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STUDIES ON ELECTROCHEMICALLY

SYNTHESIZED POLYANILINE AND ITSCOPOLYMERS

ALI PARSA

UNIVERSITI SAINS MALAYSIA

2009


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STUDIES ON ELECTROCHEMICALLY SYNTHESIZEDPOLYANILINE AND ITS COPOLYMERS

by

ALI PARSA

Thesis submitted in fulfillment of the requirementsfor the degree of

Doctor of Philosophy

September 2009


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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.


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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



(c) Vapour phase deposition 201.4.3 Poly-ortho-phenylenediamine 21



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


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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 ofPyrrole 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


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LIST OF TABLES

Page

3.1 The Epa/Epc values of redox peaks in different supportingelectrolytes

49

3.2 The effect of counter anions to two polyaniline oxidationprocesses

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 phosphatebuffer and in methanol

81

3.6 The conductivities of polymers are prepared in para-toluenesulphonic acid (PTSA)

114


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LIST OF FIGURES

Page

1.1 Examples of inherently conducting polymers. 3

1.2 Resonance structure of aniline and initial intermediate speciesproduced during aniline oxidation.

7

1.3 Different forms of polyaniline. 9

1.4 Formation of a polyaniline salt following oxidation andprotonation 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 polyanilinedoped 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 radicalcation).

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 duringelectropolymerization 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 potentialis cycled between 0.4 and +1.0 V (vs. Ag/AgCl) at a scan rateof 100 mVs1 for the 1st-21st cycle.

47

3.2 The cyclic voltammograms obtained duringelectropolymerization of aniline in 1 M H3PO4 containing (a)0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2.

51


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3.12 SEM images of polyaniline microstructure synthesizedelectrochemically in 1 M H3PO4 and 0.06 M Ca3(PO4)2, a)Flower-like inorganic salt crystal prepared on the polyanilinesubstrate; b) morphology of polyaniline; c and d) EDXspectrum of flower and polyaniline.

72

3.13 SEM images of polyaniline microstructure synthesizedelectrochemically in 1 M HCl and 0.5 M CaCl2, a) Flower-likeinorganic salt crystal prepared on the polyaniline substrate; b)EDX spectrum of flower.

73

3.14 SEM images of poly-ortho-phenylenediamine microstructuresynthesized electrochemically in 1 M H3PO4 and 0.06 MCa3(PO4)2, a) Flower-like inorganic salt crystal prepared on the

poly-ortho-phenylenediamine substrate; b) EDX spectrum offlower.

74

3.15 SEM images of copolymer ofortho-phenylenediamine andaniline microstructure synthesized electrochemically in 1 MH3PO4 and 0.06 M Ca3(PO4)2, a) Flower-like inorganic saltcrystal prepared on the copolymer substrate; b) EDX spectrumof flower.

75

3.16 The suggested reaction mechanisms for polyanilineelectropolymerized in presence of (a) Ca3(PO4)2 and (b) CaCl2(in which polyaniline has more Q structures in the polymer

backbone).

77

3.17 UVvis spectra of 8.0 105 M DPPH radicals in MeOHcontaining 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 inpresence of (a) Ca3(PO4)2 and (b) CaCl2 upon reaction with 2mL 0.08 x 103 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, containingCa3(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 containingCa3(PO4)2 and CaCl2, respectively. The scan rate is 100 mVs

1.

83


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3.21 The CVs for the growth of poly-ortho-phenylenediaminefrom a 50 mM ortho-phenylenediamine solution in 1 M H3PO4containing (a) 0.5 M CaCl2, and (c) 0.06 M Ca3(PO4)2. Thevoltammogram is cycled between Eap0.7 and +0.9 V (vs.Ag/AgCl), scan rate of 100 mVs1 and up to 10 cycles. The (b)

and (d) are CVs for the growth of polyaniline from a 50 mManiline solution in 1 M H3PO4 containing 0.5 M CaCl2 and 0.06M Ca3(PO4)2, respectively. The voltammogram is cycled

between Eap 0.4 and +1.0 V (vs. Ag/AgCl), scan rate 100mVs1 and up to 10 cycles.

85

3.22 The CVs and SEM images of copolymerization of 50 mManiline solution and 50 mM ortho-phenylenediamine solution in1 M H3PO4 containing (a) 0.06 M Ca3(PO4)2 and (b) 0.5 MCaCl2. The voltammogram is cycled between Eap 0.7 and+0.9 V (vs. Ag/AgCl), scan rate of 100 mVs1 and up to 10

cycles.

87

3.23 Baseline corrected FTIR spectra (2000400 cm1 region) ofpoly-ortho-phenylenediamine electropolymerized in 1 MH3PO4 containing (a) 0.06 M Ca3(PO4)2 and (b) 0.5 M CaCl2.

89

3.24 Baseline corrected FTIR spectra (2000400 cm1 region) ofcopolymer of aniline and ortho-phenylenediamine in 1 MH3PO4 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(anilinecoortho-phenylenediamine)/compositegraphite electrode in 1 M H3PO4 (pH=1).

92

3.26 a) LSVs of different concentrations of ascorbic acid atpoly(Anicoortho-phenylenediamine)/composite graphiteelectrode in 1 M H3PO4 (pH=1). Concentrations of ascorbicacid from (a) to (j) are 550 mM, respectively. b) The plot ofIPa vs. concentration. (n=5)

94

3.27 a) CVs of 5 mM ascorbic acid at poly(anilinecoortho-phenylenediamine)/composite graphite in 1 M H3PO4 (pH=1) at

different scan rates. Scan rate from (a) to (j) are 10, 20, 30,40, 50, 60, 70, 80, 90 and 100 mV s1, respectively. b) The plotof Ipa vs.

1/2. (n=5)

95

3.28 CVs of supporting electrolyte consisting of 25 mM ascorbicacid at pH values of (a) 1, (b) 3, (c) 5, (d) 7 and (e) 9 at

poly(anilinecoortho-phenylenediamine)/composite graphiteelectrode at 100 mVs1.

97

3.29 CVs of aniline in 0.2 M PTSA in (a) presence and (b) absenceof KCl. The scan rate is 100 mVs1.

100


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3.30 Baseline corrected FTIR spectra of polyanilineelectropolymerized in 0.2 M PTSA in (a) presence and (b)absence of KCl.

102

3.31 UVvis 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 in0.2 M PTSA on 2B pencil composite graphite in (a) presenceand (b) absence of KCl.

105

3.33 CVs of pyrrole in 0.2 M PTSA in (a) presence and (b) absenceof KCl. The scan rate is 100 mVs1 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 synthesizedelectrochemically in 0.2 M PTSA on 2B pencil compositegraphite 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 50mM pyrrole solution in 0.2 M PTSA in (a) presence and (b)

absence KCl. The scan rate is 100 mVs1

and up to 10 cycles.

113

3.37 Baseline corrected FTIR spectra (2000400 cm1 regions) ofcopolymer 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 pyrroleprepared in 0.2 M PTSA in (a) presence and (b) absence ofKCl.

118

3.39 The SEM images of copolymerization of 50 mM aniline

solution and 50 mM pyrrole solution in 0.2 M PTSA on 2Bpencil composite graphite in (a) presence and (b) absence ofKCl; (a1 and b1) SEMs recorded at respective highermagnifications.

120


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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


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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


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KAJIAN TERHADAP SINTESIS ELEKTROKIMIA POLIANILINA DANKOPOLIMERNYA

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


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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.


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STUDIES ON ELECTROCHEMICALLY SYNTHESIZED POLYANILINEAND 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 aqueouspara-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


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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 ofKCl.

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(anilinecoortho-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(anilinecoortho-phenylenediamine) modified composite graphite), indicating

easier electrode process of ascorbic acid on the later.


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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


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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 - 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 electrons system [20].


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Figure 1.1: Examples of inherently conducting polymers.


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1.3. Application of conducting polymers

Researches show that conducting polymers exhibit conductivity from the

semiconducting range (~105 S cm1) right up to metallic conductivity (~104 S cm1).

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


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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


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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.


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Figure1.2:R

esonancestructureofanilineandinitialintermediatespeciesproducedduringaniline

o

xidation.


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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).


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Figure 1.3: Different forms of polyaniline.


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Figure 1.4: Formation of a polyaniline salt following oxidation and protonation ofbases.


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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 cm1) [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.


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Figure1.5:Variousstatesofoxidationandprotonatio

nofpolyaniline.


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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 ofpara-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.


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14

Figure 1.6: Polymerization mechanism of aniline [45].


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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


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17

Figure 1.7: Various oxidation states of polypyrrole.


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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 1010 to 1011 S cm1 [51]. This could be then doped with halogenic

electron acceptors such as bromine and iodine to achieve a stable conductivity of

105 S cm1 [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.


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].


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19

Figure 1.8: Polymerization mechanism of pyrrole[58].


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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.


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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


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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 forortho-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 andpara-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-

phenylenediaminebecause no oxidative polymer ofortho-phenylenediaminecan 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-

phenylenediamineis usually the lowest and the yield ofpara-phenylenediamine the

highest [67].


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Polarographic investigations ofortho-phenylenediaminewere reported with gold,

graphite, and platinum electrodes in HCl and buffer (pH=14) solutions, and the

oxidation potential, critical oxidation potential, and polarographic half-wave

potential ofortho-phenylenediamine were given first by Lord and Rogers in 1954

[68]. Parker and Adams examined the anodic polarography of three

phenylenediamines (ortho, meta andpara) 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,


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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).

pan studies_on_electrochemically sẼ in

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