1

CHAPTER 1

INTRODUCTION

1.1 Polymers

A polymer may be defined as a substance built up of repeating chemical

units held together by chemical bonds. A high polymer is one in which the number

of repeating units is in excess of about 1000. This number is termed as “Degree of

Polymerization (DP)”. The molecular weight of a polymer is often given by the

product of the molecular weight of the repeating units and DP.

A macromolecule is the term interchangeably used for polymers, more

often of biological origin. For example insulin, a protein-like hormone, is made up

of several different amino acids and can be called as a macromolecule but not

strictly as a high polymer. Not all the substances can be polymerized. It is

necessary that a chemical substance can act as monomer only if, it has at least bi-

functionality (which could be due to double bond or two reactive functional

groups).

Many properties of polymeric materials depend on the microscopic

arrangement of their molecules. Polymers can have an amorphous or

semicrystalline (Partially crystalline) structure. Amorphous polymers lack order

and are arranged in a random manner, while semicrystalline polymers are partially

organized in orderly crystalline structures.

Thermosets, which are densely cross-linked in the form of a network,

degrade but not soften on heating; on prolonged heating however, charring of

polymers is caused. They cannot be reshaped and reused. They are usually hard,

strong and more brittle. They cannot be reclaimed from wastes. Due to strong

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bonds and intra and interchain cross-links, they are insoluble in almost all organic

solvents. Thermoplastics, which do not contain cross-links, soften and melt upon

heating readily because, secondary forces between the individual chains are broken

easily by heat or pressure. They can be reshaped and reused. They are usually soft,

weak and less brittle. These can be reclaimed from wastes. They are usually

soluble in suitable solvents.

If a material is subjected to high-strain deformation, it deforms permanently

(plastic deformation) and ultimately fails. For sufficiently low stresses and

strains, the polymeric material behaves as a linear elastic solid. The stress-strain

behaviour of a polymeric material depends on various parameters such as

molecular characteristics, microstructure, strain-rate and temperature. The elastic

behaviour of polymer is more complicated due to the chain-like structure of the

macromolecules. A polymer chain resists stretching because, it reduces its entropy.

The associated restoring force is elastic and it is the underlying cause for the

mechanical behaviour of elastomers (e.g. rubber elasticity). _ Polymeric materials

due to their macromolecular (long-chain) structure are expected to have high

viscosities. Creep, flow and plastic deformation in polymeric materials results from

the irreversible slippage, decoupling and disentanglements of polymer chains (or

groups of chains in semicrystalline polymers). Strain hardening results from the

high orientation and alignment of polymer chains at high strains.

Polymers can be classified as linear, branched or cross-linked polymers.

High density polyethylene is a linear polymer, while low density polyethylene is a

branched polymer. Natural rubber has two configurational forms. The cis form has

less density and is available as latex. The other form is the trans form and is brittle

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and hard (gutta percha). Further, the natural rubber on vulcanization develops

crosslinking and becomes processable.

Polymers are also classified as organic, elementoorganic and inorganic

polymers. Organic polymers have chains consisting of C-C linkages and have,

apart from carbon atoms, hydrogen, oxygen, nitrogen, sulfur and halogen atoms in

the side chains. Elementoorgainic polymers include (i) macromolecules whose

chains are composed of carbaon as well as heteroatoms (except N, S, O, and

halogen atoms) and (ii) inorganic chains in which side groups contain carbon

atoms directly linked to chain. Inorganic polymers are polymers containing no

carbon atoms but have Si-Si, Si-O, N-PX, P-P and B-O linkage e.g. polysilanes,

polysiloxanes, polyphosphazenes, polyphosphoric acid or polyphosphates and

polyboron oxides. Inorganic polymers have been studied to a little extent so far

and it is difficult to provide a classification. However inorganic polymers possess

superior thermal, electrical and mechanical properties over the organic polymers.

Polymers in which the side branches are present in every unit, being joined

by different chemical groups with the main polymer chain are known as comb-like

polymers. Branched polymers which resemble a star by their structure are known

as star like polymers. Collinear double chain polymers are known as ladder

polymers.

Polymers can also be classified as (i) natural or (ii) synthetic. The common

natural polymers include polysaccharides (starch, cellulose, gums etc), proteins

(gelatin, albumin, enzymes, and insulin), polyisoprenes (natural rubber, gutta

percha) and nucleic acids (RNA and DNA). Natural polymers are sometimes also

called „biopolymers‟ or „biological macromolecules‟.

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Polymers can also be classified further as fibers, plastics, resins and rubbers

based on the nature and extent of secondary valence forces and mobility among the

constitutional repeat units. Plastics, fibers and elastomers possess different physical

properties. While fibers are crystalline materials, elastomers are amorphous.

Plastics lie between these extremes cases in physical properties viz. modulus and

elongation.

Polymers may be charged or uncharged. Charged polymers have some free

functional groups. Charged polymers that are soluble in water are called as

“polyelectrolytes”. Water insoluble charged polymers are often called “ionomers”.

Polymers have now become indispensable materials for us and it is difficult

today to think of daily life without them. Virtually modern synthetic polymers have

replaced the use of metals in many cases. Polymers are in fact engineering

materials and are fast replacing meals in every application. Some of the advantages

and disvantages in using polymers in place of metals are listed as follows. The

advantages of polymers are, (i) they are light weight, (ii) they have good thermal

/electrical insulation capacity, (iii) they are resistant to corrosion effects and are

chemically inert, (iv) they have easy workability and their fabrication costs are

low, (v) they have good strength, dimensional stability and toughness, (vi) they

are transparent in appearance, have good dyeability and possess decorative surface

effects and (vii) they absorb the mechanical shocks and show resistance to abrasion

effects. The major disvantages however are, (i) polymers are high cost materials,

(ii) they are easily combustible, (iii) they have poor ductility, (iv) they deform

under applied load, (v) they have low thermal stablility and (vi) polymers em-

brittle at low temperatures.

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One of the most severe disadvantages that have been of recent concern in

using plastic/polymeric materials is associated with the problem of waste disposal.

Several thousands of polymers are now available to use now. Very few polymers

however are used in producing articles for domestic and speciality applications. All

commercially available polymers are often produced in smaller quantities and are

used for highly specialized applications. Instead of getting a new monomer for the

production of polymers, the trend is to mix two polymers (polyblends) or a

polymer with some non-polymeric material (composites).

Polyblends are just physical mixtures (heterogeneous), often designed to

improve properties like processibility, mechanical strength, abrasion resistance and

flame retardance. The term engineering plastics or performance plastics refers to a

group of polymeric materials possessing the following characteristics: plasticity at

some stage of processing, high load bearing capacity, high mechanical strength,

rigidity, abrasion resistance, dimensional and thermal stability, light weight and

high performance properties which permit them to be used in the same manner as

metals, alloys and ceramic materials.

The other thrust aspect of polymer industry has been to produce speciality

polymers for specialized applications. Many industries are carrying out trials for

synthesizing highly conducting polymers, novel conjugated polymers with light

emitting and photo conducting properties. Efforts are also on to synthesize polymer

tubes based on polycatenanes and polytoxanes. Supramolecular chemistry has also

been applied in joining small chemical molecules via noncovalent bonds to

develop large physical structures which can be used as polymer tubes for

entrapping biomolecules. Multibranched polymers with a main core having a

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trifunctionality and a first generation dendrimer (with up to six to none end groups)

are also being in the process of commercialization.

Polymers, in general, do not conduct electricity, therefore they are

insulating materials. Consequently, the fact that, some polymers can be made

conductive is quite attractive. Conducting polymers contain extended π conjugated

systems: single and double bonds alternating along the polymer backbone. This

helps to create a conductive pathway in the polymer. Conducting polymers are

attracting more and more attention from the scientific because they are chemically

stable, environmentally viable and have corrosion resistant properties. Owing to

their versatility various commercial applications of conducting polymers in areas

such as rechargeable batteries, electrolytic capacitors, gas sensors and

electrochromic displays are envisaged.

Recently, it has been widely reported that conducting polymers have

excellent corrosion resistance. Corrosion of metals has been a persisting problem

in every society and hence it is an important area of study. Research has shown

that conductive polyaniline and polypyrrole coatings are very good corrosion

inhibitors.

1.2. Conducting polymers

Traditionally, most of the polymers are insulators with very desirable

properties such as lightweight, processability, durability and low cost. By

designing molecular structures, chemists have developed new polymeric materials,

which exhibit electrical conductivities comparable to metals while retaining the

advantages of polymers. This type of work has been recognised by the

international scientific community in the 21st century. The Nobel prize in

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Chemistry for the millennium year 2000 was awarded to A. J. Heeger, A. G.

MacDiarmid and H. Shirakawa in recognition of their research work into the

development of plastic as a conductor. They demonstrated and reported [1, 2 and

3] that, polyacetylene, an intrinsically insulating polymer, could become highly

conducting on treatment with chemicals such as I2, BF4 etc. By this process, they

established that the conductivity of polyacetylene could be increased by several 10

folds of magnitude. These materials possess the electrical properties of metals and

the advantages of polymers, a blend that led to the birth of novel materials, namely

conducting polymers.

The quantum of research on highly conducting polymers (both

experimental and theoretical) has increased exponentially in the last twenty five

years. Many research papers describing new results in this field are being

published. The increased activity in this area is apparent from the publication of a

separate international journal, Synthetic Metals, which is devoted exclusively to

the articles of conducting polymer materials.

J.Molina et.al. [4] electropolymerized aniline on conducting textiles of

polyester covered with polypyrrole (ppy) and anthraquinone sulphonic acid

(AQSA) obtaining a double conducting polymer layer. Morphology was studied by

SEM. EDX analyses have been performed to study zonal composition of the

samples. X-ray photoelectron spectroscopy (XPS) was employed to determine the

doping level of polyaniline films and the oxidation state of the sample.

Y.A. Udum et.al. [5] electropolymerized 8, 11 bis (4-hexyl thiophen-2-yl)

acenaphtho [1, 2-6] quinoxaline (HTAQ) and characterized it by CV, UV-VIS-NIR

Spectroscopy and colorimetry. The electroactive polymer has fast switching time

8

and high optical contrast. Spectro electrochemistry also showed that the polymer is

capable of being switched from bluish-green and a highly transmissive green upon

p-type doping.

M. Rohwerder et.al. [6] proposed a mechanism for corrosion protection of

conducting polymers. It was shown that the protection is due to switching from

mixed anion release and cation incorporation during the reduction of the

conducting polymer.

Serhatvaris et.al. [7] synthesized a new soluble conducting polymer using

the monomers 2,5-di(4-methyl thiophen-2-yl)-1-(4-nitrophenyl)-1H- pyrrole, 2-(4-

methyl thiophen-2-yl)-5-(3-methylthiophen-2-yl)-1-(4 nitrophenyl)-1H-pyrrole and

2,5-di(3-methyl-thiophen-2-yl)-1-(4 nitrophenyl)-1H-pyrrole were synthesized.

The resulting polymers were characterized via CV, FTIR, NMR, SEM and UV-Vis

spectroscopy. Spectro electrochemical analysis of polymer revealed the switching

ability of the polymers.

M.Barth et.al. [8] synthesized electrically conducting polyaniline doped

with hetero polyanions (HPA) of keggin structure. Cyclic votammetry studies

showed reversible redox systems from the polymer itself and from the immobilised

HPA.

M.S. Silvestein et.al. [9] synthesized plasma polymerized thiophene and

studied the dependence of molecular structure and properties on the polymerization

conditions. The undoped films exhibited non-linear current-voltage behaviour,

typical of schottky metal-semiconductor barriers with breakdown at reverse bias.

Iodine doping yielded ohmic IV behaviour, reflecting the formation of a

conducting iodine percolation network.

9

A.L.Sharma et. al. [10] synthesized and characterized polynitrosoaniline by

SEM, TGA, DSC, X-ray diffraction analysis and found that the polymer possessed

lesser crystallinity as compared with emeraldine base.

M.D.Migahed et.al. [11] synthesized polypyrrole composite films using

iron (III) chloride and characterized them by SEM, UV-Vis, IR and wide angle X-

ray diffraction spectroscopy.

M.I.Redondo et.al. [12] carried out FTIR study on chemically synthesized

poly (N-methylpyrrole). FTIR spectrum of the pellet is very different from that of

the powder showing that there has been a pressure-induced conformational change.

B.C. Roy et.al. [13] synthesized aniline initiated poly(m-aminobenzene

sulfonic acid) using ammonium persulfate as oxidant and characterized it by IR,

UV, TG-DSC, 1H NMR, SEM, and ESR spectroscopic techniques. The ESR

spectral study showed that the polymer is strongly paramagnetic.

A.Siove et.al. [14] synthesized poly (3, 6 –carbazole) by oxidative

polymerization in chloroform using ferric chloride as oxidant. The resulting

polymer was characterized by 1H,

13 C NMR, SEC, DSC and UV-Vis

spectroscopy.

1.2.1 Bonding requirement for conductivity

Bonding requirement of conductivity reveals that, the metallic conductivity

arises in a material when the valence electrons are completely delocalized and

move almost freely through the crystal lattice [15]. But polymers generally consist

of saturated carbon atoms, covalently bonded to each other and the valence

electrons in such molecules are all shared between two bond-forming atoms and

held tightly by their nuclei. Such a saturated backbone of polymers with localized

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electrons is incapable of providing either electrons as charge carriers or a path for a

charge carrier movement through the chain. Hence polymers behave as electrical

insulators. It is, therefore, apparent that extensive delocalization of electrons in the

polymer backbone is necessary for a polymer to behave as an electrical conductor.

This delocalization of electrons may occur through the interaction of -bonded

electrons in a chain of extended conjugation or by a similar interaction of -

electrons with non-bonded electrons of electron-rich hetero atoms (e.g., S, N etc.)

in the backbone.

1.2.2 Doping induced conductivity of organic polymer

Dopants are either strong oxidising or strong reducing agents

accepting/delivering electrons from/to polymers, respectively. On doping,

therefore, either positive or negative charge carriers are created in polymers.

Polymer + Dopant (Acceptor) Oxidation Polymer+ - Dopant

-

Polymer + Dopant (Donor) Reduction Polymer- - Dopant

+

The dopant introduces electrons or holes in conduction and valence bands

respectively. In this process, the dopant itself gets ionized and further localization

of state due to charge defects (kinks), folds and chain-end of polymer occurs. It is

likely that such a dopant will bring order in a disordered polymer chain [16-18].

The mechanism of conductivity through doping in inorganic materials (e.g.

semiconductors) and organic polymers is different [19]. Doping of inorganic

materials generates either holes in the valence band or electrons in the conduction

band, whereas, doping of an organic polymer results in the formation of

configurational charge defects viz., solitons, polaron or bipolarons (described in

the following section) in the polymer chain [20].

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1.2.3 Formation of charge defect

Polyacetylene serves as an example to explain the formation of charge

defect. Scheme 1.1 shows the structures of two italics forms of polyacetylene

(a and b). Both cis and trans polyacetylene are essentially equivalent and have the

same thermodynamic stability thereby giving the degenerate ground state. This

degeneracy of the ground state gives rise to the possibility of structural charge

defect (kink) in chain (c) where there is a change in the conjugation sequence.

(a) (b)

.(c)

Scheme 1. 1 Structural charge defect (kink) formation in trans polyacetylene

1.2.4 Formation of soliton

In the polymer chain, when charge defect occurs, a single unpaired electron

exists (although the overall charge remains zero) with the result of a new state

(energy level) created in the band gap, i.e., in between the valence and conduction

bands. That means, the unpaired electron resides in separate non-bonding orbital.

This neutral defect state is known as soliton and is singly occupied and therefore,

has a spin of ½ [21, 22]. The soliton energy level is accommodated with 0, 1 or 2

electrons and thus the soliton is neutral, positively or negatively charged

respectively. Scheme1.2 illustrate the three types of solitons. Neutral solitons (a)

have spin but no charge, whereas the charged solitons (b and c) have no spin but

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charge. Therefore the solitons have the unusual property of separating spin and

charge.

CB CB CB

VB VB VB

Charge 0 Charge +ve Charge -ve

spin 1/2 spin 0 spin 0

S+ S

-

(a) (b) (c)

So

Scheme 1. 2 Different types of solitons

1.2.5 Ring-torsional solitons

In 1991, a new type of soliton, known as ring-torsional soliton [23] was

proposed for the leucoemeraldine base form of polyaniline. The leucoemeraldine

base has degenerate ground states with respect to ring torsions around the carbon-

nitrogen bonds (Scheme 1. 3).

Adjacent rings in leucoemeraldine base rotate in opposite directions but by

an equal amount. This type of ring-torsional degeneracy in leucoemeraldine base

leads to a possibility of a novel type of non-linear excitation in the system

corresponding to the formation of soliton defect, in the alternation of upward and

downward rotations of the rings. The ring-torsional solitons are found to optimally

extend over about three rings. The ring-torsional solitons tend to increase the band

gap slightly and no charge-density fluctuations are found.

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N

N

N

N

H

H

H

H

N

N

N

N

H

H

H

H

Scheme 1. 3 Degenerate ground states of polyaniline

(Leucoemeraldine base) with respect to ring torsion

1.2.6 Formation of polaron and bipolaron

Polaron and bipolaron formations from solitons are proposed to take place

as shown in scheme 1. 4 taking trans polyacetylene for illustration. During doping,

a charge transfer directly between doping agent and valence and conduction bands

of polyacetylene occurs and this process produces an ion-radical in the chain.

Separation of ion-radical components along the chain into one charged and one

neutral solitons is possible, leading to a more stable arrangement. This more stable

ion pair is called polaron and its formation is accompanied by the creation of two

energy levels symmetrically above and below the middle level of the band gap.

When the doping is further continued, the polarons begin to interact with each

other and produce bipolarons. In bipolaron, two unpaired electrons are united

leaving behind two charged solitons.

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+

O

-e

oxidative

doping

+

O

+

O

Polaron

-e further doping

+

+

BipolaronCharged solitons

Scheme 1. 4 Polaron and bipolaron formations in trans polyacetylene

1.2.7 Charge transport in conducting polymers

Addition of dopants generates charged species (polarons and bipolarons)

within the polymer, which are mobile enough to conduct electric charges [24]

through the bulk of the polymer either by moving along the chain (intrachain

transport) or by hopping from one polymer chain to another as a result of redox

reactions between neighbouring polymer chains (interchain transport). The

electrical conductivity is proportional to the product of the number and the

mobility of charge carriers. Charge mobility is usually facilitated through enhanced

molecular and structural orders.

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1.2.8 Development of organic conducting polymers

Table A. Different classes of conducting polymers with selected dopants

Conductivity (S/cm)

Polymer Structural formula Dopants Virgin state Doped state

Trans-

polyacetylene

I2

AsF5

4.4 x 10-5

1.6 x 102

4.0 x 102

Cis-

polyacetylene

I2

AsF5

1.7 x 10-9

5.5 x 102

1.1 x 103

Polypyrrole

N

H

n

BF4-

ClO4-

1.0 x 10-8

1.0 x 102

2.0 x 102

Polythiophene

S n

BF4-

ClO4-

1.0 x 10-7

1.0 x 101

2.0 x 101

Polyfuran

O n

BF4-,

ClO4-

--

1.0 x 102

Polyaniline

N

H

n

H2SO4 1.0 x 10-10

0.2 x 101

Polyphenylene

n

AsF5 1.0 x 10-12

4.0 x 102

The discovery of charge defects or carriers in polymer chains was

important to demonstrate the uniqueness of highly conducting polyacetylene,

polypyrrole, polyaniline etc., and it also led to a sudden spurt in research directed

towards the study of new conducting polymer materials. The different classes of

conducting polymers with conductivities equal to metal are reported in table A.

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1.2.9 Properties of conducting polymers

1. Conducting polymers exhibit anisotropic, quasi-one dimensional

properties. For e.g. the conductivity is much greater in one direction than in others.

In most of the linear conducting polymers, the conduction is greatest along the

chain direction.

2. Conducting polymers are amorphous in nature. Of the several polymers

in literature, only polyacetylene and polythiophene show a certain degree of

crystallinity.

3. Switching property: This is the most interesting and useful property of

conducting polymers. Many applications are based on this property. Conducting

polymers can be switched between two oxidation states of widely differing

conductivity by varying the applied voltage.

4. Electrical Conductivity: Doping is responsible for conduction. The

conduction is partly ionic (doping) and partly electronic (due to Л conjugation).

The factors affecting electrical conductivity of conducting polymers are:

a. Nature of the dopant (chemical reactivity)

b. Doping level (% concentration of dopant)

c. Process of doping (method) - Electrochemical method gives the highest

value.

d. Conditions of polymer synthesis.

e. Monomer concentration, electrode substrate, pH, concentration of doping

agent, temperature of polymerization, polymerization potential

f. Film morphology / degree of crystallinity

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5. Redox property/ Electroactivity: Redox property or electroactivity is

the ability to get oxidised or reduced reversibly under electrochemical conditions.

This is responsible for the switching property. The redox property throws light on

the mechanism of polymerization, conductivity, nature of intermediates formed,

stability and polymer degradation. Most of the applications are based on the redox

property.

6. Optical property: Optical properties of conducting polymer films deal

with the electronic excitation. The optical properties are important for

understanding the electronic structures. In general, an insulating polymer is

transparent (light coloured) whereas on doping, a conducting polymer films

typically absorbs in the visible region. In other words, conducting polymers show

reversible colour changes in solutions when the applied potential is varied. This

property is called electrochromism. Secondly the optical band gap is important

characteristics whose magnitude governs electronic and optical properties of a

conducting polymer. A reduction in optical band gap increases the conductivity of

the polymers.

1.2.10 Applications of conducting polymers

One of the greatest advantages of organic polymers over inorganic

materials is their flexibility, since they can be chemically modified and easily

shaped according to the requirement of a particular service. This feature makes the

electroactive organic polymers to cover a broad spectrum of applications from

solid-state technology to biotechnology and from antistatic coating to smart

molecular devices [25] including solid-state rechargeable polymer batteries [26].

Notable potential applications of conducting polymers are sensors, photocells,

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solar batteries, photography, electrostatic shielding, and electronic devices [27,

28]. Chart 1 displays the applications of conducting polymers in various

fields [29].

Chart 1. 1 Applications of conducting polymers

CONDUCTING

POLYMERS

CONDUCTING COMPOSITES

SUPER CONDUCTORS

JOSEPHSON JUNCTION COMPUTER LOGIC HIGH FIELD MAGNET GENERATOR

NON-LINEAR

OPTICAL

FREQUENCY DOUBLER

ELECTRO

CHROMIC

FERRO MANETISM

DISPLAY DEVICES

MAGNETIC

RECORDING

CONDUCTIVE SURFACE

EMI/ESD

SOLID STATE

SENSORS

SUPER CAPACITORS CONNECTORS

METAL

PLASTIC BATTERIES

PHOTOCONDUCTORS

PIEZOELECTRIC

SOLID STATE PHOTOCHEMICAL

REACTIONS

LED. PHOTOCOUPIERS TRANSDUCERS

OPTICAL STORAGE

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1.3 Copolymerization

Copolymerization may be defined as the polymerization of two or more

monomers together to obtain a polymer which contain more than one kind of

monomeric units in its chain [30, 31]. This technique facilitates to incorporate a

combination of different properties in one material. The well known examples are

vinyl chloride –vinyl acetate and butadiene-styrene copolymers. In the last fifty

years, the homo polymers of butadiene and chloroprene have been completely

replaced by the copolymers formed from these groups as well as styrene-

acrylonitrile and other acrylate monomers.

1.3.1 Why Copolymers?

Copolymers have a combination of properties in one material, for example

crystalline and amorphous structures, hydrophilic and hydrophobic, flexibility and

toughness, abrasion resistance and elasticity etc., which are seldom present in

homopolymers. Some important copolymers were created early in the days of

plastics. For instance, common household glue products (similar to Elmer‟s) can be

made by copolymerizing ethylene and vinyl acetate monomers to form ethylene

vinyl acetate (EVA). Likewise, the combination of ethylene and acrylic acid

monomers yields the class of polymers called ethylene acrylic acid (EAA). When

EAA is mixed with a metallic salt, the class of copolymers called ionomers is

formed. These copolymers have advantages over the homopolymers butadiene and

chloroprene, since they possess high abrasion resistance and oxidation stabilities.

Copolymers can be tailored depending on the designed properties by changing the

monomers, monomer compositions and techniques of copolymerization. Hence

copolymers found extensive uses in various fields of applications in different ways.

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Ionomers are much tougher than either pure polyethylene or pure

polyacrylic acid. Perhaps, the most important copolymers are made by the

combination of styrene, butadiene, and acrylonitrile monomers to make the

common thermoplastic resin ABS. Various combinations of the monomers in ABS

will produce widely different materials.

The three pure polymers (made when only one monomer is polymerized)

are represented by the three corners of the triangle and the combination of all three

monomers to make ABS is shown in the middle of the triangle. Increasing the

concentration of one of the monomers relative to the others will result in a polymer

that has properties dominated by the high concentration material and is represented

by moving in the direction of the polymer made from that pure monomer. For

instance, pure styrene is a hard, clear and brittle material that is called crystal PS

(polystyrene). If a small amount of butadiene monomer is added during the

polymerization, the resulting polymer becomes tougher and is called HIPS (high

impact polystyrene).

If a new polymer is made with even more butadiene, that polymer would be

quite elastomeric and would be called SBR (styrene butadiene rubber). Likewise,

mixtures of styrene and acrylonitrile result in properties that are intermediate

between those of pure polystyrene and pure acrylonitrile. This chart, while specific

to ABS, represents the basic principle underlying all copolymers.

Copolymers are formed by mixing at least one or more monomer types.

This rather cumbersome definition is needed because some polymers are made

with just one monomer type and some are made from two polymer types. In either

case, if an additional monomer type is added, a copolymer is produced. Invariably,

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the combination of monomers to make a copolymer which has some properties

expected from all the monomer types based upon their physical and chemical

interactions.

1.3.2 Classification of copolymers

A linear polymer consists of a long chain of monomers. A branched

polymer has branches covalently attached to the main chain. Cross-linked polymers

have monomers of one chain covalently bonded with monomers of another chain.

Crosslinking results in a three-dimensional network; the whole polymer is a giant

macromolecule. Elastomers are loosely cross-linked networks while thermosets are

densely cross-linked networks.

Another classification of polymers is based on the chemical type of the

monomers. Homopolymers consist of monomers of the same type; copolymers

have different repeating units. Furthermore, depending on the arrangement of the

types of monomers in the polymer chain, the copolymers can have the following

classification:

In random copolymers, two or more different repeating units are distributed

randomly

Alternating copolymers are made up of alternating sequences of the

different monomers

In block copolymers, long sequences of a monomer are followed by long

sequences of another monomer

Graft copolymers consist of a chain made from one type of monomer with

branches of another type

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1.3.3 Monomer Reactivity ratios:

The monomer reactivity ratios, r 1 and r2 are the ratios of the rate constant

for a given radical adding to its own monomer to the rate constant for its adding to

the other monomer. Thus r 1 >1 means that the radical M 1 prefers to add to M 1; and

r 1<1 means that it prefers to add to M 2. From the values of reactivity ratios,

copolymerization behaviour can be predicted, i.e.

a) When r 1 = r2 = 0, propagation reaction types M 11 and M 22 are possible

and the monomers will form an „alternating copolymers‟.

b) When r 1 = r2 = 1, all the four types of propagation reactions are equally

possible and this case is considered as an ideal „copolymerization‟ or

azeotropic copolymerization.

c) When r 1 >1 and r 2 < 1, the propagation reaction types M 11 and M 21 will be

preferred to types M 12 and M 22.

d) When r 1 <1 and r 2 >1, the behaviour will be opposite to case (c).

e) When r 1 <1 and r 2 < 1, the propagation type M 12 and M 21 are preferred to

types M 11 and M 22 .

The reactivity ratios (r1 and r2 ) for a pair of monomers ( M 1 and M 2 ) in a

copolymer system can be determined by various methods [32-35].

1.3.4 Synthesis of Copolymers

Copolymers can be synthesized by one of the following techniques:

(i) Chemical polymerization

(ii) Electrochemical polymerization

(iii) Photochemical polymerization

(iv) Metathesis polymerization

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(v) Emulsion polymerization

(vi) Inclusion polymerization

(vii) Solid-state polymerization

(viii) Plasma polymerization

(ix) Pyrolysis

(x) Soluble precursor polymer formation

Among all the above categories, chemical polymerization [36-38] is the

versatile method for preparing large amount of conducting copolymers. Chemical

oxidative polymerization [39] is followed by oxidation of monomers to cation

radical and their coupling to form di cations and repetition of this process generates

a polymer.

Electrochemical polymerization [40, 41] can be carried out

potentiostatically by using a suitable power supply (Potentiostat / Galvanostat).

Generally, potentiostatic conditions are recommended to obtain thick films. Free

standing as well as self-supporting copolymer films of desired thickness or

geometry can be obtained.

Photochemical polymerization [42] takes place in the presence of

sunlight/light having suitable energy. This technique utilizes photons to initiate the

polymerization reaction in the presence of photosensitizer.

Plasma polymerization is a technique used for preparing ultra thin uniform

layers (50-100A ). Electric glow discharge is used to create low temperature cold

plasma.

24

Metathesis polymerization is unique, differing from all other

polymerizations. In this technique, all the double bonds in the monomer remain in

the polymer also.

Nevertheless, copolymers have also been synthesized by other techniques

such as chain polymerization, step polymerization, chemical vapour deposition

etc., however, most of these techniques consume and involve the use of costly

chemicals.

1.3.4.1 Chemical synthesis of copolymers

Chemical synthesis of copolymers has attracted considerable interest

because it produces polymers in bulk form and it is a valuable method for the

preparation of polymers on insulating surfaces. Moreover it was shown that

chemical synthesis yields a polymer having a higher molecular weight [43].

However, there has been comparatively little work on the chemical

synthesis of copolymers [44]. A.S.Sarac et al., [45] synthesized the random

copolymer of N-methyl pyrrole and N-ethylcarbazole by chemical polymerization

method in presence of ceric ammonium nitrate in acetonitrile. Some nitrate groups

are incorporated in the polymeric structure when acetonitrile is used. A series of

polypyrrole-graft-poly ( -caprolactone) copolymers have been synthesized by

Mecerreyes et al. [46] via the oxidative chemical polymerization of pyrrole and

pyrrole-end functional monomers.

In recent years, the synthesis of conducting copolymers gained more

importance because, copolymer materials offer considerable processing advantages

over comonomeric molecules. Since, copolymer films can be generated from

solution deposition techniques, they are usually glassy and they have good

25

mechanical and optoelectronic properties [47] in addition to excellent corrosion

resistance properties at room temperature [48,49].

P.S. Antonel et.al. [50] synthesized copolymer of aniline and m-chloro

aniline (MCA) in variable ratios by chemical oxidation in HCl medium.

Copolymer composition was determined by elemental analysis and XPS.

Copolymers with high MCA: Ani ratio formed materials with properties very

different from polyaniline.

P.Rajakumar et.al. [51] copolymerized o-toludine and metanilic acid on

mild steel surfaces by potentiostatic and potentiodynamic mehods. The redox

behaviour of the copolymer has been studied using CV technique and also

characterized by UV-Vis, FTIR and TGA analysis.

P.Savitha et.al. [52] synthesized soluble poly (o-/m-toluidine-co-o-

nitroaniline) by emulsion polymerization. The copolymers show comparatively

higher conductivity, better solubility and higher thermal stability than the

homopolymers.

G. Cakmak et.al. [53] prepared conductive copolymers of polyaniline,

polypyrrole and poly (dimethylsiloxane) with different compositions and their

properties were compared. The films were characterized by SEM, FTIR, CV, TGA

and DSC.

M.Jithunsa et.al. [54] reported the syntesis of 4(5) vinylimidazole-co-

acrylic acid used as polymer electrolyte membrane for fuel cells (PEFC). These

polymers were characterized by FTIR, 1HNMR, DSC, TGA and WAXD.

T.Yasuda et.al. [55] prepared a new five membred ring heteroaromatic

copolymers composed of 1-alkyl-1H-1, 2, 4-triazole and thiophene or bithiophene

26

by palladium catalysed polycondensation. These polymers were characterized by

GPC, NMR and XRD. The optical properties as well as the electrochemical

properties of the copolymer in solution and films were determined.

R.Oliver et.al. [56] synthesized a copolymer electrochemically from N-

methyl pyrrole and 3, 4 ehylenedioxythiophene on steel electrodes by

chronoamperometry in acetonitrile with LiClO4 oxidant. They were characterized

by elemental analysis and their electrochemical properties have been examined

and compared with those of the homopolymers.

S.Koyuncu et.al. [57] studied the electrochemical, optical and

electrochromic properties of imine polymer containing thiophene and carbazole

units by chemical oxidation using FeCl3 oxidant. They were characterized by UV-

Vis, FT-IR, 1H,

13C NMR and SEM. It was shown that, the conductivities were

increased by iodine doping.

L.O.Peres, et. al. [58] synthesised and investigated a series of new phenyl

based conjugated copolymers by vibration and photoluminescence spectroscopy.

Infrared and Raman spectra were used to check the chemical structure of the

compounds. The copolymers exhibit blue emission.

G.Nie et.al. [59] synthesised and characterized a new soluble conducting

copolymer of 5-cyanoindole and 3, 4 ethylene dioxy thiophene in acetonitrile

containing tetra butylammoniumtetrafluoroborate. The electrochemical properties

of the copolymers were studied by cyclic voltammetry. This novel copolymer has

good redox activity, good thermal stability and high conductivity. The fluorescence

spectra indicated that the copolymer is a good blue-light emitter.

27

B.Yigitsoy et.al. [60] synthesized a copolymer of 2, 5-di (thiopheny-2-yl)-

1-p-tolyl-1H pyrrole (DTTP) with 3, 4 ethylenedioxy thiophene (EDOT) and

characterized by CV, FTIR, SEM, conductivity measurements and

spectroelectrochemistry. The copolymer film has distinct electrochromic

properties. It has four colours (chestnut, khaki, camouflage green and blue).

Double potential step chronoamperometry experiment showed that the copolymer

has good stability and fast switching time.

Y.Pang et.al. [61] carried out the electrochemical synthesis,

characterization and electrochromic properties of poly(3-chlorothiophene) and its

copolymer with 3-methyl thiophene by potentiodynamic and galvanostatic

polarization methods. The copolymer showed the colour change between deep red

and greenish blue in fully reduced and oxidised states.

R. Mc Connel et.al. [62] carried out the chemical synthesis of polyfuran

and co-polymers using a mild oxidizing agent. The synthesized copolymers were

characterized by IR, 1H NMR, CV and ESR.

N.Li et.al. [63] carried out the direct electrochemical copolymerization of

pyrrole and tetrahydrofuran in various monomer ratios. The characterization

results showed that the comonomers generated true copolymers rather than blends

of the two homopolymers with increased electrical conductivity and have better

flexibility than pure polypyrrole.

Sayyah et.al. [64] studied the electro copolymerization of a binary mixture

of 2-chloroaniline and 2-amino-4-(4-methoxy phenyl) thiazole at various reaction

conditions and monomer concentrations. The obtained copolymer films were

characterized by IR, 1H NMR, UV-Visible, GPC and CV.

28

Extensive studies on the catalytic behaviours of monomeric [65] and

polymeric [66] imidazole have been reported. Sebille and coworkers [67, 68] have

developed a new coating technology for protein separations using poly

(vinylimidazole) and poly (N-vinylpyrrolidone) block copolymers.

Jilde et.al. [69] applied this technology for the synthesis of non-porous

silica-based strong anion exchangers containing vinyl-imidazole-N-vinyl

pyrrolidone copolymers. They found that the conditions for separation were

optimized by varying the copolymer composition and the amount adsorbed on the

non-porous silica supports.

N-vinylimidazole-4-amino styrene copolymer as a new tailor-made steric

stabilizer for polyaniline colloids was synthesized by free-radical precipitation

copolymerization of monomers in benzene at 70 C using 2, 2‟-

azobisisobutyronitrile as an initiator [70].

Ishida et.al. [71, 72] formed silane ( -methacryloxypropyltrimethoxysilane)

- modified poly (N-vinylimidazole) copolymer coating and they observed that the

coatings have good corrosion protection and adhesion promotion capabilities for

copper substrate in severe environments.

Umesh and coworkers [73] studied chemical synthesis, spectral

characterization and electrical properties of poly (aniline-co-m-chloroaniline) in

HCl medium. They found that the copolymer exhibits excellent solubility in DMF,

DMSO and THF. Also they observed from the spectroscopic analysis that, aniline

and m-chloroaniline units are distributed along the copolymer chain.

Q. Mei et.al. [74] showed that the polymers formed by the addition of 8-

hydroxy quinoline to aluminium isopropoxide or to zinc acetate plays an important

29

role in the ongoing challenge to develop efficient electroluminescent materials due

to their thermal stability, high fluorescence and excellent electron-transporting

mobility.

T.Yasuda et.al. [75] studied the synthesis, characterization and optical

properties of the new five-membered ring heteroaromatic copolymers composed of

1, 2, 4-triazole/thiophene by palladium catalyzed polycondensation. They observed

from the NMR spectroscopy that, the copolymers formed had a regio-random

molecular structure and from the XRD data that, they have a -stacked structure in

solid state.

N.Ranieri et.al. [76] copolymerized acrylic and methacrylic monomers

bearing pyrrolyl, thienyl and terthienyl groups and with various amounts of butyl

acrylate and butyl methacrylate. They found that the heterocyclic side groups of

the resulting copolymer behaved as initiators and allowing the polythiophene chain

to grow from the side-groups that leads to the formation of graft copolymers.

Turgay et.al. [77] synthesized the copolymers containing 2-imidazolinium

and 1, 4, 5, 6-tetrahydropyrimidinium by free radical polymerization in DMF at

moderate temperature and observed that they were found to exhibit antibacterial

activities against Escherichia coli.

Shantilal et.al. [78] carried out the free radical copolymerization of methyl

methacrylate and styrene with N-(4-carboxyphenyl) maleimide (CPMI) in THF

solvent at 80 C. A series of copolymers were prepared using different monomer

ratios. They found that the initial and final decomposition temperatures of the

copolymers increased with increasing the amount of CPMI monomer.

30

B.Muller and coworkers [79] found that the corrosion reaction of Al & Zn

in aqueous alkaline media can be inhibited by the addition of copolymers like

maleic acid-styrene-3-acrylic esters and 2- methacrylic esters.

Mircea et.al. [80] synthesized poly (N-ethyl 3, 7-phenothiazinediyl-co-

acetylene) by chemical synthesis using a Grignard reagent and NiCl2.2PPh3 as

catalyst. They found that the copolymer formed is soluble in common organic

solvents and has an electrical conductivity of 10-7

-10-6

-1

cm-1

.

Rami Reddy et.al. [81] synthesized a copolymer of 3-hydroxy-4-

benzoylphenyl methacrylate with methylmethacrylate in methyl ethyl ketone using

benzoyl peroxide as the initiator. They found that the copolymers were thermally

stable, optically active and soluble in common organic solvents and were

characterized by IR, 1H-NMR,

13C-NMR, CV and Gel Permeable

Chromatography.

M.Jithunsa et.al. [82] carried out the synthesis of proton transfer poly

(acrylicacid-co-4(5)-vinyl imidazole and characterized by FTIR, WAXD and

Raman spectroscopy.

E.Hamciuc et.al. [83] prepared a novel series of nitrile-containing

polyimide-polydimethyl siloxane copolymers and analyzed the thermal stability by

TGA and DSC and the surface morphology by scanning electron microscopy.

J.Hedgewald et.al. [84] reviewed and reported the synthesis of N-(3-amino

propyl) pyrrole and N-(2-carboxyethyl) pyrrole using FeCl3 and (NH4)2S2O8 as

oxidants. The resulting polymers were characterized by 1H NMR, ATR-FTIR and

MALDI-TOF-MS.

31

Isbelvega et.al. [85] chemically modified some inexpensive commercial

polymers such as polyacrylamide and synthesized copolymers containing 1, 3

oxazole pendant groups. Molecular structures were confirmed by IR and 13

C-NMR and the thermal properties were studied by TGA and DSC techniques.

S.Martwiset et.al. [86] synthesized and characterized random copolymer,

terpolymers of 1, 2, 3 triazole containing acrylates and poly (ethylene glycol) ether

acrylate (PEGMEA). Proton conductivity measurements were made using

impedance spectroscopy.

Frederick et al. [87] found that poly-N-vinylimidazole and poly-4(5)-vinyl

imidazole copolymers are more effective anti-oxidants than the most commonly

used corrosion inhibitors for copper at elevated temperatures. Copolymer inhibitor

gives excellent coverage with good penetration on to recesses and blind holes.

M. Lebrini et.al. [88] synthesized a new corrosion inhibitor, namely,

polyphosphate derivative of guanidine and urea copolymer (PGUC) and

investigated its inhibiting action on the corrosion of Armco iron in 1M HCl at 30

oC by electrochemical impedance spectroscopy (EIS). The experimental results

revealed that PGUC is an efficient inhibitor and the inhibition efficiency increases

with increase in inhibitor concentration. This inhibitor can be used as a broad

spectrum inhibitor against both Gram positive and Gram negative bacteria.

1.3.5 Applications of Copolymers

Copolymers acquired prime importance in various avenues of industrial

applications [89-92]. The major fields of applications of these copolymers are in

leather, textile and building industries. In leather industry, copolymers have been

used for the formulations of base and top coats for leather. In addition to this, the

32

copolymer forms a part of adhesive formulations. In textile industry, the

copolymers are used as sizing and finishing agents and also in anti-shrinkage

treatments. The other applications of these copolymers are: antistatic finishes,

canvas finishes, glaze finishes, flame proofing, water and oil repellency,

transparent coating, electro conductive coatings, manufacture of tyres and other

mechanical rubber goods etc.,

Some copolymers are widely used as binders in protective coatings because

of their excellent durability, water white colour and transparency. They have a

combination of excellent flexibility toughness and resistance to chemical fumes,

alkalies, acids and water. So, they are used as primers for automotive finishes,

clean lacquers for polished metals like copper, bronze and aluminium and enamels

for household appliances like washing machines, refrigerators etc.

1.4 SCOPE AND OBJECTIVES

1.4.1 Preamble

The conducting polymer is an interdisciplinary field incorporating unique

challenges, difficulties and opportunities. The macromolecular structural

modification can lead to reduce the band gaps of conduction and valence band and

hence increases the intrinsic conductivity of conducting copolymers. With this all

round development in the basic aspects of conductivity, stability, processability

and mechanical strength, the conducting copolymers are now at the threshold of

revolutionary technology of electronic materials.

Ever since the discovery of the phenomenon of conductivity in conjugated

polymers by the three material stalwarts in 1977, there has been a tremendous

growth in research activity and publications on conducting polymers. Flourishing

33

as a novel type of materials, they possess a great scope of application in diversified

fields like electronics, electrical storage devices, capacitors, sensors, smart

materials, drug carriers, heart valve etc. Though the essential requirement for all

these applications is the conductivity, other characteristics, especially of material

and polymer origin are crucial and decide the efficiency, durability and cost factor

of the specific application/device of the conducting polymer.

In this respect, heterocyclic conducting polymers have emerged as

promising materials outweighing others, owing to their unique physicochemical

characteristics, environmental stability and easy synthesis. Hence heterocyclic

compounds have secured a focus point from many researchers of conducting

polymers.

1.4.2. Polyheterocyclics–Unique in its class and choice for the present work

Polyheterocyclics are conjugated conducting polymers having unique

molecular characteristics and hence they find a wide spectrum of applications. The

chief characteristics which make the polyheterocyclics distinct are:

i) The presence of imine nitrogen moieties.

ii) Protonation-dependent doping and conductivity without change in total

number of conduction electrons.

The chemically flexible imine nitrogen of poly heterocyclics and the

corresponding alteration in its physico-chemical properties such as redox state,

colour, structure and the charge carrier, hold the key for its entire applications.

Though many of its material characteristics are favourable for technological

applications, certain chemical properties, notably its processability, poor solubility

in common organic solvents, plasticity and long term stability with retention of

34

conductivity still pose problems and do not fructify their potential applications.

Hence, much research effort is needed to further improve its characteristics with

tailor-made properties so as to suit the end applications. To find remedies to these

problems is the task of the present attempt.

1.4.3. Objectives and work plan

The objective of the present work is very clear. Improvement in material

and polymer characteristics of the versatile conducting heterocyclic polymer by

chemical means (copolymerization) in order to fetch more practical applications is

the goal and objective.

The objectives of the present investigation are

1. Synthesis of poly imidazole, poly carbazole, poly pyridine and poly 8-

hydroxy quinoline

2. Synthesis of (imidazole-carbazole), (imidazole-pyridine), (imidazole-8

hydroxy quinoline), (carbazole-pyridine), (carbazole- 8 hydroxy quinoline),

(8 hydroxy quinoline-pyridine) copolymers.

3. Structural characterization of the polymers by UV-Vis, FTIR, 1HNMR and

XRD techniques.

4. Determination of copolymer composition by using 1HNMR technique.

5. Determination of reactivity ratios by Fine-man and Kelen-Tudos methods.

6. Thermal characterization of the polymer by TGA and DSC techniques.

7. Electrical conductivity of the polymers by four probe technique.

8. Electrochemical characterization by cyclic voltammetry.

35

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