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STUDIES ON ELECTROCHEMICALLY SYNTHESIZED POLYANILINE AND ITS
COPOLYMERS
ALI PARSA
UNIVERSITI SAINS MALAYSIA 2009
STUDIES ON ELECTROCHEMICALLY SYNTHESIZED POLYANILINE AND ITS COPOLYMERS
by
ALI PARSA
Thesis submitted in fulfillment of the requirements for the degree of
Doctor of Philosophy
September 2009
ii
ACKNOWLEDGEMENT
I would like to express my deep gratitude and respects to my supervisor
Associate Professor Sulaiman Ab Ghani for his keen interest and valuable guidance,
strong motivation and constant encouragement during the course of this study. I am
indebted to him for suggesting me this area of research on the electrochemistry of
conducting polymers. That is to say, I have understood electrochemistry better now
than I previously was and will certainly continue in this research area next.
My sincere thanks to all my laboratory colleagues, especially, to Mr. Illyas
Md Isa for his assistance during my study.
I also thank all academics and technical staffs of the School of Chemical
Sciences, USM for their advice and help.
Last but not least to my family for their understandings during the course of
this study. Their praying has helped me to get the strength to finalize this study.
iii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
TABLE OF CONTENTS iii
LIST OF TABLES v
LIST OF FIGURES vi
LIST OF ABBREVIATION xi
ABSTRAK xiii
ABSTRACT xv
CHAPTER 1 : INTRODUCTION
1
1.1 History of conducting polymers 1
1.2 Types of conducting polymers 1
1.3 Application of conducting polymers 4
1.4 Syntheses of selected conducting polymers 4
1.4.1 Polyaniline 5
(a) Chemical synthesis 13
(b) Electrochemical synthesis 15
1.4.2 Polypyrrole 16
(a) Chemical synthesis 18
(b) Electrochemical synthesis 18
(c) Vapour phase deposition 20
1.4.3 Poly-ortho-phenylenediamine 21
(a) Chemical synthesis 21
(b) Electrochemical synthesis 23
1.5 Doping of conducting polymers 26
1.5.1 Types of doping 30
1.5.2 Types of doping agents (Dopants) 31
1.5.3 Doping techniques 32
1.5.4 Effect of doping on conductivity 32
iv
1.6 Conducting copolymers 33
1.6.1 Copolymer structures 33
1.6.2 Electrochemical copolymerization 36
1.7 Objectives of study 39
CHAPTER 2: EXPERIMENTAL
40
2.1 Materials 40
2.2 Equipment & Procedure 40
2.2.1 Electrochemical synthesis and characterisation 40
2.2.2 Conductivity measurement 43
CHAPTER 3: RESULTS AND DISCUSSION
46 3.1 Electrochemical Synthesis of Polyaniline 46
3.1.1 Conductivity of polyaniline 57
3.1.2 Structural analysis 59
3.1.3 Morphological study 65
3.1.4 Antioxidant activity of polyaniline 76
3.2 Electrocopolymerization of Aniline and ortho- Phenylenediamine
84
3.2.1 Morphological study 86
3.2.2 Structural analysis 88
3.2.3 Electrocatalytic oxidation of ascorbic acid 91
3.3 The Aqueous Electrosyntheses of Homo and Copolymers of Pyrrole and Aniline in a Binary Electrolyte System
98
3.3.1 Electropolymerization of aniline 98
3.3.2 Electropolymerization of Pyrrole 106
3.3.3 Electrocopolymerization of aniline and Pyrrole 112
CHAPTER 4: CONCLUSIONS 121
REFERENCES 124
Addendum 142
v
LIST OF TABLES
Page
3.1 The Epa/Epc values of redox peaks in different supporting electrolytes
49
3.2 The effect of counter anions to two polyaniline oxidation processes
52
3.3 The conductivities of polyaniline formed in different media 58
3.4 The FTIR absorption bands 61
3.5 The formal potentials (Eo′) of various compounds in phosphate buffer and in methanol
81
3.6 The conductivities of polymers are prepared in para-toluene sulphonic acid (PTSA)
114
vi
LIST OF FIGURES
Page
1.1 Examples of inherently conducting polymers. 3
1.2 Resonance structure of aniline and initial intermediate species produced during aniline oxidation.
7
1.3 Different forms of polyaniline. 9
1.4 Formation of a polyaniline salt following oxidation and protonation of bases.
10
1.5 Various states of oxidation and protonation of polyaniline. 12
1.6 Polymerization mechanism of aniline. 14
1.7 Various oxidation states of polypyrrole. 17
1.8 Polymerization mechanism of pyrrole. 19
1.9 Polyaniline-like and Ladder-like structures of poly-ortho- phenylenediamine.
25
1.10 Structure of polyaniline by the p-type doping (X= Cl¯, Br¯, PO4
3¯, SO42¯).
28
1.11 n-Doping mechanism of emeraldine base type polyaniline doped with KH ("a" is the number of charge or cataion).
29
1.12 Types of Copolymer Structures. 35
1.13 Reaction scheme for copolymerization, (A+۠•and B+ ۠• are radical cation).
37
2.1 Model for the four probe resistivity measurements. 44
2.2 Circuit used for resistivity measurements (Four Probe Set-up). 45
3.1 The cyclic voltammograms obtained during electropolymerization of aniline in (a) 1 M H3PO4 and 1 M H3PO4 containing: (b) 0.06 M Ca3(PO4)2; (c) 0.2 M ZnSO4; (d) 0.5 M ZnCl2; (e) 0.5 M CaCl2; and (f) 0.5 M KCl. The potential is cycled between −0.4 and +1.0 V (vs. Ag/AgCl) at a scan rate of 100 mVs−1 for the 1st-21st cycle.
47
3.2 The cyclic voltammograms obtained during electropolymerization of aniline in 1 M H3PO4 containing (a) 0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2.
51
vii
3.3 Cyclic voltammetric responses of deposited polyaniline films in monomer-free solution at two different Eap ranges (−0.2 V to +1.0 V and −0.3 V to +1.0 V) at a scan rate of 100 mVs−1 in (a) 1 M H3PO4, and 1 M H3PO4 containing (b) 0.06 M Ca3(PO4)2, (c) 0.2 M ZnSO4, (d) 0.5 M ZnCl2, (e) 0.5 M CaCl2, and (f) 0.5 M KCl.
54
3.4 CVs of aniline in 1 M H3PO4 containing 0.5 M CaCl2 at (a) 1st cycle, (b) 1st to 3rd cycle and (c) 20th cycle. The scan rate is 100 mVs−1.
56
3.5 Baseline corrected FTIR spectra (2000–400 cm−1 region) of polyaniline electropolymerized from (a) 1 M H3PO4, and 1 M H3PO4 containing (b) 0.5 M CaCl2 (c) 0.5 M KCl (d) 0.5 M ZnCl2 (e) 0.06 M Ca3(PO4)2 and (f) 0.2 M ZnSO4.
60
3.6 UV-vis spectra of polyaniline solution in DMF. The polymers were deposited from 50 mM aniline in (a) 1 M H3PO4 and 1 M H3PO4 containing (b) 0.5 M CaCl2, (c) 0.5 M KCl, (d) 0.5 M ZnCl2, (e) 0.2 M ZnSO4 and (f) 0.06 M Ca3(PO4)2.
64
3.7 a) SEM image of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 on 2B pencil composite graphite b) The respective EDX analysis.
67
3.8 SEM images of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.5 M CaCl2, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) morphology of polyaniline; c and d) EDX spectrum of flower and polyaniline.
68
3.9 SEM images of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.5 M KCl, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) morphology of polyaniline; c and d) EDX spectrum of flower and polyaniline.
69
3.10 SEM images of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.5 M NaCl, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) morphology of polyaniline; c and d) EDX spectrum of flower and polyaniline.
70
3.11 SEM images of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.5 M ZnCl2, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) morphology of polyaniline; c and d) EDX spectrum of flower and polyaniline.
71
viii
3.12 SEM images of polyaniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.06 M Ca3(PO4)2, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) morphology of polyaniline; c and d) EDX spectrum of flower and polyaniline.
72
3.13 SEM images of polyaniline microstructure synthesized electrochemically in 1 M HCl and 0.5 M CaCl2, a) Flower-like inorganic salt crystal prepared on the polyaniline substrate; b) EDX spectrum of flower.
73
3.14 SEM images of poly-ortho-phenylenediamine microstructure synthesized electrochemically in 1 M H3PO4 and 0.06 M Ca3(PO4)2, a) Flower-like inorganic salt crystal prepared on the poly-ortho-phenylenediamine substrate; b) EDX spectrum of flower.
74
3.15 SEM images of copolymer of ortho-phenylenediamine and aniline microstructure synthesized electrochemically in 1 M H3PO4 and 0.06 M Ca3(PO4)2, a) Flower-like inorganic salt crystal prepared on the copolymer substrate; b) EDX spectrum of flower.
75
3.16 The suggested reaction mechanisms for polyaniline electropolymerized in presence of (a) Ca3(PO4)2 and (b) CaCl2 (in which polyaniline has more Q structures in the polymer backbone).
77
3.17 UV–vis spectra of 8.0 × 10−5 M DPPH radicals in MeOH containing 1 M H3PO4 after 30 min exposure to 0.3 mg polyaniline electropolymerized in the presence of (a) Ca3(PO4)2, (b) CaCl2 and (c) neither.
78
3.18 Plot of absorbance against amount of polyaniline prepared in presence of (a) Ca3(PO4)2 and (b) CaCl2 upon reaction with 2 mL 0.08 x 10−3 M DPPH in MeOH containing 1 M H3PO4. (n=5)
79
3.19 LSVs of polyaniline prepared in presence of (a) Ca3(PO4)2 and (b) CaCl2, taken in phosphate buffer, pH 7, containing Ca3(PO4)2 and CaCl2, respectively, The scan rate is 100 mVs−1.
82
3.20 LSVs of polyaniline prepared in presence of (a) Ca3(PO4)2 and (b) CaCl2, taken in methanol and 1 M H3PO4 containing Ca3(PO4)2 and CaCl2, respectively. The scan rate is 100 mVs−1.
83
ix
3.21 The CVs for the growth of poly-ortho-phenylenediamine from a 50 mM ortho-phenylenediamine solution in 1 M H3PO4 containing (a) 0.5 M CaCl2, and (c) 0.06 M Ca3(PO4)2. The voltammogram is cycled between Eap −0.7 and +0.9 V (vs. Ag/AgCl), scan rate of 100 mVs−1 and up to 10 cycles. The (b) and (d) are CVs for the growth of polyaniline from a 50 mM aniline solution in 1 M H3PO4 containing 0.5 M CaCl2 and 0.06 M Ca3(PO4)2, respectively. The voltammogram is cycled between Eap −0.4 and +1.0 V (vs. Ag/AgCl), scan rate 100 mVs−1 and up to 10 cycles.
85
3.22 The CVs and SEM images of copolymerization of 50 mM aniline solution and 50 mM ortho-phenylenediamine solution in 1 M H3PO4 containing (a) 0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2. The voltammogram is cycled between Eap −0.7 and +0.9 V (vs. Ag/AgCl), scan rate of 100 mVs−1 and up to 10 cycles.
87
3.23 Baseline corrected FTIR spectra (2000–400 cm−1 region) of poly-ortho-phenylenediamine electropolymerized in 1 M H3PO4 containing (a) 0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2.
89
3.24 Baseline corrected FTIR spectra (2000–400 cm−1 region) of copolymer of aniline and ortho-phenylenediamine in 1 M H3PO4 containing: (a) 0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2.
90
3.25 CVs of 25 mM ascorbic acid at (a) bare composite graphite and (b)poly(aniline–co–ortho-phenylenediamine)/composite graphite electrode in 1 M H3PO4 (pH=1).
92
3.26 a) LSVs of different concentrations of ascorbic acid at poly(Ani–co–ortho-phenylenediamine)/composite graphite electrode in 1 M H3PO4 (pH=1). Concentrations of ascorbic acid from (a) to (j) are 5–50 mM, respectively. b) The plot of IPa vs. concentration. (n=5)
94
3.27 a) CVs of 5 mM ascorbic acid at poly(aniline–co–ortho-phenylenediamine)/composite graphite in 1 M H3PO4 (pH=1) at different scan rates. Scan rate υ from (a) to (j) are 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s−1, respectively. b) The plot of Ipa vs. υ1/2. (n=5)
95
3.28 CVs of supporting electrolyte consisting of 25 mM ascorbic acid at pH values of (a) 1, (b) 3, (c) 5, (d) 7 and (e) 9 at poly(aniline–co–ortho-phenylenediamine)/composite graphite electrode at 100 mVs−1.
97
3.29 CVs of aniline in 0.2 M PTSA in (a) presence and (b) absence of KCl. The scan rate is 100 mVs−1.
100
x
3.30 Baseline corrected FTIR spectra of polyaniline electropolymerized in 0.2 M PTSA in (a) presence and (b) absence of KCl.
102
3.31 UV–vis spectra of polyaniline in DMF. polyaniline is deposited from 50 mM aniline in 0.2 M PTSA in (a) presence and (b) absence of KCl.
104
3.32 a) SEM images of polyaniline synthesized electrochemically in 0.2 M PTSA on 2B pencil composite graphite in (a) presence and (b) absence of KCl.
105
3.33 CVs of pyrrole in 0.2 M PTSA in (a) presence and (b) absence of KCl. The scan rate is 100 mVs−1 and up to 10 cycles.
107
3.34 Baseline corrected FTIR spectra of polypyrrole electropolymerized in 0.2 M PTSA in (a) presence and (b) absence of KCl.
109
3.35 SEM images of polypyrrole microstructure synthesized electrochemically in 0.2 M PTSA on 2B pencil composite graphite in (a) presence and (b) absence of KCl; (a1 and b1) SEMs recorded at respective higher magnifications; (a2 and b2) The respective EDX analysis.
111
3.36 The CVs of copolymerization of 50 mM aniline solution and 50 mM pyrrole solution in 0.2 M PTSA in (a) presence and (b) absence KCl. The scan rate is 100 mVs−1 and up to 10 cycles.
113
3.37 Baseline corrected FTIR spectra (2000–400 cm−1 regions) of copolymer of aniline and pyrrole prepared in 0.2 M PTSA in (a) presence and (b) absence of KCl.
116
3.38 XPS spectra of N1s for copolymer of aniline and pyrrole prepared in 0.2 M PTSA in (a) presence and (b) absence of KCl.
118
3.39 The SEM images of copolymerization of 50 mM aniline solution and 50 mM pyrrole solution in 0.2 M PTSA on 2B pencil composite graphite in (a) presence and (b) absence of KCl; (a1 and b1) SEMs recorded at respective higher magnifications.
120
xi
LIST OF ABBREVIATIONS AA Ascorbic acid
Ani Aniline
B Benzenoid
CG Composite Graphite
CVs Cyclic Voltammograms
DMF N,N – Dimethylformamide
DPPH 1,1-diphenyl-2-picrylhydrazyl
Eap Applied Potential
EB Emeraldine Base
EDX Energy Dispersive X-ray
Ep Peak Potential
Epa Anodic Peak Potential
EQCM Electrochemical Quartz Crystal Microbalance
ES Emeraldine Salt
ESCA Electron Spectroscopy for Chemical Analysis
Ipa Anodic Peak Current
LEB Leucoemeraldine Base
LSV Linear Sweep Voltammetry
MeOH Methanol
mPDA meta-Phenylenediamine
OFN Oxygen-Free Nitrogen
oPDA ortho-Phenylenediamine
PAni Polyaniline
xii
PDAs Phenylenediamines
PE Protonated Emeraldine
PNB Pernigraniline Base
PoPDA Poly-ortho-phenylenediamine
pPDA para-Phenylenediamine
PPDAs Polyphenylenediamines
PPy Polypyrrole
PTSA para-Toluene Sulphonic Acid
Py Pyrrole
Q Quinoid
SCE Saturated Calomel Electrode
SEM Scanning Electron Microscopy
XPS X-ray Photoelectron Spectroscopy
xiii
KAJIAN TERHADAP SINTESIS ELEKTROKIMIA POLIANILINA DAN KOPOLIMERNYA
ABSTRAK
Pengelektropolimeran oksidaan anilina di dalam asid fosforik di atas grafit
komposit pensil 2B telah disempurnakan dengan menggunakan beberapa garam
takorganik yang dipilih sebagai elektrolit penyokong. Garam-garam ini menentukan
darjah kekonduksian polianilina yang terbentuk mengikut turutan CaCl2 > KCl >
ZnCl2 > ZnSO4 > Ca3(PO4)2. Kehadiran samada 0.06 M Ca3(PO4)2 atau 0.2 M ZnSO4
di dalam sintesis polianilina telah menyebabkan anjakan puncak redoks sebanyak
250 mV ke keupayaan negative. Anjakan puncak ini hanya dipengaruhi oleh jenis-
jenis anion yang ada bukan nya kation. Kejadian ini memudahkan pembentukan
kopolimer Ani dan orto-fenilindiamina.
Morfologi lapisan nipis homopolimer polianilina juga telah diperiksa dengan
mikroskop imbasan elektron pemancaran medan dan penyebaran tenaga pembelauan
sinar-X. Keputusan menunjukkan morfologi permukaan homopolimer bergantung
kepada elektrolit yang digunakan. Morfologi permukaan juga menentukan
pembentukan hablur seperti bunga yang berbeza setiap garam takorganik.
Sintesis homopolimer di dalam garam yang berbeza menambah keupayaan
antioksidan. Polianilina dalam medium fosfat adalah agen penurunan yang kuat,
berupaya mengaut radikal 1,1-difenil-2-pikrilhidrazil dalam metanol. Ia nya dua kali
lebih berkesan sebagai antioksidan daripada polianilina dalam medium klorida.
Penyediaan anod homo dan pengkopolimeran anilina dan pirola di atas grafit
komposit di dalam asid para-toluena sulfurik akueus dengan dan tanpa kalium
klorida (KCl) telah dilakukan. Kehadiran KCl di dalam medium telah meningkatkan
xiv
sifat struktur, optik dan morfologi polimer yang dihasilkan. Spektroskopi
fotoelektron sinar-X membuktikan sintesis poli(anilina-ko-pirola) dengan kehadiran
KCl mempunyai hampir setara unit pirol dan anilina di dalam tulang belakang
polimer berbanding sintesisnya tanpa kehadiran KCl.
Anjakan negatif inheren puncak redoks polianilina telah juga dilakukan di
dalam pengkopolimeran anilina dengan orto-fenilinadiamina di atas grafit komposit.
Grafit komposit terubahasuai poli(anilina-ko-pirola) menunjukkan gerak balas
pengelektromangkinan terbaik terhadap pengoksidaan asid askorbik. Keupayaan
puncak anod asid askorbik telah beranjak dari +0.5 V vs. Ag/AgCl (pada grafit
komposit) ke +0.1 V vs. Ag/AgCl (pada grafit komposit terubahsuai poli(anilina-ko-
pirola)), menunjukkan proses elektod yang mudah bagi asid askorbik pada grafit
komposit terubahsuai.
xv
STUDIES ON ELECTROCHEMICALLY SYNTHESIZED POLYANILINE AND ITS COPOLYMERS
ABSTRACT
Oxidative electropolymerization of aniline in phosphoric acid on 2B pencil
composite graphite was accomplished using some selected inorganic salts as
supporting electrolytes. These salts determined the degree of conductivity of
polyaniline formed, in the order of CaCl2 > KCl > ZnCl2 > ZnSO4 > Ca3(PO4)2. The
presence of either 0.06 M Ca3(PO4)2 or 0.2 M ZnSO4 in the synthesis of polyaniline
have caused the shifting of its redox peaks as much as 250 mV to the negative
potential. The shifting of these peaks was only influenced by type of anions presence
not the cations. This incident has prompted the formation of copolymer aniline and
ortho-phenylenediamine.
The morphology of the homopolymer polyaniline films was also examined by
a field emission scanning electron microscope and energy dispersive X-ray
diffraction. The results showed that surface morphology of the homopolymer was
dependent on the electrolytes used. The surface morphology also dictated each
inorganic salt into different bloom-like crystal formations.
The homopolymer synthesised in different salts has its antioxidant capacity
increased. The phosphate medium polyaniline was a strong reducing agent, capable
of scavenging 1,1-diphenyl-2-picrylhydrazyl radical in methanol. It was twice more
effective as an antioxidant than the chloride medium polyaniline.
The anodic homo and copolymerization of aniline and pyrrole on composite
graphite in aqueous para-toluene sulphonic acid with and without potassium chloride
(KCl) were investigated. The presence of KCl in the medium has improved the
structural, optical and morphological properties of the resulting polymers. The X-ray
xvi
photoelectron spectroscopy confirmed that poly(aniline-co-pyrrole) synthesized in
the presence of KCl has nearly equal units of both pyrrole and aniline in the
polymeric backbone as compared to the one synthesized in absence of KCl.
The inherent negative shift of redox peaks of polyaniline has also been
investigated in the copolymerization of aniline and ortho-phenylenediamine on
composite graphite. The poly(aniline–co–ortho-phenylenediamine) modified
composite graphite showed excellent electro catalytic response towards the oxidation
of ascorbic acid. The anodic peak potential of ascorbic acid has been shifted from
+0.5 V vs. Ag/AgCl (at bare composite graphite) to +0.1 V vs. Ag/AgCl (at
poly(aniline–co–ortho-phenylenediamine) modified composite graphite), indicating
easier electrode process of ascorbic acid on the later.
1
CHAPTER 1
INTRODUCTION
1.1. History of conducting polymers
Conducting polymers are relatively new class of materials whose interesting
metallic properties were first reported in 1977, with the discovery of electrically
conducting polyacetylene [1]. The importance of this discovery was recognized in
2000 when the Nobel Prize for Chemistry was awarded to the scientists who
discovered electrically conducting polyacetylene in 1977 [2]. Since the discovery of
polyacetylene, there has been much research into conducting polymers and many
new conducting polymers have been synthesized. The most important, and common
of these are polypyrrole [3], polyaniline [4], polythiophene [5] etc. There have been
many potential applications suggested for these materials, including sensors [6, 7],
electro-chromic devices [8, 9], corrosion inhibitor [10, 11], supercapacitor [12, 13],
electromagnetic shielding [14, 15], polymeric batteries [16], polymeric actuators [17,
18], etc. These wide ranges of applications are possible in part due to the ability to
alter the electrochemical, optical, chemical and mechanical properties of conducting
polymer by changing the monomer and or dopant incorporated into the polymer.
1.2. Types of conducting polymers
Conducting polymers can be classified mainly into three types:
a) Intrinsically / inherently conducting polymers.
b) Conducting polymer composites.
c) Ionically conducting polymers.
An organic polymer that possesses the electrical, electronic, magnetic and
optical properties of a metal while retaining the mechanical properties, processibility
2
etc. commonly associated with a conventional polymer, is termed an “Intrinsically
Conducting Polymer” more commonly known as “synthetic metal” [19].
The most common examples of intrinsically / inherently conducting polymers
are Polyacetylene, Polyaniline, Polypyrrole, Polythiophene, Poly(p-phenylene),
Poly(phenylene vinylene) etc. The Figure 1.1 shows some of the conjugated
polymers, which have been studied as intrinsically conducting polymers.
The unique electronic properties of the conjugated polymers are derived from
the presence of π - electrons, the wave functions of which are delocalized over a long
portion of the polymer chain when the molecular structure of the backbone is planar.
It is therefore necessary that there are no large torsion angles at the bonds, which
would decrease the delocalization of the π – electrons system [20].
3
Figure 1.1: Examples of inherently conducting polymers.
4
1.3. Application of conducting polymers
Researches show that conducting polymers exhibit conductivity from the
semiconducting range (~10−5 S cm−1) right up to metallic conductivity (~104 S cm−1).
With this range of electrical conductivity and low density coupled with low cost
polymeric conductor pose a serious challenge to the established inorganic
semiconductor technology. There are mainly two groups of applications for organic
conducting polymers which are briefly described below:
Group I: These applications just use the conductivity of the polymers. The
polymers are used because of either their lightweight, biological compatibility for
ease of manufacturing or cost. Electrostatic materials, Conducting adhesives,
Electromagnetic shielding, Printed circuit boards, Artificial nerves, Antistatic
clothing, Piezoceramics, Active electronics (diodes, transistors), Aircraft structures.
Group II: This group utilizes the electroactivity character property of the
materials. Molecular electronics, Electrical displays, Chemical, Biochemical and
Thermal sensors, Rechargeable batteries and Solid electrolytes, Drug release
systems, Optical computers, Ion exchange membranes, Electromechanical actuators,
'Smart' structures, Switches.
1.4. Syntheses of selected conducting polymers
Various methods [21] are available for the synthesis of conducting polymers.
However, the most widely used technique is the oxidative coupling involving the
oxidation of monomers to form a cation radical followed by coupling to form di-
cations and the repetition leads to the polymer. Electrochemical synthesis is rapidly
becoming the preferred general method for preparing electrically conducting
polymers because of its simplicity and reproducibility. The advantage of
electrochemical polymerization is that the reactions can be carried out at room
5
temperature. By varying either the potential or current with time the thickness of the
film can be controlled.
Electrochemical polymerization of conducting polymers is generally
employed by: (1) constant current or galvanostatic; (2) constant potential or
potentiostatic; (3) potential scanning/cycling or sweeping methods. Standard
electrochemical technique which employs a divided cell containing a working
electrode, a counter electrode and a reference electrode generally produces the best
films. The commonly used anodes are chromium, gold, nickel, palladium, titanium,
platinum and indium-tin oxide coated glass plates. Semi-conducting materials such
as n-doped silicon [22], gallium arsenide [23], cadmium sulphide and semi-metal
graphite [24] are also used for the growth of polymer films. Electrochemical
synthesis can be used to prepare free standing, homogeneous and self doped films.
Besides this, it is possible to obtain copolymers and graft copolymers. Polyazulene,
polythiophene, polyaniline, polycarbazole and several other polymers have been
synthesized using this approach.
1.4.1. Polyaniline
The largest use of aniline and its derivatives is in dye production. They are
thus important ingredients in color industry. Every year, thousands of new colors and
shades are generated and tested in industry. Polyaniline is one of the oldest artificial
conducting polymers [25] and its high electrical conductivity among organic
compounds has attracted continuing attention [26]. Aromatic amines are also
involved in the search for potentially interesting new compounds in materials science
[27].
The anodic oxidation of aniline to the final product, polyaniline, is quite complex
and there is still a controversy as regards the reaction pathway. The main question to
6
be answered concerns the chemical nature of the intermediates. For instance, it was
widely believed even in early papers [28] that, in acidic media, the polymerization
process involves a coupling of aniline radical cation that is formed in the initial
oxidation step. The cation, being an acid, would be stabilized in acidic conditions.
Initial intermediate species produced during aniline oxidation (Figure 1.2) have been
known to be head-to head, tail-to-tail and head-to-tail type linkages whose molar
ratio depends on pH of the medium. Aniline couples mainly tail-to-tail, and head-to-
tail under acidic conditions. However the head-to-head coupling reported at high pH
values.
7
F
igur
e 1.
2: R
eson
ance
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and
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term
edia
te sp
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8
Polyaniline is composed of aniline repeat units connected to form a
backbone. The existence of a nitrogen atom lying between phenyl rings allows the
formation of different oxidation states (doping) that can affect its physical properties.
In general, three distinct forms are available, depending on the degree of oxidation of
the nitrogens (Figure 1.3):
(a) A leucoemeraldine base is a fully reduced form which contains only benzene
rings in the polymer chain.
(b) An emeraldine base is a half oxidized form, where both benzene and quinoidal
rings are present.
(c) A pernigraniline base is a fully oxidized form.
The conducting emeraldine salt form can be obtained by oxidative doping of
leucoemeraldine base or by protonation of emeraldine base (Figure 1.4).
9
Figure 1.3: Different forms of polyaniline.
10
Figure 1.4: Formation of a polyaniline salt following oxidation and protonation of bases.
11
Electrically conducting polymers in their pristine and doped states have been
the materials of great interest for their applications in modern technologies. Among
all conducting polymers polyaniline has a special representation, probably due to the
fact that new applications of polyaniline in several fields of technology are expected.
Although polyaniline is one of the most important conducting polymers around,
detailed study on its structure and properties only begun in the 1980s [29-35].
Generally, it is accepted that polyaniline is a redox polymer. Figure 1.5a
shows this blend where polyaniline has both the reduced benzenoid unit (y) and the
oxidized quinoid unit (1 – y) [36]. Polyaniline may exist in several oxidation states,
i.e. from the completely reduced leucoemeraldine state (Figure 1.5b), where 1 – y =
0, to the completely oxidized pernigraniline state, where 1 – y = 1. The half-oxidized
emeraldine base state, where 1 – y = 0.5, is composed of an alternating sequence of
two benzenoid units and one quinoid unit [37, 38]. Even though each of these three
states of polyaniline possesses quite interesting physical and chemical properties,
they, actually, are insulators. However, the insulator blue emeraldine base may be
conducting if it is doped with protic acids and become green protonated emeraldine
or emeraldine salt with dc conductivity in the metallic region (ca. 1~5 S cm−1) [39].
The later may also be obtained via a redox doping process of either its corresponding
reduced leucoemeraldine or oxidized pernigraniline, by either a chemical or an
electrochemical step in acid conditions (Figure 1.5b). The deprotonation of
emeraldine salt revert this to emeraldine base.
12
Figu
re 1
.5: V
ario
us st
ates
of o
xida
tion
and
prot
onat
ion
of p
olya
nilin
e.
13
Polyaniline is prepared either by chemical or electrochemical oxidation of
aniline under acidic conditions. The method of synthesis depends on the intended
application of the polymer. Whenever thin films and better patterns are required, an
electrochemical method is preferred.
1.4.1(a) Chemical synthesis
Chemical polymerization of aniline has been carried out in an acidic solution
[40-44]. This acidic condition provides the solubilization of the monomers as well as
the formation of emeraldine salt as a conducting polyaniline. Figure 1.6 depicts a
chemical polymerization mechanism of aniline in acidic solution using an oxidizing
agent. Aniline monomer forms the anilinium ion in acidic medium and chemical
polymerization results in the formation of protonated, partially oxidized form of
polyaniline. The initial step involves formation of the aniline radical cation. The next
step is followed by the coupling of N- and para-radical cations with consecutive
rearomatization of the dication of para-aminodiphenylamine. The oxidation process
of the diradical dication makes the fully oxidized pernigraniline salt form of
polyaniline due to the high oxidizing power of the oxidant such as ammonium
persulfate ((NH4)2S2O8). Whereas “head-to-tail” coupling is predominant, some
coupling in the ortho-position also occurs, leading to conjugation defects in the final
product.
14
Figure 1.6: Polymerization mechanism of aniline [45].
15
After consuming all the oxidant, the unreacted aniline monomer reduces the
pernigraniline to produce the green emeraldine salt as the resultant polymer. From
the viewpoint of color change in solution, the formation of para-
aminodiphenylamine reflects pink, and the formation of protonated pernigraniline
becomes deep green. Green emeraldine salt precipitates after reduction of
pernigraniline in the final step. In the termination step, green emeraldine salt forms
as a conductive polyaniline form and it can be converted to emeraldine base with an
alkaline solution or an excess of water. The emeraldine base can be transformed to
two non-conductive polyaniline such as the completely oxidized pernigraniline and
reduced leucoemeraldine depending on oxidation states. Imine sites of the
emeraldine base are easily protonated in acid condition, which results in the
formation of green emeraldine salt as a conducting polyaniline [46]. The
conductivity of polyaniline was affected by degree of protonation and oxidation. In
addition, the structural and conformational factors derived from polymerization
condition also affect the conductivity of polyaniline.
1.4.1(b) Electrochemical synthesis
Electrochemical synthesis of polyaniline is a radical combination reaction
and is diffusion controlled. Electropolymerization is generally carried out in aqueous
protonic acid medium and this can be achieved by any one of the methods given
here:
i) Galvanostatic: constant current in the range of 1-10 mA.
ii) Potentiostatic: at constant potentials −0.7 to 1.1 V versus SCE.
iii) Sweeping the potential: between two potential limits −0.2V to +1.0V vs. SCE.
In the case of Ani electropolymerization, the radical cation of aniline
monomer is formed on the electrode surface by oxidation of the monomer [40, 47-
50]. This process is considered to be the rate-determining step. Radical coupling and
16
elimination of two protons make mainly paraformed dimers. Chain propagation
proceeds with oxidation of the dimer and aniline monomer on the electrode surface.
In this step, the radical cation of the oligomer couples with a radical cation of aniline
monomer. In the final step, polyaniline is doped by the acid present in solution. The
growth of polyaniline has been considered to be self catalyzed. This means that the
polymers are formed at the higher rate as the more monomers are deposited onto the
polymer surface. It involves the adsorption of the anilinium ion onto the oxidized
form of polyaniline, followed by electron transfer to form the radical cation and
subsequent reoxidation of the polymer to its most oxidized state.
1.4.2. Polypyrrole
Polypyrrole is an inherently conductive polymer due to excluded π
conjugation of electrons, which is stabilized by the heterocyclic group. Polypyrrole is
an especially promising conductive polymer for commercial applications, owing to
its high conductivity, good environmental stability and ease of synthesis. It is easy to
prepare by standard electrochemical techniques and its surface charge characteristics
can easily be modified by changing the dopant anion that is incorporated into the
material during synthesis.
Conducting polypyrrole can be prepared by various methods such as
chemical, electrochemical, vapor phase etc. The different two oxidation states of
polypyrrole are shown in the Figure 1.7.
17
Figure 1.7: Various oxidation states of polypyrrole.
18
1.4.2(a) Chemical synthesise
Polypyrrole has been prepared in the presence of various oxidizing agents
like sulfuric acid, nitrous acid, quinones or ozone. The material obtained was in the
insulating range 10−10 to 10−11 S cm−1 [51]. This could be then doped with halogenic
electron acceptors such as bromine and iodine to achieve a stable conductivity of
10−5 S cm−1 [52].
In recent times, polypyrrole is synthesized chemically in conducting state, because
the polymer oxidation occurs with oxidant salts acting as dopant agents. Several
metallic salts such as FeCl3, Fe(NO3)3, K3[Fe(CN)6], CuCl2 and CuBr2, have been
employed for the polymerization of pyrrole [53-55]. Ferric salts are the most
commonly used oxidants for the chemical synthesis of highly conductive polymer
complexes.
Figure 1.8 describes a typical mechanism for chemical oxidation
polymerization of pyrrole. In the initiation step, radical cations (C4NH5+) are
generated by the oxidation of pyrrole monomer. A radical-radical coupling occurs
between two radical cations, and forms a dimer with deprotonation, leading to a
bipyrrole. The bipyrrole is reoxidized and couples with other radical cations. This
process is repeated consecutively during the propagation step. The termination takes
place due to the nucleophilic attack of water molecules or impurities in the polymer
chains.
1.4.2(b) Electrochemical synthesis
It is reported that [56] the electrosynthesis of polypyrrole film proceeds via
the oxidation of pyrrole at the platinum electrode to produce an unstable P-radical
cation which then reacts with the neighboring pyrrole species. The cyclic
voltammograms of these solutions show an irreversible peak for the oxidation of
pyrrole at +1.2 V versus the saturated calomel reference electrode (SCE) [57].
19
Figure 1.8: Polymerization mechanism of pyrrole [58].
20
The mechanism of the overall reaction for the formation of fully aromatized
product is very complicated and involves series of oxidation and deprotonation steps.
In practice polypyrrole films are prepared by the electro-oxidation of pyrrole in one
compartment cell equipped with platinum working electrode, gold wire counter
electrode and a saturated calomel reference electrode. A wide variety of solvents and
electrolytes can be used as the electrical resistance of the solution is high and the
nucleophilicity does not interfere with the polymerization reaction. These conditions
can be accomplished by selecting solutions where the electrolyte is highly
dissociated and which are slightly acidic. Films of various thicknesses can be
prepared by changing the current density. Many other authors have investigated the
process of electrochemical deposition of conducting polypyrrole [59].
1.4.2(c) Vapour phase deposition
This technique, although known for depositing polypyrrole films since about
a decade ago [60]. The gas phase preparation of polypyrrole has been described by
Chen et al. [61]. Conducting polymer films were prepared in a similar manner to the
chemical vapor deposition method [60]. The polypyrrole films were prepared in two
ways i.e. by vapor phase polymerization using H2O2 or HCl, or by chemical vapor
deposition of polypyrrole doped with FeCl3. Bhattacharya and Misra [62] have used
polyvinyl chloride-cupric chloride complex film as base polymer and polypyrrole
was vapor phase polymerized on this. Wang et al. [63] have also made polypyrrole
films by this method. They used polyethyleneoxide and FeCl3, which were first
dissolved in a suitable solvent, from which, films were cast on glass slides. These
were dried and exposed then exposed to pyrrole vapors. The sample showed in-situ
growth of polypyrrole films, which also developed crystalline order.
21
1.4.3. Poly-ortho-phenylenediamine
The polymers of aromatic diamines including phenylenediamines,
diaminonaphthalenes, diaminoanthraquinones, benzidine, naphthidine, and
diaminopyridine have received increasing attention. These monomers are very
susceptible to oxidative polymerization via oxidation of one or both amino groups to
give linear polyaminoaniline, linear polyaminonaphthylamine, ladder polyphenazine,
and phenazine unit containing polymers. It is believed that investigations on the
aromatic diamine polymers are more attractive since they exhibit more novel
multifunctionality than polyaniline and polypyrrole. The polymers have shown
apparently different characteristics versus widely researched conducting polymers
such as polyaniline and polypyrrole in the application of electrocatalysis,
electrochromics, sensors, electrode materials, heavy metal ion complex, and
detection [64-66], although the polymerization mechanism and properties of the
polymers have not been definitely reported. It should be appreciated that the
aromatic diamine polymers possess good multifunctionality partially due to one free
amino group per repetitive unit on the polymers. Most of the polymers of the
aromatic diamines have been prepared by electrochemical polymerization, but only a
small number of polymers were obtained through chemically oxidative
polymerization.
1.4.3(a) Chemical synthesis
Thus far, only a few reports have described the formation of polymeric
products by chemical oxidation of aromatic diamines. However, relatively few
reports have appeared on the synthesis and structure of the polymers from aromatic
diamines prepared by chemically oxidative polymerization. The oxidative
polymerization was generally accomplished as follows: first, phenylenediamine
monomers and oxidant were separately dissolved in acidic aqueous solutions. Then
22
the polymerization reaction was initiated by steadily adding oxidant solution drop
wise into the monomer solution at 25-30 °C for meta-phenylenediamine and para-
phenylenediamine in common inorganic or organic acidic aqueous solutions and 118
°C for ortho-phenylenediamine only in glacial acetic acid. The drop wise addition of
oxidant solution is beneficial for obtaining polymers with a relatively high molecular
weight and narrow molecular weight distribution because the oxidation
polymerization of the aromatic diamines is highly exothermic. Finally, the reaction
continued for at least 12 h. The resulting powder product was filtered and dried at
temperatures lower than 60 °C. A drying temperature of higher than 60 °C might
lead to cross-linking of the polymers. The acids used for the preparation of aromatic
diamine polymers mainly include HCl and glacial acetic acid. The effective oxidants
are persulfate, iodine, H2O2, ceric ammonium nitrate, and Cu(NO3)2. Among them,
the persulfates including Na+, K+, and NH4+ persulfates are commonly employed as
oxidizing agents. Note that the oxidant is not necessary to the chemical oxidative
polymerization of meta-phenylenediamine and para-phenylenediamine. For
example, meta-phenylenediamine and para-phenylenediamine in oxidant-free acetic
acid aqueous solution can polymerize into polymers with the characteristics of
semiconductors at polymerization yields of 15% and 60%, respectively. However,
the oxidant is required for the chemically oxidative polymerization of ortho-
phenylenediamine because no oxidative polymer of ortho-phenylenediamine can be
obtained even if reaction at flux temperature in acetic acid aqueous solution is
allowed to proceed for 8 h. Moreover, the oxidative polymerization yield of ortho-
phenylenediamine is usually the lowest and the yield of para-phenylenediamine the
highest [67].
23
1.4.3(b) Electrochemical synthesis
Polarographic investigations of ortho-phenylenediamine were reported with gold,
graphite, and platinum electrodes in HCl and buffer (pH=1–4) solutions, and the
oxidation potential, critical oxidation potential, and polarographic half-wave
potential of ortho-phenylenediamine were given first by Lord and Rogers in 1954
[68]. Parker and Adams examined the anodic polarography of three
phenylenediamines (ortho, meta and para) on a rotating Pt electrode using a current-
scanning technique in 1956 [69]. They observed a linear relationship between
limiting current and para-phenylenediamine concentration. The
electropolymerization mechanism of three phenylenediamines was first suggested by
Elving and Krivis in 1958 [70]. They proposed that the oxidation mechanism of three
phenylenediamines depends on the phenylenediamine monomer structure and
solution pH value [71].
Poly-ortho-phenylenediamine can be prepared electrochemically in acid,
neutral and alkaline solutions and is very stable in both aqueous solutions and air
[72, 73]. The ease of dissolution of poly-ortho-phenylenediamine in organic solvents
such as dimethyl sulfoxide makes it one of the so-called soluble electroactive
polymers and thus provides more flexibility in the studies of its structures [73-76].
Electrochemical techniques, insitu FTIR spectroscopy [73, 77], resonant Raman
spectroscopy [78], radiometry [79], the electrochemical quartz crystal microbalance
(EQCM) [80] and other methods have been employed to study the
electropolymerization mechanism, to characterize the polymer films and to develop
various applications of poly-ortho-phenylenediamine.
During the electropolymerization process, polymer film is deposited on the
electrode surface and the formation of soluble oligomers might occur. The local
concentration of the oligomers near the electrode surface might be very high,
24
depending on the solubility and the diffusivity of the oligomers. The polymer films
formed might be very porous.
Generally, ortho-, meta-, and para-phenylenediamines undergo oxidative
polymerization to give phenazine ring containing ladder polymers with free amino
groups [72, 75, 76]. Rothwell and co-workers [72] proposed a ladder-like,
asymmetrical “quinoid” structure with phenazine-like units, while Yano and
Yamasaki [74] suggested a ring-opened 1,4-substituted benzenoid-quinoid structure.
The mechanisms of electropolymerization were very complicated in nature [72, 74].
poly-ortho-phenylenediamine film should possess a mixture of ladder-like and ring-
opened structures (Figure 1.9).
