1

11

IInnttrroodduuccttiioonn 1.1. NANOMATERIALS

1.1.1. Introduction

Many properties of solids depend on the size of the test specimen. The bulk properties of

the materials are the average properties of micro- or macro- scale test specimens. Micro-

and macro-scale range particles are ordinarily studied in traditional fields of physics such

as mechanics, electricity, magnetism, optics etc. The size of the objects under study may

range from mm to km and the properties that we associate with these materials are

averaged properties such as density, elastic modulus, resistivity, magnetization and so on.

When the measurements are made on the specimens of nanometer range, many properties

of the material are observed to be different from the bulk properties such as mechanical,

ferroelectric and ferromagnetic properties [1].

1.1.2. Nano Size and its Impact on the Properties of Materials

The nano size ranges usually from 1 to 100 nm (10-9 to 10-7 m), below this there is atomic

scale around 0.1 nm (10-10 m) followed by the nuclear scale (10-15 m) [1-3]. To perceive

and address the concept of size of specimens, Figure.1.1 presents the comparison of sizes

of various objects from living and non living systems [2,3].

The word nano originated from Greek word “nanos” which means “dwarf” [4,5].

“Nanotechnology” deals with the structure, properties and applications of the matter in

the range of 10-9 to 10-7 m. The words “nanoparticles” and “nanotechnology” are

relatively new. However, nanoparticles themselves had been around and studied long

before these words were coined. For example, many of the beautiful colors of the

2

Figure.1.1. Size comparison of various things from living and non living world [2,3].

3

stained glass windows are a result of the presence of small metal oxide clusters in the

glass having a size comparable to the wavelength of light. The particles of different sizes

(nanoscale level) scatter different wavelengths of light imparting different colors to the

same glass. Nanoparticles are generally considered to be the clusters of atoms or

molecules bonded together with a radius <100 nm. A nanometer (nm) is 10-9 m or 10 Å,

so particles having a radius ≤1000 Å can be considered to be nanoparticles [1,6]. Some

typical nanomaterials available in the market or those could be synthesized are given in

Table-1.1.

The definition based on size is not truly satisfactory because it does not really distinguish

between molecules and nanoparticles. In truth, there is no clear distinction between

them. They can be built by assembling individual atom or subdividing bulk materials.

What makes nanoparticles so interesting and endows them with unique properties is that

their size is smaller than the critical length that characterizes many physical phenomena.

Generally, physical properties of materials can be characterized by some critical length or

a scattering length. If the size of the particles is less than this characteristic length, it is

possible that new physics and chemistry may be required to evolve the understanding of

the properties of the same material [1].

Nanoparticles exhibit so called dimensional effects when their structural parameters are

commensurable in at least one direction with the correlative radius of one or another

chemical or physical phenomenon. They are characterized by quantum size effects. The

classical physical laws are replaced by the rules of quantum mechanics. When the size of

a solid or liquid particle diminishes down to 100 nm and lower, quantum mechanical

effects start to become more and more noticeable. The effects are displayed in the

variation of the quantum-crystalline structure of the particles and their properties. Three

reasons at least, responsible for these effects are mentioned here.

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Table-1.1. Typical Nanomaterials available in the market or that can be synthesized [5].

Types Size Range Materials

Nanocrystals and clusters

(quantum dots)

1-10 nm in diameter Metals, semiconductors,

magnetic materials,

Langmuir-Blodgett films

Other nanoparticles 1-100 nm in diameter Ceramic oxides

Nanowires 1-100 nm in diameter Metals, semiconductors,

oxide, sulfides, nitrides

Nanotubes 1-100 nm in diameter Carbon, layered metal,

chalcogenides

2D arrays of nanoparticles A few nm2 to 1 µm2 Metals, semiconductors,

magnetic materials,

polymer films

Surfaces and thin films 1-100 nm thick Various materials

3D structures (superlattices) A few nm in all three

dimensions

Metals, semiconductors,

magnetic materials,

consolidated material,

nanostructured materials

Nanoparticles in polymers 1-100 nm Metal-polymer

nanocomposites

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1. The size of the particles corresponds with one of the several fundamental values

of the substance or characteristic length of some processes in it that evoke

dimensional effects.

2. High specific surface and raised surface energy of nanoparticles at a limited

number of atoms and uncompensated electronic links affects their lattice and

electronic sub systems.

3. The diverse and extreme conditions of their formation (high or low temperatures

and process rates, exposure to powerful radiation sources etc.) transform

nanoparticles into a non-equilibrium state. These factors determine the specifics

of the atomic structure of separate nanoparticles and of the atomic and crystalline

structure of nanomaterials as whole [5].

1.1.3. Zinc Oxide Nanoparticles

Zinc oxide, wide band gap II-VI compound semiconductor, has a stable wurtzite structure

with lattice spacing a=0.325 nm and c=0.521 nm. Zinc oxide is on the borderline

between a semiconductor and an ionic material [7,8].

It has attracted intense research effort for its unique properties and versatile applications

in transparent electronics, ultraviolet (UV) emitters, piezoelectric devices, chemical

sensors and spin electronics [9-18]. Invisible thin film transistors (TFTs) using zinc

oxide as an active channel have achieved much higher field effect mobility than

amorphous silicon TFTs [19-21]. These transistors can be widely used for display

applications.

Zinc oxide has been proposed to be a more promising UV emitting phosphor than gallium

nitride (GaN) because of its higher exciton binding energy (60 meV). This leads to a

reduced UV lasing threshold and yield higher UV emitting efficiency at room

temperature [22]. Piezoelectric zinc oxide thin film has been fabricated into ultrasonic

transducers arrays operating at 100MHz [23]. Bulk and thin film of zinc oxide have

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demonstrated high sensitivity for toxic gases [24-27]. Based on these remarkable

properties and the motivation of device miniaturization, large efforts have been focused

on the synthesis, characterization and device application of zinc oxide nanomaterials.

It is worth noting that as the dimension of the semiconductor materials continuously

shrinks down to nanometer or even smaller scale, some of their physical properties

undergo change known as the “quantum size effects” [28,29]. For example, quantum

confinement increases the band gap energy of quasi-one dimensional (Q1D) zinc oxide,

which has been confirmed by photoluminescence [28], band gap of zinc oxide

nanoparticles also demonstrates such size dependence [29]. X-ray absorption

spectroscopy and scanning photoelectron microscopy reveals the enhancement of surface

state with downsizing of zinc oxide nanorods [30].

1.2. CONDUCTING POLYMERS

1.2.1. Introduction

Polymer is a generic name given to a vast number of materials of high molecular weight.

These materials exist in countless form and numbers because of very large number and

type of atoms present in their molecules. The word ‘polymer’ is combination of two

Greek words ‘poly’ which means ‘many’ and ‘mers’ meaning ‘units/parts’. Thus, a

polymer may be defined as long chain molecule produced by repeated joining of small

chemical units (monomers) by covalent bonds. In some cases, the repetition is linear

while in others may be branched or cross linked [31-33].

Primitive human being exploited naturally available polymers as materials of basic need

such as clothing, shelter, food weapons writing etc. However, modern polymer industry

is believed to come to existence with the discoveries in the modification of some natural

polymers [34].

The word ‘polymeric’ was first used by Jacob Berzelius, a Swedish chemist [35]. The

studies in the organic chemistry dates back to 18th century but the studies of molecular

basis of polymer science are the achievements of 20th century. Herman Stuadinger

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proposed the concept of ‘macromolecules’ during 1920s, he was awarded Nobel Prize in

chemistry in 1953 for his contribution in field of macromolecules [36].

Until four decade ago, it was believed that polymers are electrically insulator due to their

high electrical resistance and was very much exploited as insulator for their property.

The idea that these polymers could be made electrical conductor was very absurd that

time. However, as the time passed and many new requirements came to being, it was

recognized that if the electrical conduction could be added to other properties of

polymers, very useful and advanced materials could be produced. Research to get

electrical conduction was believed to have begun in late 1950s.

However, work of Shirakawa et. al. [37] in 1970s really gave a great enthusiasm in the

search of conducting polymers. In fact, these conducting polymers did not successfully

replace conventional polymer in every sphere of utilization, but novel applications have

been found for them like application is electroluminescent devices, plastic batteries and

various sensors etc.

Polymers are insulators of electricity because they neither have a large number of charge

carriers (free electrons or holes) nor an orbital system to make the charge-carriers mobile

(conjugated backbone of the polymer), the two essential components for polymer to

manifest the charge-conduction process. That is why they have largely been used as

substitute for structural materials such as metals and alloys, wood, ceramics etc. Until

the last few decades, polymers remained unsuccessful in replacing metals and semi-

conductors in electrical and electronic applications due to their insulating properties [38,

39].

The term “conducting polymers” came to existence in past decades, when MacDiarmid,

Shirakawa and Heeger [36] with their group discovered the electrical conduction in

“polyacetylene” during seventies which led to the introduction of a new class in the

organic polymers “electrically conducting polymer”. The sudden impetus to the research

of this field was the discovery of metallic electrical conduction in polyacetylene when

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exposed to iodine. Therefore, polyacetylene became the first conjugated organic polymer

to show metallic electrical conductivity when treated with gaseous halogens during last

seventies [40]. Structures of commonly used conducting polymers are given in

Figure.1.2.

The development of conduction in organic molecules started as early as in 1960 when the

TTF:TCNQ (Tetrathiafulvalence:Tetracyanoquinodimethane) and its derivatives were

prepared inspired by the Little’s [41,42] idea of superconduction in organic polymers.

The discovery of low temperature superconduction in polysulphurnitride [(SN)x] [43] in

1975 triggered active research in the field of conduction in polymers.

Organic polymers are usually insulators and therefore it may be understood that the

conducting polymers must possess some special features. The relationship between the

structure of polymer and ability to allow the movement of electrical charges along the

backbone is the key to unfold the mysteries of this field. The presence of single and

double bonds in conjugation i.e. polymer contains π-conjugated structure, is the intrinsic

feature which provides an orbital system to delocalize the electrons or holes. The

electrons or holes have to be provided extrinsically by a process typically known as

“doping” in a poor analogy to silicon technology [38,45].

Thus, to be classified as a conducting polymer, a polymer must possess the following

essential features [46]:

1. Presence of extended conjugation which provides a great degree of delocalization

of π-electrons in the molecules.

2. As pristine conjugated polymers do not contain intrinsic charge-carriers, charge

carriers must be provided with an extrinsic process, called “doping”.

The doping in conjugated polymers is a charge-transfer reaction resulting into the partial

oxidation (or less frequently reduction) of conjugated polymers. The process involves

9

Figure.1.2. Structure of some conducting polymers.

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the addition of dopant up to 50%. In contrast, the doping involves the substitution of

some of the host atoms within the lattice by dopant atoms at ppm level in inorganic

semiconductors.

In conducting polymer larger volume is occupied by amorphous regions, however, a

certain degree of crystallinity is also present. These regions consist of localized and

delocalized states. The delocalized π-electrons are highly polarizable. The ability of

electronic delocalization of π-electrons of conjugated polymers provides them a highway

for the charge mobility along the polymer chain. The delocalization of π-electrons

depends on the extent of disorder (interchain and intrachain). The disorder induced

delocalization plays a dominant role in metal-insulator transition and transport properties

of polymer. Moreover, the structure of the polyconjugated chain, interchain/interaction

disorder and doping level determine the stability of charge-carriers such as solitons,

polarons and bipolarons and free charge-carriers in conducting polymers. Hence, a wide

range of behavior from metallic to insulating regime could be observed in the transport

properties of such materials [36,46].

The initial attempts on producing organic polymers with π-conjugated structure led to

rigid, infusible, insoluble and intractable powders which were difficult materials to

process and characterize [47].

The two key findings led to a dramatic growth in conducting polymer research. First was

the discovery by Shirakawa et. al. [48] that the free standing semiconducting films of

polyacetylene could be prepared. The second was the increase in the electrical

conductivity of these films to several orders of magnitude on treatment with some

oxidizing/reducing agents. This treatment, causing the transition from semiconductor to

conductor, actually induces charge-carriers by a process called “doping” [39].

In the last two decades, we have observed an explosive growth in the use and dependence

of digital and electronic devices by people. The development of the newer and smarter

materials for modern devices became the need of the time. A new thrust on the

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development of such materials was witnessed world wide including that for conducting

polymers. The most important advantage of conducting polymers is their tailorable

electrical, electronic and magnetic properties from insulator through semiconductor to

conductor while foreseeing high temperature superconduction in polymers in the time to

come. The obvious advantages may include: ease of preparation, low temperature

fabrication, tailorable mechanical properties, good environmental stability, low capital

investment etc. [49].

These are sometimes referred as “Synthetic Metals” and their wide range of electrical,

electronic and magnetic properties suggest their potential applications in many fields

such as solar cells [50], rechargeable solid state batteries [51,52], optical storage devices

[53-54], EMI shielding [55], electrochromic display devices [56-58], light emitting

diodes [46,59], electroluminescence [60,61], Schottky diodes [62,63] etc.

1.2.2. Polyaniline (PANI)

Conducting polymers especially polyaniline has experienced several phases of

developments since the reports of preparation and variable oxidation states in 1896.

However, research into and application of conducting polymers expanded tremendously

after the publication by Shirakawa, MacDiarmid and Heeger [64].

Polyaniline has been extensively studied because of it is easy to prepare by oxidative

polymerization technique [65], its good environmental stability [66] biocompatible

toward certain cells [77] and its electrical property can be modified by p-doping (i.e.

changing oxidation states) or by acid-base chemistry (affecting protonation states).

Several factors contribute to much lower than its expected conductivity. It was predicted

that only 10-3 of available charge carriers in doped PANI actually contribute to its

observed conductivity (≈102 Scm-1), and if all such carriers actually contributed, its room

temperature conductivity would be comparable to that of copper and silver (≈105 Scm-1)

[68]. However, due to coiling and interaction with surrounding solvents [69], self

association and self doping [70], entanglement and polydispersity of polymer chains, the

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closeness of packing of individual molecule in the solid state is very far below optimum

which leads to a mixture of better and less ordered domains. This is compounded by the

fact that, as suggested by MacDiarmid et. al. [71], PANI acts as dynamic block

copolymer, with an internal structuring of domains of different oxidation states. To add

confusion and further complicate the picture for PANI, it was recently shown that PANI

doped with camphorsulphonic acid exhibited conductivity of 103-102 Scm-1, exhibiting

metallic behavior upon changing the temperature [72].

1.2.3. Electrical Conduction in Conducting Polymers

Conjugated polymers can more easily be oxidized or reduced than any conventional

polymer. It is because of the presence of π-conjugated structure and the electrical

conduction is obtained through “doping” leading to the generation of charge-carriers in

the form of free electrons or holes. The charge carriers are usually delocalized over the

conjugated polymer chain [73]. The transport of the charge-carriers along a conjugated

backbone can be described by the Band Model as has been done for metals and

semiconductors. Besides this intrachain conduction incorporating very high intrinsic

conductivity to the doped conjugated polymers, several hopping and tunneling processes

are also in operation for non-intrinsic (interchain and interfiber) conduction processes.

1.2.3.1. Band theory

The free electron model is very useful for explaining the electrical conduction in simple

metals, for example alkali metals. It proposes that the electrical conduction in such

metals is due to the presence of electrons in the valence shell which are free to move

throughout the volume of the metal. However, a more suitable model was needed to

explain the electrical properties of different types of insulators, semiconductors and

conductors which led to the development of Band Theory. This theory is an expansion

of the free electron model [74,75].

The electrons are supposed to be nearly free according to the band theory, however, they

weakly interact with the crystalline lattice composed of ions. This results in to well

defined energy bands in which there is continuum of allowed states. These energy bands

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are separated by an energy gap that has no allowed states [76]. The low energy band is

called as valence band (or bonding level) and higher energy band is called as conduction

band (or antibonding level) while the energy gap that is separating highest occupied

molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is referred

as “Band Gap” [77] which represents forbidden zone for electrons as shown in

Figure.1.3.

In other words, Band Theory explains the differences in the characteristic features of

conductors, semiconductor and insulators. As the molecular orbitals begin to hybridize, a

valence band where the electrons are tightly held by atoms to which they correspond is

formed. A conduction band where the electrons are free to move throughout the material

is also formed. There is an energy gap (forbidden zone) between the top of the valence

band and bottom of the conduction band called Band Gap (Eg). The magnitude and

shape of the energy gap separating the top of the filled (or valence) band and the bottom

of the empty (or conduction) band are the defining characteristics of metals,

semiconductors and insulators [78,79].

In electrical conductors, the valance band and conduction bands overlap and therefore,

there is no energy gap between the two. Another possibility is that the valence band is

partially filled with electrons while the conduction is empty. The electron of the valance

band can move freely throughout the volume of material. This is the characteristic of all

metals and often referred as metal-like conduction. Insulators have large band gap (more

than 1.5 eV) between the valance band and conduction band which forbids electron

transport from valence band to conduction band. Therefore, there are no electrons in the

conduction band of insulators necessary for conduction. Low energy of band gap (less

than 1.5 eV) is the characteristic feature of the semiconductors which could easily be

overcome by photon absorption or temperature elevation [79].

1.2.3.2. Hopping and tunneling

The precise mechanism of charge transport in conducting polymers is still not very well

understood. Conjugated polymers possess a complex morphological distribution of

14

Figure.1.3. Band structure of metals, semiconductors and insulators.

15

crystalline and amorphous regions. Therefore, this complex situation poses a great

hindrance to trace the movement of charge-carriers in various domains [80]. Solitons and

bipolarons constitute the majority of charge-carriers. There is an intermediate range of

electronic energy state in which mobility is very low. Conduction is possible only if

electrons are excited to higher state with greater mobility. Conduction via localized

electrons implies direct jumps across or tunnel through an energy barrier from one site to

the next. Several mechanisms for conduction and charge transport have been reported,

however, the relative importance of these mechanisms depends upon the shape of the

barrier and availability of the thermal energy [81].

The movement of charge along a specific chain has to be considered as well as the

movement from one to another polymer chain and from crystalline to amorphous domain

or vice versa. Inter-soliton hopping mechanism is proposed technique to explain such

conduction. The solitons move around by exchanging electrons with the nearby charged

solitons [82].

In hopping and tunneling mechanism, the charge carriers hop/tunnel from one localized

state to another within energy band gap [83-85]. The energy for hopping to take place is

provided by the phonon at non-zero temperature. Electrons hop from one energy state to

next by absorbing phonons. At zero temperature, the conductivity is zero in those

conducting polymer where the hopping mechanism is dominant. As the temperature

increases, more and more electrons absorb phonons and start hopping [86].

1.2.3.3. Percolation theory

Hammershley and Broadbent [87] in 1957 gave the idea of percolation theory. It was

introduced to show how the random properties of the medium influence the spread of

“fluid” through it.

The landmark studies by Gurland in 1966 [88] and Malliaris and Turner in 1971 [89] on

the metal-polymer systems showed that there must exist a relationship between the

degree of contiguity or connectedness and electrical conduction. For conduction in the

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filled polymer systems, conductive pathways of conducting filler particles must be

formed throughout the matrix that allows the electrons to freely move from one side of

the material to another [90].

The electrical conductivity of a composite material is thus characterized by its

dependence on conducting filler volume fraction. As the amount of the conducting filler

is increased, its particles begin to come in contact with each other. At a certain volume

fraction (Vc) of the conductive filler “percolation threshold”, continuous paths are

formed throughout the volume of the composite for the electrons to travel from one part

to other. There are three approaches to describe conduction by this theory viz. statistical,

thermodynamic and structure oriented percolation [91].

Percolation statistics quantitatively relates the volume fraction of the conductive filler to

the electrical conductivity of the composite [88,92]. In this approach, three distinct

regions are defined as shown in Figure.1.4. Region “A”, where the volume fraction of

conductive filler is less than Vc and average number of contacts per particle (say m) is

less than one. The conductivity is almost constant with increasing filler contact in this

region. The region “B” is called as the critical region starting at Vc from where the

conductivity increases rapidly with increasing volume fraction of conductive filler. At

Vc, m is equal to one and, therefore, the probability of forming an infinite chain becomes

non-zero. According to the statistical percolation model, the conductivity (σ) in the

critical region is given by a power law relationship as shown in Equation-1.1 below [93]:

σ = σo (V - Vc)t (1.1 )

where σ is the electrical conductivity of composite at volume fraction V, σo is the

electrical conductivity of pure conductive filler, V is the volume fraction of the

conductive filler, Vc is the critical volume fraction of the conductive filler and t is the

empirically determined power law index (which typically ranges from 1.5 to 1.6).

Finally, the critical region (region B) ends with the onset of region C when m becomes

equal to 2, conductivity levels off and becomes approximately constant once again.

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Figure.1.4. Statistical percolation model for conducting composites.

18

Mamunya et. al. [94] recognizing the effects of both the conductive filler and polymer

matrix on conductivity proposed to include for the interaction between the filler particles

and matrix in the statistical percolation theory. Though the statistical percolation does

give the correct shape of conductivity curve, it fails to explain the difference in

conductivity for different polymer-filler systems at the same volume fraction. This led to

thermodynamic percolation model based on overall interfacial energy, the surface

tensions and the interaction between filler and polymer. This model assumes the kinetics

of the adsorption of the polymer melt on the particulate surface is equal to the adsorption

process in low viscosity fluids. The filler particles are unevenly distributed in flat

agglomerates if they are below the critical volume and with the increase in filler

concentration, the filler particles agglomerate together. The interfacial energy of the

particles will force the filler to form a three dimensional network as the phases come to

thermodynamic equilibrium.

Lux [95] has reported that no doubt this model provides a stronger theoretical basis for

conductivity than statistical percolation model but its predictions do not correspond to

experimental data.

Theories and models based on statistical and thermodynamic percolation do not fully

explain conduction in polymer composite while describing the shape of the conductivity

curve. A structure oriented percolation model using an effective medium approach was

proposed by Yoshida [96] for ceramic-metal conductive composite. This model has been

successful in predicting the behavior of the randomly mixed composites where the micro-

structure is known. Another type of structure oriented model used a fractal approach.

Zhang et. al. [97] used this approach to describe conductivity of composites. The filler

particles are assumed to form aggregates inside the polymer matrix which are of fractal

dimension. The fractal dimension reflects the connectedness of the filler and is directly

correlated to the probability of forming a continuous conductive network.

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1.2.4. Doping of Conducting Polymers

The conductivity of conjugated polymers can be increased by many orders of magnitude

by “doping”. Doping involves the removal or addition of electrons from the backbone of

conjugated polymers i.e. oxidation or reduction of the backbone. Oxidative doping

occurs when electrons are transferred from conjugated polymer backbone to dopant. It

may also be termed as p-type doping. The electrons are transferred to conjugated

polymer backbone from dopant in case of reductive doping. It is also known as n-type

doping [76].

Thus the conducting polymer backbone stores +ve charge in case of oxidative and –ve

charge in case of reductive doping. In both the cases, two scenarios may arise. The

polymer may either lose electrons from its valence band leading to the formation of holes

in the valence band or gain electrons in the conduction band. The charge whether +ve or

–ve then delocalize over a section of polymer chain. This delocalization of charge

decreases the ionization energy and increases electron affinity [73].

The doping mechanism in conjugated polymers is rather similar to the intercalation

process in two dimensional layered structures such as graphite. The dopant ions (p-type:

BF4-, ClO4

-, I3-, etc. and n-type: Na+, Li+, Rb+ etc.) can either oxidize the polymer to

create the positive charges on the conjugated polymer backbone (p-type doping) or

reduce the polymer to create negative charges on the conjugated polymer backbone

depending on the redox process involved [46].

Doping of conjugated polymers and composites involve random dispersion of dopant in

molar concentrations in disordered structures of entangles chain and fibrils. When a

conjugated polymer is doped, charge carriers such as solitons, polarons, bipolarons and

free carriers are generated. The carrier concentration, to a great extent, depends on the

doping level, structure of the conducting polymer chain, inter-chain interactions,

disorders etc. The reaction between oxidant (p-type doping by an acceptor) and reductant

(n-type doping by a donor) with conjugated polymer have been observed to cause a

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dramatic increase in electrical conductivity. A general equation for p-type doping of a

conjugated polymer by FeCl3 may be given by the following chemical reactions:

(π-Polymer) + 2 FeCl3 (π-Polymer)+ + FeCl-4 + FeCl2 (1.2)

(π-Polymer)+ + 2 FeCl3 (π-Polymer)2+ + FeCl-4 + FeCl2 (1.3)

The maximum level of doping in conjugated polymers can be as high as 50% which

corresponds to one dopant per two monomer residues. The distribution of dopant may

not be uniform due to the complex morphology of the polymer matrix which consists of

both crystalline and amorphous regions. Hence, both the structural and the doping

induced disorders play major role in the charge transport in these systems [46,98].

1.2.5. Techniques of Doping of Conjugated Polymers

The conductivity of the conducting polymers can be increased by “doping”. Doping is

simply addition/removal of electrons. Oxidative doping occurs if electrons are

transferred from conducting polymer backbone to dopant, it is also called as p-doping.

The conducting polymers store charge due to oxidative or reductive doping. In both the

cases two scenarios may be possible. The polymer may either lose an electron from its

band or localize charge decrease and electron affinity.

There are two main types of doping processes that converts a conducting polymer from

its insulating or semi-conducting state to conducting form. The first one is chemical or

electrochemical reductive/oxidative, redox doping and the second is protonic acid doping.

The polymers which contain π-conjugated backbone can be oxidized or reduced with

chemical/electrochemical processes which changes the number of electrons associated

with the polymer backbone. This change in the number of electrons during redox doping

produces a net charge on polymer backbone. This net charge is then stabilized by

presence of “dopant counterions” in vicinity of the charged backbone. This may lead to a

large increase in the observed conductivity of the polymers [76].

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1.2.5.1. Chemical doping by charge-transfer

The initial discovery of the ability to dope conjugated polymers involved charge-

transfer/redox chemistry i.e. oxidation (p-type doping) or reduction (n-type doping). This

can be illustrated with the following examples [99].

p-type doping

(π-Polymer) + 3y/2 I2 (π-Polymer)+y:y I-3 (1.4)

n-type doping

(π-Polymer) + y Na+Naphthalide- (π-Polymer)+y:y Na+ + y Naphthalene (1.5)

1.2.5.2. Electrochemical doping

Complete doping to the highest possible dopant concentration yields reasonably highly

conducting materials. However, attempts to obtain intermediate doping levels often

result into inhomogeneous doping. Electrochemical doping was invented to overcome

this problem where the electrodes supply the redox charges to conducting polymer while

ions diffuse into the polymer electrodes from the nearby electrolyte for electro-neutrality.

The doping level is determined by the voltage between the conducting polymer electrode

and counter-electrode at electrochemical equilibrium or by the amount of electronic

charge passed during the process. A particular doping level is precisely achieved by

setting the electrochemical cell at the corresponding applied voltage and waiting as long

as necessary for the system to come to an electrochemical equilibrium as indicated by the

current through the cell going to zero. Electrochemical doping can be illustrated by the

following examples [100]:

p-type doping:

(π-Polymer) + [x Li+BF4-]electrolyte [(π-Polymer)x+:(x BF4

-)] + x Li+ (1.6)

n-type doping:

(π-Polymer) + [x Li+BF4-]electrolyte [(π-Polymer)x-:(x Li+)] + x BF4

- (1.7)

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This technique is used for doping of polymers obtained by electrochemical or other

methods as well as for redoping or further doping. In this process, only ionic dopants are

used as electrolytes dissolved in polar solvents [36,76].

1.2.5.3. Self doping

Self doping does not require any external doping agent. The ionizable groups attached to

the polymer chain act as dopant for the polymers [81].

1.2.5.4. Radiation doping

The semi-conducting polymer chain is locally oxidized and nearby chain is reduced by

photo-absorption and charge separation i.e. electron-hole pair creation and separation into

free charge-carriers:

π-Polymer (π-Polymer)+x + (π-Polymer)-x (1.8)

where x is the number of electron-hole pairs. In case of the photo-excitation, the

photoconductivity is transient and lasts only until excitation are either trapped or decay

back to ground state. In contrast, the induced electrical conductivity is permanent in case

of chemical or electrochemical doping until the charge carriers are purposely

removed/destroyed by undoping. High energy radiations such as γ-rays, electron beam

and neutron radiation are used for doping of polymer by neutral dopants. For example,

γ -rays irradiation in the presence of SF6 gas or neutron radiation in presence of I2 has

successfully been used to dope polythiophene [46].

1.2.5.5. Doping by acid-base chemistry

Polymers containing π-conjugated system may also be doped by non-redox means in

certain situations. In these cases, the number of electrons that are associated with the

conducting polymer backbone does not change during doping process. However, the

energy levels are rearranged during doping. One way in which to dope without involving

redox chemistry is to use protonic acids. Protonic acid doping has been observed in

polyacetylene in which the conductivity increased by some four orders of magnitude

[46].

23

In real sense, polyaniline is the first example of the doping of an organic polymer to

highly conducting regime by this process. Protonic acid doping is extensively used when

converting polyaniline from the non-conducting to conducting form [101].

Protonation by acid-base chemistry leads to an internal redox reaction and converts the

non-conducting emeraldine base into highly conducting emeraldine salt. The chemical

structure of the emeraldine base is somewhat similar to an alternating block copolymer.

Upon protonation of the emeraldine base leading to the formation of emeraldine salt, the

proton induced spin unpairing mechanism leads to a structural change with one unpaired

spin per repeat unit without any change in the number of the electrons. This remarkable

conversion of polyaniline from non-conductor to conductor causes a 9-10 orders of

magnitude increase in the conductivity. This has very well been described in the

literature but it is not well understood from the point of basic theory [102].

1.2.6. Charge-Carriers in Conducting Polymers

When conducting polymers are doped charge carriers are injected onto the polymer

backbone. For example, p-doping removes electrons from top of the valence band

creating holes, while n-type doping add electron to the bottom of the conduction band. It

is also possible that charge carries may become delocalized over the polymer backbone

creating soliton, polarons and bipolarons [103]. When trans-polyacetylene is doped, the

phasing of the double bond may be reversed on either side of the dopant site as seen in

Figure.1.5.

Since the energy of the system is same regardless of the phasing, trans-polyacetylene has

double degenerate ground state, however, the charge may not be simply localized over (-

CH-) group on the chain but are delocalized over several repeat units with the magnitude

of the charge diminishes as the distance from the counterion increases. When a double

degenerate conducting polymer has a charge delocalized over several repeat units, a

“soliton” is formed as in Figure.1.6.

24

Figure.1.5. Phase reversal of double bonds that may occur when trans- polyacetylene is doped at dopant site, D.

Figure.1.6. Formation of solitons in polytacetylene.

25

There are two other types of charge-carriers also known to exist. They occur in polymers

that do not have a doubly degenerate ground state. These are called as “polarons” and

“bipolarons”. A polaron contains radical ion pair while a bipolaron consists of dication

or dianions depending on the type of doping. A polymer has polaron at one doping level

and bipolaron at another doping level. Alternatively, a polymer may contain significant

amount of both polaron and bipolaron in equilibrium with each other as can be seen in

Figure.1.7.

Solitons are elementary excitations in polyacetylene chain. The discontinuities occur

when a carbon atom shares a single bond with the neighboring carbon atom [104,105].

The formation of discontinuities causes the formation of additional electronic states in

band gap. The formation of solitons does not lead to conduction in polymers. The

solitons does not lead to conduction in polymer. The solitons hop from one site to the

other along the length of a single polymer chain (intra-chain hopping). When a vast

number of solitons occur, there is soliton-hopping across the polymer chains (inter-chain

hopping). This hopping conduction varies with a higher power on temperature [106].

For lightly and moderately doped samples, variable range hopping mechanism

contributes to conduction [107].

Polarons are formed by lattice deformations which cause the appearance of additional

electronic states in the band gap. Polarons have both charge and spin and their motion

contributes to charge transport along the polymer chains. When the polarons of opposite

spin combine, they form bipolarons with no spins. Bipolarons may also contribute to

charge transport and conduction [108].

For conducting polymers which have a degenerate ground state, the conduction

mechanism involves formation of solitons (Figure.1.6). In this case, the charge and the

free radical are not bound with each other but they are separate on the conjugated

polymer chain. This separation results into the formation of independent domain. These

domains may enclose two phases with opposite orientations and similar energy. Hence,

the solitons can also be neutral sometimes. Soliton formation creates new energy levels

26

Figure.1.7. Schematic representation of formation of polarons and bipolarons with increase in dopant concentration (X) and its impact on electrical conductivity and spin concentration of the conducting polymers.

27

in the middle of energy gap. At higher doping levels, solitons interact with each other to

form band which may overlap with the valence band as well as conduction band to

increase conductivity [109].

1.2.7. Dedoping or Undoping or Electrical Neutralization or Electrical

Compensation in Conducting Polymers

Dedoping or Undoping is an important aspect and can be well understood as reverse of

doping or simply electrical neutralization of doped polymer. The dedoping agents diffuse

into the polymer matrix and neutralize the charge of the system by charge-transfer

reaction. The process may involve reaction between the dedoping agent and carbonium

ion/carbanion and/or dopant leading to the neutralization by the charge-transfer complex.

The commonly used dedoping agents for p-type doped conjugated polymers include

ammonia, water; hydrazine etc. and the chemical reactions for the process may be given

by the following equations.

8 NH3 6 NH4+ + 6 e- + N2 (1.9)

6 H2O 4H3O+ + 4 e- + O2 (1.10)

(π-Polymer)x+ x e- (π-Polymer) (1.11)

Dedoping may also be affected by thermal treatment as observed in case of

polythiophene. Kinetics of dedoping may be studied by several techniques which could

be the change in electrical conductivity, X-ray diffraction pattern, optical spectrum etc.

which may be interpreted into the depletion of the extent of doping using certain

assumptions [110].

1.3. POLYMER NANOCOMPOSITES

The word composites is used in the technical sense to describe a product that arise from

the incorporation of some basic structural material in the form of particles, whiskers,

28

fibers etc. into another substance called as matrix. The main characteristics sought in an

additive that in turn gets conferred on the composites are elastic rigidity, tensile strength,

appropriate electrical and magnetic properties. Thus, a polymer matrix composite may be

defined as combination of one or more materials with a polymer matrix to produce a

material with desired properties from individual components [111].

1.3.1. Introduction

Polymer-inorganic particle composites evolved as the structural materials date back to

1960s [112], usually reinforced by micrometer-scale fillers particles into polymer

matrices. They have found large-scale applications for decades in automobile,

construction, electronics and several other consumer products. Composites gained

enhanced properties due to incorporation of micro-meter scale fillers such as higher

strength and stiffness compared with pure polymer [113].

However, the improvement in a particular property achieved by these traditional

composite formulations involved compromises i.e. improvement achieved on the cost

some other desirable property. For example, stiffness is obtained on the cost of

toughness as well as the optical clarity. Recently, nanoscale filled polymer composites

known as “nanocomposites” gave a new way to overcome the limitations of traditional

counterparts. An idea suggested by current micromechanics theories [114] is that the

effective properties of polymer microscale filled composites rely on constituents, volume

fraction of components, shape and arrangement of fillers and the characteristics of the

polymer/filler interface.

Based on this idea, properties of polymer composites are thus independent of the size of

fillers. Obviously, this may not be correct for polymer composites containing nano size

fillers. The most prominent difference of such nanocomposites compared with their

traditional counterparts is the small size of the filler particles which could bring added

specific effects [115]

29

The transition in the studies on the composites with new perceptions from microscale to

nanoscale fillers experienced a serious start in 1980s by Toyota engineers in the quest to

develop new and advanced materials. They modified silica clay (montmorillonite) via an

ion exchange process followed by in-situ polymerization of ω-caprolactam within the

clay matrix. Ring opening polymerization resulted into nylon-6 reinforced well-

exfoliated clay plates creating a “polymer-inorganic particles nanocomposite” [112].

These montmorillonite clay nanocomposites showed a great enhancement in their

mechanical and thermal properties at a very low clay loading level of ~4% which is much

lower than that of their micro/macro counterparts [116].

1.3.2. Classification of Nanocomposites

Nanocomposites are the polymer matrices containing nanoparticles and/or clusters of

nanoparticles randomly distributed within the polymer matrices. Thus the nanoparticles

in such composite serve as the dispersed phase while the polymer matrix serves as

dispersion medium. Nanoparticles may be perceived as the particles microencapsulated

in the polymer shell. Recently, composites whose nanoparticles are localized only on the

surface of the particles, fibers, films of the polymer have been reported. The

nanocomposites containing nanoparticles having 3D, 2D and 1D structures have also

been used.

The nanocomposites may be classified into the following four types depending upon the

nature of dispersed phase and dispersion medium [117]

Inorganic-organic nanocomposites: for example, metal or semiconductor

nanoclusters/particles dispersed in a polymer matrix (such as poly methyl

methacrylate and block copolymers).

Organic-inorganic nanocomposites: for example, nanoparticles of organic dyes or

biopolymers dispersed in an inorganic matrix (such as silica, titania or alumina).

Inorganic-inorganic nanocomposites: for example, gold nanoparticles dispersed in

the matrix of silica.

30

Organic-organic nanocomposites: for example, nanoparticles of organic dyes

dispersed in the matrix of polymethylmethacrylate.

The silicate/clay polymer nanocomposites may be classified on the basis of their structure

as follows [118] and may be represented as in Figure.1.8.

Phase-separated composites: If the polymer is unable to intercalate between the

inorganic layers such as silicate sheets, the obtained nanocomposite may be

considered as phase-separated composite. Their properties stay in the same range as

those of traditional microcomposites.

Intercalated nanocomposites: When a single or more than one extended polymer

chains are inserted in a regular fashion between the silicate layers as a result of which

a well-ordered multilayer morphology is built up with alternating polymeric and

inorganic layers. Thus, obtained composites may be called as intercalated

nanocomposites.

Flocculated nanocomposites: Conceptually, this type is the same as the intercalated

nanocomposites. Sometimes hydroxylated edge of the silicate layers interact

resulting flocculation and therefore such composites are termed as flocculated

nanocomposites.

Exfoliated nanocomposites: In these nanocomposites, silicate layers are uniformly

and completely dispersed individually as nanoscale platelets in the polymeric matrix

[119]. This gives rise to the exfoliated/delaminated structure and therefore named so.

Normally, the inorganic clay content of an exfoliated nanocomposite is lower than

that of an intercalated nanocomposite.

1.3.3. Properties of Nanocomposites

The improvement in desirable properties and the emergence of newer properties in the

materials using nanoscale reinforcement motivated the explosive study of the properties

of nanocomposites and their applications as follows:

31

Figure.1.8. Structures of nanocomposites formed by dispersion of clay in polymer matrices.

32

The primary reason for adding inorganic fillers particles to polymer matrices is to

improve their mechanical performance. For example, the addition of high-modulus

fillers increases the modulus and the strength of a polymer. In traditional composites,

unfortunately, this often comes at the cost of a substantial reduction in ductility, and

sometimes in impact strength, because of stress concentrations caused by the fillers.

Well-dispersed nanoparticles, on the other hand, can improve the modulus and strength

and maintain or even improve ductility because their small size does not create large

stress concentrations [120].

Numerical simulations predict tensile moduli on the order of 1 TPa for carbon nanotubes,

making them perhaps the ultimate high stiffness filler material. Numerical simulations

and experimental findings also suggest large elastic (recoverable) strains for nanotubes.

Other forms of nanoreinforcements such as nanoclays and graphite nanoplatelets also

have high modulus values for nanocomposites i.e. high stiffness enhancement.

Strength- and stiffness-to-weight ratios: Given the exceptional mechanical properties

and low densities associated with the typical nanoreinforcements, nanocomposites

with high volume fractions of nanoinclusions may result into the strength and

stiffness to weight ratios, unachievable with traditional composite materials. This

offers a substantial weight savings for weight-critical applications. Hybrid multiscale

composites where nanoreinforcement is added to the matrix material in traditional

micro-sized fibrous composites have also been proposed.

Multifunctionality: In addition to their outstanding mechanical properties, nanotubes

and other forms of nano-reinforcement have also been shown to have exceptional

electrical and heat-related properties. This suggests that the materials may be

designed to meet mechanical requirements besides some other secondary material

properties. For example, low volume (weight) fractions of nano-reinforcement have

been used to enhance the electrical conductivity, to increase the working temperature

and to improve the barrier and diffusion properties of polymers (primarily for

platelet-shaped reinforcement) besides improving the mechanical properties.

33

Electrical and optical properties: The electrical and optical properties of polymer

nanocomposites are exciting areas of research. This becomes particularly important

because of the possibility of creating composites of unique combinations of

functionalities such as electrically conducting composites with good wear resistance

that are optically clear as well. Such properties can result because the nanoparticles

display their solid-state physical properties when embedded in transparent matrices.

Optical composites have been defined as composites consisting of optically active

nanoparticles embedded in a transparent host material, often a polymer [121].

Optical composites take advantage of the optical properties of materials that are

difficult to grow in the form of single crystals or that require protection from the

environment and give them the ease of processing afforded by many polymers. In

addition, nanoscale particles are required to achieve specific optical properties in

materials for some specific applications while the role of polymer matrix is just to

hold the particles together and to provide processibility. For example, high-grade

optical composites of properties otherwise obtainable only in optical glasses became

accessible through the use of polymer molding techniques [120].

The initial results on nanoparticle reinforcement of polymers were inconsistent with

respect to the changes in fundamental thermal and mechanical properties of the resultant

composites. It quickly became apparent that a myriad of parameters were influencing

overall nanocomposite performance such as dispersion and distribution of nanoparticles,

load transfer from nanoparticles to matrix, geometric arrangement of nanoparticles,

nanoscale mechanical response of the nanoparticles, interphase polymer formation and

properties as well as the chemical modification of the nanoparticles etc. Initially, the

most significant factor was nanoparticle dispersion where reducing nanoparticle

clustering led to the better dispersions and increased the surface area of contact between

polymer and nanoparticles. With improved processing methods leading to better

dispersed systems, the increase in moduli of nanocomposites is now more consistent;

however, the improvements in the strength and toughness remain more or less elusive.

34

To address the novel properties of polymer nanocomposites, the development and use of

synthetic methods having control over the particle size distribution, dispersion and

interfacial interactions are critical. Synthetic techniques for nanocomposites are quite

different from those for conventional microscale-filled composites and creating one

universal technique for developing polymer nanocomposites is impossible due to the

physiochemical differences between each system. Each polymer system may require a

special set of processing conditions to be formed and different synthetic techniques in

general could yield nonequivalent results [122].

Despite numerous challenges, considerable research has been done to develop

appropriate synthetic techniques for making good polymer nanocomposites in the

literature.

1.3.4. Preparation of Nanocomposites

There are several ways of dispersing nanoparticles in polymer matrix. Few of them are

discussed below [120]:

1.3.4.1. Direct/melt mixing

Direct mixing takes advantage of well established polymer processing techniques. This

method involves mechanically blending organically modified clays/nanoparticles with

polymer matrix. The nanocomposite is formed by the addition of swollen and pretreated

layered silicate to the polymer melt [123]. For example, polypropylene and nanoscale

silica have been mixed successfully in a two-roll mill, but samples with more than 20 wt.

% of filler could not be drawn. Melt mixing is the fastest method for introducing new

nanocomposites to the world market since it can take full advantage of well-built polymer

processing equipments including extruders or injectors. Figure.1.9 (A) presents a simple

flow diagram for the melt mixing process.

1.3.4.2. Solution mixing

This is a very simple method that involves the dispersion of nanoparticles in a solution of

polymer followed by drying of the solvent. This method is relatively easy and

35

commercially available nanoparticles can be used for preparation of nanocomposites by

this method. Figure.1.9 (B) gives an idea of producing nanocomposites by solution

mixing process.

However, this method requires extensive mixing to disperse the nanoparticles

homogeneously in the polymer matrix. Special care has to be taken to avoid flocculation

or agglomeration that may sometimes occur resulting into inhomogeneous distribution of

nanoparticles within the polymer matrix. A better dispersibility could be achieved by

pretreating the surface of nanoparticles to make it compatible with the polymer matrix.

This treatment induces ease in movement of the nanoparticles within the polymer matrix

[124]. If the homogeneity of the nanoparticles dispersion is not a critical requirement,

this method of preparation is very promising. However, the method is not so promising if

the homogeneity of the nanocomposite is a critical requirement. The problem with such

inhomogeneous nanocomposites in optical applications may include the disadvantageous

scattering leading to reduced performance of the material [120]

1.3.4.3. In-situ polymerization

In-situ polymerization method is very useful method for the preparation of high

performance polymer nanocomposites containing inorganic nanoparticles. In this

method, nanocomposite is synthesized by introducing the monomer, into the organically

modified clay and then polymerizing it in-situ [125] or nanoparticles are dispersed in the

monomer or monomer solution and the resulting suspension is polymerized by standard

polymerization methods as described in Figure.1.10 (A). One of the fortunate aspects of

this method is the potential to graft the polymer onto the surface of the nanoparticles.

Many different types of nanocomposites have been produced by in-situ polymerization

technique. The key to in-situ polymerization is the appropriate dispersion of the

nanoparticles in the monomer/monomer solution. This often requires modification of the

surface of the nanoparticles because the settling process is more favored than the

dispersion in a viscous solution/melt [120].

36

Figure.1.9. Flow diagrams for the preparation of nanocomposites by (A) melt blending and (B) solution mixing processes.

Figure.1.10. Flow diagrams for the preparation of nanocomposites by (A) in-situ polymerization and (B) emulsion polymerization processes.

37

1.3.4.4. Emulsion polymerization

M. H. Noh et. al. [126] reported a new approach based upon one step emulsion

polymerization for the preparation of polymer nanocomposites containing inorganic

nanoparticles. It involves addition of surfactant along with inorganic nanoparticles under

vigorous stirring condition. Monomer is fed with the initiator. Emulsion polymerization

proceeds under vigorous agitation condition. The reaction mixture is cooled to room

temperature. The final product is obtained after filtration and washing several times with

water. It is then dried under reduced pressure. A flow diagram for emulsion

polymerization technique is as shown in the Figure.1.10 (B).

1.4. DEGRADATION AND STABILIZATION

Degradation is an irreversible change, similar to the phenomenon of metal corrosion.

Chemical degradation of polymer is a very important phenomenon, which affects the

performance of all polymeric materials in day to day life. In practice, any change in the

polymer structure or properties relative to the initial or desirable properties is called

“degradation”. Thus the degradation is a generic term for any number of reactions that

are possible in a polymer leading to the loss of some of its desirable properties [127].

Therefore, degradation of polymers involves several physical and/or chemical processes

accompanied by small structural changes which lead to significant deterioration of the

quality of the polymeric materials and finally to the loosening of its functionality and

property [128].

There are many external causes of deterioration of polymeric materials such as heat,

light, mechanical stress, oxygen, ozone, atmospheric pollutants etc. along with other

factors effective at the time of processing. Also, the presence of reactive sites in the

polymer (e.g. superoxide, defects, chemically reactive groups etc.) may degrade the

polymer properties with or without combination of external factors [129].

Knowledge of mechanism of polymer degradation has led to the development of more

efficient polymer stabilizers [130] for high performance products on the one hand and the

38

development of sensitizers to produce environmental friendly degradable plastics [131],

on the other hand. Thus, the fundamental knowledge of degradation behavior of

polymers is of utmost importance because it may be desirable in one application and

undesirable in some other application. If undesirable, it has to be controlled else can play

havoc with a polymer performance leading to safety hazards such as premature failures,

fire, toxicity etc. If desirable, thus harnessed properly, it may offer new and better

materials as per the constraints of environmental regulations.

There are two main factors, which affect the intrinsic degradation of conjugated polymers

viz. reactivity of polymer backbone and reactivity of dopant. The pristine conjugated

polymers have been reported to contain electronic spins leading to inter and intra chain

reactions between these reactive sites which can alter the chemical structure even when

they are pure, affecting their dopability and, hence, the electro-activity. The oxidative

degradation of most pristine polymers proceeds via the chemical reactions of peroxy

radicals.

A proposed mechanism for the degradation of polyaniline in dilute acidic medium may

be given as in Figure.1.11 adapted from the degradation pathway for p-

aminodiphenylamine.

Sunlight consists of IR and visible radiations, apart from high energy UV radiation (200-

380 nm) of the electromagnetic spectrum. The conjugated bonds, present in conducting

polymer, undergo n-π*, π-π* and σ-σ* transition very easily, leading to formation of free

radicals on exposure to sunlight. The UV radiation contains enough energy to cause C-C,

C-N and C-O hemolytic bond fission. Thus produced free radicals can react with

atmospheric oxygen leading to oxidation accompanied by depletion of chain length of the

polymer.

The long conjugated backbone of conducting polymers could sustain defects such as free

radicals. Such states can readily be oxidized.

39

Figure.1.11. Proposed mechanism for the degradation of polyaniline [132].

40

The ability of a polymer to retain its useful properties is defined as the stability and the

preventive measures undertaken to inhibit the degradation processes, are collectively

known as “polymer stabilization”.

For the purpose of increasing durability of polymeric materials by protecting from

environmental factors or by reducing the rate of degradation process, the substance called

“stabilizers” can be incorporated into polymer matrix. There have been some

disadvantages in using single additive system such as compatibility, migration with low

molecular weight stabilizers, immobility with high molecular weight stabilizers,

yellowing with phenolic antioxidants. The combined effect of screeners, quenchers, UV

absorbers and antioxidants, which are synergistic towards one another, can provide

effective protection against degradation [133]. To overcome the difficulties of

evaporation and migration, the higher molecular weight or polymeric stabilizers can be

introduced. The polymeric stabilizers can be prepared by following two ways [127]:

Grafting of stabilizer moieties onto polymer backbone.

Synthesis of polymerizable monomers anchored with stabilizer moieties.

These sensitizers or stabilizers are generally added during preparation and processing of

polymeric materials as it is done mostly for preparing nanocomposites. Figure.1.12

represents the various techniques available for following/monitoring the degradation as

well as stabilization [128].

Of late, conductive polymers are being used as electrode materials in electrical storage

devices. i.e. in non-rechargeable (primary) and rechargeable batteries (secondary)

batteries. Besides, their high electrical conductivity, they also have selectivity to

electrode reactions, low catalyst activity toward side reactions, sufficient mechanical

strength, fabricability, low cost etc. the electrode materials must also possess high

stability toward degradation of reaction during the passage of current or the storage.

41

Figure.1.12. General techniques used in the study of polymer degradation [128].

42

The degradation of electrode materials leads to instability in electrode potential with

time that take place due to diffusion processes occurring at the electrodes and other side

reactions such as cross linking, chemical reactions of dopant ions with polymer etc. For a

commercial battery, a good shelf life (capacity to retain its charged state) as well as

ability to repeated charging and discharging many hundred times is a prequalification. It

has been observed that electro-active polymers are distinct from traditional electrode

materials as electrically conducting polymers neither deteriorate nor deposit during

charging and discharging processes and, hence they may be expected to give long life

storage systems. The durability studies of polymeric electrodes may be done

galvanostatically or potentiostatically to evaluate their life in battery applications [134-

136].