.
Industrial applications of polyaniline // Synthetic Metals. - 1999. - 101. - P. 703 – 704.
136. Bessiere A., Duhamel C., Badot J.-C., Lucas V., Certiat M.-C. Study and
optimization of a flexible electrochromic device based on polyaniline // Electrochimica
Acta. – 2004. – 49. – P. 2051 – 2055.
137. Lu J.L., Liu N.J., Wang X.H., Li J., Jing X.B., Wang F.S. Mechanism
and life study on polyaniline anti-corrosion coating // Synthetic Metals. – 2003. – 135 –
136. – P. 237 - 238.
138. Rout T.K., Jha G., Singh A.K., Bandyopadhyay N., Mohanty O.N.
Development of conducting polyaniline coating: a novel approach to superior corrosion
resistance // Surface and coatings technology. – 2003. – 167. – P. 16 - 24.
139. Muhammad-Tahir Z., Alocilja E.C. A conductometric biosensor for
biosecurity // Biosensors and bioelectronics. – 2003. – 18. – P. 813 - 819.
140. Nicolas-Debarnot D., Poncin-Epaillard F. Polyaniline as a new sensitive
layer for gas sensor // Analytica Chimica Acta. – 2003. – 475. – P. 1 - 5.
141. Pandey P.C., Singh G. Tetraphenylborate doped polyaniline based novel
pH sensor and solid-state urea biosensor // Talanta. – 2001. – 55. – P. 773 – 782.
142. Neoh K.G., Tay B.K., Kang E.T. Oxidation and ion migration during
synthesis and degradation of electroactive polymer – nylon 6 composite films //
Polymer. - 2000. - 41. - P. 9 – 15.
143. Byun S.W., Im S.S. Physical properties and doping characteristics of
polyaniline – nylon 6 composite films // Polymer. - 1998. - Vol. 39, 2. - P. 485 –
489.
243
References
144. Byun S.W., Im S.S. Transparent and conducting nylon 6-based
composite films prepared by chemical oxidative polymerization // Synthetic Metals. –
1993. – 57. – P. 3501 – 3506.
145. Hong K.H., Oh K.W., Kang T.J. Polyaniline-Nylon 6 composite fabric
for ammonia gas sensor // J. Appl. Pol. Sci. – 2004. – 92. – P. 37 - 42.
146. Toppare L. Synthesis and characterization of conducting polymers and
their composites // Tr. J. of Chemistry. – 1997. – 21. – P. 30 - 34.
147. Basheer R.A., Hopkins A.R., Rasmussen P.G. Dependence of transition
temperatures and enthalpies of fusion and crystallization on composition in
polyaniline/nylon blends // Macromolecules. – 1999. – 32. – P. 4706 - 4712.
148. Галюс З. Теоретические основы электрохимического анализа. – М.:
Мир, 1974. – 552 с.
149. Электродные процессы в растворах органических соединений / под
ред. Б.Б.Дамаскина. - М.: Изд-во Моск. ун-та, 1985. – 312 с.
150. Laska J., Zak K., Pron A. Conducting blends of polyaniline with
conventional polymers // Synthetic Metals. – 1997. – 84. – P. 117 - 118.
151. Пуд А.А., Фатеева Е.Ю., Шаповал Г.С. Электрохимическая
полимеризация анилина в полиамидной и поливинилспиртовой матрицах //
Теоретическая и экспериментальная химия. – 2002. – 38. – C. 33 – 36.
152. Pud A.A., Rogalsky S.P., Shapoval G.S., Korzhenko A.A. The
polyaniline/poly(ethylene terephthalate) composite. 1. Peculiarities of the matrix aniline
redox polymerization // Synthetic Metals. - 1999. - 99. - P. 175 – 179.
153. Annis B.K., Wignall G.D., Hopkins A.R., Rasmussen P.G., Basheer
R.A. Structural investigation of polyaniline/nylon 6 blends by small-angle neutron
scattering and small- and wide-angle X-ray scattering // Journal of polymer science:
Part B: Polymer Physics. – 1997. – 35. – P. 2765 – 2774.
154. Неницеску К.Д. Органическая химия. – М.: Изд-во иностранной
литературы, т.2. - 1963. – 1048 с.
155. Якименко Л.М. Производство хлора, каустической соды и
неорганических хлорпродуктов. – М.: Химия. – 1974. - 600 с.
156. Фрумина Н.С., Лисенко Н.Ф., Чернова М.А. Хлор. – М.: Наука, 1983.
– 199 с.
157. MacDiarmid A.G., Xia Y.N., Wiesinger J.M. Methods for preparing
conductive polyanilines // US patent 1995. - IC6 C08F 6/00.- No.5403913.
244
References
158. Николаев А.Ф. Синтетические полимеры и пластические массы на их
основе. – М.-Л.: Химия. – 1964. - 784 с.
159. Фридрихсберг Д.А. Курс коллоидной химии. – Л., Химия. – 1984. -
368 с.
160. Albuquerque J.E., Mattoso L.H.C., Balogh D.T., Faria R.M., Masters J.G.,
MacDiarmid A.G. A simple method to estimate the oxidation state of polyanilines //
Synthetic Metals. – 2000. – 113. – P. 19 - 22.
161. Vilcnik M., Zigon M., Zupan M., Sebenik A. Influence of polymerization
parameters on the molecular weight of polyaniline // Acta Chem. Slov. – 1998. – 45. –
P. 173 – 183.
162. Antropov L.I. Theoretical electrochemistry. – M.: Mir Publishers, 1972. –
568 p.
163. Diaz A.F. Chemically modified electrodes and conducting polymers //
Organic electrochemistry. An introduction and a guide / edited by Lund H. and Baizer
M.M. – New York, Marcel Dekker. – 1991. – P. 1363 – 1396.
164. Berrada K., Quillard S., Louarn G., Lefrant S. Polyanilines and substituted
polyanilines: a comparative study of the Raman spectra of leucoemeraldine, emeraldine
and pernigraniline // Synthetic Metals. – 1995. – 69. – P. 201 - 204.
165. Pereira da Silva J.E., Cordoba de Torresi S.I., Temperini M.L.A.
Polyaniline conformational studies in conductive blends using resonance Raman
spectroscopy // Synthetic Metals. – 2001. – 119. – P. 331 - 332.
166. Tagowska M., Palys B., Jackowska K. Polyaniline nanotubules – anion
effect on conformationa and oxidation state of polyaniline studied by Raman
spectroscopy // Synthetic Metals. – 2004. – 142. – P. 223 - 229.
167. Bernard M.C., Bich V.T., Hugot-Le Goff A. Resonant Raman
identification of the polaronic organization in PANI // Synthetic Metals. – 1999. – 101.
– P. 811 - 812.
168. Zuo F., Angelopoulos M., MacDiarmid A.G., Epstain A.J. ac Conductivity
of emeraldine polymer // Phys. Rev. B. – 1989. – 39. – P. 3570 - 3578.
169. Pinto N.J., Shah P.D., Kahol P.K., McCormick B.J. Dielectric constant and
ac conductivity in polyaniline derivatives // Solid State Communications. – 1996. – 97.
– P. 1029 – 1031.
170. Lian A., Besner S., Dao L.H. Broadband dielectric and conducting
properties of poly(N-alkylanilines) // Synthetic Metals. – 1995. – 74. – P. 21 - 27.
245
References
171. Han M.G., Im S.S. Dielectric spectroscopy of conductive polyaniline salt
films // J. Appl. Polym. Sci. – 2001. – 82. – P. 2760 – 2769.
172. Dutta P., Biswas S., De S.K. Alternating-current conductivity and
dielectric permittivity of polyaniline doped with β-naphthalene sulphonic acid // J.
Phys.: Condens. Matter. – 2001. – 13. – P. 9187 - 9196.
173. Luthra V., Singh R., Mansingh A. Effect of protonic acids on the dielectric
spectroscopy of polyaniline // Synthetic Metals. – 2001. – 119. – P. 291 - 292.
174. Jonscher A.K. Dielectric relaxation in solids. – Chelsea Dielectric Press. -
1996.
175. Dielectric spectroscopy of polymeric materials: fundamentals and
applications / Runt J.P., Fitzgerald J.J. – American Chemical Society, Washington DC.
– 1997. – 485 p.
176. Houzé F., Meyer R., Schneegans O., Boyer L. Imaging the local electrical
properties of metal surfaces by atomic force microscopy with conducting probes //
Appl. Phys. Lett. – 1996. – 69. – P. 1975 – 1977.
177. Shirakawa H., Lowis E.J., MacDiarmid A.G. et al. Synthesis of
electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x
// J. Chem. Soc. Chem. Commun. - 1977. - 16. - P. 578 - 580.
178. Genies E.M., Tsintavis C. Redox mechanism and electrochemical
behavior of polyaniline deposits // J. Electroanal. Chem. - 1985. - Vol. 195, 1. - P.
109 - 128.
179. Yang C.-H. Electrochemical polymerization of aniline and toluidines on
a thermally prepared Pt electrode // J. Electroanal. Chem. – 1998. – 459. – P. 71 - 89.
180. Kobavashi T., Yoneyama H., Tamura H. Oxidative degradation pathway
of polyaniline film electrodes // J. Electroanal. Chem. - 1984. - Vol. 177, 1-2. - P.
293 - 297.
181. Orata D., Buttry D.A. Determination of ion populations and solvent
content as functions of redox state and pH in polyaniline // J. Am. Chem. Soc. - 1987. -
Vol. 109, 12. - P. 3574 – 3578.
182. Алпатова Н.М., Овсянникова Е.В., Казаринов В.Е. Роль
образования зародышей проводящей фазы в редокс-превращениях полианилина //
Электрохимия. – 1996. - т.32, 5. - С. 631 – 634.
246
References
183. Zotti G., Cattarin S., Comisso N. Cyclic potential sweep
electropolymerization of aniline. The role of anions in the polymerization mechanism //
J. Electroanal. Chem. - 1988. - Vol. 239, 1-2. - P. 387 – 397.
184. Байрачный Б.И., Васильченко А.В., Ляшок Л.В. Электроосаждение
проводящего полимера – полианилина из водных растворов // Электрохимия. –
1994. - т. 30, 5. – С. 694 – 696.
185. Hatchett D.W., Josowicz M., Janata J. Comparison of chemically and
electrochemically synthesized polyaniline films // J. Electrochem. Soc. - 1999. - Vol.
146, 12. - P. 4535 – 4338.
186. Hussain A.M.P., Kumar A. Electrochemical synthesis and
characterization of chloride doped polyaniline // Bull. Mater. Sci. – 2003. - Vol. 26,
3. – P. 329 – 334.
187. Rabiej S., Ostrowska-Gumkowska B., Wlochowicz A. Investigations of
the crystallinity of PA-6/SPS blends by X-ray diffraction and DSC methods // European
Polymer Journal. – 1997. – 33. – P. 1031 – 1039.
188. Van Kleveren D.W. Properties of polymers. – London. – 1997. – 654 p.
189. Gospodinova N., Terlemezyan L. Conducting polymers prepared by
oxidative polymerization: polyaniline // Prog. Polym. Sci. – 1998. – 23. – P. 1443 –
1484.
190. Material safety data sheet from Arkema (France) and Good Fellow
(France).
191. Gill M.T., Chapman S.E., DeArmitt C.L., Baines F.L., Dadswell C.M.,
Stamper J.G., Lawless G.A., Billingham N.C., Armes S.P. A study of the kinetics of
polymerization of aniline using proton NMP spectroscopy // Synthetic Metals. – 1998.
– 93. – P. 227 - 233.
192. Tzou K., Gregory R.V. Kinetic study of the chemical polymerization of
aniline in aqueous solutions // Synthetic Metals. – 1992. – 47. – P. 267 - 277.
193. Madathil R., Ponrathnam S., Byrne H.J. Evidence of a redox equilibrium
assisted chain propagation mode for aniline polymerization: in situ spectral
investigation in dodecylbenzene sulfonic acid based system // Polymer. – 2004. – 45. –
P. 5465 – 5471.
194. Ayad M.M., Germaey A.H., Salahuddin N., Shenashin M.A. The
kinetics and spectral studies of the in situ polyaniline film formation // Journal of
colloid and interface science. – 2003. – 263. – P. 196 – 201.
247
References
195. Wei Y., Jang G.-W., Chan C.-C., Hsueh K.F., HAriharan R., Patel S.A.,
Whitecar C.K. Polymerization of aniline and alkyl ring-substituted anilines in the
presence of aromatic additives // J. Phys. Chem. – 1990. – 94. – P. 7716 – 7721.
196. Wei Y., Sun Y., Tang X. Autoacceleration and kinetics of
electrochemical polymerization of aniline // J. Phys. Chem. – 1989. – 93. – P. 4878 -
4881.
197. Mav I., Zigon M. Synthesis and NMR characterization of a novel
polyaniline derivative. The copolymer of 2-methoxyaniline and 3-
aminobenzenesulfonic acid // Polymer bulletin. – 2000. – 45. – P. 61 - 68.
198. Mav I., Zigon M. A kinetic study of the copolymerization of substituted
anilines by 1H NMR // Polymer International. – 2002. – 51. – P. 1072 – 1078.
199. Snauwaert P., Lazzaroni R., Riga J., Verbist J.J., Gonbeau D. A
photoelectron spectroscopic study of the electrochemical processes in polyaniline // J.
Chem. Phys. – 1990. – 92. – P. 2187 - 2193.
200. Mazeikiene R., Malinauskas A. Kinetic study of the electrochemical
degradation of polyaniline // Synthetic Metals. – 2001. – 123. – P. 349 – 354.
201. Bremer L.G.B., Verbong M.W.C.G., Webers M.A.M., van Doorn
M.A.M.M. Preparation of core-shell dispersions with a low tg polymer core and a
polyaniline shell // Synthetic Metals. – 1997. – 84. – P. 355 - 356.
202. Abell L., Pomfret S.J., Adams P.N., Monkman A.P. Thermal studies of
doped polyaniline // Synthetic Metals. – 1997. – 84. – P. 127 - 128.
203. Beadle P.M., Nicolau Y.F., Banka E., Rannou P., Djurado D. Controlled
polymerization of aniline at sub-zero temperatures // Synthetic Metals. – 1998. – 95. –
P. 29 – 45.
204. Sulimenko T., Stejskal J., Krivka I., Prokes J. Conductivity of colloidal
polyaniline dispersions // European Polymer Journal. – 2001. – 37. – P. 219 – 226.
205. Ahmed S.M. Mechanistic investigation of the oxidative polymerization
of aniline hydrochloride in different media // Polymer Degradation and Stability. –
2004. – 85. – P. 605 – 614.
206. Stejskal J., Kratochvil P., Jenkins A.D. The formation of polyaniline and
the nature of its structures // Polymer. – 1996. – 37. – P. 367 – 369.
207. Zotti G., Schiavon G. Spectroelectrochemical determination of polarons
in polypyrrole and polyaniline // Synthetic Metals. - 1989. - Vol. 30. - P. 151 – 158.
248
References
208. Томилов А.П., Майрановский С.Г., Фиошин М.Я., Смирнов В.А.
Электрохимия органических соединений. – Л.: Химия. - 1968. – 592 с.
209. Wang X.-H., Geng Y.-H., Wang L.-X., Jing X.-B., Wang F.-S. Thermal
behaviours of doped polyaniline // Synthetic Metals. – 1995. – 69. – P. 265 - 266.
210. Pandey S.S., Gerard M., Sharma A.L., Malhotra B.D. Thermal analysis
of chemically synthesized polyemeraldine base // J. Appl. Pol. Sci. – 1999. – 75. – P.
149 – 155.
211. Traore M.K., Stevenson W.T.K., McCormick B.J., Dorey R.C., Wen S.,
Meyers D. Thermal analysis of polyaniline. Part I. Thermal degradation of HCl-doped
emeraldine base // Synthetic Metals. – 1991. – 40. – P. 137 – 153.
212. Matveeva E.S., Calleja R.D., Parkhutik V.P. Thermogravimetric and
calorimetric studies of water absorbed in polyaniline // Synthetic Metals. – 1995. – 72.
– P. 105 – 110.
213. Han M.G., Lee Y.J., Byun S.W., Im S.S. Physical properties and thermal
transition of polyaniline film // Synthetic Metals. – 2001. – 124. – P. 337 – 343.
214. McNally T., Mutphy W.R., Lew C.Y., Turner R.J., Brennan G.P.
Polyamide-12 layered silicate nanocomposites by melt blending // Polymer. – 2003. –
44. – P. 2761 - 2772.
215. Rodrigues P.C., Akcelrud L. Networks and blends of polyaniline and
polyurethane: correlations between composition and thermal dynamic mechanical and
electrical properties // Polymer. – 2003. – 44. – P. 6891 - 6899.
216. Das B., Kar S., Chakraborty S., Chakraborty D., Gangopadhyay S.
Synthesis and characterization of polyacrylamide-polyaniline conductive blends // J.
Appl. Pol. Sci. – 1998. – 69. – P. 841 – 844.
217. Rodrigues P.C., Muraro M., Garcia C.M., Souza G.P., Abbate M.,
Schreiner W.H., Gomes M.A.B. Polyaniline/lignin blends: thermal analysis and XPS //
European Polymer Journal. – 2001. – 37. – P. 2217 – 2223.
218. Schmidt V., Domenech S.C., Soldi M.S., Pinheiro E.A., Soldi V.
Thermal stability of polyaniline/ethylene propylene diene rubber blends prepared by
solvent casting // Polymer Degradation and Stability. – 2004. – 83. – P. 519 – 527.
219. Gangopanhyay R., De A., Ghosh G. Polyaniline-poly(vinyl alcohol)
conducting composite: material with easy processability and novel application potential
// Synthetic Metals. – 2001. – 123. – P. 21 - 31.
249
References
220. Kulszewicz-Bajer I., Zagorska M., Niziol J., Pron A., Luzny W. Esters
of 5-sulfo-i-phthalic acid as new dopants improving the solution processibility of
polyaniline: spectroscopic, structural and transport properties od the doped polymer //
Synthetic Metals. – 2000. – 114. – P. 125 - 131.
221. Gazotti W.A.Jr., Faez R., Paoli M.-A. De Thermal and mechanical
behaviour of a conductive elastomeric blend based on a soluble polyaniline derivative //
European Polymer Journal. – 1999. – 35. – P. 35 – 40.
222. Leyva M.E., Soares B.G., Khasrgir D. Dynamic-mechanical and
dielectric relaxations of SBS block copolymer: polyaniline blends prepared by
mechanical mixing // Polymer. – 2002. – 43. – P. 7505 – 7513.
223. Pud A.A., Tabellout M., Kassiba A., Korzhenko A.A., Rogalsky S.P.,
Shapoval G.S., Houze F., Schneegans O., Emery J.R. The poly(ethylene-terephtalate)-
polyaniline composite: AFM, DRS and EPR investigations of some doping effects // J.
Mater. Sci. – 2001. – 36. – P. 3355 – 3363.
224. Tabellout M., Fatyeyeva K., Baillif P.-Y., Bardeau J.-F., Pud A.A. The
influence of the polymer matrix nature on the dielectric and electric properties of
conductive polymer composites based on polyaniline // J. Non-Cryst. Solids. – 2005. –
351. – P. 2835 – 2841.
225. Zhang Q., Jin H., Wang X., Jing X. Morphology of conductive blends of
polyaniline and polyamide-11 // Synthetic Metals. – 2001. – 123. – P. 481 – 485.
226. Korzhenko A.A., Tabellout M., Emery J.R., Pud A.A., Rogalsky S.,
Shapoval G.S. Dielectric relaxation properties of poly(ethylene-terephtalate)-
polyaniline composite films // Synthetic Metals. – 1998. – 98. – P. 157 – 160.
227. Furukawa Y., Ueda F., Hyodo Y., Harada I., Nakajima T., Kawagoe T.
Vibrational Spectra and structure of polyaniline // Macromolecules. – 1988. – 21. – P.
1297- 1305.
228. Quillard S., Louarn G., Lefrant S., MacDiarmid A.G. Vibrational
analysis of polyaniline: a comparative study of leucoemeraldine, emeraldine, and
pernigraniline bases // Phys. Rev. B. – 1994. – 50. – P. 12496 - 12508.
229. Bernard M.-C., Hugot-Le Goff A. Raman spectroscopy for the study of
polyaniline // Synthetic Metals. – 1997. – 85. – P. 1145 – 1146
230. Boyer M.I., Quillard S.,Rebourt E., Louarn G., Buisson J.P., Monkman
A., Lefrant S. Vibrational analysis of polyaniline: a model compound approach // J.
Phys. Chem. B. – 1998. – 102. – P. 7382 – 7392.
250
References
231. Pereira da Silva J.E., Temperini M.L.A., Cordoba de Torresi S.I.
Relation between structure and homogeneity of polyaniline in blends by infrared and
Raman spectroscopies // Synthetic Metals. – 2003. - 135-136. – P. 133 – 134.
232. Pereira da Silva J.E., Temperini M.L.A., Cordoba de Torresi S.I.
Secondary doping of polyaniline studied by resonance Raman spectroscopy //
Electrochimica Acta. – 1999. – 44. – P. 1887 - 1891.
233. Laska J. Protonation/plasticization competitions in polyaniline doped
with bis(2-ethylhexyl) hydrogen phosphate // Synthetic Metals. – 2002. – 129. – P. 229
- 233.
234. Lindfors T., Ivaska A. Raman based pH measurements with polyaniline
// J. Electroanal. Chem. – 2005. – 580. – P. 320 - 329.
235. The Handbook of infrared and Raman characteristic frequencies of
organic molecules / Lin-Vien D., Colthup N.B., Fateley W.G., Grasselli J.G. –
Academic Press. – 1991. – 503 p.
236. Stuart B.H. Polymer crystallinity studied using Raman spectroscopy //
Vibrational Spectroscopy. – 1996. – 10. – P. 79 - 87.
237. McCrum N.G., Read B.E., Williams G. Anelastic and Dielectric Effects
in Polymeric Solids. – Dover Publications, Inc., New York, 1991. – 616 p.
238. Pelster R., Nimtz G., Wessling B. Fully protonated polyaniline: hopping
transport on a mesoscopic scale // Phys. Rev. B. – 1994. – 49. – P. 12718 - 12723.
239. Papathanassiou A.N., Grammatikakis J., Sakkopoulos S., Vitoratos E.,
Dalas E. Localized and long-distance charge hopping in fresh and thermally aged
conductive copolymers of polypyrrole and polyaniline studied by combined TSDC and
dc conductivity // J. Phys. Chem. Solids. – 2002. – 63. – P. 1771 – 1778.
240. Pinto N.J., Acosta A.A., Sinha G.P., Aliev F.M. Dielectric permittivity
study on weakly doped conducting polymers based on polyaniline and its derivatives //
Synthetic Metals. – 2000. – 113. – 77 – 81.
241. Bengoechea M.R., Aliev F.M., Pinto N.J. Effects of confinement on the
phase separation in emeraldine base polyaniline cast from 1-methyl-2-pyrrolidinone
studied via dielectric spectroscopy // J. Phys.: Condens. Matter. – 2002. – 14. – P. 1 -
10.
242. Pingsheng H., Xiaohua Q., Chune L. Electric and dielectric properties of
polyaniline // Synthetic Metals. – 1993. – 57. – P. 5008 - 5013.
251
References
243. Probst M., Holze R. Time- and temperature-dependent changes of the in
situ conductivity of polyaniline and polyindoline // Electrochimica Acta. – 1995. – 40.
– P. 213 - 219.
244. Joo J., Prigodin V.N., Min Y.G., MacDiarmid A.G., Epstein A.J.
Phonon-induced nonmetal-metal transition of a doped polyaniline // Phys. Rev. B. –
1994. – 50. – P. 12226 - 12229.
245. Chen S.-A., Lian C.-S. Conductivity relaxation and chain motions in
conjugated conducting polymers: neutral poly(3-alkylthiophene)s // Macromolecules. –
1993. – 26. – P. 2810 – 2816.
246. MacDiarmid A.G., Epstein A.J. The concept of secondary doping as
applied to polyaniline // Synthetic Metals. – 1994. – 65. - P. 103 – 116.
247. Zuppiroli L., Bussac M.N., Paschen S., Chauvet O., Forro L. Hopping in
disordered conducting polymers // Phys. Rev. B. – 1994. – 50. – P. 5196 – 5203.
248. Kuo C.-T., Chen C.-H. Characterization of polyaniline doped with
diphenyl phosphate // Synthetic Metals. – 1999. – 99. – P. 163 – 167.
249. Prokes J., Krivka I., Sulimenko T., Stejskal J. Conductivity of
polyaniline films during temperature cycling // Synthetic Metals. – 2001. – 119. – P.
479 - 480.
250. Abthagir P.S., Saraswathi R., Sivakolunthu S. Aging and thermal
degradation of poly(N-methylaniline) // Thermochimica Acta. – 2004. – 411. – P. 109 -
123.
251. Beaume F., Laupretre F., Monnarie L., Maxwell A., Davies G.R.
Secondary transitions of aryl-aliphatic polyamides. I. Broadband dielectric
investigation // Polymer. – 2000. – 41. –P. 2677 - 2690.
252. Rocha I.S., Mattoso L.H.C., Malmonge L.F., Gregorio R.Jr. Effect of
low contents of a polyaniline derivative on the crystallization and electrical properties
of blends with PVDF // J. Polym. Sci.: Part B: Polym. Phys. – 1999. – 37. – P. 1219 –
1224.
253. Thiéblemont J.C., Planche M.F., Petrescu C., Bouvier J.M., Bidan G.
Stability of chemically synthesized polypyrrole films // Synthetic Metals. – 1993. – 59.
P. 81 – 96.
254. Cheah K., Forsyth M.,Truong V.-T., Olsson-Jacques C. Mechanisms
governing the enhanced thermal stability of acid and base treated polypyrrole //
Synthetic Metals. – 1997. – 84. – P. 829 - 830.
252
References
255. Dey A., De S., De A., De S.K. Characterization and dielectric properties
of polyaniline-TiO2 nanocomposites // Nanotechnology. – 2004. – 15. – P. 1277 –
1283.
256. Stauffer D. Introduction to percolation theory. – London: Taylor and
Francis Ltd. – 1985.
257. Gingold D.B., Lobb C.J. Percolative conduction in three dimensions //
Phys. Rev. B. – 1990. – 42. – P. 8220 - 8224.
258. Epstein A.J., MacDiarmid A.G. Polyanilines: from solitons to polymer
metal, from chemical curiosity to technology // Synthetic Metals. – 1995. – 69. – P. 179
- 182.
259. Joo J., Long S.M., Puoget J.P., Oh E.J., MacDiarmid A.G., Epstein A.J.
Transport of the mesoscopic state in partially crystalline polyaniline // Phys. Rev. B. –
1998. – 57. – P. 9567 – 9580.
260. Gould R.D., HAssan A.K. A.c. electrical properties of thermally
evaporated thin films of copper phthalocyanine // Thin Solid Films. – 1993. – 223. – P.
334 – 340.
261. Kudoh Y. Properties of polypyrrole prepared by chemical
polymerization using aqueous solution containing Fe2(SO4)3 and anionic surfactant //
Synthetic Metals. – 1996. – 79. – P. 17 - 22.
262. Chattopadhyay D., Mandal B.M. Methyl cellulose stabilized polyaniline
dispersions // Langmuir. – 1996. – 12. – P. 1585 - 1588.
263. Bae W.J., Jo W.H., Park Y.H. Preparation of polystyrene/polyaniline
blends by in situ polymerization technique and their morphology and electrical
properties // Synthetic Metals. – 2003. – 132. – P. 239 – 244.
264. McManus P.M., Cushman R.J., Yang S.C. Influence of oxidation and
protonation on the electrical conductivity of polyaniline // J. Phys. Chem. – 1987. – 91.
– P. 744 – 747.
265. Planes J., Wolter A., Cheguettine Y., Pron A., Genoud F., Nechtschein
M. Transport properties of polyaniline-cellulose-acetate blends // Phys. Rev. B. – 1998.
– 58. – P. 7774 – 7785.
266. Tsotra P., Friedrich K. Thermal, mechanical, and electrical properties of
epoxy resin/polyaniline-dodecylbenzenesulfonic acid blends // Synthetic Metals.- 2004.
– 143. – P. 237 - 242.
253
References
267. Singh R., Arora V., Tandon R.P., Chandra S., Mansingh A. Charge
transport and structural morphology of HCl-doped polyaniline // J. Mater. Sci. – 1998.
– 33. – P. 2067 – 2072.
268. Luthra V., Singh R., Gupta S.K., Mansingh A. Mechanism of dc
conduction in polyaniline doped with sulfuric acid // Current applied physics. – 2003. –
3. – P. 219 – 222.
269. Jousseaume V., Bonnet A., Morsli M., Cattin L. Comparative study of
electrical properties of polyaniline films and polyaniline-polystyrene blends // Eur.
Phys. J.: Appl. Phys. – 1999. – 6. – P. 7 - 12.
270. Wolter A., Rannou P., Travers J.P., Gilles B., Djurado D. Model for
aging in HCl-protonated polyaniline: structure, conductivity, and composition studies //
Phys. Rev. B. – 1998. – 58. – P. 7637 - 7647.
271. Turcu R., Brie M., Leising G., Niko A., Tosa V., Mihut A., Bot A.
Correlation between the electrochemical synthesis conditions and the optical properties
of polypyrrole // Synthetic Metals. – 1997. – 84. – P. 825 – 826.
254
Abstract
Two types of composites based on polyaniline (PANI) and polyamide (PA) were
elaborated by oxidative polymerisation: surface conducting polymer films with a subsurface
conducting layer and bulk conducting polymers in powder and film forms. In order to
determine the synthesis-structure-properties relationship, different physico-chemical methods
of investigation (cyclic voltammetry, dielectric relaxation spectroscopy, thermogravimetry,
microscopy, Raman spectrometry,….) were used. It is shown that the rate of the aniline
polymerization process inside the polymer matrix is determined by the matrix’s ability to
swell in the reaction media and by the rate of oxidant and monomer diffusion into the reaction
zone. Three well defined polymerization stages (induction period, propagation stage and
termination stage) are observed and the kinetic parameters of the process are calculated, that
enable the control of the polymerization process. It is confirmed that independently from the
presence or absence of the polymer matrix (film or dispersed media) the mechanism of aniline
polymerization does not change and the polymerization process follows an autocatalytic
mechanism. The thermal and mechanical properties of the composites are improved as
compared with pure PANI. The degradation temperature of the composite is enhanced by
∼100°C. The use of organic acid-dopants leads to better mechanical characteristics due to
their triple role (of a dopant, compatibilizer and plasticizer) in comparison with inorganic
ones. The layered and clustered structure of the surface conducting composites is revealed by
atomic force microscopy (AFM) and optical microscopy. The thickness of the conducting
layer is found to be dependent on the polymer matrix structure. This organisation is traduced
by the appearance of interfacial polarisation relaxation processes related to the conductive
properties of the PANI clusters. The core-shell (PA grains-PANI) structure of the bulk
conducting composites, as demonstrated by Resonance Raman Spectrometry, results in a high
conductivity value with a low percolation threshold for the powder form of the composites.
An interfacial polarisation relaxation process related to the composite structure is pointed out
which relaxation frequency increases with increasing PANI content and, hence, with
conductivity. Therefore, the obtained composites owning the mechanical properties of the
matrix and the electrical properties of PANI are good candidates for practical application such
as antistatic materials, materials for capacitors, sensors, etc.
Key words: polyaniline, polyamide, composite, conductivity, dielectric relaxation,
percolation, kinetics of polymerization, voltammetry, Raman spectrometry, AFM.
Résumé: Parmi les conducteurs organiques, la polyaniline (PANI) est le polymère
qui présente une grande stabilité environnementale et une grande conductivité électrique.
Cependant, elle présente également des inconvénients tels que ses faibles propriétés
mécaniques qui représentent un verrou technologique pour de nombreuses applications. Pour
y remédier, une des méthodes consiste à les associer avec des polymères conventionnels pour
former des composites ayant à la fois les propriétés mécaniques de la matrice et les propriétés
électriques de PANI. Aussi, deux types de composites conducteurs à base de PANI et de
polyamide (PA) ont été élaborés: les films composites conducteurs en surface et les
composites conducteurs en volume sous forme de poudres et de films. La synthèse rationnelle
et la conception de ces matériaux nécessitent l’établissement de relation synthèse-structure-
propriétés.
L’étude électrochimique et chimique a montré que la vitesse de polymérisation de
l’aniline est déterminée par la capacité de gonflement de la matrice dans le milieu réactionnel
et par la vitesse de diffusion de l’oxydant et du monomère. Trois stades de polymérisation ont
été clairement identifiés pour lesquels les paramètres de la cinétique ont été calculés
permettant ainsi le contrôle du processus autocatalytique de polymérisation.
Les études thermique et mécanique ont montré que les propriétés de ces composites
sont voisines de celles du polymère hôte pour de faibles taux de PANI. La structure en double
couche des films conducteurs en surface, d’une part, et l’organisation de PANI sous forme
d’amas conducteurs dispersés dans un milieu isolant, d’autre part, ont été mises en évidence
par spectrométrie Raman et par microscopie. Cette organisation se traduit, sur les propriétés
diélectriques, par l’apparition de relaxations de polarisation d’interface dont les fréquences de
relaxation sont corrélées avec la conductivité de la surface du film. Il a été montré que la
percolation des amas conducteurs, qui détermine les propriétés électriques et diélectriques du
composite, dépend de la structure de la matrice.
Les composites conducteurs en volume sous forme de poudre, de part leur
structuration en “core-shell” (PA/PANI), présentent des conductivités voisines de celles de
PANI avec des seuils de percolation très bas (<1 wt.% ). La structure “core-shell” s’est
traduite par l’apparition d’une relaxation de polarisation d’interface dont la fréquence
augmente avec le taux de PANI et de ce fait avec la conductivité.
De part les propriétés électriques, thermiques et mécaniques obtenues, ces
composites présentent de grandes potentialités d’application dans différents domaines.
Mot-clés: Polyaniline, polyamide, composite, cinétique de polymérisation, percolation,
voltammétrie, spectrométrie Raman, microscopie, spectroscopie diélectrique, conductivité.