Chapter 1INTRODUCTION

The present study deals with the effect of gamma irradiation and ion

implantation on optical, thermal, electrical and structural properties of

Poly(methyl methacrylate) polymer. In this chapter, starting with the brief

introduction of polymers emphasizing the fundamental concepts and the

unique features of polymers; the effect of ion implantation and gamma

irradiation on the properties of polymers has been briefly discussed. Finally,

the importance and broad objectives of the present study are highlighted.

2

Introduction

1.1 fundamentals of polymers

Polymer science was born in great industrial laboratories of the world to develop

new kinds of materials which could meet the basic thirst of more and more

promising substitutes of woods and metals. Very soon, the versatile characteristics

of polymers, low cost of their fabrication, easy availability, tunability to suit

specific requirements etc, aroused the interest of researchers worldwide in

polymeric materials. Today, polymers have invaded almost every aspect of our

daily life. In every step, we come across the things which are the fruits of polymer

science.

Chemically, polymers are long chain molecules of very high molecular weight.

More frequently, referred to as ‘macromolecule’, a polymer is built up by the

repeating union of much smaller and simpler chemical unit called ‘monomer’.

The number of this repeating unit present in the polymeric chain is specified by

‘degree of polymerization (DP)’. The molecular weight of polymer is determined

by the product of the molecular weight of its monomer and its degree of

polymerization [Mark et al. 1992; Rosen 1993; Young & Lovell 2002; Billmeyer

2005; Hiemenz & Lodge 2007].

Polymers, on account of the giantness of their size and molecular weight, behave

significantly different from low molecular weight compounds. Many factors

influence the ever widening and enormous range of properties of polymers

[Chapiro 1962; Young & Lovell 2002; Fink 2004; Billmeyer 2005]. A few of them

may be listed as:

chemical composition of the monomer unit

degree of polymerization

spatial organization

skeletal structure

processing parameters

3

Introduction

extent of impurities

presence of side groups with the main chain, etc.

The various possible combinations of these factors result in an extensively

diverse range of the polymers significantly different from each other in a number

of ways.

1.2 categorization of polymers

Polymers can be categorized in many ways (figure 1.1) which depend upon

several parameters such as their origin, morphology, polymerization techniques,

skeletal structure, thermal response etc.

1.2.1 Origin

Considering their origin, the vast variety of polymers can broadly be categorized

under two divisions namely – natural and synthetic.

(a) Natural polymers

Polymers which can be obtained from natural sources are termed as natural

polymers. DNA, RNA, proteins, natural rubber etc. are a few examples of

natural polymers.

(b) Synthetic polymers

Polymers which can be prepared in the laboratory from smaller molecules are

known as synthetic polymers. e.g. Polyacrylates.

1.2.2 Morphology

The nature of molecules present in the main chain backbone gives rise to two

categories of polymers – organic and inorganic polymers.

(a) Organic polymers

Organic polymers are made up of long chains mainly consisting of carbon

atoms joined together or separated by some hetero-atoms such as oxygen or

nitrogen. These polymers are derivatives of petroleum mainly and are also

obtained from plants, animals and micro-organisms. e.g. Polypropylene.

4

Introduction

Figure 1.1: Classification of polymers.

5

Introduction

b) Inorganic polymers

Inorganic polymers have atoms of both organic as well as inorganic elements

in their main chain. Generally, these are synthetic polymers such as

polysiloxanes, polygermanes etc.

1.2.3 Polymerization techniques

Polymerization is defined as the process of synthesizing the polymers from their

monomers. To undergo polymerization, a monomer should essentially have bi-

functionality, which may arise from one double bond or a minimum of two

reactive functional groups. The two major types of polymerization may be named

as

a) addition polymerization

b) condensation polymerization

Generally, unsaturated monomers undergo addition polymerization while the

monomers containing two functional groups undertake condensation

polymerization.

(a) Addition polymerization

This is also known as chain polymerization and characterized by the self-

addition of monomers to each other with no byproducts. The polymer and its

monomer have identical elemental composition.

For example: 5 CH2=CH2 [-CH2-CH2-]5

(b) Condensation polymerization

This step reaction polymerization is characterized by abridgment taking place

between two poly-functional groups to synthesize large molecules along with

the possible elimination of smaller molecules such as water.

For example:

4 HO-R-COOH HO-[-RCOO-]4H + 3H2O

6

Introduction

1.2.4 Skeletal structure

The extent of networking present in the chains of polymers forms another basis of

categorizing polymers as linear, branched or cross-linked polymers.

(a) Linear polymers

In this type of polymers, monomers join together to form long straight chains

(Figure 1.2). This type of polymers is characterized by high densities, high

tensile strengths and high melting points. For example: high density

polyethylene.

Figure 1.2: Schematic structure of linear polymer.

(b) Branched chain polymers

In this type of polymers (Figure 1.3), monomers are irregularly packed

resulting in comparatively low tensile strengths and melting points than

linear polymers. For example: low density polyethylene.

Figure 1.3: Schematic structure of branched chain polymer.

(c) Cross-linked polymers

The polymer chains, here, are joined together to form three dimensional

network structure (figure 1.4). These polymers are characterized by their

hardness, rigidity and brittleness. For example: bakelite.

.

Figure 1.4: Schematic structure of cross-linked polymer.

7

Introduction

1.2.5 Thermal response

Thermal behaviour of polymers defines yet another category of polymers as:

(a) Thermoplastic polymers

The polymers which take up new shapes upon applying heat and pressure

are commonly known as thermoplastics. In general, this type of polymers

has linear structure. For example: polyethylene. These can again be

subcategorized as

i) Crystalline polymers: This type of polymers exhibit long range order in

their structure. In contrast to low molecular weight crystalline

compounds, in a polymer, there are small segments showing crystalline

order in their structure. These segments are called as crystallites and

hence polymers are known to have polycrystalline nature. The

crystallites are firmly held by intermolecular forces and do not show any

molecular mobility even at fairly large temperatures. Hence, crystalline

polymers do not show ‘glass transition temperature (Tg)’.

ii) Amorphous polymers: A polymer which does not exhibit any long

range order in their structure is known as amorphous polymer. This type

of polymers shows a peculiar behaviour upon heating. At the glass

transition temperature (Tg), they show a transition from the glassy state

to rubbery state, due to the onset of the segmental motion in the

polymeric chains.

(b) Thermosetting polymers

The polymers which do not flow or cannot melt irreversibly upon heating

are called thermosetting polymers. For example: bakelite.

(c) Elastomers

Some polymers vulcanize into rubbery products upon heating. This type of

polymers is known as Elastomers. These are characterized by their good

mechanical strength and elongation properties. For example: rubber.

8

Introduction

1.3 Peculiar properties of polymers

Polymers have become most versatile materials suitable for ever widening

applications [Chapiro 1962; Thompson 2001; Fink 2004] in various fields of

science and technology by virtue of their extraordinary properties. Some of these

may be enlisted as:

An enormous range of repeating units alongwith a vast mechanism of

bonding structure provide an unlimited variety of polymers.

These are light weight materials which provide an excellent combination of

flexibility, easy workability etc.

Their bio-compatibility, low toxicity and a range of mechanical properties

similar to those of tissue materials make them appropriate for medical

applications.

They have good thermal and electrical insulation capacity and are resistant

to corrosion effects.

These are accessible in extensively diverse forms such as conducting, non-

conducting, light emitting polymers etc.

Their properties can be modified through various treatments like chemical

doping, annealing, irradiation etc.

Besides these intrinsic properties, polymers can be tuned to desired characteristics

for specific applications [Darraud et al. 1997; Ruck et al. 1997; Hong et al. 2001;

Lee et al. 2001; Thompson 2001; Liu et al. 2002; Singh 2002; Fink 2004; Singh et

al.2005; Kondyurin & Bilek 2008; Hadjichristov et al. 2009; Kumar et al 2011 ].

For the fabrication of novel types of electrodes, rechargeable batteries, assembly

of electronic components, optical switches etc [Cotts & Reyes 1986; Salaneck et

al. 1991; Chandrasekhar 1999; Adhikari & Majumdar 2004; Skompska et al.

2007], conducting polymers provide a good choice. Semi-conducting polymers

offer promising materials for a wide range of applications in many areas such as

9

Introduction

flat and flexible image sensors, multimedia displays [Osaka et al. 2010] etc. In

bio-medical applications also, polymers are being increasingly used as artificial

joints, in food industry [Kondyurin & Bilek 2008] etc. Fluorescent polymers

[Barashkov & Gunder 1994], along with their applications as materials for lasers,

are also frequently used as ionizing recording materials, luminescent solar

concentrators etc. Irradiated polymers [Fleischer et al. 1975; Durrani & Bull

1987; Ruck et al. 1997; Whitman et al. 1997; Cottin et al. 1999; Bhattacharya

2000; Apel 2003; Fink et al. 2005; Rouif 2005; Jagielski et al. 2007] are

extensively used in charged particle detection, tunable refractive index devices,

waveguides fabrication, luminescent devices, electronic and automobile

components etc.

1.4 Ionizing radiation interaction with polymers

Ionizing radiations comprise of all kinds of electromagnetic or corpuscular

radiations having energies considerably greater than bond dissociation energy

[Chapiro 1962; Fink 2004]. Upon irradiation, various chemical changes take place

in the polymeric medium which may be categorized as:

i. Cross-linking which may be defined as the formation of different

bonds between molecules.

ii. Chain-scissioning which is the irretrievable cleavage of bonds

resulting in disintegration of molecular chains.

iii. Free radicals formation which are the unsaturated groups behaving as

chemically reactive sites.

Generally, gamma and heavy ion radiations are employed as ionizing radiations

for modifying the properties of the polymeric materials. Hence, a study of

mechanisms of interaction of these ionizing radiations and subsequent

modifications induced in the polymeric material is highly essential. Figure 1.5

highlights the modes of interaction of these radiations, associated parameters and

10

Introduction

Figure 1.5: Overview of radiation induced effects in polymers.

11

Introduction

the induced structural changes in the polymeric material.

1.4.1 Gamma radiations

Gamma rays, while traversing through the medium, transfer their energy [Evans

1955; Chapiro 1962 & 1988; Leo 1994] through various processes such as:

i. Photoelectric absorption

ii. Compton scattering

iii. Pair production

In all the three processes, the energy is transferred to the electrons of the medium

which, in turn, produce secondary electrons as a consequence of ionization

process. As the gamma photons do not lose their energy in a continuous manner

and are simply attenuated, therefore, the entire bulk structure of the material gets

modified after gamma irradiation.

1.4.2 Heavy ion radiations

The major processes through which an incident heavy ion transfers its energy to

the target medium [Fink 2004] are:

i. Electronic energy loss

ii. Nuclear energy loss

The electronic energy loss occurs as a result of excitation and ionization processes

cropping up in the target medium as a consequence of inelastic collisions between

the incident charged particle and the target electrons. This type of interaction

dominates for MeV heavy ions. For the incident ions having energies in keV, the

incident ion collides elastically with the target nucleus and displaces it from its

mean position. Such energy losses are termed as nuclear energy losses. In both

types of energy loss, the incident ion, to a first approximation, losses energy

continuously along its trajectory. Depending upon the incident ion parameters, the

near surface layers, deeper layers or the entire bulk gets modified.

All types of radiations induce changes in the polymeric material at molecular level

12

Introduction

through various processes of bond cleavage, chain scissioning, cross-linking, free

radical formation etc [Chapiro 1962; Ichikawa & Yoshida 1990; Rosenberg et al.

1992; Sinha et al. 2001; Saad et al. 2005; Saqan 2007]. These modifications at the

molecular level alter the structural properties [Schmitz 1996; Lee 1999; Fink 2004;

Saqan 2007] of the polymeric material which again become responsible for the

alterations in optical, electrical, thermal, mechanical etc behaviour of the polymer.

1.5 Optical behaviour of polymers

The optical behaviour of polymers attains special attraction because of their

applicability in various optical and opto-electronic devices. Various factors

[Efimov 1995; Fox 2010; Sharma et al 2011; Kumar et al 2011] such as

transmission, reflection, absorption and scattering of the photons incident on the

polymeric material are the key guiding factors deciding the optical properties of

polymers at the macroscopic level. These factors help in determining various

optical parameters like optical energy gap, refractive index, Urbach’s energy etc.

These parameters are of prime importance in determining the applications of

polymeric materials in various optical and optoelectronic devices. At the

microscopic level, these are strongly related to the molecular arrangements, degree

of crystallinity, bonding structure, polymeric composition etc.

1.5.1 Optical energy gap

The optical behaviour of low molecular weight solids can be easily understood on

the basis of periodicity of lattice [Kittel 2004], successfully explained by the band

theory. However, in polymers, the formation of ideal bands is interrupted by many

factors such as giant polymer chain network, degree of crystallinity, presence of

non-periodic sequences etc. The absence of long range order in polymers renders

Bloch theorem inapplicable and leaves the crystalline momentum ħk undefined.

Hence, in polymers, valence and conduction bands are not sharply defined; rather,

these states are characterized by extended states formed as a consequence of

13

Introduction

joining of various electronic states in molecular orbitals [Dissado & Fothergill

1992; Biederman 2004; Fink 2004]. The basic features of electronic states found

in most polymers may be listed as [Bamford 1985]:

i. The absence of sharp band edges and tailing of density of states in the gap.

ii. Localization of electronic states in the region where density of states is low.

iii. The existence of broad bands in the gap, in contrast to discrete levels as in

the case of low molecular weight solids, through which a hopping

conduction may occur.

Figure 1.6 presents a schematic illustration of 2-dimentional band model for the

density of states in polymers. This is, generally, known as Mott-CFO model

[Biederman 2004]. As is clearly shown in the figure, Ev and Ec represent the

boundaries of valence and conduction bands respectively, consisting of localized

and extended states. The energy difference between these two is termed as the

mobility gap Emob and is equivalent to the band gap of crystalline materials. The

optical energy gap Eopt is represented by the gap EAEB and is determined by the

optical absorption edge.

The analysis of optical spectra proves to be one of the most powerful tools to

determine the optical energy gap in polymers. From the absorption edge as

observed in optical absorption spectroscopy and using Tauc’s relation [Tauc et al.

1966; Das et al. 1999; Mathai et al. 2002; Sun et al. 2002; Raja et al. 2003;

Biederman 2004; Migahed & Zidan 2006], optical energy gap Eopt may be

determined using the relation

αhν ~ (hν – Eopt)γ ……………(1.1)

where α is the absorption coefficient and hν is the energy of the incident photon

corresponding to the fundamental edge of the absorption spectrum. The parameter

γ is connected to the distribution of density of states in the transport gap (in band

tails). The value of the exponent γ [Fink 2004] can be taken to be 1/2 for allowed

14

Introduction

Figure 1.6: Two dimensional band model for a polymer, with shaded portion as a region of localized states.

15

Introduction

direct transitions, 1 for transitions between density of states at band edges, 3/2 for

forbidden direct transitions, 2 for indirect transitions with phonons, and 3 for

transitions between valence and conduction band-edge tails.

1.5.2 Refractive index

Refractive index (n) is one of the key parameters which determine the applicability

of a material in opto-electronic devices. This may be defined as the ability of a

material medium to bend light as it passes through it. At the molecular level

[Brydson 1995; Wahlstrom 2000; Fink 2004], it is related to the density, molecular

weight, polarizability etc of the material medium. For normal incidence of light,

refractive index ‘n’, over a wide wavelength range, can be determined from the

relation [Migahed & Zidan 2006]

= 4( − 1) − − + 1− 1 … … … … … … (1.2)where r is reflection coefficient and K = αλ/4π is extinction coefficient with α as

absorption coefficient at wavelength λ.

1.6 thermal behaviour of polymers

It is well known that heat treatment can alter various properties like physical,

structural, dielectric, optical, electrical etc of the material [Shaban 1995; El-

Shahway 1997; Liu et al. 2002; Singh et al.2005; Lee et al. 2007]. Heat gets

assimilated into the materials through any of the three processes: conduction,

convection or radiation. Heat is incorporated to a polymeric system mainly

through conduction mechanism [Bamford 1985; Kittel 2004]. It increases the

vibrational energy of the constituent atoms or molecules. Through atomic bonding,

these vibrations are coupled to the adjacent atoms. With increase in temperature,

the relative amplitude of these vibrations increases resulting in an enhancement of

16

Introduction

the segmental motion of the polymeric chains. . This may affect either the main

chain linkage or substituent atoms or side chains. Main chain scission often gives

rise to the free radicals and can occur almost equally at random or at weak linkage.

The end groups are often considered to be the most suitable sites for thermal

degradation. The micro radicals formed out of main chain scission may again

reunite in several ways giving rise to the cross linked structures.

All these rearrangements of molecules and bonds due to heat treatment give rise to

structural changes in polymers which in turn, are responsible for the changes in

their optical, mechanical, electrical, thermal, dielectric properties etc. The extent

of such changes depends strongly on the energy being transferred through heating

and molecular structure of the polymer. In order to get an insight of the effect of

the heat treatment on the properties of polymers, it is essential to analyze the

thermal behavior of the polymers.

Thermal analysis of polymers

Thermal analysis techniques are used in a wide range of disciplines, from

pharmacy and foods to polymer science, materials and glasses. The wide range of

possible measurements made through such techniques provide fundamental

information on the properties of material under test and hence, these techniques

find increasing applications both in basic characterization of materials and in the

development in industrial research.

Thermal analysis of polymers relates to the measurement of a specific property of

the polymer as a function of temperature [Jellinek 1978; Anslyn et al. 2006]. It

involves a group of techniques in which the property under study of the sample is

monitored against time or temperature in a specified atmosphere with the

temperature of the sample programmed. Among such techniques i.e. derivative

thermal analysis, differential thermal analysis, thermogravimetric analysis etc, in

the present study, we have employed Thermogravimetric Analysis (TGA)

17

Introduction

technique. In this analytical technique, the mass of the sample is monitored

against time or temperature with respect to the temperature of the sample. This is a

powerful technique used to study the thermal degradation behaviour of polymers,

which helps in determination of the influence of the polymer morphology on the

thermal stability, the optimum temperature of operation and the activation energies

related to the processes of degradation [Jellinek 1978; Anslyn et al. 2006].

Thermogravimetric analysis has been proved to be reasonably rapid and precise

method for the determination of kinetic parameters [Horowitz and Metzger 1963;

Flynn et al. 1966; Kalsi et al. 1995; Mallikarjun 2004; Nouh et al. 2004; Singh et

al. 2009] such as activation energy, frequency factor, entropy of reaction, free

energy etc related to the degradation processes. In the present study, we have

determined the important kinetic parameters i.e. activation energy and frequency

factor, a brief description of which is as follows:

i. Activation energy: It is defined as the minimum amount of energy

required to initiate a reaction [Bamford 1985]. Basically, activation energy

represents the height of potential barrier separating two minima of potential

energies of reactants and the products respectively of a reaction. Majority

of the reactions involving neutral molecules cannot take place

spontaneously until they acquire the energy to stretch, bend or distort one

or more bonds. This critical amount of energy corresponds to the activation

energy [Menzinger et al 1968]. This energy is assimilated to the reactants

either through intermolecular collisions or by thermal excitation of a bond

stretching vibration. The reactants, after acquiring this energy reach a

sufficient high quantum level, which is known as activated complex. This

activated complex decays rapidly to form the products. As the products are

formed, the activation energy is returned in the form of vibrational energy

which is quickly degraded to heat.

ii. Frequency factor: It is also known as the pre-exponential factor. It relates

to the frequency of the molecules that collide [Bamford 1985] with the

18

Introduction

correct orientation and enough energy to start a reaction. It influences the

rate of reaction through the frequency of molecular collisions and varies

with reaction conditions like temperature, concentration etc.

1.7 electrical behaviour of polymers

Despite the extensive range of polymer composition and structures that have been

investigated over the period, there is no satisfactory theoretical model for

conductivity in polymers. The reason being, polymers possess a heterogeneous

degree of inter- and intra molecular order [Cotts et al 1986]. Polymers possessing

polycrystallites in their structures have intermediate degrees of order as compared

to crystalline or amorphous solids. Owing to their structure, polymers are basically

insulators. Despite this, a low degree of DC conductivity has been observed in

some polymers which play an important role in understanding the electronic

structure of polymers.

Electrical conductivity in polymers

Conduction in polymers arises due to the following two types of conductivity:

i. Electonic conductivity

In general, the valence electrons in polymers are bound in sp3 hybridized

covalent bonds. These σ-bonded electrons do not contribute to conductivity

in polymers. However, conducting polymers have backbones of contiguous

sp2 hybridized carbon centers. One valence electron on each center resides

in a pz orbital, which is orthogonal to the other three σ –bonds [Bamford

1985]. The electrons in these delocalized orbitals have high mobility, when

the material is "doped", by oxidation which removes some of these

delocalized electrons. Thus, the conjugated p-orbitals form a one-

dimensional electronic band, and the electrons within this band become

mobile when it is partially emptied.

19

Introduction

ii. Ionic conductivity

The basic sources of ions in polymers may be identified as [Cotts et al 1986]:

a) Ionized particles: These are normally bound to the polymer backbone.

e.g. pendant carboxylic or sulfonic acid groups, amines, amides groups

etc. These are usually temperature dependent. Their concentration

increases with an increase in temperature.

b) Inadvertent impurities: These include catalysts residues, degradation

products and dopants that are intended to enhance conductivity.

c) Water: Almost all polymers absorb 0.1 to 1% of water which by itself

or with ionizable groups or impurities can greatly enhance the observed

conductivity.

Electric conduction arising out of the movement of any of the above described two

charge carriers i.e. electrons or ions, follows the basic equation [Blythe et al 1979;

Bamford 1985; Cotts et al 1986] given as:

σ = qnµ ……………..(1.14)

where σ is the conductivity, q is the charge, n denotes the concentration of the

charge carriers and µ represents the mobility of the charge carriers. Depending on

the specific mechanism involved, all the three variables are dependent on the

environment and can be modified to some degree. Various electrical conductivity

phenomena in polymers can be explained by introducing the inhomogeneous

transport of charge through ordered regions with periodic differences in their

composition. These may, in general, be described as:

i. Band transport: This relates to the transport of the charge carriers in

extended states. As already described in section 1.5, polymers are

characterized by having localized and extended states in their valence and

conduction bands. If the charge carriers acquire sufficient energy to jump

through the extended states in valence band to the conduction band, it can

20

Introduction

give rise to electrical conductivity [Blythe 1979; Takahashi 1994; Wintle

1994; Wise et al. 1998; Teruyoshi 2003]. But, the lack of extended states in

the giant network of polymeric chains excludes this type of transportation

of charge carriers in polymers.

ii. Hopping transport: This relates to the transport of charge carriers in

localized states. Figure 1.7 represents schematically the hopping transport

mechanism of charge carriers in polymers.

Figure 1.7: Schematic representation of the hopping mechanism of charge carriers in polymers.

Thermally excited charge carriers exhibit this type of transport. Hopping transport

requires the electrons to execute discrete jumps across an energy barrier through

space from one site to another. In addition to hopping, the electron can also tunnel

through the barrier [Blythe 1979]. To hop between sites, the electron must acquire

21

Introduction

sufficient thermal energy to surmount the barrier while for tunneling the site

separation must be small enough for the tail of the electron wavefunction to cross

through it. A very weak temperature dependent mobility of charge carriers in

polymers confirms the hopping character of charge transportation.

1.8 Broad objectives of the present study

Polymers, due to their excellent inherent properties, have become most versatile

materials in today’s era. It is well known that various physical as well as chemical

properties of polymers can be tailored by suitable treatments such as chemical

doping, ion implantation, gamma irradiation etc for specific applications. We can

tune bulk, surface or near surface properties of polymers by choosing the proper

technique and monitoring the related external parameters. Thus, polymers find

extensive applications in various fields such as electronic and opto-electronic

devices (solar cells, LEDs, LCDs, optical switches, optical waveguides etc),

automobile parts, aerospace technologies, medical science etc [Hioki et al. 1983;

Darraud et al. 1994; Ruck et al. 1997; Lee et al. 2001; Hong 2001; Singh 2001;

Hong et al. 2002; Liu et al. 2002; Fink 2004; Singh et al.2005; Kondyurin et al.

2008; Hadjichristov et al. 2009; Kumar et al 2011; Hubbard et al 2012].

Some studies related to the induced modification of one or the other property of

various polymers through different treatments are available in the literature [Lee

1999; Wang 2000; Fink 2004; Manso et al. 2007; Kondyurin 2008; Radwan et al.

2008; Ahmed et al. 2009; Hadjichristov et al. 2009; Te Nijenhuis 2009; Abdul-

Kader et al. 2010; Girolamo et al. 2011; Himanshu et al. 2011; Niklaus & Shea

2011; Sinha 2012]. Prompted with such studies, in the present work, we have

carried out a systematic study on the modification of bulk properties as an effect of

gamma irradiation and on the near surface properties as a result of ion

implantation, of an important polymer Poly(methyl methacrylate) (PMMA).

22

Introduction

Selection of PMMA, in the present study, is based on its unique characteristics as

material like high transparency, good tensile strength and hardness, chemical

inertness, thermal stability etc [Billmeyer 2005]. Due to such extraordinary

features, this polymer finds extensive role as material in many important

applications [Hong et al. 2001; Kuo et al. 2003; Tatro et al. 2003; Koval 2004;

Hadjichristov et al. 2008; Dorranian et al. 2009; Lin et al. 2010]. Keeping in

mind the importance of optical, thermal, electrical and structural behaviour for the

utility of the polymer as material, we have concentrated on the change in these

properties of PMMA as an effect of ion implantation and gamma irradiation.

The important aspect of the present study is that it couples the irradiation

induced changes in structural behaviour of PMMA at the microscopic level to

the resulting modifications in its optical, thermal and electrical properties at the

macroscopic level.

The optical performance of the polymers, being the key guiding factor, deciding

their applicability in different applications in optical and opto-electronic devices, it

is mandatory to have an accurate knowledge of various optical parameters such as

optical energy gap, refractive index etc. [Lu et al. 1996; Townsend 1987; Biersack

et al. 1990; Winder et al. 2002; Kondyurin et al. 2008; Hadjichristov et al. 2009;

Ahmed et al. 2010; Kumar et al. 2011; Hubbard 2012].

Thermogravimetric analysis is a promising tool to investigate the thermal

decomposition of polymers and to assess their relative thermal stability. The

unique thermogram of a particular sample and the thermogravimetric data can be

exploited for the determination of various kinetic parameters related to the

degradation process of the sample. The precise knowledge of the decomposition

process and the mode of decomposition under high temperature is highly essential

to optimize the applicability of a polymer. The onset temperature of degradation

gives an idea of highest processing temperature that can be used, whereas, the

23

Introduction

study of different kinetic parameters helps in identification of the degradation

mechanism [Horowitz and Metzger 1963; Flynn et al. 1966; Kalsi et al. 1995;

Mallikarjun 2004; Nouh et al. 2004; Singh et al. 2009]. Thus, thermal behavioral

study plays a crucial role in optimizing the usage of polymers in various fields.

The polymers, in general, are inherent electrical insulators. The basic insulating

properties of polymers make them suitable in a number of applications.

But, there are certain areas such as electrets microphones which demand very low

conductivity materials. These can be met with fluorinated polymers. A careful

grooming of the polymers by suitable techniques makes them fit for highly

specialized electrical applications [Blythe 1979; Takahashi 1994; Wintle 1994;

Wise et al. 1998; Teruyoshi 2003]. The polymers with improved electrical

behaviour are important to a wide range of industries like automotives, aerospace,

building products, marine, packaging and goods.

Motivated with the importance of optical, thermal and electrical behaviour of the

polymers after different treatments (as already mentioned), in the present study,

we have focused on a systematic study of the optical behaviour, electrical

conduction and thermal response of gamma irradiated and ion implanted PMMA.

The observed changes have been tried to be correlated with the induced structural

changes revealed through Fourier Transform Infrared (FTIR) and Raman

spectroscopic techniques.

The broad objectives of the present study were:

1. To irradiate the samples of PMMA at different gamma doses with

maximum up to ~1600 kGy.

2. To implant the PMMA samples to 100 keV N+ and Ar+ ions at various

fluences with maximum upto 5x1016 ions/cm2.

24

Introduction

3. To study the optical response, thermal stability and DC electrical

conduction behaviour of ion implanted PMMA polymeric samples.

4. To study the optical response, thermal stability and DC electrical

conduction behaviour of gamma irradiated PMMA polymer.

5. To reveal the induced structural changes in these polymers as a result of ion

implantation and gamma irradiation through FTIR and Raman

spectroscopic techniques.

6. To understand the mechanism involved in the observed changes in

electrical and optical response of these polymers due to ion implantation in

terms of the Linear Energy Transfer (LET) by the implanted ions.