Introduction.......

1

CHAPTER – I

1.1. Introduction

Polymers are the giant molecules or macromolecules formed by the repeating

units of several simple molecules. Polymers by the virtue of their light weight and greater

ease of fabrication are continuing to replace metals in several areas of application.

Polymers have long been used as insulating materials. This insulating property of, most

of the polymers represent a significant advantage for many practical applications of

plastics. During the last 20 years, however, organic polymers characterized by good

electrical conductivity have been found. Due to their low specific weight, good

processability and resistance to corrosion and the exciting prospects for plastics

fabricated into electrical wires, films or electronic devices, these materials have attracted

the interest of both industrial and academic researchers in domain ranging from

Chemistry to Solid State Physics and Electrochemistry. The close interaction between

scientists from diverse background has been a significant factor in the rapid development

of the field of conducting polymers.

The discovery of doping in conducting polymers has led to further dramatic

increase in the conductivity of such conjugate polymers to values as high as 105 Scm

-1 .

Discovery and development of conducting polymers has opened up new frontiers in

Materials Chemistry and Physics. This new generation of polymers combines the

mechanical properties and processablity of traditional polymers with electrical, optical

properties which are unknown earlier. The enormous technological potential that , this

rare combination offers is beginning to be trapped.

Introduction.......

2

1.2. Conducting Polymers

A polymer filled with conductive materials such as carbon black, metal flakes or

fibers, and so on is known as conductive polymer or the polymers whose backbones (or

pendant groups) are responsible for the generation and propagation of charge carriers.

Conducting polymers are classified into two ways depending on

1) Nature of polymer and dopant (figure 1.1) and

2) Conduction mechanism (figure 1.2).

1.2.1. Classification based on nature of polymer and dopant

Figure 1.1 Classification for conducting polymer based on nature of matrix & dopant

In conjugated polymers the long chain carbon compounds contains alternate

single and double bonds that leads to one unpaired electron per carbon atom [1]. In

charge transfer polymers, the orbital on adjacent molecules are overlapped to form

continuous one-dimensional bands. The charge carriers in this system are provided by the

electron transfer between the electron donors (D) and acceptor (A) molecules. In this

case, the conductivity arises from the ion migration between coordination sites repeatedly

Conducting polymers

Conjugated

conducting polymer Charge Transfer

Polymers

Ionically

Conducting

polymers

Conductively

filled polymers

Introduction.......

3

generated by the local motion of polymer chain segments [2]. Therefore, a desirable

polymer host must possess: a) electron-donating atoms or groups for the coordinate bond

formation with cations, b) low bond rotation barriers for an easy segmental motion of the

polymer chain, and c) an appropriate distance between coordinating centers for multiple

inter-polymer bonding with cations.

In the case of conductively filled polymers, the conductivity is introduced

through the addition of the conducting components in various polymer materials

including both amorphous and crystalline polymers which can be made electrically

conducting.

1.2.2. Classification based on conduction mechanism

Figure 1.2 Classification for conducting polymer based on conduction mechanism

1.2.3. Conducting Polymer Composites

Conducting polymer composites are mixture or blends of conductive particles and

polymers. Various conductors have been used in different forms together with large

number of conducting and engineering plastics. These can be injection molded into

Conducting polymers

Conducting

polymer

composites

Organometallic

polymeric

conductors

Polymeric

charge transfer

complexes

Inherently

conducting

polymers

Introduction.......

4

desired shapes. Various conductive fillers have been tried such as carbon blacks, graphite

flakes, fibers, metal powders etc. The electrical conductivity of the polymer is decided by

the volume fraction of the filler. A transition from insulating to non-insulating behavior is

generally observed when volume fraction of conductive filler in the mixture reaches a

threshold of about 25%. The various polymers, which have been used as major matrix,

are typically PC, PET, PP, Nylon, PVC, HDPE etc.

1.2.4. Organometallic Polymeric Conductors

These types of conducting materials are obtained by adding organometallic

groups to polymer molecules. In this type of materials, the d-orbital of metal may overlap

π–orbital of the organic structure and thereby increases the electron delocalization. The

d - orbital may also bridge adjacent layers in crystalline polymers to give conducting

property to it. Metallophthalocyanines and their polymers fall in this class of polymeric

material [3]. These polymers have extensively conjugated structures. The bridge

transition metal complexes form one of the stable systems exhibiting intrinsic electrical

conductivities without external oxidative doping.

Polyferrocenylene is also an example of this type of polymer. These materials

possess strong potential for future applications such as antistatic foils, molecular wires

and fibers in xerography.

Introduction.......

5

Figure 1.3 Structure of polyphthalocyanines

1.2.5 Polymeric Charge Transfer Complexes

Polymeric Charge Transfer Complexes (CTC) are formed when acceptor like

molecules are added to the insulating polymers. There are many charge transfer

complexes reported in the literature, e.g. CTC of tetrathaifulvalene (TTF) with bromine,

chlorine etc [4]. The reason for high conductivity in polymeric charge transfer complexes

and radical ion salts are still somewhat obscure. It is likely that in polymeric materials,

the donor – acceptor interaction promotes orbital overlap, which contributes to alter

molecular arrangements and enhances electron delocalization.

1.2.6 Inherently Conducting Polymers

Research in the field of inherently conducting polymer started nearly three

decades ago when Shirakawa and his group found drastic increase in the electrical

conductivity of polyacetylene films when exposed to iodine vapor [5]. Following this

breakthrough, many small conjugated molecules were found to polymerize, producing

Introduction.......

6

conjugated polymers, which were either insulating or semiconducting in the oxidized or

doped state. These conjugated polymers are studied as the intrinsically conductive

polymers. The electronic properties of these polymers are due to the presence of π-

electrons and the wave functions of which are delocalized over long portions of polymer

chain when the molecular structure of the backbone is planar [6]. Hence it is necessary

that there are no torsion angles at the bonds, which would decrease the delocalization of

the π-electron system. Some of the examples of conjugated polymers are shown in the

figure 1.4 below and the features, which differentiate, conjugated polymers from

conventional polymers are as follows:

Figure 1.4 Schematic representations of conjugated polymers

Band gap Eg (electronic band gap) is small (~ 1 to 3.5 eV) with corresponding to

low excitations and semiconducting behavior.

Can be oxidized or reduced through charge transfer reactions with atomic or

molecular dopant species.

Introduction.......

7

Net charge carrier mobilities in the conducting state are large enough and because

of this, high electrical conductivity is observed.

Quasiparticle, which under certain conditions, may move relatively freely through

the material.

The electrical and optical properties of these kinds of materials depend on the electronic

structure and on the chemical nature of the repeated units. The electronic conductivity is

proportional to both density and the drift mobility of the charged carriers. The carrier

drift mobility is defined as the ratio of the drift velocity to the electric field and reflects

the ease with which carriers are propagated. To enhance the electrical conductivity of

polymers, an increase in the carrier mobility and the density of the charge carriers is

required [7].

1.3. Conjugated Conducting Polymers

1.3.1. Structure and Properties

Conducting polymers(CP) are extensively conjugated molecules: they have

alternating single and double bonds. In these molecules, electrons are able to move from

one end of the polymer to the other through the extended π-orbital system [8]. Hence CPs

is known to be either semiconductors or conductors, which are related to how bands and

shells of electrons form within a compound. In view of the electronics of CPs, the Band

theory is employed to explain the mechanisms of conduction in CPs. The theory

originates from the formation of energy bands in polymer materials from discrete orbital

energy levels found in single atom systems. In this regard, it is vital to review band

theory [9].

Introduction.......

8

1.3.2 Band theory of CPs

The physical chemistry approach to explanation of band theory is to relate it to the

quantum theory of atomic structures. The first major success of quantum theory was its

explanation of atomic spectra, particularly that of the simplest atom, hydrogen [10].

Quantum mechanics introduced an important concept which explained that atoms could

only occupy well-defined energy states and for isolated atoms the energy states were very

sharp [11]. The spectral emission lines which resulted correlated to electrons jumping

from one allowed energy state to another and this gave rise to correspondingly narrow

line widths.

Figure 1.5 Formation of bands in a conducting solid in the 3rd period and overlap

between the valence and conduction bands.

Introduction.......

9

In a crystalline solid, atoms cannot be viewed as separate entities, because they

are in close proximity with one another, and are chemically bonded to their nearest

neighbor [12]. This leads to the concept that an electron on an atom sees the electric field

due to electrons on other atoms and the nature of the chemical bond implies that electrons

on close-neighbour atoms are able to exchange with one another, causing the broadening

of sharp atomic energy states into energy ‘bands’ in the solid [13].

This can be illustrated using an example below in figure 1.5, that depicts 3p and

3s electron shells for a single metallic atom in the third period of the periodic table that

overlap to become bands that overlap in energy (Atkins, 2002) [14]. The association of

these bands is no longer solely with single atoms but rather with crystal as a whole. In

other words, electrons may appear with equal probability on atoms anywhere else in the

crystal.

The energy band that results from the bonding orbitals of a molecule is known as

the valence band, while the conduction band is as a result of the antibonding orbitals of

the molecule. The width of individual bands across the range of energy levels is called

band width. The valence band (VB) represents the highest occupied molecular orbital

(HOMO) and the conduction band (CB) represents the lowest unoccupied molecular

orbital (LUMO) [38]. The gap between the highest filled energy level and lowest unfilled

energy level is called band gap (Eg). This band gap represents a range of energies which

is not available to electrons, and this gap is known variously as ‘the fundamental energy

gap’, the ‘band gap’, the ‘energy gap’, or the ‘forbidden gap[8] The level of electrons in a

system which is reached at absolute zero is called the Fermi level (Fg) [15]. It has been

demonstrated that in order to allow the formation of delocalized electronic states, CPs

Introduction.......

10

molecular arrangement must be conjugated [16]. The delocalization of the electronic

states relies on the resonance stabilized structure of the polymer. The size of the energy

band gap depends on extend of delocalization and the alternation of double and single

bonds. Moreover the size of the energy band gap will determine whether the CP is metal,

semiconductor or insulator [17]. Combining the concepts explained in both atomic and

molecular orbital theory, the electronic properties of metals, semiconductors, and

insulators can be differentiated with reference to the energy band gap as shown in figure

1.6 below.

Figure1.6 Energy band diagram demonstrating band gaps

In metals there is no range of energies which is deemed unavailable to electrons,

which simply means that forbidden gap or band gap in metals is Eg = 0 eV. Hence metals

always have a partially filled free-electron band, because the conduction and valence

bands overlap. Hence the electron can readily occupy the conduction band. Insulators

have a band gap which is larger than 3 eV [18], the energy gap between VB and CB is

too large, hence the electron is not able to make that jump to detach from its atom, in

Introduction.......

11

order to be promoted to the valance band. Consequently they are poor electrical

conductors at ambient temperatures. Insulators can be defined as materials in which the

valence bands are filled and the forbidden energy gap between valence band and

conduction band is too great for the valence electrons to jump at normal temperatures

from VB to the CB [19].

1.3.3 Solitons, Polarons, Bipolarons and Band Structures of Intrinsically

Conducting Polymers

The band structure of trans-(CH)x, assuming an idealized linear one dimensional

molecules, can be regarded as being developed as shown diagrammatically in table 1.1, in

which the bonding molecular orbitals are ¼ orbitals and antibonding molecular orbitals

are ¼ orbitals. The formation of filled ¼ band (valence band) can be regarded as , by

joining together of (CH) units containing an unpaired P¼ electron.

Addition of electrons to the conduction band or removal of electrons from the

valence band cause a change in the energy level of these bands which results in the

formation of new bands. Since trans-(CH)x has a doubly degenerate ground state, i.e., the

energy of the molecule is the same regardless of the phasing of the double bonds, one

new band is formed in the middle of band gap when the number of electrons in the ¼

system of the molecule is changed. Removal of electron from valence band (oxidation of

the ¼ system, p-doping) results in the formation of an empty band, i.e., positive “soliton”

band in the band gap. Addition of electron to the conduction band (reduction of the ¼

system, n-doping) results in the formation of a filled band, i.e., negative “soliton” band in

the band gap [20].

Introduction.......

12

If a conjugated polymer such as poly(paraphenylene) does not have a degenerate

ground state, two new bands will be formed in the band gap when the number of electron

in the ¼ system is changed. The upper band in the band gap lies under the bottom of

conduction band while the lower band lies above the top of valence band. If the lower

band is half filled (p-doping), it is called positive “polaron” band. If the upper band is

half filled (n-doping), it is called negative “polaron” band. When both lower band and

upper band are empty (p-doping), it is called positive “bipolaron” band [21]. When both

lower and upper bands are filled (n-doping), it is called negative “bipolaron” band. A

given polymer may consist of polarons at one doping level and bipolarons at a different

doping level or consist of significant amounts of polarons and bipolarons with each other

under certain conditions.

Polypyrrole is one of the most attractive polymers to come from a new class of

materials which have special electrical properties. These properties originates from the

fact that polypyrrole is an intrinsic conducting polymer and can be synthesized to have

conductivities up to 1000 Scm-1

which approaches the conductivity of metals. Most

practical polypyrroles have conductivities in the range of 1 – 100 Scm-1

. This research

opens new ways of producing polypyrrole and how polymerization conditions affect the

final properties. Further research explores methods of combining polypyrrole with other

Introduction.......

13

polymers, and applications in novel devices. Polypyrrole is popular in research over other

conductive polymers because it is relatively easy to synthesize and is chemically stable.

Table 1.1 presents both physical and chemical terms of the defects mentioned above.

Physical terms Chemical terms

Non-doped state

Undisturbed

conjugation

Neutral soliton Free radical

Positive soliton Carbocation

Negative soliton Carbanion

Positive polaron Radical cation

Negative polaron Radical anion

Positive bipolaron Carbodication

Negative

bipolaron Carbodianion

The material is commonly prepared by either a one step electro-oxidation process

to make thin films or chemical polymerization to get powder.

Potential application of polypyrrole which utilizes its electrical properties are

numerous and will enhance as the development of this material and other ‘synthetic

material’ continues. Traditional materials used in electrical applications (metals and

silicon-based semiconductors) may be replaced by polymers in the future due to special

Introduction.......

14

advantages in physical properties, cost or ease of production. Conducting polymers have

been used in Lithium batteries as a positive electrode for 25 years and inroads of

replacing other battery components has been steady since then. In November 1996

researchers from John Hopkins University announced the creation of an ‘all-plastic’

battery which drew the attention of CNN and spurred a write-up in the issue of Scientific

American.

Since then it has been found that about a dozen of different polymers and polymer

derivatives undergo transition to conducting state when doped with a weak oxidation or

reducing agent. They are all various conjugated polymers. The early conjugated polymers

were unstable in air and were not capable of being processed. The recent research in this

area has been focused towards the development of highly conducting polymers with good

stability and acceptable processing attributes.

1.3.4. Charge storage

One of the early explanations of conducting polymers used band theory as

method of conduction, According to this a half filled valance band would be formed

from a continuous delocalized π – system. This would be an ideal condition for

conduction of electricity. However, it turns the polymer can more efficiently lower

its energy by band alteration ( alternating short and long bonds ), which introduces

a band width of 1.5 eV making it a high energy gap semiconductor. The polymer is

transferred into a conductor by doping it with either an electron donor or electron

acceptor. This is reminiscent of doping of silicon based semi-conductors where silicon is

doped with either arsenic or boron. However, while the doping of silicon produces a

Introduction.......

15

donor energy level close to the conduction band or an acceptor level close to the valance

band, this is not the case with conducting polymers. The evidence for this is that the

resulting polymers do not have a high enough concentration of free spins, as determined

by electron spin spectroscopy.

Initially the free spins concentration increases with concentration of dopant. At

large concentrations, however, the concentration of free spins levels becomes maximum.

To understand this it is necessary to look into the way how the charge is stored along the

polymer chain and its effect.

The polymer may store charge in two ways. In an oxidation process, it could

either lose an electron from one of the bands or it could localize the charge over a small

section of the chain. Localizing the charge causes a local distribution due to change in

geometry, which costs the polymer some energy. However, the generation of this local

geometry decreases the ionization energy of the polymer chain and increases its electron

affinity making it more able to accommodate the newly formed charges. This method

increases the energy of the polymer less than it would if the charge was delocalized and,

hence, takes place in preference of charge delocalization. This is consistent with an

increase in disorder detected after doping by Raman Spectroscopy. A similar scenario

occurs for a reductive process.

Typical oxidizing dopants used include iodine, arsenic pentachloride, iron (III)

chloride and NOPF6. A typical reductive dopant is sodium naphthalide. The main criteria

is its ability to oxidize or reduce the polymer without lowering its stability or whether or

not they are capable of initiating side reaction that inhibit the polymers ability to conduct

Introduction.......

16

electricity. An example of the latter is the doping of a conjugated polymer with bromine.

Bromine is too powerful an oxidant and adds across the double bonds to from sp3

carbons. The same problem may also occur with NOPF6 if left too long.

Conjugated polymers with a degenerate ground state have a slightly different

mechanism. As with Polypyrrole, polarons and bipolarons are produced upon oxidation.

However, because the ground state structure of such polymers are twofold generate, the

charged cation are not bound to each other by a higher energy bonding configuration and

can freely separate along the chain. The effect of this is that the charged defects are

independent of one another and can form domain walls that separate two phases of

opposite orientation and identical energy. These are called solitons and can some times be

neutral. Solitons produced in polyacetylene are belived to be delocalized over about 12

CH units, with the maximum charge density next to the dopant counter ion. The bonds

closer to the defect, show less amount of bond alteration than the bonds away from the

center. Soliton formation results in the creation of new localized electronic states that

appear in the middle of the energy gap. At high doping levels, the charged solitons

interact with each other to form a soliton band which can eventually merge with the band

edges to create true metallic conductivity.

1.3.5. Charge Transport

Although solitons and bipolarons are known to be the main source of charge

carriers, the precise mechanism is not yet fully understood. The problem lies in

attempting to trace the path of the charge carriers through the polymer. All of these

polymers are highly disordered, containing a mixture of crystalline and amorphous

Introduction.......

17

regions. It is necessary to consider the transport along and between the polymer chains

and also the complex boundaries established by the multiple number of phases. This has

been studied by examining the effect of doping, temperature, magnetism and the

frequency of the current used. These tests show that a variety of conduction mechanisms

are used. The main mechanism used is by movement of charge carriers between localized

sites or between solitons, polaron or bipolaron states. Alternatively, where

inhomogeneous doping produces metallic island dispersed in an insulating matrix,

conduction is by movement of charge carriers between highly conducting domains.

Charge transfer between these conducting domains also occurs by thermally activated

hopping or tunneling. This is consistent with conductivity being proportional to

temperature.

1.3.6. Stability

There are two distinct types of stability. Extrinsic stability is related to

vulnerability to external environmental agent such as oxygen, water and peroxides. This

is determined by the polymers susceptibility of charged sites to attack by nucleophiles,

electrophiles and free radical. If a conducting polymer is extrinsically unstable then it

must be protected by a stable coating.

Many conducting polymers, however, degrade over time even in dry, oxygen free

environment. This intrinsic instability is thermodynamic in origin. It is likely to be caused

by irreversible chemical reaction between charged sites of polymer and either the dopant

counter ion or the p-system of an adjacent neutral chain, which produces an sp3

carbon,

Introduction.......

18

breaking the conjugation. Intrinsic instability can also come from a thermally driven

mechanism which causes the polymer to lose its dopant. This happens when the charge

sites become unstable due to conformational changes in the polymer backbone. This has

been observed in alkyl substituted Polythiophenes.

1.3.7. Processability

Conjugated polymers may be made by a variety of techniques, including cationic,

anionic, radical chain growth, co-ordination polymerization, step growth polymerization

or electrochemical polymerization. Electrochemical polymerization occurs by suitable

monomers which are electrochemically oxidized to create an active monomeric and

dimeric species which react to form a conjugated polymer backbone. The main problem

with electrically conductive plastics stems from the very property that gives it its

conductivity, namely the conjugated backbone. This causes many such polymers to be

intractable, insoluble films or powders that cannot melt. There are two main strategies to

overcoming these problems. These are, to either modify the polymer so that it may be

more easily processed, or to manufacture the polymer in its desired shape and form.

There are, at this time, four main methods used to achieve these aims.

The first method is to manufacture a malleable polymer that can be easily

converted into a conjugated polymer. This is done when the initial polymer is in the

desired form and then, after conversion, is treated so that it becomes a conductor. The

treatment used is most often thermal treatment. The precursor polymer used is often made

to produce highly aligned polymer chain, which are retained upon conversion. These are

Introduction.......

19

used for highly oriented thin films and fibers. Such films and fibers are highly

anisotropic, with maximum conductivity along the stretch direction.

The second method is the synthesis of copolymers or derivatives of a parent

conjugated polymer with more desirable properties. This method is the more traditional

one for making improvements to a polymer. What is done is to try to modify the structure

of the polymer to increase its processability without compromising its conductivity or its

optical properties. All attempts to do this on polyacetylene have failed as they always

significantly reduced its conductivity. However, such attempts on Polythiophenes and

polypyrroles proved more fruitful. The hydrogen on carbon - 3 on the thiophene or the

pyrrole ring was replaced with an alkyl group with at least four carbon atoms in it. The

resulting polymer, when doped, has a comparable conductivity to its parent polymer

whilst be able to melt and it is soluble. A water soluble version of these polymers has

been produced by placing carboxylic acid group or sulphonic acid group on the alkyl

chains. If sulphonic acid groups are used along with built-in ionizable groups then such

system can maintain charge neutrality in its oxidized state and so they can effectively

dope themselves. Such polymers are referred to as “self doped” polymers. One of the

most highly conductive derivatives of polythiophene is made by replacing the hydrogen

on carbon - 3 with a –CH2-O-CH2CH2-O-CH2CH2-O-CH3. This is soluble and reaches a

conductivity of about 1000 Scm-1

upon doping.

The third method is to grow the polymer into its desired shape and form. An

insulating polymer impregnated with a catalyst is fabricated into its desired form. This is

then exposed to the monomer, usually a gas or a vapour. The monomer then polymerizes

Introduction.......

20

on the surface of the insulating plastic producing a thin film or a fiber. This is then doped

in the usual manner. A variation of this technique is electrochemical polymerization with

the conducting polymer being deposited on an electrode either at the polymerization stage

or before the electrochemical polymerization. This technique may be used for further

processing of the conducting polymer. For instance, by stretching aligned band to

polyacetylene / polybutadiene, the conductivity increases by 10 fold, due to the higher

state of order produced by this deformation.

The final method is the use of Longmuir – Blodgett technique to manipulate the

surface active molecules into highly ordered thin films whose structure and thickness are

controllable at the molecular layer. Amphiphilic molecules with hydrophilic and

hydrophobic groups produces monolayer at the air-water surface interface of Longmuir –

Blodgett films. This is then transferred to a substrate creating a multiple structure

comprised of molecular stacks which are normally about 2.5 mm thick. The main

advantage of this technique is its unique ability to allow control over the molecular

architecture of the conducting films produced. It can be used to create complex multiple

structures of functionally different molecular layers. By producing alternating layers of

conductor and insulator, it is possible to produce highly anisotropic film which is

conducting within the plane of the film, but insulating across it. The stability and

processing attributes of some conducting polymers are given in the following table1.2.

Introduction.......

21

Table1.2 : The stability and processing attributes of some conducting polymers

Polymer Conductivity (Ω-1

cm-1

) Stability

(Doped state)

Processing

Possibilities

Polyacetylene 103

– 105

Poor Limited

Polyphenylene 1000 Poor Limited

PPS 100 Poor Excellent

PPV 1000 Poor Limited

Polyaniline 10 Good Good

Polythiophenes 100 Good Excellent

Polypyrroles 100 Good Good

1.4. Applications of Conducting polymers

The extended π – systems of conjugated polymer are highly susceptible to

chemical or electrochemical oxidation or reduction. These alter the electrical and optical

properties of the polymer, and by controlling this oxidation and reduction, it is possible to

precisely control these properties. Since these reactions are often reversible, it is possible

to systematically control the electrical and optical properties with a great deal of

precision. It is even possible to switch from a conducting state to an insulating state.

Introduction.......

22

There are two main groups of applications for these polymers. The first group

utilizes their conductivity as its main property. The second group utilizes electro activity.

They are shown below.

Group – 1 Group – 2

Electrostatic materials Molecular electronics

Conducting adhesives Electrical displays

Electromagnetic shielding Chemical and biochemical sensors

Printed circuit boards Rechargeable batteries and solid electrolytes

Artificial nerves Drug release systems

Antistatic clothing Optical computers

Thermal sensors Ion exchange membranes

Piezoceramics Electromechanical actuators

Active electronic switches Smart structure

Aircraft structures

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. These materials are used as Electrostatic materials, Conducting

adhesives, Electromagnetic shielding, Printed circuit boards, Artificial nerves, Antistatic

clothing, Piezoceramics, Active electronics (diodes, transistors) and Aircraft structures.

Introduction.......

23

Electrostatic materials: By coating an insulator with a very thin layer of conducting

polymer it is possible to prevent the building of static electricity. This is particularly

important, where such a discharge is undesirable. Such a discharge can be dangerous in

an environment with inflammable gases and liquids and also in the explosives industry.

Conducting adhesives: By placing monomer between two conducting surfaces and

allowing it to polymerize and it is possible to stick them together. This is a conductive

adhesive and is used to stick conducting objects together and allow an electric current to

pass through them.

Electromagnetic shielding: Many electrical devices, particularly computers, generate

electromagnetic radiation, often at radio and microwave frequencies. This can cause

malfunctions in nearby electrical devices. By coating the inside of the plastic casing with

a conductive surface, this radiation can be absorbed.

Printed circuit boards: Many electrical appliances use printed circuit boards. These are

copper coated epoxy-resins. The copper is selectively etched to produce conducting lines

used to connect various devices. These devices are placed in holes cut into the resin. In

order to get a good connection , the holes need to be lined with a conductor. This process

is being replaced by the polymerization of a conducting plastic. If the board is etched

with potassium permanganate solution , a thin layer of manganese dioxide is produced

only on the surface of the resin. This will then initiate polymerization of a suitable

monomer to produce a layer of conducting polymer.

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24

Artificial nerves: Due to the biocompatibility of some conducting polymers they may be

used to transport small electrical signals through the body, i.e. act as artificial nerves.

Aircraft structures: Modern planes and spacecraft are often made with lightweight

composites. This makes them vulnerable to damage from lightning bolts. By coating

aircraft with a conducting polymer the electricity can be directed away from the

vulnerable internals of the aircraft.

Group II: This group utilizes the electro activity character property of the materials. The

materials include 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.

Rechargeable batteries: Batteries were one of the first areas where conducting polymers

promised to have a commercial impact [22]. A number of conducting polymers such as

polyacetylene, polyaniline and other polyheterocycles have been used as electrode

materials for rechargeable batteries. Trivedi et al had studied extensively on rechargeable

batteries using conducting polymers [23].

Sensors: Since electrical conductivity of conducting polymers varies in the presence of

different substances, these are widely used as chemical sensors or as gas sensors. In its

simplest form, a sensor consists of a planar interdigital electrode coated with conducting

polymer thin film. If a particular vapor is absorbed by the film and affects the

conductivity, its presence may be detected as a conductivity change. Interdigited

electrodes covered by a PPy layer have been tested by Miasik et al [24].

Introduction.......

25

Electrochromic devices: The phenomenon of electrochromism can be defined as the

change of the optical properties of a material due to the action of an electric field. The

field reversal allows the return to the original state. Conjugated polymers that can be

repeatedly driven from insulating to conductive state electrochemically with high contrast

in color are promising materials for electrochromic device technology. Conjugated

polymers have an electronic band structure. The energy gap between the valence band

and the conduction band determines the intrinsic optical properties of the polymers. The

color changes elicited by doping are due to the modification of the polymer band

electronic structure. The electrochromic materials have been employed in large area

display panels. In architecture, electrochromic devices are used to control the sun energy

crossing a window. In automotive industry rearview mirrors are a good application for

electrochromic system. With oxidation, polypyrrole turns from yellow to black whereas

polythiophene turns from red to blue [25].

Electromechanical Actuators: Conducting polymers also change volume depending on

their oxidation state. Therefore it is possible for conducting polymers to convert electrical

energy into mechanical work. Conducting polymer actuators were proposed by

Baughmann and coworkers [26]. Oxidation induced strain of polyaniline [38] and

polypyrrole based actuators has been reported [27]. The first self contained actuators

were reported by MacDiarmid et al [28].

Drug release systems: Another application for conducting polymers is controlled release

devices. Ions [29- 30] can be selectively released, as well as biologically active ions such

as adenosine 5-triphosphate (ATP) [31, 32] and Heparin. Ion transport is an interesting

Introduction.......

26

way to deliver ionic drugs to certain biological systems. One can deliver selective ions

depending on the requirement.

Catalyst: Conducting polymers show redox property; therefore these are expected to

behave as redox catalyst. Several reports have been found in the literature on

modification of conducting polymers and their use as catalyst for small organic

molecules. Conducting polymers in their various oxidation states interconvert each other,

which permits to construct redox cycle for catalytic reactions.

Much research will be needed before many of the above application will become

a reality. The stability and processability both need to be substantially improved if they

are to be used in the market place. The cost of such polymers must also be substantially

lowered. However, one must consider that, although conventional polymers were

synthesized and studied in laboratories around the world, they did not become

widespread until years of research and development had been done. In a way, conducting

polymers are at the same stage of development as their insulating brothers were some 50

years ago. Regardless of the practical applications that are eventually developed for them,

they will certainly challenge researchers in the years to come with new and unexpected

phenomenon. Only time will tell, whether the impact of these novel plastics be as large as

their insulating relatives.

1.5. A brief history of conducting polymers

Introduction.......

27

There are multiple reviews in the history of the conducting polymers. In the mid-

19th century, Let herby reported the electrochemical and chemical oxidation products of

aniline in acidic media, noting that the reduced form was colourless but the oxidized

forms were deep blue. In the early 20th century, German chemists named several

compounds "aniline black" and "pyrrole black" and used them industrially. Classically,

such polymer "blacks", their parent compound polyacetylene, and their co-polymers were

called "Melanins" [33]. In the 1950s, researchers reported that polycyclic aromatic

compounds formed, semi-conducting charge-transfer complex salts with halogens. While

these compounds were technically not polymers, this indicated that organic compounds

can carry current. While organic conductors were previously intermittently discussed, the

field was particularly energized by the prediction of superconductivity following the

discovery of BCS theory [34-35].

In 1963, Bolto and co-workers reported conductivity in iodine-doped

polypyrroles. This Australian group eventually claimed to reach resistivities as low as

0.03 ohm-cm with other conductive organic polymers. This resistivity is roughly

equivalent to present-day efforts. The 1964 monograph Organic Semiconductors cites

multiple reports of high-conductivity oxidized polyacetylenes, some with resistivity as

low as .001 ohm-cm [36-38].

Subsequently, De Surville and co-workers reported high conductivity in a polyaniline

[39]. Likewise, in 1980, Diaz and Logan reported films of polyaniline that could serve as

electrodes [40].

Introduction.......

28

Similarly, much early work on the physics and chemistry of conductive polymers was

done under the melanin rubrick. This was because of the medical relevance of this

material. For example, in the 1960s Blois et al. showed semiconduction in melanins, as

well as further defining their physical structures and properties [41]. Nicolaus et al.

further defined the conductive polymer structures [42]. Classically, all polyacetylenes,

polypyrroles and polyanilines are melanins, The simplest melanin can be considered the

acetylene-black from which it is possible to derive all the others. Substitution does not

qualitatively influence the physical properties like conductivity, colour, EPR, which

remain unaltered [43].

In 1974, McGinness and co-workers described an "active" organic-polymer electronic

device, a voltage-controlled bistable switch. This device used DOPA-melanin, a well-

characterized self-doping copolymer of polyaniline, polypyrrole, and polyacetylene. The

"ON" state of this device exhibited low conductivity with switching, with as much as five

orders of magnitude shifts in current. Their material also exhibited classic negative

differential resistance [44].

In 1977, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa reported similar high

conductivity in oxidized iodine-doped polyacetylene. This research earned them the 2000

Nobel prize in Chemistry "For the discovery and development of conductive polymers.

Some reviewers have questioned the Nobel citation's discovery assignment. Thus, Inzelt

notes that, while the Nobelists deserve credit for publicising and popularizing the field,

conductive polymers were ", produced, studied and even applied " [45, 46] well before

their work.

Introduction.......

29

1.6. Polypyrrole

1.6.1. A Brief History

As was first reported by Italian chemists, pyrrole monomer is very readily

polymerized to give a black conducting powder [47]. This chemistry is particularly facile,

taking place with a large number of oxidizing agents, and can even be observed taking

place on the outside of bottles of pyrrole down which the monomer has been allowed to

flow. The resulting conducting powders have been referred to as pyrrole black for many

years. Oxidation of the powders with KMnO4 has been shown to lead predominantly to

the 2, 5-dicarboxylic acid, and this has been interpreted as evidence that the

polymerization leads to an ∝, ∝' – bonded polymer [48]. The polymerization can take

place electrochemically [49] as well as chemically. Chemically, pyrrole can take be

oxidatively polymerized in both solution and vapour phase [50]. Though chemical

oxidation usually leads to powders, films can be obtained by allowing the oxidation to

take place at a solid or liquid surface [51, 52]; however, these chemically prepared films

are of poor quality. In some cases there is no evidence that these chemically prepared

films have much in common with what is now meant by polypyrrole-indeed, they are not

even conducting [53]. Thus, even though a chemical preparation of polypyrrole films

remains a desirable goal, presently chemical synthesis, described in sec 2.2, provides the

only satisfactory route.

1.6.2. Pyrrole – Based Polymers

Introduction.......

30

The attractiveness of the polypyrrole system stems from several factors. Although

initially the most important factor was undoubtedly the chemical and thermal stability of

these polymers relative to (SN)x and (CH)x, the ease of preparation was also appealing.

Key too has the ability to prepare derivatives which had a range of conductivities [54], a

situation which contrasted with the attempts to prepare such derivatives of (SN)x [55].

Further, encouraging features of the pyrrole system were the degrees of freedom

available to modify the electrical and physical properties by restoring to its derivatives [

56], copolymers [57, 58], or particular anions [59] in order to achieve any desired matrix

of polymer properties. All these attributes encouraged us to believe that the effort to

understand these somewhat intractable polymers would be worthwhile. These

experimental efforts along with those of other groups and more recently the attention of

theoreticians [60, 61] has changed our level of appreciation of this material from that of a

conducting curiosity to that of a highly characterized material, exposing new insights into

the whole field of conducting polymers, from electrochemical switching to unifying

theories of conductivity of conjugated polymers involving polarons and bipolarons [62 –

67]. Table 1.3 gives some of the parameters which have been experimentally or

theoretically determined for polypyrrole; included for comparison are the corresponding

values for polyacetylene and Polyphenylene [62 – 68].

Table. 1.3 : Comparison of various parameters for polypyrrole, poly-para-phenylene and

polyacetylene

Polymer Ionization

Potential

Band

gap

Electron

affinity

Oxidation

potential

Reduction

potential

Width of

highest

occupied π

Introduction.......

31

(eV) (eV) (eV) V vs. SSCa V vs. SSC

a band (eV)

Polypyrrole 3.8 3.0

(2.5)b

0.8 -0.6(-0.2)b -3.6 3.8

Poly-para-

phenylene 5.6 3.2 2.3 1.2 -2.1 3.5

Polyacetylene 4.7 1.5 3.2 0.4 -1.1 6.5

aSSC, Standard sodium calomel electrode.

bData in parenthesis are experimental [68 – 70]; other data are theoretical [69].

1.6.3. Introduction to Polypyrrole.

In the last 5 years , a great deal has been reported about conducting polymers and

increasing attention has been paid to those derived from heterocyclic monomers [71– 72].

Although most of the studies have been devoted to polypyrroles, extensive work on

polythiophenes [73], polycarbazoles [74], polyquinolines [75] and polyphthalocyanines

[76] has also been described. Except for the polyquinolines, which are n type and highly

unstable, the other polymers derived from heterocyclics are p type and remarkably stable

in air over extended periods of time. This work will focus on the polymers derived from

pyrrole and will emphasize the progress that has been made to understand and

characterize these complex systems and also the progress which has been made to

improve those physical properties which this author believes would be crucial if large-

scale applications of these polymers were to materialize.

Until recently, polymers and electrical conduction were thought to be mutually

exclusive. However, this view was proved to be incorrect with the synthesis of

Introduction.......

32

conducting polymers in the 1970’s. Unusual properties of these polymers have led to

extensive research resulting in better understanding and numerous commercial

applications. Conductive polymers can be made by filling an insulating polymer matrix

with conducting particles such as carbon black, metal flakes, or metallised fibres, or by

chemical and electrochemical synthesis methods to produce intrinsically conducting

polymers. The conductivity of the former is provided by the filler material, and the

function of the polymer matrix is to hold the material together in one piece. These

conductive composites often replace metals when light weight, toughness, shape ability

and corrosion resistance are required for the application. However, a considerably high

concentration of the conducting filler is required to achieve acceptable levels of electrical

conductivity, thus giving rise to poor mechanical properties in these composites.

Conductivity in these materials is not an intrinsic property of the polymer chains but a

property of the material as a whole.

The term “Intrinsically Conducting” refers to a polymer the conductivity of which

is a property originating from its own electronic structure. A common feature of

intrinsically conducting polymers (ICP) is the alternation of the double and single carbon

bonds along the polymer backbone, referred to as π – bond conjugation. The conductivity

is due to four conditions in their molecular organization: namely, the existence of charge

carriers, an overlap of molecular orbitals to aid carrier mobility, π – bond mobility and

charge hopping between polymer chains [77].

Introduction.......

33

Intrinsically conducting polymers possesses the unique property of wide ranging

modification of their conductivity by the variation of electrolyte dopant anion

concentration during electrochemical polymerization. Undoped conjugated polymers are

insulating. However, conductivity can be increased by incorporating dopant counterions

during polymerization. Small concentration of the dopant anion results in semi-

conducting polymer with significant band gaps, whereas high dopant concentrations give

rise to highly conducting polymers. That is why highly doped conducting polymers are

often referred to as “synthetic metals” [78].

Although unstable, the most conductive polymer is polyacetylene. Conductivities

up to 104 Scm

-1 have been reported by Shirakawa et. al. [79]. Pure polyacetylene is the

most semi-conducting. Conductivity is achieved by chemical doping with an oxidizing

agent such as iodine. The most stable polymers among ICPs are polyheterocycles

(polypyrrole and polythiophene). These polymers consist of five-membered cyclic ring

molecules with nitrogen or sulphur heteroatom. Pyrrole or thiophene monomers are

ideally linked at ∝ - ∝' positions (lowest energy bonding) which provides free π – bond

mobility.

Most ICPs are unprocessible. Therefore, physical properties of the polymer are

determined at the synthesis stage. For example, the electrical, dielectric, microwave and

morphological properties of the polymer can be tailored by adjusting synthesis

parameters such as dopant and monomer concentration, dopant type, synthesis time,

synthesis temperature and electrolyte pH. Ideally, the electrical properties of a metal

Introduction.......

34

would combine with the chemical and mechanical properties of a thermoplastic to

produce a processible, tough and highly conducting polymer. Till recently, most

conducting polymers are unprocessible and possess poor mechanical properties when

compared with conventional materials. However, significant developments have been

made in the synthesis of soluble derivatives of ICPs and in the in-situ synthesis in

conventional thermoplastics [77].

Interest in the development of conducting polymers such as Polyaniline,

polypyrrole, polythiophene, Polyphenylene etc., has increased tremendously during the

last decade because of their electrochromic properties for use in batteries, electronic

devices, functional electrodes, electrochromic devices, optical switching devices, sensors

and so on [80-84]. Conducting polymers can be prepared by chemical or electrochemical

polymerization. In the chemical polymerization process, monomers are oxidized by

oxidizing agents or catalysts to produce conducting polymers [85-86]. The advantage of

chemical synthesis is that it offers mass production at reasonable cost. On the other hand,

the electrochemical method involves the direct formation of conducting polymers with

better control of polymer film thickness and morphology, which makes them suitable for

use in electronic devices.

Polypyrrole is by far the most extensively studied conducting polymer since

monomer pyrrole is easily oxidized, water-soluble, commercially available, and possesses

good environmental stability, good redox properties and high electrical conductivity [87].

The structure of polypyrrole is shown in following Figure. 1.7

Introduction.......

35

N

H

N

H

H H

N

H

N

H

H H

N

H

N

H

H H

N

H

H H

Figure. 1.7 Chemical Structure of polypyrrole

Interest in the development of energy sources for electric vehicles has stimulated

research into electrochemical capacitors, to be used in combination with fuel cells or

indeed internal combustion engines to release and store energy during acceleration and

breaking, i.e. within approximately 10 s. The super-capacitor needs to have a high-power

but not necessarily a high-energy density. The essential requirement is for fast

electrochemistry. This can be achieved either ‘non-faradeically’ by double layer charging

of the large internal surface of activated carbons, or by the ‘double insertion’ (i.e. doping)

of ions and electrons into thin layers of electro active material [88].

This electroactive material could be a conducting polymer such as polypyrrole.

When the polymer is being oxidized anodically, it becomes p-doped and in the case of

polypyrrole, one additional electron can be removed for every third monomer (C4H3N)

unit in the chain. In effect, the seven electrons must be removed for every three monomer

units being deposited. The residual positive charge on the polymer is balanced by the

negative charge on a dopant anion. This essentially faradaic process imparts a large

Introduction.......

36

capacitive response: the electrode potential in polypyrrole increases more or less linearly

with the state or charge as would also be the case for a double layer device [88].

Since the first electrochemical preparation of polypyrrole by Dall’Olie et. al.,

considerable improvement in the mechanical properties has been achieved. These

improvements stem from the work of Diaz and Hall [89], who showed that the

mechanical properties of polypyrrole toluenesulfonate were significantly superior to other

forms of polypyrrole. In many respects, the mechanical properties of these films are

comparable to regular insulating polymers or carbon-loaded polymers of similar

conductivity [89]. The mechanical properties of these polypyrrole tolueneslfonate films

can be improved even further by growing them on vitreous carbon electrodes [90, 91].

These electrodes permit the use of high voltages and high currents, which allows growth

of thick films in relatively short times.

Hotta et. al. [92] have shown that films of polypyrrole hexafluoroarsenate grown

at elevated temperatures from tetrebutylammonium hexafluoroaresenate solutions in

dimethylsulfate, have improved mechanical properties. Recently, Lindsey and Street [93]

have demonstrated that polypyrrole can be deposited within the matrix of several

swellable polymer to form a conducting composite. For instance, polypyrrole sulfate can

be electrochemically deposited from an aqueous electrolyte solution into a

polyvinylalcohol film which has been spun on to a metal electrode and then partially

crosslinked to reduce its solubility while still permitting to swell.

Introduction.......

37

Although the mechanical properties and stability would not seem to present an

obstacle, there are no commercial applications of these polymers at the moment. Serious

attention has been given to their application as the active element in display devices [94]

which would take advantage of the color change that accompanies electrochemical

switching of the polymer between the conducting and insulating states. Further

improvements in contrast ratio, switching speed, and long-term stability are required.

Polypyrrole appears to be seriously considered as a battery material; its lower open

circuit voltage is attractive relative to polyacetylene, but its nonfibrous structure is a

disadvantage. A number of workers pointed out that polypyrrole can serve as a pacifying

layer for semiconductors, particularly n-type semiconductors in photo-electrochemical

solar cells, where a thin layer of the polymer effectively inhibits photo corrosion [95 -

98]. A polypyrrole membrane has been used as an ion gate, which offers lower resistance

to ion transport when the polymer is in the neutral form and a higher resistance when in

the oxidized form [98]. Bull et. al. [99] have shown that conducting pyrrole polymers can

be used as catalysts. They incorporated tetrasulfonated iron phthalocyanine as the anion

of a polypyrrole film and demonstrated that the films, on glassy carbon electrode,

catalyzed the reduction of O2 at potentials 250 to 800 mV less negative that at bare glassy

carbon or glassy carbon electrodes coated with polypyrroles containing non catalytic

anions.

Introduction.......

38

Plastic LEDs Micromotors Optical Storage

Batteries Photocopiers Transducers Lithography

Conductivity Photoconducting Piezoelectric Photochemical Reactions

Coducting Solid State

Composites Sensers

Supercapacitors

Conductive

Surface

EMI/ESD

Membranes

paration of Gases)

Nonlinear Optics Electrochromic Ferromagnetsm

Harmonic Generators Display Devices Magnetic Recording

Introduction.......

39

Figure 1.8: Chart showing the various known and envisaged application of polypyrrole

In view of the progress that has been made in improving the properties of

polypyrrole, it would be surprising if applications for these polymers did not eventually

appear.

1.7. Introduction to Transition Metal Oxides.

Recently effort has been made to understand the electrical conductivity and

dielectric behavior of fly ash [100 – 101] and it was observed that these materials possess

very high relative dielectric constant of the order of 104. Such a high dielectric constant

is one of the important parameter in capacitor fabrication, and microwave absorption

applications. The DC resistivity behavior of fly ash clearly shows a phase transition at

temperature 713 K [102]. This special feature can be utilized for temperature sensor

applications.

Transition metal oxides constitute the most fascinating class of materials,

exhibiting a variety of structures and properties [103]. The metal oxygen bond can vary

anywhere between highly ionic to covalent or metallic. The unusual properties of

transition metal oxides are clearly due to the unique nature of the outer d-electrons. The

phenomenal range of electronic and magnetic properties, exhibited by transition metal

oxides is noteworthy. Thus, the electrical resistivity in oxide materials spans the wide

range of 10-10

to 1020

Ω cm. We have oxides with metallic properties (e.g. RuO2 RuO3) at

Introduction.......

40

on end of the range and oxides with highly insulating behavior (e.g. BaTiO3) at the other

as shown in fig. 1.9. (a) & (b)

Figure 1.9.(a) Rutile structure Figure 1.9.(b) Perovskite structure

(highly metallic) (highly insulating)

There are also oxides that transverse either these regimes with changes in

temperature, pressure or composition (e.g. V2O5, La1-xSrxVO3). Interesting electronic

properties also arise from charge density wave (e.g. K0.3MoO3), charge ordering (e.g.

Fe3O4) and defect ordering (e.g. Ca2Mn2O5, Ca2Fe2O5). Oxides with diverse magnetic

properties anywhere from ferromagnetism (e.g. CrO2, La0.5Sr0.5MnO3) to anti-

ferromagnetism (e.g. NiO, LaCrO3. α-Fe2O3) are known. Many oxides posses switchable

orientation states as in ferroelectric (e.g. BaTiO3, KNbO3) and ferroelastic [e.g. Gd2

(MoO4)3] materials. Then, there is a variety of oxides bronzes showing a gamut of

property [104].

Introduction.......

41

The unusual properties of transition metal oxides that distinguish them form

different phases are due to several factors:

1. Oxides of d-block transition elements have narrow electronic bands, because of the

small overlap between the metal d-orbital and the oxygen p-orbital. The bandwidths

are typically of the order of 1-2 eV (rather the 5-15 eV as in most metals).

2. Electron correlation effects play an important role, as expected because of the

narrow electronic bands. The local electronic structure can be described in terms of

atomic like states [e.g. Cu+ (d

10), Cu

2+ (d

9) and Cu

3+ (d

8) for Cu in CuO] as in the

Heitler-London limit.

3. The polarizability of oxygen is also of importance. The divalent oxide ion O2-

does

not exactly describe the state of oxygen and configurations such as O- have to be

included especially in the solid state which gives rise to polaronic and bipolaronic

effects. Species, such as O- which are oxygen holes with a p

5 configuration instead

of filled p6 configuration of O

2-, can be made mobile and correlated.

4. Many transition metal oxides are not truly three-dimensional but also have low-

dimensional features [105]

Among the transition metals oxides, zinc oxide (ZnO) [106], aluminum oxide

(Al2O3), titanium oxide (TiO2) [107], tin oxide (SnO2) [108], tungsten oxide (WO3),

Vanadium oxide (V2O5), cerium oxide (CeO2), iron oxide (Fe2O3), cobalt oxide (Co3O4)

[109] etc. are mostly widely known oxides and industrially employed transition metal

oxides since last fifty years. The cause of these oxides to have become important both

scientifically and industrially include their applications for sound and picture recording,

data storage, humidity and gas sensors, conducting composite super capacitors,

Introduction.......

42

electrochromic display devices, etc. In the present study the following transition metal

oxides are used.

Zirconium dioxide(ZrO2)

Molybdenum (VI) oxide(MoO3)

Titanium dioxide (TiO2)

1.7.1. Zirconium dioxide (ZrO2)

It is sometimes known as zirconia, is a white crystalline oxide of zirconium. Its

most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral,

baddeleyite. The high temperature cubic crystalline form, called 'cubic zirconia', is rarely,

if ever, found in nature, but is synthesized in various colours for use as a gemstone. The

cubic crystal structured variety cubic zirconia is the best-known diamond simulant.

Zirconium dioxide is one of the most studied ceramic materials. Pure ZrO2 has a

monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at

increasing temperatures. The volume expansion caused by the cubic to tetragonal to

monoclinic transformation induces very large stresses, and will cause pure ZrO2 to crack

upon cooling from high temperatures.

Zirconium dioxide can occur as a white powder which possesses both acidic and

basic properties. Zirconia is also an important dielectric material that is being

investigated for potential applications as an insulator in transistors in future

nanoelectronic devices.

1.7.2. Molybdenum Oxide (MoO3)

Introduction.......

43

It is an yellow solid with the chemical formula MoO3. This compound is produced

on the largest scale of any molybdenum compound. It occurs as the rare mineral

molybdite. Its chief application is as an oxidation catalyst and as a raw material for the

production of molybdenum metal. The oxidation state of Molybdenum in this oxide is +6.

In the gas phase, three oxygen atoms are double bonded to the central molybdenum atom.

In the solid state, anhydrous MoO3 is composed of layers of distorted MoO6 octahedra in

an orthorhombic crystal. The octahedra share edges and form chains which are cross-

linked by oxygen atoms to form layers. The octahedra have one short molydenum-

oxygen bond to a non-bridging oxygen.

1.7.3. Titanium dioxide (TiO2)

Titanium dioxide, also known as titanium (IV) oxide or titania, is the naturally

occurring oxide of titanium, chemical formula TiO2. When used as a pigment, it is called

titanium white.

The naturally occurring oxides can be mined and serve as a source for commercial

titanium. The metal can also be mined from other minerals such as ilmenite orleucoxene

ores, or one of the purest forms, rutile beach sand. Star sapphires and rubies get their

asterism from rutile impurities present in them. Titanium dioxide (B) is found as a

mineral in weathering rims on tektites and perovskite and as lamellae in anatase from

hydrothermal veins and has a relatively low density [110-112].

1.8. Introduction to Microwaves

Introduction.......

44

Microwave is descriptive term used to identify electromagnetic waves in

frequency spectrum ranging approximately from 1 GHZ to 30 GHZ . This corresponds to

wavelengths from 30 cm to 1 cm . In 1888 , Heinrich Hertz was the first to demonstrate

the existence of electromagnetic waves by building an apparatus that produced and

detected microwave has not only been an interesting and challenging academic

endeavour and it has led to several useful applications which are as follows.

1. Telephone networks

2. Broadcast and Television systems

3. In RADAR to detect the aircraft

4. To measure the pollutants in polluted areas

5. Microwave energy can be used for heating

6. Microwave oven for cooking and other useful purposes

7. Microwave dryers used for printing, textiles and other useful purposes

Microwaves exhibit another interesting feature in molecular , atomic and nuclear

systems , which display various resonance phenomena , when placed in periodic

electromagnetic fields . Several of these resonance absorption lie in the microwave

frequency range. The resonance absorption is due to rotational transitions in the

molecules and the absorption spectra provide information on the molecular structure and

intra molecular energies. Thus microwaves become a very powerful experimental tool for

the study of some of the basic properties of the materials . Besides , scientific research ,

absorption of microwaves by molecular resonance is well suited for various industrial

measurements

Introduction.......

45

Microwaves (30 M Hz – 30 G Hz) have become very important for today’s

human life and which are extensively used in today’s civilian and military

communication systems as well as in domestic and industrial appliances. Due to the

extensive expansion of the wireless communication networks the microwave instruments

are become the part of day to day life. Mean while the microwave radiations are

capable of producing harmful effects to the human body organs if exposed for a

Considerable time. These effects include increase in heart beats weakening of immune

systems, rearrangement of proteins including DNA increasing possibility of leukemia,

sterility, cataract cancer etc. Therefore stricter environmental stipulations are nowadays

being enforced [113]. The electromagnetic compatibility (EMC) is an essential

requirement to be fulfilled by the electronic devices/systems for the fast development in

information and communication technology with high packing density of circuits in

electronic devices. Electromagnetic interference (EMI) is a disturbance on a

electronically controlled systems. For medical industry EMI become a matter of crucial

concern. The rapid growths in cellular phones & wireless devices have further added to

problem of EMI. The common problems due to EMI are malfunctioning of devices

formation of false/ghost images inconsistent radar signals etc. therefore operation of

cellular phones & other wireless devices are prohibited in hospitals, bank, ATM,

airplanes, at some specific time & place. Because of these problems microwave absorbers

are gaining immerse importance in controlling the wave pollution [114] – [117] and

ensures the undisturbed functioning of the equipment in presence of internal

electromagnetic waves. Appropriate microwave absorbing materials in appropriate places

Introduction.......

46

in electronic equipment controls the excessive self-emission of electromagnetic waves

and ensures the undisturbed functioning of equipment. Achieving these EM wave

conditions is referred as Electro-magnetic compatibility (EMC). Radar signature

reduction is another area where these microwave absorbers are employed for effective

counter measures against radar surveillance [118], [119]. Microwave absorbers stick on

metallic target, absorb radar microwaves and prevent them from returning back to the

transmitting/receiving antenna at the radar unit. Thus defeating the detection by radar.

Microwave absorbing material has got extensive demands due to these reasons. Many of

the microwave absorbers are being tried and out of them conducting polymers (CP) based

absorbers are getting momentum because of their relatively high absorption combined

with very light weight, more over they are appear to be one of the few materials capable

of dynamic (switch able) microwave absorption. Various investigations are being carried

out in order to improve the absorption characteristics of the CP based microwave

absorbers.

1.9. Literature Review

Ogasawara et. al. [120] has studied the preparation of and the effect of stretching

polypyrrole films were studied in an attempt to enhance the electrical conductivity ,

where the films were prepared by anodic oxidation of pyrrole in propylene carbonate,

containing 1% water and tetraethyl ammonium per chloride as the electrolyte.

Introduction.......

47

Stevan Armes [121] has reported the optimum reaction conditions for the

polymerization of pyrrole by iron (III) chloride in aqueous solution by electrochemical

synthesis.

Hagiwara et. al. [122] compared the structure and properties of polypyrrole

films prepared by potentiostatic method with those prepared by the galvanostatic

method.

Tian et. al. [123] presented a detailed vibrational analysis of the infrared and

Raman spectra of doped and crystalline polypyrrole where the theory of the effective

conjugation coordinate is applied and fully justifies the observed spectra.

Rosa et. al. have reported [124] the electrochemical redox mechanism of a

dodecyl sulfate – doped polypyrrole and studied in detail the electrogravimetry using a

quartz crystal microbalance. In order to distinguish the nature of the inserted / deinserted

species, different salts used as aqueous electrolytes.

Yuri A Dubitsky et. al. [125] has shown that Conducting polypyrrole –

polyvinyl chloride and polypyrrole – cellulose acetate composite films have been

prepared by a simple method of opposite diffusion polymerization.

Hauber. Et. al. [126] has studied the interaction between silver as an electrode

material and the surface of polypyrrole films as a prototype material of stable conducting

Introduction.......

48

polymers in a comparative investigation by electron and electrical impedance

spectroscopy. It is found that silver forms a stable interface on the two dimensional

polypyrrole films.

Butterworth et. al. [127] has described the Synthesis of colloidal polypyrrole –

magnetite – silica nano composites . Firstly, silica-coated magnetite particles were

prepared by the aqueous deposition of silica onto ultrafine magnetite particles via

controlled hydrolysis of sodium silicate. Then pyrrole was chemically polymerized using

oxidants in presence of these silica-coated magnetite particles to yield colloidal

dispersions of polypyrrole – magnetite – silica particles.

The processes of the preparation of highly conducting polymer composites of

polymethyl methacrylate and polypyrrole composites have been studied by

M. Omastova et. al. [128]. The composites were prepared by chemical modification

method resulting in a network – like structure of polypyrrole embedded in the resulting

polymer matrix. Water was used as a solvent. The electrical conductivity of moulded

samples were prepared and characterized.

B. Faye et. al. [129] has synthesized Side-end and side-on liquid crystal pyrrole

monomers and polymers and found that an original way of polymerization allows to

obtain in-situ orientated polymers and also Each polymers receives either a planar or an

homeotropic orientation of its mesogens.

Introduction.......

49

S. Kuwabata et. al. [130] have prepared charge discharge properties of V2O5

polypyrrole composites as positive electrode materials in rechargeable lithium batteries.

Kalaycioglu et. al. [131] reported the synthesis of Poly(2-(N-

pyrrole)ethylvinylether) from poly(2-chloroethyl- vinylether) via phase catalysis reaction

and Graft copolymers of polypyrrole / poly(2-(N-pyrrole)ethylvinylether) by

electrochemical methods. The chemical structure of poly(2-(N-pyrrole)ethylvinylether)

was also investigated by several spectroscopic and thermal methods .

Bhat. et. al. [132] has reported Electrochemical polymerization of pyrrole in a

solution containing dissolved polyvinyl alcohol produces a homogeneous , free

standing, flexible and conductive polymer film .

Nano composites of polypyrrole and iron oxide using simultaneous gelation and

polymerization process by Komilla et. al. [133] where varing the amount of pyrrole

monomer added to a solution containing iron nitrate as precursor and 2-methoxy ethanol

as solvent.

The electro deposition of Polyaniline / polypyrrole composites coating on

aluminum was successfully performed by using cyclic voltammetry by Gouri Smitha et.

al. [134] where Oxalic acid was used as the electrolyte.

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50

Lee et. al. [135] has prepared electrically conducting composites by chemical

oxidative polymerization using polypyrrole and polycarbonate or sulfonated

polycarbonate in chloroform, where the pyrrole was protonated and polymerized using

iron (III) chloride.

Synthesis and characterization of pyrrole based chiral liquid crystals have been

presented by Y Chen et. al. [136]. The thermotropic and electrochemical properties of

and polymerization of a series of N – substituted pyrrole monomers bearing mesogenic 4

– substituted azobenzenes attached as a pendent group via alkyl group, have been

discussed by Y Chen et. al. [137].

A Ohlan et. al [138] reported that polyphenyl amine with barium ferrite

nanoparticles has shown high shielding effectiveness due to absorption SEA of 28.9 dB

99.9 %, which strongly depends on dielectric loss, magnetic permeability, and volume

fraction of barium ferrite nanoparticles. The high value of SEA suggests that these

composites can be used as a promising radar absorbing materials.

Qiao-ling Li, et. al [139] has prepared BaTiO3 powders by sol-gel method by

cotton template. Polypyrrole is prepared by chemical oxidation route in the emulsion

polymerization system. Then BaTiO3-polypyrrole composites with different mixture

ratios are prepared by as-prepared material. The structure, morphology, and properties of

the composites are characterized with Infrared spectrum, X-ray diffraction, scanning

electron microscope, and network analyzer. The complex permittivity and reflection loss

Introduction.......

51

of the composites are measured at different microwave frequencies in S-band and C-band

(0.03–6 GHz) employing vector network analyzer model PNA 3629D vector. The effect

of the mass ratio of BaTiO3 to polypyrrole on the microwave loss properties of the

composites is investigated. A possible microwave absorbing mechanism of BaTiO3-

polypyrrole composite is proposed. The BaTiO3-polypyrrole composite can find

applications in suppression of electromagnetic interference and reduction of radar

signature.

Seyed Hossein Hossein, et.al [140] has synthesized Conductive polypyrrole

(PPy) - manganese ferrite (MnFe2O4) nanocomposites with core-shell structure in situ

polymerization in the presence of dodecyl benzene sulfonic acid (DBSA) as the

surfactant and dopant and iron chloride (FeCl3) as the oxidant. The structure and

magnetic properties of manganese ferrite nanoparticles were measured by using powder

X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. Its

morphology , microstructure, and DC conductivity of the nanocomposite were

characterized by scanning electron microscopy (SEM), Fourier transform infrared

spectroscopy (FTIR), and four-wire technique, respectively. The microwave-absorbing

properties of the nanocomposite powders dispersed in resin acrylic coating with the

coating thickness of 1.5mm were investigated by using vector network analyzers in the

frequency range of 8–12GHz. A minimum reflection loss of −12 dB was observed at

11.3GHz.

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52

Bin Hao et al [141] reported that the electrical and dielectric properties of the

polypyrrole were measured by four-point technique and impedance/materials analyzer.

The results revealed that reaction time, reaction temperature, the types of doping agent,

and the molar ratio of initiator to pyrrole monomer (nAPs/nPy) had an important effect

on electrical and dielectric properties of the PPy. It showed that the PPy doped with

phosphoric acid and with nAPs/nPy = 1 at 10°C for 12 h had better performance of

electrical conductivity and dielectric loss. The PPy sample with 2 mm thickness had a

minimum reflection loss value of −19.68 dB at approximately 16 GHz and an available

bandwidth of 6.2 GHz in the range of 8–18 GHz.

1.9. Aim of the study

There are several reports available in literature related to various studies in

polypyrrole blends. But the reports on polypyrrole composites are scarce. Since to tailor

the various electrical and microwave properties of polypyrrole, synthesis of new

composites of polypyrrole, with better dielectric, ferroelectric and magnetic properties are

the need of the hour. Hence the author has tried to tailor the properties of polypyrrole

composites by the selection of MoO3, ZrO2 and TiO2 in polypyrrole.

Therefore this work is oriented towards the better understanding of basic

electrical and microwave properties in polypyrrole - composites. These parameters

which have been studied here may provide better route for technological applications in

near future.

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