Radiation Damage Effects in Solids [1 ed.] 9783038262121, 9783037858318

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Radiation Damage Effects in Solids

Edited by Hardev Singh Virk

Radiation Damage Effects in Solids

Special topic volume with invited peer reviewed papers only.

Edited by

Hardev Singh Virk

Copyright  2013 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of the contents of this publication may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Trans Tech Publications Ltd Kreuzstrasse 10 CH-8635 Durnten-Zurich Switzerland http://www.ttp.net

Volume 341 of Defect and Diffusion Forum ISSN print 1012-0386 ISSN cd 1662-9515 ISSN web 1662-9507 (Pt. A of Diffusion and Defect Data – Solid State Data ISSN 0377-6883)

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Editor’s Note Public interest and concern about radiation damage effects has increased during recent times. Nuclear radiation proved to be a precursor for the study of radiation damage effects in solids. In general, all types of radiation, e.g. X-ray, gamma ray, heavy ions, fission fragments and neutrons produce damage effects in materials. Radiation damage latent tracks in solids find applications in nuclear and elementary particle physics, chemistry, radiobiology, earth sciences, nuclear engineering, and a host of other areas such as nuclear safeguards, virus counting, ion track filters, uranium exploration and archaeology. Radiation dosimetry and reactor shielding also involve concepts based on radiation damage in solids. This special volume consists of ten Chapters, including Review and Research Papers on various topics. The first three Chapters are focussed on Ion Irradiation Effects on Polymers. The effect of various radiations in a polymer is more complex and intense, compared to that in other materials, in view of the more complex structure and low bonding energies. The review by S. Banerjee et al. focuses on irradiation effects due to low energy ions (LEIs) and swift heavy ions (SHIs) on electro-active and engineering polymers, since gamma- and electron-beam-irradiations have been more widely studied and reviewed. The authors have discussed in detail the mechanism of Ion-Polymer interaction. N.L. Singh et al. have reported a detailed characterization of the physical properties of Ni nanoparticles embedded in a polymethyl methacrylate (PMMA) amorphous matrix in Chapter 2. They report observations from UV-visible, DSC, FTIR, XRD, SEM, AC electrical, dielectric and magnetization measurement studies. Ramola and Chandra have reviewed their investigations on ion beam induced modifications in conducting polymers in Chapter 3. Mohapatra and Tomar have contributed the fourth Chapter on spectroscopic investigations of radiation damage in glasses used for immobilization of radioactive waste. Their investigations reveal that borosilicate based glass formulations have been found suitable for vitrification of high level nuclear waste (HLW) generated during the reprocessing of spent nuclear fuel from nuclear reactors. These glasses possess desirable properties like high chemical, mechanical, thermal and radiation stability for HLW storage. Kalpana Sharma et al. have focused on energy loss calculations for swift heavy ions with Z= 3-29 in different polymeric absorbers using a new approach for effective charge parameterization in Chapter 5. In Chapter 6, Zagorskiy et al. discuss “Ion Track Matrices: Porous Structure, Deposition of Metals and Emission Properties of Obtained Replicas”. They conclude that  the obtained ensembles of nanowires could be used as the effective templates for emission of molecules in mass-spectrometry. Ferrites are important magnetic materials having vast applications because of their high resistivity and low eddy current losses. Hexagonal ferrites (SrFe12O19) are widely known as technical materials having applications in a number of electronic and/or magnetic devices. In Chapter 7, Nital and Rajshree investigate effect of swift heavy ion irradiation on structural and magnetic properties of strontium hexaferrites. Abdul Kader et al. report  the effect of 150 keV N2 ions beam bombardment on the optical and mechanical properties of ultra-high

molecular weight polyethylene (UHMWPE) in Chapter 8. They conclude that the conductivity of the bombarded UHMWPE surface is greater than that of the bulk material. This confirms the role of the ion beam technique as one of the better candidates for fabricating electronic devices. S.K. Tripathy reports “Irradiation Induced Changes in Semiconducting Thin Films” in Chapter 9. This review article aims to present an overview of the advancement of research in the modification of glassy semiconducting thin films using different types of radiations (light, proton and swift heavy ions). A detailed study of different types of irradiation induced effects on the optical, electrical and structural changes of chalcogenides glasses has been carried out. Chapter 10: Applications of Thermoluminescent Dosimeters (TLDs) in Radiation Dosimetry is contributed by KVR Murthy. This paper reports on personnel dosimetry, environmental dosimetry, clinical dosimetry, retrospective dosimetry, and applications of TLD in medicine. Absorbed dose measurements in radiotherapy and in diagnostic radiology are discussed in detail. TLD applications in radiation oncology constitute a separate section of this Chapter. Brachytherapy sources, diagnostic X-rays and mammography are other areas of interest included in this Chapter. The role of TLDs in personal monitoring of radiation workers is of paramount interest, based on ICRP recommendations. Hardev S. Virk

Table of Contents Editor's Note Ion Irradiation Effects in some Electro-Active and Engineering Polymers Studies by Conventional and Novel Techniques S. Banerjee, M. Deka, A. Kumar and U. De Swift Heavy Ion Induced Modification in Physical Properties of Poly Methylmethacrylate (PMMA)/Nickel (Ni) Nanocomposites N.L. Singh, C. Gavade and P.K. Khanna Ion Beam Induced Modifications in Conducting Polymers R.C. Ramola and S. Chandra Spectroscopic Investigations of Radiation Damage in Glasses Used for Immobilization of Radioactive Waste M. Mohapatra and B.S. Tomar Energy Loss for Swift Heavy Ions in Polymers: A New Approach for Effective Charge Parameterization K. Sharma, Neetu, Anupam and S. Kumar Ion Track Matrices: Porous Structure, Deposition of Metals and Emission Properties of Obtained Replicas D. Zagorskiy, S. Bedin, V. Oleinikov, V. Korotkov, V. Kudryavtsev and B. Mchedlishvili Effect of Swift Heavy Ion Irradiation on Structural and Magnetic Properties of Strontium Hexaferrites N.R. Panchal and R.B. Jotania Effect of N2 Ion Bombardment on Optical and Mechanical Properties of Ultra-High Molecular Weight Polyethylene (UHMWPE) A.M. Abdul-Kader, Y.A. El-Gendy, A.A. Al-Rashdi and A.M. Salem Irradiation Induced Changes in Semiconducting Thin Films S.K. Tripathi Applications of TLDs in Radiation Dosimetry K.V.R. Murthy

1 51 69 107 129 143 155 169 181 211

Defect and Diffusion Forum Vol. 341 (2013) pp 1-49 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/DDF.341.1

Ion Irradiation Effects in some Electro-active and Engineering Polymers: Studies by Conventional and Novel Techniques Somik Banerjee1,2,a, M. Deka1,b, A. Kumar1,c and Udayan De3,d 1

Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur-784028, Assam, India 2

Girijananda Chowdhury Institute of Management and Technology (GIMT), Kundarbari, Dekargaon, Tezpur-784501, Assam, India 3

Kendriya Vihar C-4/60, VIP Road, Kolkata 700052, India

a

[email protected], [email protected], [email protected], d [email protected] (corresponding author)

Keywords: Swift Heavy Ion Irradiation, Ion Beam Modification, Conducting Polymers, Polymer Electrolytes, Ionic Conductivity, Engineering Polymers

Abstract. The effect of various radiations to a polymer is more complex and intense, compared to that in other materials, in view of the more complex structure and low bonding energies (5 – 10 eV for covalent bonds of the main carbon chain). Since the energy delivered to the polymer in most irradiations (including even beta and gamma rays of 1 to 10 MeV) exceeds this energy by many orders of magnitude, there is a high risk of radiation damage to all kind of polymers. However, engineering polymers (PC, PMMA, PVC, etc. and newer ones) as well as electro-active and other functional polymers (conducting polymers, polymer electrolytes) are finding ever increasing applications, often as nanocomposites, e.g. chemical and biomedical applications, sensors, actuators, artificial muscles, EMI shielding, antistatic and anticorrosion coatings, solar cells, light emitters, batteries and supercapacitors. Critical applications in spacecrafts, particle accelerators, nuclear plants etc. often involve unavoidable radiation environments. Hence, we need to review radiation damage in polymers and encourage use of newer tools like positron annihilation spectroscopy, micro-Raman spectroscopy and differential scanning calorimetry (DSC). Present review focuses on irradiation effects due to low energy ions (LEIs) and swift heavy ions (SHIs) on electro-active and engineering polymers, since gamma- and electron-beam-irradiations have been more widely studied and reviewed. Radiation damage mechanisms are also of great theoretical interest. Contents 1. Introduction 2. Ion-polymer interaction 2.1 Structural specialties of polymers and damage mechanisms 2.2 Choice of ion beam and sources 2.3 Low energy ion (LEI) irradiation of polymers 2.4 Swift heavy ion (SHI) irradiation of polymers 2.4.1 Latent tracks in polymers 3. Modification of polymers by ion beams 3.1 Modification of engineering polymers 3.2 Modification of electroactive and functional polymers 3.2.1 Modification of conjugated polymers 3.2.2 Modification of polymer electrolytes 4. Different techniques for analysis of radiation damage in polymers 4.1 Basic and Conventional techniques

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4.2 Newer analytical techniques 4.2.1 Positron Annihilation Spectroscopy (PAS) 4.2.2 Micro-Raman spectroscopy (RS) 4.2.3 Differential Scanning Calorimetry (DSC) 4.2.4 Dielectric Relaxation Spectroscopy 4.2.5 Modern tools for characterizing track diameter and damage 4.2.5.1 Elastic Recoil Detection Analysis (ERDA) 4.2.5.2 Quadrupole Mass Analyzer (QMA) 5. Ion irradiation effects in specific application of polymers 5.1 Polymeric sensors 5.2 Batteries and supercapacitors 5.3 Biomedical and health sciences 5.4 Nuclear reactors and Space Applications 5.5 Microelectronics 5.6 Nanopatterning 6. Summary References 1. Introduction Irradiation of materials with different ion beams in different energy regimes can induce different types of variations in their physico-chemical properties, which cannot always be obtained by other thermodynamics-limited processes like chemical doping, annealing etc. and, as such, researchers around the world have investigated the effects of ion irradiations upon different types of materials. Among the different categories of materials, polymers specifically are highly sensitive to radiation, and the effects of irradiation in polymers are more complex and intense as compared to other materials [1-4]. The intense radiation effects observed in polymers can be attributted to their complex structures and low bonding energies (5 – 10 eV for covalent bonds of the main carbon chain, for example). There is a high risk of radiation damage to all kind of polymers since the energy delivered to the polymer in most irradiations (including even β and γ rays of 1 to 10 MeV) exceeds their bonding energy by many orders of magnitude. However, radiation-polymer interaction is quite different for swift heavy ion (SHI) or low energy ion (LEI) irradiation as compared to electron and γ-ray irradiations [2, 3]. Polymers may be broadly classified into two categories: (a) engineering or structural polymers, which are materials with exceptional mechanical properties such as stiffness, toughness, and low creep that make them valuable in the manufacture of structural products like gears, bearings, doors, parts of electrical and electronic devices, and auto parts, and (b) functional polymers that are polymers with advanced optical and/or electronic properties and applications as active components. Electroactive polymers (EAPs) are functional polymers. Advantages of functional polymers are low cost, ease of processing and a range of attractive mechanical characteristics for functional organic molecules. One can adjust properties while keeping material usage low. This opens interesting environmental perspectives. Electroactive polymers (i.e. semiconducting conjugated polymers, polymer electrolytes), stimuli-responsive polymers, biomimetic polymers and supramolecular metallopolymers are some of the examples of functional polymers [5]. Both engineering and functional polymers have a great deal of technological importance and applications in sensors, actuators, artificial muscles, EMI shielding, antistatic and anticorrosion coatings, biomedical applications, solar cells, light emitters, batteries and supercapacitors, machine/auto parts, etc. [6-20], often as composites. The importance of polymers in science, engineering and technology is enhanced by the opportunity that their properties can be modified in a controlled fashion by several industrially easy techniques. Processing of polymers by radiation is now a standard technique and a growing field as well.

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Irradiation can modify the molecular structure in polymers in a controlled way leading to changes in their chemical, electronic, electrical, tribological and optical properties [1, 4]. Ionizing radiations with low energies such as the e-beams, γ-radiations, x-rays, UV-radiations as well as low energy ion (LEI) beams have been used since 1950s for polymer modifications and these polymers have been used for different applications such as wire insulations, heat-shrink products, curing of coatings and inks, production of tubing, pipes and automobile tyres, enhancing processibility of polymers for incorporation in coatings and inks, sterilization, etc. [3, 21-28]. In biomedical science, hydrogels cross-linked and sterilized by irradiation have been successfully commercialized for application in burns and wounds [29-31]. Other emerging applications include contact lenses and superabsorbant materials [32, 33]. Irradiation induced grafting of polymers have also found tremendous applications in biomedical sciences since this technique can be used to modify the surface properties of polymers such as wetability, chemical resistance, biocompatibility, etc., which are important properties for biomedical applications [3, 34-38]. Employing ionizing radiations for recycling of polymeric waste products is an exciting field of research because of environmental and economic considerations [39-41]. In recent years, irradiation of polymer surfaces and thin films with swift heavy ions (SHIs) has been a particularly active area of research considering the promising application of these highly energetic ionizing radiations for the production of porous ion-track membranes with track diameters ranging from a few nanometers to micrometers [42-47]. Ion beam lithography is a very promising approach to obtain submicron features on polymeric materials [48, 49]. Energy released by swift heavy ions in polymers produces a “high energy density chemistry” (hot chemistry) and results in final products, which are structurally much different from those generated by low energy loss particles (electrons, photons etc.). As compared to conventional radiations, the linear energy transfer (LET) (eV/nm) for swift heavy ions is much higher than conventional radiations resulting in higher and nonlinear chemical yield [4, 50]. In addition, heavy ions can modify a large number of polymer properties such as structural and optical properties, hardness, solubility, molecular weight, electrochemical stability, electrical and mechanical strength [51-60]. Ion irradiated polymers for application areas such as in nanolithography, sensors, biomedical and health sciences, batteries and supercapacitors also have the potential of commercialization. The only obstacle in the commercialization of swift heavy ion irradiation techniques for polymer modification is the cost associated with such a sophisticated technique and the lack of efficient manpower. Heavy ion irradiation of polymers for property modification or enhancement is a costly affair. It is not viable industrially at present, except for certain critical applications. In spacecrafts and nuclear plants, the effects of radiation environments upon engineering and functional polymers cannot be avoided. So, it is extremely important to build a better insight into the physics behind radiation damage of polymers and to encourage the application of sophisticated techniques for characterizing radiation damage. Analytical techniques such as Positron Annihilation Lifetime Spectroscopy (PALS), micro-Raman spectroscopy (RS), Differential Scanning Calorimetry (DSC), dielectric spectroscopy, Residual Gas Analysis (RGA), on-line ERDA for ion track radius measurements, Quadrupole Mass Analyzer [QMA] for on-line track analysis etc. can provide very important information regarding radiation damage. Further, with the development of in-situ and online characterization facilities in most of the accelerator centres around the world, the investigation of radiation induced damage in polymers has become even more exciting. Thus the use of these sophisticated tools for analysis of radiation damage in polymers must be encouraged. The present work is an attempt to review the current status of research and development in this field with a stress upon the modern characterization tools that have been used by scientists for studying the radiation induced modifications and damage in engineering and functional polymers.

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2. Ion-Polymer interaction Processing and analysis of materials by ion beams are crucial tools for a wide variety of applications and technologies. The basic requirement for performing any study in this field is the availability of energetic ions, which can be obtained in many ways [61] discussed in following subsections. Interaction of energetic ions with materials is of great physical importance and a great deal of theoretical and experimental research has been carried out to bring out a proper explanation of the observed phenomena. The most important parameter that controls the extent of damage and modifications in materials due to interaction with ion beams is the Linear Energy Transfer (LET) or energy loss per unit path length for ionization (eV.nm-1). Ion bombardment (irradiation) can modify the physico-chemical properties of materials in primarily two ways [1, 62, 63]: (a) through the incorporation of the ion species in the irradiated material (ion implantation including doping - a chemical effect), and (b) by creation of defects in the materials (radiation damage or defect effect). The former mechanism is predominant for low energy ion beams (LEIBs) with low value of LET, while the latter is generally observed when one employs swift heavy ions (very high LET) for irradiating the material [62, 63], with ion range much larger than the sample thickness. Thus, it is evident that the nature of irradiation induced effects in materials depends upon the amount of energy deposited in the material per unit volume/length. However, the defect effect is more important in surface modification of polymers than for metals and ceramics, and may mask the doping effects. Energetic ions interact with matter by losing their energy in the matter predominantly by two different mechanisms: the ‘nuclear energy loss (Sn)’ and the ‘electronic energy loss (Se)’. Nuclear energy loss (Sn) arises from ‘momentum-transfer’ between the energetic ion and the target nuclei, which causes atomic displacements and phonons [64]. When the colliding particle imparts energy greater than displacement threshold energy (Ed) to a target atom it gets displaced. Ed is the energy that a recoil atom requires to overcome the binding forces and to move a distance more than the atomic spacing away from its original site. Since the nuclear collision occurs between two atoms with electrons around their nuclei, the interaction of an ion with a target nucleus is treated as the scattering of two screened particles. The incident ion primarily undergoes nuclear energy loss (Sn) at low energies (~ 1 keV/nucleon) [65]. Electronic energy loss (Se), on the other hand, is a result of electromagnetic interaction between the positively charged ion and the target electrons, and dominant at high energies (> 100 keV/nucleon) [65]. Electronic energy loss can be explained by primarily two mechanisms: one mechanism is called glancing collision (inelastic scattering, distant resonant collisions with small momentum transfer) and the other is known as knock-on collision (elastic scattering, close collisions with large momentum transfer) [64]. Both glancing and knock-on collisions transfer energy in two ways: electronic excitation and ionization. Ion-polymer interaction is more complex due to their high radiation sensitivity; polymers undergo modifications at the molecular level upon ion irradiation [66-69]. These molecular level modifications such as electronic excitation, ionization, chain scissions and cross-links as well as mass losses result in irreversible macroscopic variations in the physico-chemical properties of polymers which are quite different from the type of variations induced by e-beam or γ radiations [70]. Polymers can succumb to permanent damage mainly in the form of chain scission by displacements of atoms from polymer chains. Chain-scissioning events caused by the displacement of target atom as a whole from its original location are mainly ascribed to the nuclear energy losses of the ions in the polymer. Since, nuclear energy loss is an independent damage, the probability of simultaneous creation of two radicals in adjoining chains is small and thus the possibility of crosslinking is negligible. In case, the scission takes place in the polymer back-bone (main chain), the polymer may degrade yielding lower molecular weight products, which often exhibits better solubility and processibility [64, 68-70].

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On the other hand, if the ion beam causes electronic excitation or removes valence electrons (ionization) via electronic energy loss, ‘free radicals’ may be generated in the polymer chains [71]. This can lead to cross-linking or chain-scissioning events in the polymer. Initially, the electronic energy loss along the trajectory of the particle was considered to be a continuous process. However, because of quantization of energy levels and the existence of discrete potential barriers for excitation and ionization processes, the electronic energy loss in polymers is discontinuous and is referred to as ‘spur’ [60]. For polymers, the spur energy is generally in the range of 30-40 eV, which can lead to the production of one ion or radical pair and result in the formation of several active chemical species like cations, anions, radicals, secondary electrons etc. along the polymeric chains. Due to the Coulomb forces among these vibrant chemical species, the polymer chains undergo segmental motion and vivacious bond stretching leading to bond-cleavage as well as crosslinking. If, however, the linear energy transfer (LET) of the ion in the polymer is low, spurs developed are far-flung and independent. Thus, the deposited energy tends to be confined in one chain leading to chain-scission. With increasing LET, spurs may superimpose, resulting in high radical concentration gradient. Thus, the probability for the existence of two radical pairs in two neighboring chains is enhanced and cross-linking is made possible that may eventually transform the polymer into a three dimensional network of an infusible, insoluble gel. If the reaction of the free radicals generated due to irradiation remains incomplete, the chemical reactivity of the polymer is enhanced promoting oxidation and brittleness of the polymer chains and reducing wear resistance of the polymer. However, it is very difficult to predict, on the basis of energy considerations alone [72], whether any particular bond will undergo cleavage and generate free radicals upon ion irradiation. In general, the energy that is spent by the ions for ionizing the target is 20 eV or more, which is much higher than the bond energies of the commonly occurring bonds in organic molecules such as H-CH (~4.3 eV), CH3-CH3 (~3.7 eV), F-CH (~4.6 eV) etc. With the energy available for ionizations, almost all types of bonds in polymers can undergo cleavage. However, experimental results show that cleavage occurs in certain selected bonds only, which primarily depends upon the chemical structure of the polymer.

Besides cross-linking and main chain scissioning events, irradiation of most polymers also results in gas evolution and creation of unsaturated bonds. Common gases that are evolved from polymers upon ion irradiation include H2, CH4, CO, CO2, HCl, H2O, C2H2, CF3, C2F4, etc. [72, 73]. Gases are released due to the elimination of an atom or molecular fragment from the polymer and bear the signature of the macromolecular structure. In fact, it was observed that the chemical nature of the gases that evolved out of the polymer after irradiation depended strongly upon the side-chains, e.g., the evolution of HCl from poly(vinyl chloride) in which the Cl-atom resides in the side-chain and not in the main-chain. It was observed that branched structures were more susceptible to ion irradiation. Thus, even if C-C bond energy is less as compared to C-H or C-Cl, it generally does not suffer bond cleavage upon ion irradiation, if it constitutes the main backbone chain of the polymer. Even if there are C-C bond cleavages in the main chain of the polymer due to ion irradiation, they

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are healed by recombination of the chain ends, since the two polymeric fragments diffuse out very slowly and hence the chance of recombination persists. Another chemical modification that is observed in polymers upon ion irradiation is the creation of unsaturated bonds i.e., double or triple bonds from single bonds upon ion irradiation. The creation of double bonds is a result of abstraction of hydrogen or HCl gas from the target upon ion irradiation. Typical reactions [reactions (1) and (2)] that occur in case of poly(vinyl chloride) [PVC] upon ion irradiation are shown below. It can be noticed that ion irradiation leads to extraction of HCl from PVC by breaking the C-Cl and C-H bonds which results in an unsaturated double bond between carbon atoms. Further, the allylic chlorine loosely bound to the carbon atom easily cleaves away to yield another HCl molecule and a subsequent double bond in the main chain of PVC and the process continues depending upon the irradiation dose [72, 73]. Thus, it is possible to have cross-linking as well as main-chain scission in engineering as well as electroactive polymers employing ion irradiation techniques. However, it has been observed that the relative magnitude of chain-scissioning and cross-linking depends on the structure of polymer and certain parameters such as fluence, beam current, charge state and LET of the ion used for irradiation. Phenomena like cross-linking or chain-scissions in polymers are responsible for the change in their optical, electrical, mechanical and other properties [1, 56, 74]. The properties of polymers can thus be engineered as per need with ion–polymer interaction by correlating the induced changes in the properties of the polymers with the energy loss process. This requires the knowledge of the details of energy loss in polymers and associated fluctuations. From fundamental point of view, this area is quite important because this is the basic process involved in ion-solid interaction. In the next sub-section, we shall provide an overview of certain structural specialities of engineering and electroactive polymers and the associated damage mechanisms. 2.1 Structural specialties of Polymers and Damage Mechanisms. It is very important to have a proper understanding of the structural details of polymers to understand their radiation damage and probable modifications. Several engineering polymers, both aliphatic and aromatic have been irradiated to reveal damage mechanisms in polymers upon irradiation. Based upon the results, these polymers have been divided into two groups: (I) cross-linking type polymers, and (II) chain-scission type polymers. An empirical rule for differentiating cross-linking polymers from chain-scission ones based upon their molecular structure was figured out by studying the effects of ion irradiation upon different engineering polymers [1, 75]. Although not entirely satisfactory, this rule can indeed be used to differentiate between certain cross-linking and chain-scission polymers. Figure 1 shows the generalized structure used for discriminating between chain-scission and crosslinking polymers. The figure also elucidates the molecular structures of some polymers belonging to the two groups (I) and (II) which follow the general rule. If the structure of a vinyl polymer is such that each carbon atom of the main chain carries at least one hydrogen atom or either X or Y is hydrogen, the polymer cross-links and thus belongs to group (I). Whereas if a tetra-substituted carbon is present in the monomer chain, i.e., neither X nor Y is hydrogen, the polymer exhibits chain-scission and may be placed in group (II) [75]. This may be explained by the fact that the carbon bond in the main chain could be weakened by the presence of the tetra-substituted carbon atom, since they cause a strain in the molecule by a steric repulsion effect. Indeed, a correlation has been found between the ease of cross-linking and the heat of polymerization of the given vinyl monomer. It has also been found that cross-linking in polyethylene does not occur in the lamellar crystal interior but in the tie-molecules that connect crystalline and amorphous regions. This rule can also be sometimes applied for explaining cross-linking and chain scissioning effects in engineering polymers with hetero-atoms in their main chain. Polymers such as polyethylene oxide (PEO), polyamides exhibit cross-linking since if one considers only the vinyl part of their monomer unit, then all the carbon atoms present contain at least one hydrogen atom such as in Nylon-6 shown

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in Fig. 1. However, there are certain anomalies regarding this rule and its applicability specifically for halogenated polymers. As can be observed from Fig. 1, poly(vinyl chloride) [PVC] is a crosslinking polymer whereas poly(vinylidene chloride) is a chain scission type polymer which can be as poly(vinylidene fluoride) [PVDF] and even polytetrafluoroethylene (TEFLON), since these polymers do not endure remarkable chain scission even though according to the rule they should [75, 76]. The presence or absence of oxygen environment is also sometimes very important when it comes to cross-linking or chain-scissioning events in polymers. Oxygen can easily react with the free radicals generated in the polymer due to irradiation and can cause further degradation of the polymer. Crosslinking polymers such as poly(propylene) and poly(vinyl chloride) suffer chain-scissioning effects when they are irradiated in the presence of air. The same case is observed for TEFLON, which in vacuum environment is highly radiation resistant but degrades enormously if irradiated in presence of air [72, 75].

Fig. 1 Generalized structure considered for differentiating cross-linking (Group-I) and chainscission (Group-II) polymers; followed by the molecular structures of seven cross-linking polymers and four chain-scission polymers. Figure 2 shows the chemical structure of some of the most common conjugated polymers that have been irradiated. Above-mentioned prediction of cross-linking or chain-scissioning from molecular structure, fairly successful for engineering polymers, is difficult in case of electroactive polymers,

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as there is no generalized empirical rule for electroactive polymers. Electroactive polymers such as conjugated polymers have somewhat more complex structure than normal engineering polymers because of the altering bond conjugation along the main chain. These polymers are also mechanically less stable than engineering polymers and have poor processibility. Cross-linking and chain scissioning events have been observed in electroactive polymers also but their relative magnitudes have been found to depend strongly upon the ion fluence and the nature of ion selected for irradiation [77]. At lower ion fluences cross-linking of polymer chains have been the predominant mechanism, whereas at higher ion fluences the polymer backbone degrades by chain scission. In nanostructured conjugated polymers (a special class of EAPs), increasing fragmentation has been observed with increasing irradiation dose. This phenomenon has been attributed to the amorphization of the polymer within the core of the ion track because of the high electronic energy deposition, which causes the nanostructures to fragment. As the ion fluence increases, the tracks overlap and the fragmentation increases [78].

Fig. 2 Molecular structures of some common electroactive polymers: (a) Polyacetylene, (b) Polythiophene, (c) Polypyrrole, (d) Poly (p-phenylene vinylene), (e) Poly(para-phenylene), and (f) Polyaniline 2.2 Choice of Ion Beam and Sources. The purpose of the irradiation experiment and the polymeric sample chosen, guide the selection of ion beam – its species, energy, intensity (current density) and time of irradiation. Availability of energetic ions is a basic requirement for any experimental study to understand the process of radiation damage. Energetic projectiles such as alpha-particles, fission fragments, neutrons, protons, tritons etc., can be generated by using radioactive atomic sources or by simple nuclear reactions [60, 61, 79]. However, generation of mono-energetic ions with energy ranging from, say, keV to GeV, needed different accelerators. Development of such accelerators and irradiation infrastructure form a major part in radiation research. Various ion sources and particle accelerators have been developed [80, 81] since the start of ion acceleration with CockroftWalton high voltage units (1930) and van de Graff generators (1930). Van de Graaff generators can build up voltage by mechanical transport of charge using a conveyor belt up to a maximum of 10 to 20 MV. Starting with acceleration of negative ions towards the positive HV terminal (based on van de Graff generator and placed at the centre of the machine), mid-way inversion of ion charge and re-acceleration as positive ions gave Tandem van der Graaff accelerator (Fig. 3). Pelletron Accelerator is an improved version of Tandem Accelerator, basically by replacing the earlier-mentioned belt by a strip or chain of suitable pellets. Small accelerators, mostly pelletrons, capable of generating low energy ions, like protons with maximum energy of 2 MeV or so, have been extensively used for ion implantation and surface science experiments in different materials. Larger machines like 15 UD pelletron at Inter University Accelerator Centre (IUAC), New Delhi, India, can generate a variety of swift heavy ions ranging from Hydrogen (1H) to Lead (208Pb). The highest energy beam that has been delivered at IUAC is 270 MeV 107Ag and the maximum current that has been attained is 500 pnA [82]. In addition to above-mentioned electrostatic accelerators, there are Radio-Frequency Accelerators like LINAC, RFQ, Cyclotron, Isochronous Cyclotron,

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Synchrocyclotron, Microtron and Synchrotron. As an example, the addition of superconducting linac boosters [83] for increasing the energy of heavy ions from the Pelletron accelerator at IUAC, may be mentioned. 590 MeV Proton Cyclotron at PSI, Switzerland can operate at 2 mA proton current, implying > 1 MW proton beam power. Induction linacs and betatrons create electric field by another process - magnetic induction in a longitudinal evacuated cavity in a magnetic material. Many accelerators are designed to accelerate electrons, and not ions. The Large Hadron Collider (LHC) synchrotron and storage ring system in the CERN at Geneva takes 450 GeV protons from SPS as input and accelerated these to 4000 GeV till date, before its 2013–2014 shutdown for consolidation. However, these are not meant for materials science research and cited here as an example of one of the most advanced accelerators [84].

Fig. 3 Schematic diagram of the electrostatic Tandem Accelerator. The “charged belt” is replaced by a charge-carrying chain of pellets in the Pelletron Accelerator (adopted from http://wwwwin.gsi.de/charms/Talks/CHARMS/). The choice of ion beam is also a very important parameter while studying radiation induced damage in polymers, since the linear energy transfer (LET) and the range of the ion varies according to the composition of the target and the ion energy. Generally, during radiation damage by ion beams, it is desired that the thickness of the target should be much less than the range of the ion (so that there is no implantation) and that the ion deposits fairly uniform energy all along the thickness of the target (so that the polymer suffers fairly uniform damage or modifications to avoid ambiguities). It is an advantage that ions generally have longer range in polymers (mostly made up of light atoms) and most polymeric films can be synthesized to have very small thickness. 2.3 Low Energy Ion (LEI) Irradiation of Polymers. Low energy ion (LEI) beams with energies ranging between ~ 1keV and 1 MeV have been used, for decades, in electronics industry for semiconductor doping by ion implantation [85, 86]. In case of low LEI irradiation of polymers, the nuclear energy loss (Sn) is the predominant mechanism, resulting in surface modification and

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creation of point defects in the polymer through displacement or removal of atoms from the backbone chains. Focus in ion implantation, however, is on doping, and uniform doping is often achieved by multiple irradiation by ion beams of varying energy. Undesirable defects, induced by irradiation, need to be annealed out in many industrial applications [87, 88]. 2.4 Swift Heavy Ion (SHI) Irradiation of Polymers. Modification and tailoring the properties of different materials by swift heavy ions (SHIs) with energies > 100 MeV/u have been widely studied, with limited commercial application till now. Making micro/nano-pores with ion beams is one such application [49, 89, 90]. In contrast to the interaction of energetic electrons or photons with matter, the salient feature of ion-solid interaction is the extremely high localization of the energy transferred to the target material. In this way, solids may receive from the impact of just one energetic heavy ion, the same energy density which can be compared to that of an exploding hydrogen bomb, but for a very short time (~10-17 to 10-15 s) and within a very tiny volume (~10-17 to 10-16 cm3) [60, 61]. For heavy ion irradiations, the primary events are strongly correlated to each other, while they are produced nearly at random for electron or gamma irradiations (apart at the end of the electron range where small clusters of excitation/ionization are produced). The large LET and the high concentration of ionized atoms in the track of the projectile ion characterize the high energy ion irradiation. 2.4.1 Latent Tracks in Polymers. The interaction of a swift heavy ion with some solid target materials is marked by the formation of latent tracks. Two phenomenological models have been extensively used for explaining the latent track formation in solids upon interaction with swift heavy ions: (a) Thermal spike model, and (b) Coulomb explosion model. It was reported by Norman [91] that the absorption of radiation is a quantized process obeying statistical laws. As such, the localized regions of the absorbing medium are heated to high temperatures that can cause widespread changes in materials. This model referred to as the “thermal spike model” has also been used to explain the track formation mechanism in solids [91]. However, in radiation chemistry and biology, thermal effects were not predominant, and the thermal spike model was largely ignored [92]. It was Seitz [93] who formulated the modern form of the thermal spike model and showed that it is indeed a very important mechanism when it comes to radiation related studies in any branch of science. In the thermal spike model, a heavy-ion track is considered as a linear energy burst [94] and the temperature distribution resulting from such a burst in an infinite medium is given by   r2   Q   T (r , t )  T0   (1)  exp   cd 4 xt   4 xt  where T is the temperature at the time t after the burst at a radial distance r from the axis of the track, Q is the heat released per unit length, T0 is the temperature of the medium before the burst, c, d, and x are, respectively, the heat capacity, density, and thermal diffusivity of the medium. For calculation purposes, Eq. (1) can be simplified to:  Q  (2) r 2  4 xt T  T0     cd 4 xt  T  T0 (3) r 2  4 xt and the spike can be considered to have duration, given by: (4) ts  r 2 / 4x The thermal spike model also takes into account any alteration in the temperature distribution caused by the passage of charged particles through a medium. Thermal spike model also considers the effect of changes in the local energy density on free radical yield as well as on temperature [95].

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Thermal spikes, in addition to affecting reaction rates, also generate large pressures in the rapidly heated material within the spike. These pressures, caused by the tendency of heated materials to expand, can cause plastic deformation in solids [94] and explosive growth of vapour bubbles in liquids [96]. The importance of the thermal spike model has been evaluated by the development of the model to account for the problem of defining temperature under the non-equilibrium conditions in the spike, together with the availability of new experimental data [91]. However, recent studies have shown that the thermal spike mechanism is less important in the production of tracks by heavy ions in solids than was thought to be the case [97]. Nevertheless, the thermal spike model represents an important mechanism for radiation damage, whether we are dealing with heavy ions or with intense ultrasonic or laser beams since a large fraction of the absorbed energy is converted into heat in all these cases. The Coulomb explosion model was first suggested by Fleischer et al. [98], which suggested that the resulting repulsion between the transiently ionized atoms in the solid, called a “Coulomb explosion,” can be the reason for the production of a latent track in an insulator which may be seen by chemical etching. This model predicts that a narrow cylinder of densely filled positive ions is created upon irradiation, which strongly repels one another and are ejected (Coulomb explosion) into interstitial positions surrounding a depleted core region. However, for the coulomb explosion model to be valid, the following criteria were laid down by Fleischer et al. [98]: (1) Tracks may be formed by Coulomb explosion model in materials with low mechanical strength, low dielectric constant and small inter-atomic spacing, if the electrostatic stress becomes larger than the mechanical strength. This is possible only when (5) n 2  R  Ea04 10e 2 (2) Tracks must be atomically continuous, requiring, at least one ionization per atom. Thus, n  1 is the second criterion for track formation by Coulomb explosion model. (3) The third requirement for track formation is that the availability of electrons for replacing the ejected ones in the material must be low and it must not be able to replace the ejected electrons within a time frame of less than ~ 10-13 s. Thus for a track to be formed by the Coulomb explosion model one must have: nn  ena a0  nkTt (6) where nn is the number of free electrons in the system, na is the number of ionizations per atomic plane, k is the Boltzmann constant and  n is the electron mobility. (4) The fourth and the final criterion for the Coulomb explosion model to be appropriate for explaining track formation in solids is low mobility of the holes created by the ejection of 2 electrons along the ionized track. Ideally, the hole mobility  p must be less than a0 e / tkT in order to sustain the latent tracks. However, it has been elucidated by Bringa et al. [99], that Coulomb explosion and thermal spike are basically two events that define the early and late aspects of the ionized tracks produced in some solid targets during the passage of a highly energetic ion through it. No matter whether a Coulomb explosion has really taken place in the solid, a thermal spike will eventually occur since at higher excitation densities the repulsive energy in the track produces a spike [99]. Although these two models have been used extensively to explain track formation in solids, there is still a lot of debate regarding the applicability of these models. 3. Modifications of Polymers by Ion Beams In this section we will focus upon the different types of irradiation induced physico-chemical modifications in polymers that have been observed and investigated. We shall restrict our review to some specific polymers such as PMMA, polystyrene, Makrofol, PET, PEO, Polyethylene, PES etc., which are most commonly used polymers in our day to day life and in technological applications. In

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case of functional polymers we shall present a comprehensive review of the ion irradiation effects on electro-active polymers, which includes conjugated conducting polymers and polymer electrolytes. Irradiation effects on nanostructures and nanocomposites of these polymers will also be reviewed considering the current trend of research in these fields. 3.1 Modifications of Engineering Polymers. Initial studies concerning ion irradiation of polymers was focussed on the bringing out the relation of the linear energy transfer (LET) of the ions in the polymer matrix with the physico-chemical modifications that followed as a result of irradiation. Schnabel et al. [100] reported that for polymers that exhibit chain scissioning, e.g., poly (methyl methacrylate) [PMMA] and polymethacrylonitrile [PMCN], the main chain scission yields decreased with increasing LET, whereas for cross-linking polymers, e.g., polystyrene [PS], the main-chain cross-linking yield was more or less independent of the LET. Calcagno et al. [101] reported that irradiation of polystyrene and polyimide films with heavy ions (Ar, Kr, Xe, etc.) in the energy range 25-250 keV induce a strong densification of the polymers. In 1991, Sasuga et al.[59] reported the effects of 30 MeV He2+, 80 MeV C4+ and N4+ ion irradiation effects on the mechanical properties of some aliphatic and aromatic polymers and concluded that the manner in which LET effects appeared in aromatic polymers was quite different from that for aliphatic ones. In aromatic polymers, the observed LET effect was associated with the increase of recombination probability. It was observed that for polymers comprising of both aromatic and aliphatic units such as PET, the LET effects were intermediate. Calcagno et al. [69], showed that while irradiating polymers with high energy (10 keV/amu - 1.0 MeV/amu) ions, typical ion-chain interactions that were observed involved a complexity of phenomena related to the high value of energy deposited by the ion beams into electronic excitation and/or ionization processes such as cross-links and scissions formation, which deeply affected the macromolecular structure. In some other reports, Calcagno et al. [2, 56, 63] investigated ion beam effects upon polymeric chains as a function of ion energy deposition. It was observed that at low energy deposition (1eV/atom), new bond formations with subsequent rheological modifications occurred in hydrocarbon polymers, viz., polystyrene and polyethyelene, while only a slight modification was observed in the electronic structure and no appreciable change in the stoichiometry of the polymers. With increase of the ion deposition energy to 10-50 eV/atom, the original molecular structure of the polymer got modified with both inter-chain and intra-chain cross-linking events occurring in the polymer. At very high ion energy deposition (> 50 eV/atom), an evolution from chain structure to amorphous carbonaceous material was observed. For irradiation with high-energy protons of 30-45 MeV, there was little or no effect of the linear energy transfer (LET) in terms of changes in mechanical properties such as elongation or flexural strength at break for some polymers such as polyethylene (PE), polytetrafluoroethylene (PTFE), poly(methyl methacrylate) (PMMA), epoxy matrix resin glass fibre reinforced plastic (GFRP), epoxy matrix resin carbon fibre reinforced plastic (CFRP), and polyimide matrix resin, CFRP. On the other hand, other polymers such as poly(ether sulfone) (PES) and bisphenol A type polysulfone (UPS) exhibited an LET effect for 10 MeV protons and 20 MeV He ions [102]. In case of PE and PTFE, similar mechanical degradation behaviour was found for all types of irradiations, which implied that the degradation of mechanical properties was less sensitive to the LET and depended mainly on the absorbed dose [103]. Initially, it was believed that ion irradiation of polymers leads to carbonization of the polymer matrix and the resultant product was apparently of no use and as such the research in this field was primarily restricted to the studies of linear energy transfer of different ions and their effects. However, later on it was realized that ion beams could be used for ordering materials in a controlled manner and thus the performance of the bulk or surfaces could be engineered for specific applications. In fact, nowadays radiation processing of polymers is commercially used in polymer industry. It was observed that ion beams led to the formation of new carbonaceous material with enhanced electrical conductivity and optical absorption [104]. Variation in refractive index of PMMA upon SHI irradiation was ascribed to the electronic energy loss parameter (Se) of the ion

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beam in the polymer, which opened up possibilities of engineering waveguides with ion beams for optimization and control of the loss of channeling [105]. While investigating the basic phenomena induced by SHIs in polymers, Bouffard et al. [4] suggested that the energy transfer from the ion to the target material governed the modifications induced in the material. They found that the three primary parameters required for the non-ambiguous description of radiation damage in polymers are the atomic number, the velocity of the ions and ion fluence. Balanzat et al. [106] observed that increasing the electronic stopping power produced a remarkable increase of the radiation sensitivity of polystyrene. They also investigated the effect of ion irradiation on PVDF and PE by irradiation with ions from C to Pb with energies of a few MeV/u and observed the creation of vinyl group which was attributed to a double ionization of a single monomer leading to a simultaneous C-C and C-H bond breaking possible only with SHI irradiation [50, 70]. J. C. Pivin [54] investigated the variations in hardness of poly-iso-quinoline, poly-2-vinylpiridine and polyacrylonitrile, induced by irradiations with increasing fluences of Au, Ne, C, Li, He or H ions, employing nanoindentation test. Picq et al. [67] proposed that the release of small gaseous molecules was a general phenomenon of irradiated polymers such as Polyethylene (PE), polypropylene (PP) and polybutene (PB) and could provide important information on the damage process if a reliable chemical identification of the molecules released and accurate yield values are obtained. Licciardello et al. [107] studied the ion beam effects on the surface and on the bulk of thin films of polymethylmethacrylate (PMMA) and inferred that there was a strong evidence of contemporary scission at low fluencies, whereas at higher fluences cross linking events predominate. Enhancement in surface adhesion property of polymers upon irradiation led to fabrication of metalpolymer contacts [108]. In another astounding invention, ion beam induced depolymerization of polymers leading to the unzipping of the macromolecule and the creation of hundreds of monomers per macromolecule was reported by Puglisi et al. [109]. Biswas et al. [55] studied the high electronic excitation (~10 keV/nm) induced effects on the radiochemistry and melting behavior of semicrystalline polyethylene terephthalate (PET) after irradiation with a 180 MeV Ag14+ ion. Irradiation of polymers was found useful for modification of tribological and other mechanical properties. High density polyethylene (HDPE) used for hip joint replacements exhibited remarkable improvements in their wear resistance upon ion irradiation [110]. Singh et al. [111] studied the irradiation effects of 50 MeV Li3+ ion beams in polyethylene terephthalate (PET) films with respect to their structural and electrical properties and observed that capacitance value of irradiated PET is almost temperature independent and increased with an increase of lithium ion fluence. Cui et al. [112] reported modification of biomaterials by ion beam based processes such as ion implantation and ion beam assisted deposition (IBAD) employing low energy ion beams for enhanceing their biocompatibility and medical device function. In recent years, polymeric biomaterials have taken the fore-front of research for the development and improvement of medical devices and systems. Ion implantation is a physico-chemical surface modification process that results from the impingement of a high-energy ion beam which can modify the course of different interactions that occur when polymeric materials come in contact with biological moieties. Newly designed polymers using ion-beam modification methods have been reported to improve cell adheshion, blood and tissue compatibility [113]. Modification of the chemical structure and bonding as a result of 70 MeV C5+ ion beam irradiation of CR-39 (DOP) and polyamide nylon-6 polymers were investigated at fluences of 1011, 1012 and 1013 ions/cm2 by Kumar et al. [114]. Chain scissions were predominant at primary amine site, but not observed at the carbonate site. Hama et al. [68] studied the distribution of the local transformations induced in a low density polyethylene irradiated with various ion beams using micro-FTIR system. Quamara et al. [115] investigated the effect of 50 MeV Si+ ion irradiation effects on the dielectric relaxation phenomena in Kapton-H polyimide employing TSDC spectroscopy. Tripathi et al. [116] reported atomic force microscopy studies on 900 keV Xe ion beam irradiated films of methyltriethoxysilane and phenyltriethoxysilane polymers at fluences

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varying from 5x1012 ions/cm2 to 1x1015 ions/cm2, and observed surface features decreasing in size from 70 nm to 25 nm. In polymethyl methacrylate (PMMA) polymer matrix filled with organometallic complex, 120 MeV Ni10+ irradiation enhanced the metal to polymer bonding and converted the structure of the polymer finally to a hydrogen depleted carbon network due to H2 gas emission [117]. Srivastava et al. [118] observed that 50 MeV Li ion irradiation of PVDF polymer induced a decrease in the crystallite size, while the lattice spacing, amorphization and disorder increased. For CR-39, no appreciable variation was observed in the physical structure [53]. However, the absorption characteristics indicated chain-scissioning events occurred in the polymer due to irradiation. Degradation processes in various polymers (polycarbonate, poly(ethylene terephthalate), polyimide, and polysulfone) induced by krypton and molybdenum ions of several hundred MeV were studied by infrared and mass spectroscopy. The observation of degradation products in close analogy to pyrolysis and the thermal stability of acetylene, strongly suggest that transient temperature peaks above 1500 K were involved in the track formation process [119]. The spatial distribution of defects generated in PVDF polymer by ion irradiation were investigated using FTIR spectroscopy and it was observed that the radii of damage zone increases with increasing (dE/dx)e [120]. Kumar et al. [121] irradiated 50 micron thick films of Makrofol-N using 70 MeV C5+ ions at fluences of 107, 108, 9.3x1011 and 9.3x1012 ions/cm2, and characterized the samples using FTIR, UV–Vis and differential scanning calorimetry (DSC). Modifications were observed at higher fluences. However, at low fluence the polymer exhibited no remarkable change. Kumar et al. [122] also studied the 70 MeV Carbon ion irradiation effects on Polyether sulphone (PES) foils and observed significant loss of crystallinity at higher fluences. The damage process of polymer in the energetic heavy ion tracks in polycarbonate (PC, Makrofol KG) was analyzed employing the thermal spike model. The thermal spike model of Szenes was found to be in quantitative agreement with the experimental results [123]. Swift heavy ion irradiation effects on the optical, chemical and structural properties of polypropylene were investigated as a function of fluence. The optical energy gap of the polymer reduced by 51% while new vibrational bands corresponding to O-H and C=O vibrations were observed in the irradiated polymers [124]. It was reported that luminescence yield in ion irradiated Polyvinyltoluene (PVT) was proportional to the radiation stopping power and to the absorbed dose. Radiation damage reduced the luminescence yield significantly. A reduction of about 50% is obtained with 300 keV proton and argon ions, having 300 keV energy, at a dose of about 1014 and 1013 ions/cm2, respectively [125]. Davenas et al. [126] reported that ion beam irradiation of saturated polymers leads to the formation of new carbonaceous materials exhibiting enhanced electrical conductivity and increase of the optical absorption, which shifts gradually from the near UV to the visible. Variation in optical band gap and carbon cluster sizes formed in 100 MeV Si8+ and 145 MeV Ne6+ ions irradiated polypropylene polymer were investigated by Kumar et al. [127]. It was reported that the value of optical band gap Eg decreased with ion fluence for both the types of ions, while the number of carbon atoms per conjugation length (carbon cluster size) exhibited an increasing trend. Cluster size also increased with the increase of electronic stopping power. The effects of 70 MeV C 5+ ion irradiation have been studied by Kumar et al. [128] and obtained a change in the free volume properties of polystyrene which were correlated with macroscopic properties of polystyrene. SHI irradiation was also reported to induce exciting effects at the interface of two different class of materials such as metal-polymers and polymers-semiconductors. Kaur et al. [129] studied the effect of 50 MeV Li ions on polycarbonate Makrofol KG coated on GaAs substrate and observed drastic surface modifications owing to bond scissions and cross linkages at the interface. 3.2 Modification of Electro-active and other Functional Polymers. Traditionally, polymers were thought of as insulating structural materials used for construction of non-responsive items used in our day to day life and industry. However, with the advent of electro-active polymers (EAPs), e.g. semi-conducting and conducting polymers, ion conducting polymers etc., they are now being used

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as “active” or responsive device components in battery electrolytes, sensors, actuators, lightemitters, active drug delivery devices and many such applications. Like other structural and engineering polymers, ion irradiation has drastic effects upon EAPs. Swift heavy ion (SHI) irradiation specifically has been found to modify the molecular and electronic level structure of EAPs and bring about remarkable structural, conformational and morphological changes. As a result, several physico-chemical properties of the conducting polymers were modified and their performance enhanced. Unlike engineering polymers, radiation damage studies in electro-active polymers are considerably less and there is a lot of future scope in this field. 3.2.1 Modifications of Conjugated Polymers. Conjugated polymers, a special class of electroactive polymers having C=C or C=N bond alteration in their main chain, exhibit conductivity up to metallic regimes upon doping [130-136]. Hussain et al. [137-139] were amongst the first groups to investigate the irradiation effects on conducting polymer films synthesized by electrochemical polymerization. They observed enhancement in electronic conductivity of polyaniline (PAni), polypyrrole (PPy) and poly (3-methyl thiophene) (PTh) films irradiated with Ni12+ and Si9+ ions. This was ascribed to creation of large number of charged and active chemical species, cations, anions, radicals and electrons along the ion track due the tremendous amount of electronic energy deposition [137-139]. These active chemical species gave rise to cross-linking events within the polymer chains due to Coulombic interactions and facilitated inter-chain electron hopping for conduction between two chains. The authors reported that the increase of crystallinity of the polymer films upon SHI irradiation also contributed to the increase in conductivity. SHI irradiation was also predicted to introduce defect sites in the molecular structure of the polymer chain which led to the enhancement of dc conductivity as charge accumulation in the defects produced charge carriers. However, the increase in conductivity was less in case of Si9+ as compared to Ni12+ ion irradiation, which was attributed to the lower value of electronic energy loss of Si9+ ion [137-139]. Ramola et al. [77,140] reported studies of 50 MeV Li3+ and 90 MeV C6+ ions on structural, electrical and morphological properties of free-standing PAni films. It revealed that the crystalline nature of the polyaniline (PAni) films increased with SHI fluence, followed by a decrease beyond the critical ion fluence. The conductivity increased with the formation of clusters and craters at higher fluences. 60 MeV C5+ ion irradiation effects on crystallinity and DC conductivity of poly (o-toluidine)–poly vinyl chloride blend films were investigated by Lakshmi et al. [141]. The DC conductivity and activation energy obeyed Meyer-Neldel rule. The effect of SHI irradiation on the crystallinity of the semi-crystalline conducting polymers has been worth noticing. The crystallinity of conducting polymer films was found to increase upon SHI irradiation. Collective excitations (plasmons) resulted in large excited volume and rotated the backbone bonds to adopt a variety of conformations, which on being cooled preferred the lowerenergy positions with the stereo-regular chains favoring regular helical shapes. At the same time, these regular sections of chain may pack together to form regions of crystallinity. Upon SHI irradiation the density of the polymer increased making the polymer more compact, which may produce closely packed regions by chain folding, crosslinking and formation of single or multiple helices, which contributes to production of more crystalline regions in the polymer films with increase in degree of crystallinity. It was also observed that on SHI irradiation the UV-Visible optical properties of conducting polymers were so affected that the intensity of the carrier absorption peak increased with the increase in fluence. This indicated an increase in carrier concentration in polymer films upon ion irradiation [137-139, 142]. The UV–visible absorption spectra of irradiated and pristine poly(m-toluidine)–poly vinyl chloride (PmT– PVC) polymer blend revealed that the optical density varies with fluence and the electron excitation peak at 540 nm in pristine sample was slightly red-shifted in the irradiated blends [142]. Conductivity behavior of films of polyaniline (PAni) blended with PVC were investigated after irradiation with swift heavy ions of silicon [143]. Conducting polymers also exhibited changes in their grain size upon

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irradiation with swift heavy ions. The changes observed in the morphology of the polymer films were ascribed to the grain growth, agglomeration and displacement of the polymer molecules under the impact of the incident swift heavy ions making the film surface dense and smooth [144]. During ion irradiation of nanostructures of conducting polymers, remarkable results were obtained by Banerjee et al. [145-150]. Swift heavy ion irradiation was found to enhance the antioxidant activity and biocompatibility of polyaniline nanofibers [145]. It was ascribed to the varying surface architecture of the PAni nanofibers upon irradiation. Irradiation decreased the diameter of HCl doped nanofibers from 40 nm to about 10 nm and that of CSA doped nanofibers from 50 nm to 15 nm as shown in Figure 4 (a-d). The enhancement in antioxidant activity of PAni nanofibers was associated with the reduction in their size and corresponding increase in their surface to volume ratio. The photoluminescence of conducting polymer nanocomposites is affected by SHI irradiation, as the concentration of defects in the form of the colour centres increases. Fluence dependent crossover from 1D to 3D VRH hopping mechanism was observed for PEDOT-TiO2 nanocomposites upon 90 MeV O7+ ion irradiation [151]. Hazarika et al. [152] investigated 160 MeV Ni12+ ion irradiation effects on the dielectric properties of polyaniline nanotubes.

Fig. 4 Transmission electron micrographs of CSA-doped polyaniline nanofibers: (a) before and after irradiation with 90 MeV O7+ ions at fluences (b) 3x1010, (c) 3x1011, and (d) 1x1012 ions/cm2 [144]. Adopted from [145], with permission from Elsevier. 3.2.2 Modification of Polymer Electrolytes. Since the discovery of ionic conduction in nonmetallic solids, Ag2S and PbF2, by Faraday in 1839 [153], lot of efforts have been devoted by the researchers worldwide to develop newer ion conducting materials for electrochemical devices such as high energy density batteries, supercapacitors, electrochromic windows, sensors, etc. Till now several of ion conducting materials like ionic salts, glasses, ceramics, etc. have been tried for electrochemical device applications. Modern trend towards miniaturization has invited major attention [154-156] for polymeric ionic conductors or ‘polymer electrolytes’, that are often ‘polymer-salt complexes’. The main reason for using polymeric electrolytes over other liquid and solid electrolytes lies in the free standing consistency, which allows easy handling and cell design, modularity and reliability combined with flexibility and conformability to the electrodes. Ion conducting phases, free from low molecular weight solvents and based on the dissolution of salts in suitable polymers are key components in new types of batteries for portable electronic devices and electric cars. Irradiation induced modifications in different types of polymer electrolytes have been investigated by A. Kumar and his group [157-162]. Kumar et al. [157-160] studied the effect of Li3+ and C7+ ion irradiation on P(VdF-HFP) based gel polymer electrolytes complexed with Li salts. Irradiation with swift heavy ion (SHI) showed enhancement of conductivity at lower fluences ( 1011 ions/cm2) with respect to unirradiated polymer electrolyte films. This result was attributed to the decrease of polymer crystallinity due to chain scission effect up to a particular frequency. Increase of porosity at lower fluence was also held responsible for the observed rise of ionic conductivity since better porosity implied better

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connectivity of the liquid electrolyte through the polymer. On the other hand, ion irradiation with fluence higher than 1011 ions/cm2 provided the activation energy required for cross linking and recrystallization to occur, which resulted in decrease of ionic conductivity. Study of XRD in SHI irradiated polymer electrolytes revealed that the degree of crystallinity decreases at lower fluence and increases at higher fluence [157-161]. The increase in amorphicity of the polymer gel electrolyte at lower fluence was attributed to bond breaking processes and decrease in amorphicity at higher fluence results from cross-linking processes. Effects of 90 MeV O7+ ion at four different fluences on P(VDF−HFP)–(PC+DEC)–LiClO4-dedoped (insulating) polyaniline (PAni) nanofibers polymer electrolytes have also been investigated by the same group [161]. They observed that in addition to the chain scissioning and cross-linking of polymer chains, the PAni nanofibers get phase separated forming insulating clusters upon irradiation at higher fluence. As a result, a decrease in ionic conductivity at higher fluence was observed. Recently, Deka and Kumar [162] used SHI irradiation as a tool to exfoliate polymer/clay nanocomposites. Using 90 MeV O7+ ions at a fluence of 5 x 1012 ions/cm2, they have successfully exfoliated polymer(PEO)/clay(MMT) nanocomposites, synthesized by solution intercalation technique. With increase in fluence more polymer chains enter into the galleries of MMT, which offer greater interaction between heteroatom of PEO and Na+ cations residing inside the gallery. This results in higher ionic conductivity. In case of PEO/clay nanocomposites [162] the increase of irradiation fluence caused an increase of d-spacing of (001) lattice plane of MMT. During irradiation, the temperature within the latent tracks created in the MMT galleries is quite high and the low viscous polymer diffuses into the gallery of MMT to cause higher intercalation. The higher the fluence, the larger is the time of SHI exposure, and the polymer gets more time to diffuse inside the gallery. At the highest fluence (5×1012 ions/cm2), the (001) diffraction peak is greatly diminished, indicating exfoliation of MMT layers in the nanocomposites. The intensity of the characteristics X-ray diffraction (XRD) peaks of pure PEO decreases with the increase in irradiation fluence with no changes in their 2θ positions. This could be attributed to enhanced interaction of PEO into the MMT galleries due to transient wetting of PEO upon SHI irradiation. Positron annihilation lifetime studies (PALS) on PEO-salt polymer complex revealed occurrence of scission in the polymer chains and the fragmentation of larger free volumes into smaller ones [163]. 4. Different Techniques for Analysis of Radiation Damage in Polymers In recent years, many techniques such as positron annihilation lifetime spectroscopy (PALS), micro-Raman spectroscopy (RS), differential scanning calorimetry (DSC) etc., have been newly used for studying ion irradiation effects in polymers. Nowadays, online and in-situ techniques such as online residual gas analysis, online ERDA, online quadrupole mass analysis etc., are available for studying impact of ion irradiation in various materials. In the next few sections, we will discuss such techniques with reference to polymers. 4.1. Basic and Conventional Techniques. Radiation induced damage and modifications in polymers can be studied by different analytical techniques. Some of the basic and conventional techniques that are generally used for investigating radiation damage in polymers include electron microscopy, FTIR spectroscopy, UV-Vis spectroscopy, X-ray diffraction analysis (XRD), gel permeation chromatography (GPC), mechanical property studies etc. Some advanced techniques such as ESR, XPS and scanning probe microscopy have also been used for studying irradiation induced modifications in both engineering and electroactive polymers. Both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been extensively used for characterizing irradiated polymers with a view to study the morphological variations induced by ion beams. Adla et al. [164, 165], employed TEM to capture images of latent tracks produced in different engineering polymers viz., polyimide (PI), polyethylene terephthalate

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(PET), polyvinyledene fluoride (PVdF) etc. by the impact of heavy ions. Figure 5 shows latent tracks in PET. Scanning electron microscopy (SEM), basically meant for surface study, has also been an equally important analytical tool for the investigation of morphological variations in polymers upon ion beam treatment both for engineering and electroactive polymers [144, 166]. Applications of electron microscopy for understanding ion beam induced morphological modifications in electroactive polymers have been reported by Kumar et al. [144].

Fig. 5 TEM images of ultrathin section of PET irradiated with 2640 MeV uranium ions to a fluence of 5x1010 ions/cm2 and subsequently post-stained with (A) RuO4 (1 h) and (B) OsO4 (24 h). Adopted from [165], with permission from Wiley Interscience. However, during latent track imaging in polymers by TEM, electron induced damage in polymers can easily mask the miniature ion tracks and, as such, tracks may not be visible unless they are prestained with osmium or ruthenium tetroxide prior to performing TEM experiments [164, 165]. Due to the possibility of electron induced radiation damage in polymers during TEM or SEM experiments, researchers have also used scanning probe microscopy (SPM) techniques for analyzing the morphological and topographical modifications in ion irradiated polymers. Scanning probe microscopy experiments in the non-contact regime can provide valuable information about the surface topography of the irradiated samples without damaging the samples, which may not be possible in TEM/SEM. The application of scanning probe microscopy for studying irradiation effects in polymers and other materials was reviewed by Neumann [167]. Pignataro et al. [168], reported atomic force microscopy (AFM) studies of poly(methyl methacrylate) [PMMA] upon irradiation with 5 keV Ar+ ions, whereas AFM studies of 63 keV Ar+ and 155 keV Xe+ ion irradiated polyethylene was reported by Svorcik et al. [169]. Tripathi et al. [116] reported swift heavy ion irradiation induced surface modification studies of polymers using SPM. Surface modification studies of four polymers namely, methyltriethoxysilane/phenyltriethoxysilane (MTES/PTES) based gel, triethoxisilane (TH) based gel, highly oriented pyrolytic graphite (HOPG) bulk and fullerene (C60) thin films, were carried out employing SPM. Several other conventional characterization techniques have also been widely used to study irradiation effects and radiation damage in polymers, out of which X-ray diffraction, FTIR and UVVisible absorption spectroscopy are most common. X-ray diffraction has been used by researchers for evaluating the variation in crystallinity and the effects of ion-polymer interactions upon the crystalline arrangement of polymers, while FTIR spectroscopy has been extensively used to investigate chain-scissioning and cross-linking events in polymers upon ion irradiation [67, 157162, 170]. UV-Visible spectroscopy has been used often to investigate the fluence dependent formation of carbonaceous clusters in polymers after interaction with ion beams [127, 171-174]. Absorption spectroscopy has also been employed to study the variation in optical band-gap and defect generation in both engineering and electroactive polymers [53, 127, 137-139]. Other sophisticated analytical techniques that have been regularly used for studying ion irradiation effects

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in polymers, include Residual Gas Analysis (RGA), X-ray Photoelectron Spectroscopy (XPS) and Electron Spin Resonance (ESR) [71, 73, 175-177]. Lee et al. [73] employed RGA analysis to understand the ion-polymer interactions leading to improved surface properties in polymers such as Teflon and Kapton after irradiation with ion beams of helium, nitrogen and silicon in the energy range 0.2 to 2.0 MeV. Evolution of CF and CF3 was observed from Teflon and H2, CO, and CO2, from Kapton. ESR studies have been undertaken by various groups to investigate the amount of free radicals generated in ion irradiated polymers [71, 177]. 4.2. Newer Analytical Tools for Characterizing Radiation Damage. Besides the use of conventional characterization techniques, researchers in recent years have started employing newer tools for characterizing radiation damage in polymers. Techniques such as Positron Annihilation Lifetime Spectroscopy (PALS), Micro-Raman Spectroscopy, Dielectric Relaxation Spectroscopy, Differential Scanning Calorimetry (DSC) etc., have been employed to extract very specific information regarding ion induced radiation damage. In the next few sections, we present an overview of the current status and perspectives of research in radiation damage of polymers employing some of these novel techniques. 4.2.1 Positron Annihilation Spectroscopy (PAS). Positron annihilation spectroscopy (PAS) includes techniques like Positron Annihilation Lifetime Spectroscopy (PALS), Doppler Broadened of Positron Annihilation Radiation Lineshape (DBPARL) analysis, Coincidence DBPARL technique, Angular Correlation of Annihilation Radiation (ACAR) and positron beam version of some of the above techniques. Here, PALS and DBPARL are outlined to review their use in ion irradiation studies. A positive ion vacancy in a solid is like a temporary home for a probing positron. Resulting sensitivity of positrons to lattice defects, particularly to vacancy-type defects, in condensed medium has made it possible to use it extensively as an atomic level probe to study defects in a wide variety of solids including polymers [178-180]. Swift heavy ions produce latent tracks in polymers which contains amorphous material with the highest degree of disorder. This modifies the free volume, which can be measured by positron annihilation lifetime spectroscopy and can provide very important information regarding the radiation damage in the polymer. Positron (e+), the anti-particle of electron (e- or simply e) is equivalent to an electron in mass (m) and all other aspects except the opposite charge. In most positron lifetime experiments, positrons are generated from the β+-decay of the radioactive isotope 22Na, according to the decay reaction 22 Na=22Ne + γ + νE + β+. A prompt gamma-photon of 1.276 MeV is emitted almost simultaneously with the positron, and it can be considered as the birth signal of the positron. The lifetimes of positrons generated from 22Na isotope are generally measured using a start-stop coincidence γspectrometer. During the annihilation of a positron with an electron, the total rest mass is converted into 1022 keV energy. This energy is equally distributed into two oppositely emitted photons (each of energy 511 keV in the electron-positron centre of mass) to meet the criteria of momentum conservation. The start–stop coincidence γ-spectrometer detects the 1.27 MeV photon as the start and one of the two 511 keV photons as the stop for a time to pulse height (TPH) analyzer [181]. The time difference between the occurrences of these two events is measured as the lifetime of the positron and the lifetime spectrum N(t) vs. t is recorded by a multi channel analyzer or a computer. From a proper analysis of the lifetime spectrum, one can obtain positron lifetime , or the positron annihilation rate, =1/The positron lifetime is determined by the overlap of the positron density and electron density at the annihilation site [182, 183] and annihilation rate is higher for higher number density of electrons at the site. An outline of Doppler Broadened of Positron Annihilation Radiation Lineshape (DBPARL) analysis including “Coincidence DBPARL” experiments is desirable, and given below. Positrons are thermalized soon after entering the solid sample. So, the momentum of the annihilating positron

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Radiation Damage Effects in Solids

is usually ignored, and momentum of the electron is taken as the momentum of the positronelectron system. Depending on the electron momentum component (p) in the direction of photon detection, the two 511 keV photons are Doppler shifted by an amount +pc/2 and - pc/2 in the laboratory frame (our measuring system). Tail regions with high p (i.e., away from the 511 keV peak) are formed from annihilation of the positrons with high momentum electrons (core electrons). Momentum profile of the core electrons are element specific and hence the atoms of the sample can be identified in Coincidence DBPARL. Most DBPARL investigations define few channels around 511 keV as the “central region” arising out of “low” momentum electrons of the sample. Sharpness of the 511 keV peak, called S parameter, is of interest as an indication of the electron momentum distribution in the sample. Fraction of “low” momentum electrons [184, 185] in the sample is estimated from S = (“central” area) / (full area), in N(E) vs. E energy spectrum of annihilation gammas. Positrons from a source like 22Na have an average range. Since positrons annihilate at the end of the range, the probed sample has to be thicker than the positron range to probe the sample and not the air or medium across the sample. If the increase of sample thickness makes it thicker than the range of irradiating ion, the result is implantation and poorly defined radiation damage, as already discussed. More attention should be paid in PAS probing of ion radiation damage to match these two conflicting demands on sample thickness. Open-volume defects such as vacancies reduce the electron density and hence can be probed by the observed variation of the positron lifetime or by positronium lifetime values. When a positron enters a vacancy or a vacancy like defect, it can either annihilate with an electron in the defect leading to lifetime component little longer than the bulk lifetime, or it can form a positronium (Ps) atom by getting an electron from inside the hole. Positronium atoms consist of ortho-Positronium (o-Ps, having electron positron spins parallel) atoms with 142 ns lifetime in vacuum and paraPositronium (p-Ps, having electron positron spins anti-parallel) atoms with only 124 ps lifetime in vacuum. Inside the open-volume or hole, the o-Ps can interact with the wall to pick up an electron of spin opposite to its positron, within a time determined roughly by the size of the hole, and become p-Ps to decay practically immediately (within 124 ps). So, the actual lifetime τ 3 of o-Ps in a solid (in its hole sites) is much less than 142 ns, usually of the order of 1 ns, as determined by the hole size. The o-Ps can, therefore, be very useful for probing sub-micrometer size holes. Positronium atoms consist of ortho-Positronium (o-Ps) and para-Positronium (p-Ps) atoms. The oPs can be very useful for probing sub-micrometer size holes. Larger hole implies longer time until the o-Ps collides with the hole-wall, and hence a longer τ3. There are semi-empirical expressions for hole radius in terms of τ3 [183]. In case of polymers, o-Ps annihilation is useful to reveal the dynamic free-volume structure (thermal-volume fluctuations). The irradiation of polymeric materials with swift heavy ions (SHI) results in a change of their free volume properties which have strong correlation with their macroscopic properties. Rajesh Kumar et al. [186], investigated the free volume properties of Makrofol-KG polycarbonate upon irradiation with 100 MeV Si8+ ions. They also studied the irradiation induced free volume modifications in PN6 and PES polymers and observed that the average free volume and fractional free volume obtained from long lived component, attributed to ortho-positronium (o-Ps) lifetime, decreased with increasing fluence [187]. Free volume study of 145 MeV Ne ion induced modifications in Makrofol-KG polycarbonate at various fluences was undertaken through positron annihilation lifetime measurements. Small decrease in o-Ps lifetime with the ion fluence was observed. It was observed that ion irradiation reduced the available free volume and facilitated cross-linking at high fluences [188]. Similar studies were reported for 70 MeV C5+ ion irradiated polystyrene (PS) [189]. CR-39 (DOP), a polycarbonate widely used as ion track detector and polyamide Nylon-6, a high performance plastic having a unique combination of superior mechanical, electrical, chemical and thermal properties were irradiated with 70 MeV C5+ ion beam to different fluences ranging from

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1x1011 to 3.7x1013 ions/cm2. PALS studies showed that o-Ps lifetime and the average free volume for both the polymers decreased with fluence, indicating the facilitation of cross-linking. Maximum change of 9.7% in average free volume was observed in PN-6 irradiated to 3.7x1013 ions/cm2 [190]. An interesting result was obtained from PAS and Doppler broadening spectroscopy (DBS) study of the fluence dependence of free volume in ion-irradiated conducting poly-(ethylene-oxide)–salt polymers [163]. The free volume hole radius, free volume of micro voids and fractional free volume were calculated from ortho-positronium (o-Ps) lifetime and it was observed that all the free volumes decreased for the fluence 1010 and 1011 ions/cm2 but subsequently an increase was observed with fluences of 1012 and 1013 ions/cm2. A continuous decrease in the S parameter and intermediate lifetime (2) with increasing fluence revealed the occurrence of scission in the polymer chains and the fragmentation of larger free volumes into smaller ones [163]. Doppler Broadened Positron Annihilation Radiation Lineshape (DBPARL) analysis has been used for profiling radiation damage depth in different polymers. For this, various polyimide layers [2.2– 2.6 m of hexafluoroisopropylidene bis(phthalic anhydride-oxydianiline), pyromellitic dianhydrideoxydianiline, and 3,3’-4,4’-biphenyltetracarboxylic dianhydride-p-phenylenediamine] spin-coated on silicon substrates were studied with a variable-energy positron beam by DBPARL technique. It was observed that irradiation of the polyimides with 1x1015 boron ions/cm2 at an energy of 180 keV led to a strong chemical modification of the irradiated top layer, which caused inhibition of positronium formation in the irradiated layer confirmed by lowering of S, the line-shape parameter [191]. Kobayashi et al. [192], used mono-energetic positron beams to measure radiation damage depth in poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1 MeV and 2 MeV O+ ions. It was observed that annihilation lines recorded at relatively low positron energies broadened with increasing irradiation dose, indicating inhibition of positronium (Ps) formation in the damaged regions. The positron results were compared with the results of dynamic hardness and electron spin resonance measurements [192]. Radiation damage depth-profiling of amorphous poly(aryl-etherether ketone) (PEEK) irradiated with 2 MeV Au+ and O+ ions was carried out using the slow positron Doppler broadening technique and the results were compared with TRIM. The slow positron beam was used. It was observed that in case of O+ irradiated semicrystalline PEEK, the annihilation lines recorded at relatively low positron energies became broader with increasing fluence. The region where the reduced S parameters were observed extended deeper for O + irradiated samples than Au+ irradiated ones which was found to be consistent with TRIM codes [193]. The slow-positron Doppler broadening technique has thus been effectively used for damagedepth profiling of positronium forming polymers. A very significant result was obtained by Al-Qaradaw et al. [194], from the PAS studies of ultra high molecular weight polyethylene bombarded with He+ and Ar+ ions to fluences ranging from 1x1013 to 2x1016 ions/cm2. Their studies revealed the reach, within the bulk, of the ion bombardment, by detecting the radiation defects. Positron Annihilation spectroscopy (PAS) was also employed to investigate the free volume controlled diffusion process in polymers. The diffusion of iodine was monitored in un-irradiated and irradiated polycarbonate (PC). It was observed that the diffusion process followed Fick’s law with an exponential type correlation between the fractional free volume and the diffusion coefficient [195]. Depolymerization in polymers is a special class of ion irradiation induced chemical modification that results in a catastrophic unzipping of the macromolecules. Depolymerization effect in + o poly(methylmethacrylate) [PMMA] upon irradiation with 300 keV He ions at 200 C were investigated using positron annihilation techniques by Puglisi et al. [109]. They suggested that positronium techniques have great potential for studying depolymerization effects in polymers. The reduction in the nano-cavities of the polymer upon thermal and ion beam treatment can be determined using positron annihilation techniques [109].

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4.2.2. Micro-Raman Spectroscopy (RS). Raman scattering results from incident electromagnetic radiation induced transitions in atoms/molecules/solids of the scattering medium. The transition can be rotational, vibrational, electronic, or a combination (but first-order Raman scattering involves only a single incident photon). In a Raman experiment, the sample is irradiated with monochromatic electromagnetic radiation. If the sample is transparent, most of the light is transmitted, a small fraction is elastically (Rayleigh) scattered, and a very small fraction is inelastically (Raman) scattered. The inelastically scattered light is collected and dispersed, and the results are presented as a Raman spectrum, which plots the intensity of the inelastically scattered light as a function of the shift in wave number of the radiation. Each peak in the spectrum corresponds to one or more vibrational modes of the solid. Total number of peaks in the Raman spectrum is related to the number of symmetry allowed Raman active modes. Some of the modes may be degenerate and some may have Raman intensities that are too low to be measured, in spite of their symmetry allowed nature. Consequently, the number of peaks in the Raman spectrum will be less than or equal to the number of Raman active modes. The practical usefulness of Raman spectroscopy resides largely in the fact that the Raman spectrum serves as a fingerprint of the scattering material. Raman spectroscopy not only provides basic phase identification, but subtle spectral alterations can be used to assess nano-scale structural changes and characterize micromechanical behaviour [196]. Unlike conventional Raman spectroscopy, Micro-Raman spectroscopy (RS) provides the Raman spectra of a desired micro-region of the sample which can be viewed and selected using an optical microscope in-built with the equipment. The equipment generally consists of Ar ion laser source, the power of which can be altered according to sample specifications and requirement. The exposure time and the wavelength region important for study can also be selected. In advanced versions of the RS, one can select the excitation wavelength according to choice. Generally, excitation wavelengths are in the order of 514.5 nm or 632.8 nm. However, for glossy polymeric samples infrared excitation wavelength can be used for acquiring the Raman spectra. Micro-Raman spectroscopy (RS), because of its specificity in selecting the region of study has become an important tool for studying ion irradiation induced structural and conformational modifications in polymers. RS is thus a very suitable sophisticated analytical technique for precise measurements of any structural and conformational modifications induced in polymers upon irradiation, which are Raman active. In recent years there have been some reports concerning the investigation of the ion irradiation induced structural and conformational modifications in polymers using RS. Radiation damage in engineering polymers such as PES, PS and PVC polymers have been investigated with Raman microprobe analysis, and RBS combined with in-situ residual gas analysis [197]. Conjugated polymers such as polyaniline (PAni) and polypyrrole (PPy) are Raman active. Investigation of the structural and conformational modifications in nanofibers of these polymers, due to 90 MeV O7+ irradiation, have been done [146, 149] employing RS. It was observed that the intensity of all the Raman active modes for both PAni and PPy nanofibers reduced with increasing irradiation fluence. The decrease in the intensity of the Raman active modes in the low wave number region was ascribed to the crystallinity dis-arrangement of the chains in PAni and PPy nanofibers primarily due to increase of the torsion angles of the Cring–N–Cring segments [146, 149]. The increase in the torsion angles was an after effect of the loss of -stacking among the PAni and PPy rings, leading to amorphization of the nanofibers, which was also corroborated by XRD results. The authors assumed that electronic interactions between the electron rich C–N site in the aromatic rings of PAni and PPy chains and the ion beam induced polymeric chain distortion leading to the observed conformational modifications.

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Fig. 6 90 MeV O7+ ion irradiation induced (a) benzenoid to quinoid transition in polyaniline nanofibers (b) benzoid to quinoid transition in polypyrrole nanofibers. Reprinted from [146,149], with permission from Elsevier and Taylor and Francis. Modifications in the molecular structure of the backbone chain in PAni and PPy nanofibers were also revealed by RS. A benzenoid (benzoid for PPy) to quinoid transition was a common feature that was observed in both the PAni and PPy nanofibers upon irradiation with 90 MeV O7+ ions as shown in the Figures 6a and 6b. This result obtained by RS is of considerable interest since the major properties of these conjugated polymers are in fact controlled by the benzenoid (benzoid) to quinoid ratio in their backbone chain. Further information regarding variations in the doping state and crystallinity of polyaniline and polypyrrole nanofibers/wires upon SHI irradiation were obtained by analysis of their micro-Raman spectra before and after irradiation [146, 149]. 4.2.4 Dielectric Relaxation Spectroscopy. In general, the ac electric response is a superposition of the dielectric response of the bound charges (dipoles), the effect of hopping of the localized charge carriers and the response of the molecular structure deformations due to the diffusion of charge carriers [198]. Different types of polarization mechanisms can co-exist in the same material. Besides the conductivity and dipolar polarization, there are electrode effects, interfacial effects and space charge relaxations. The correct interpretation of the experiments is obtained by verifying the reliability and physical grounding of the model by which the fitting procedure is carried out. A dielectric function is used to study the polarization mechanisms, whereas complex impedance and electric modulus are usually used to describe the conductivity relaxation mechanisms of the materials [199]. Recently, dielectric relaxation spectroscopy (DRS) has been widely employed by several research groups to investigate the radiation damage of the ac electrical response of polymeric materials and polarization mechanisms in these materials. Singh et al. [200] investigated the effects of 80 MeV O6+ ion irradiation on the dielectric response of polycarbonate (makrofol-DE) using dielectric spectroscopy. An exponential increase in ac conductivity as a function of frequency was observed to obey the Universal Joncher’s law. The dielectric constant and dielectric loss were also observed to change with the fluence. In case of polyimide irradiated with 80 MeV O6+ ions, Singh et al. reported a sharp increase in AC electrical conductivity around 20 kHz, and this frequency increased with irradiation fluence. The dielectric constant and loss factor were observed to change significantly, and was associated with scissioning of polymeric chains [201]. Similar observations were also made for 120 MeV Ni10+ ion irradiated polyvinyl chloride and PMMA with ferric oxalate as filler [202, 203]. Banerjee et al. [150] reported the effects of 90 MeV O7+ ion irradiation on the relaxation and charge transport mechanisms in hydrochloric acid (HCl) doped polyaniline (PAni) nanofibers, a conducting polymer composite, as studied by dielectric relaxation spectroscopy. They observed non-Debye

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type relaxation in PAni nanofibers with a distribution of relaxation times that was dominated by hopping of trapped charges. A very significant result was the observation of a benzenoid to quinoid structure in the PAni main chain upon SHI irradiation using dielectric relaxation spectroscopy. They also showed that the dynamic processes occurring at different frequencies in PAni nanofibers exhibit the same activation energy indicating better coupling among the localized short range dipolar motions. An increase in conductivity relaxation time upon irradiation was observed. This was attributed to the increase in the carrier hopping length, thereby impeding charge transport. Like the engineering polymers, the ac conductivity of the pristine and irradiated PAni nanofibers was also found to satisfy the universal power law. It was also shown that charge transport in both the pristine and irradiated PAni nanofibers follows the correlated barrier hopping (CBH) model with polarons as major charge carriers but there was a significant decrease in conductivity upon irradiation [151]. Effects of 160 MeV Ni12+ ion irradiation upon the structural and dielectric properties of PAni nanotubes were also investigated. In this case, an increase in the ac conductivity was observed; but beyond a fluence of 3 x 1011 ions/cm2 conductivity was found to decrease [152]. Deka and Kumar [162] investigated 90 MeV O7+ ion irradiated PEO/MMT based ion conductor, by dielectric spectroscopy. They observed that ε′ and ε″ exhibit higher values for the samples irradiated with higher fluence This was due to the fact that the intercalated and exfoliated MMT-clay structures in PEO change dramatically with the increase in ion fluence in PEO–MMT films. The increase in the complex dielectric function values with increase in ion fluence was associated with the generation of dipolar and free charges in the nanocomposites due to intercalation and exfoliation of MMT layers. Furthermore, the significant increase in ε″ and ε″ values of these materials with increase in irradiation fluence was related to the formation of a percolation structure of the nanoparticles. On the other hand, monotonous increase in ε″ with increasing ion fluence suggested enhancement in the number of free Na+ cations and their mobility in the clay galleries. At high frequencies, the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field. The polarization due to the charge accumulation decreased, leading to the observed decrease in the value of real and imaginary part of dielectric constant. They also performed the electric modulus study of PEO-MMT nanocomposites. They observed that the relaxation peak appearing in modulus formalism shifts towards higher frequency with the increase in ion fluence giving rise to decrease in the conductivity relaxation time. This was attributed to the fact that as exfoliation proceeds with irradiation, nano-size silicate flakes present an ever-expanding polymer/clay interface increasing the internal capacitance of the composite where conducting ions can accumulate. Silicate flakes act as nanocapacitors in the melt polymer. In general, the MaxwellWagner (MW) relaxation time can be viewed as an electrical RC time constant, where R is the resistance of the polymer matrix and C is the capacitance of the silicate particle. The decrease in relaxation time with increase in ion fluence in PEO–MMT nanocomposites was attributed to the extent of clay exfoliation in dynamic equilibrium [162]. 4.2.3 Differential Scanning Calorimetry (DSC). Differential Scanning Calorimetry measures differentially, heat gained or lost (per unit time), by a sample, as a function of temperature while increasing and decreasing the temperature at a pre-determined rate. So, DSC measures the specific heat in general, and more importantly the enthalpy change as a function of temperature to detect the thermal response of any physical or chemical change taking place in the sample. So, if specific information, such as glass transition temperature (Tg), enthalpy of crystallization (Hc), enthalpy of melting (Hm), melting point (Tm) etc. can be obtained by DSC for the pristine polymer before irradiation and also after radiation damage [204-206], newer insight into ion induced physicochemical modifications can be gained.

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DSC of 19 wt% after irradiation DSC of 19 wt% before irradiation

0

Heat flow per unit mass (W/mg)

25

-100

-200

Exotherm

-300

Endotherm -400

40

60

80

100

120

0

Temperature ( C )

Fig. 7 Heat flow per unit mass as a function of the sample temperature in the DSC experiment, for PEO – 19wt.% salt film. Larger-sized symbol is for the unirradiated polymer and the other is for polymer irradiated with 2.5x1010 ions/cm2 of 160 MeV Ne6+ ion beam. Adopted from [205]. Ion-conducting polymer films have been prepared by complexing non-conducting poly-(ethyleneoxide), PEO, with x fraction of NH4ClO4 salt. Since its electrical conductivity showed a maximum at x somewhere between 0.18 and 0.19, such polymer films having 17 and 19 wt% salt, have been chosen and irradiated by ion beam. Melting point (MP), identified with the endothermic minimum in the Differential Scanning Calorimetry (DSC) result, increased [205] for PEO-salt samples with and without laponite, due to irradiation by 160 MeV Ne6+ beam to 2.5 × 1010 ions/cm2 in the cyclotron at the VEC Centre, India. This irradiation-induced increase has been from 54.6 °C to 57.9 °C for the x = 19% film (Figure 7). However, 1.25 MeV  irradiation decreased [206] MP of PEO polymers as shown in Table I. Actually, a complex bimodal endotherm with minima m1 and m2 has been observed even in unirradiated samples. This lowering of MP appears to be due to γradiation induced chain-scission. It decreases the average molecular weight and hence the MP. Higher energy transfer to the polymer structure from 160 MeV Ne-ion irradiation must have led to cross-linking of chains as the predominant process, increasing the molecular weight and hence the MP. This opposite effect is remarkable, and a new result. Table 1 DSC-determined melting points m1 & m2 (in oC), measured before and after irradiation, for different salt fractions (17, 19 and 21 wt.%) with a constant laponite fraction of 5%. Composition PEO17_Lapo PEO19_Lapo PEO21_Lapo

Unirradiated m1 (oC) m2 (oC) 55.831 64.920 56.194 65.033 54.131 73.873

50 kGy γ-irradiated m1 (oC) m2 (oC) 52.000 62.268 48.215 55.015 53.088 70.179

Difference (oC) (oC) -3.83 -2.65 -7.98 -10.0 -1.04 -3.69

4.2.5 Modern Techniques for Measuring Track Dimensions and Damage. Investigation of track dimension and damage caused by ion interaction with polymers has been characterized by transmission electron microscopy, scanning probe microscopy and other analytical techniques. However, in recent years, some new and innovative techniques have also come to the picture. In the next sub-sections, we shall attempt to throw some light into the details of two such innovative techniques that have been applied to study track dimensions and the extent of damage within the latent tracks.

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4.2.5.1 Elastic Recoil Detection Analysis (ERDA). Engineering polymers are mostly insulating materials and to have a proper understanding of ion-insulator interaction, knowledge of the track diameter is extremely important. Diameter of latent tracks of heavy ions in polymers can be measured by techniques such as TEM, scanning force microscopy and other state-of-the-art surface morphology probing equipments. Recently a novel approach to estimate track diameters in polymers employing online measurement of hydrogen loss during ion irradiation by on-line elastic recoil detection analysis was reported by Mittal et al. [207]. Upon ion irradiation, hydrogen (H2) is liberated from polymers as a result of cleavage of bonds associated with H-atom due to electronic excitation of constituent atoms. Since hydrogen is the lightest gaseous molecule, it can easily diffuse and escape from the polymer bringing about a reduction in the hydrogen content in the polymer due to ion irradiation. The irradiating ions affect the polymers along its path to form a damage track, which is generally larger than its own size. Thus by investigating the hydrogen profile of the irradiated polymer by on-line ERDA, one can determine the dimension of the damage zone or the track diameter. It was observed that the concentration of H-atoms decreased with increasing fluence in polymer films due to the loss of H-atoms from the damage zones. 4.2.5.2 Quadrupole Mass Spectrometer [QMA]. It is not only the track diameter, but the variation of damage caused by ion irradiation in those tracks can also provide very important information regarding radiation damage of the polymer. This can be achieved by on-line measurements using quadrupole mass analyzer [QMA]. When a polymer film is irradiated with a swift heavy ion, various gases evolve out from the ion tracks formed in the polymer beacuse of bond cleavages caused by the electronic excitation of the atom along these damaged zones. As a result there is an initial increase in the partial pressure of the gases but as the irradiation dose is increased, the partial pressure reaches a saturation value and then starts to decrease. This phenomenon occurs because with increasing fluence ion tracks overlap and when a ion enters into a previously existing iontrack, there is no release of gases. So, the partial pressure reaches a saturation and finally starts decreasing. This decay of partial pressure of the gases as a function of ion dose can be measured by QMA and provide important details about the cylindrical damage zone from which the gases are released [208, 209]. The estimation of damage cross-section by QMA differs from that of ERDA technique in the fact that in QMA the hydrogen coming out of the tracks are measured wheras in ERDA the hydrogen left in the latent tracks are investigated. Avasthi et al., measured the changes in the partial pressure of gases H2, CH4 and CO as a function of ion fluence. It was observed that the inner damage zone produced three gases while the outer damage zone produced only hydrogen. It was concluded that the ions create heavy damage in the inner zones and the degree of damage decreases radially outwards [209]. 5. Specific Applications of Irradiated Polymers 5.1 Polymeric Sensors. Conducting polymers such as polypyrrole (PPy), polyaniline (PAni), polythiophene (PTh) and their derivatives, have been used as the active layers for different type of sensors since early 1980s [210–213]. In comparison with sensors based on metal oxides that are operated at high temperatures, the sensors made of conducting polymers have some improved characteristics. They have high sensitivities and short response time even at room temperature. Conducting polymers can be more easily synthesized at lower temperatures through chemical or electrochemical processes, and their molecular chain structure can be modified conveniently by copolymerization or structural derivations. Furthermore, many conducting polymers have good mechanical properties, which allow facile fabrication of sensors. As a result, increasing attention has been paid to the sensors fabricated from conducting polymers. The fact that conducting polymers exhibit changes in color, conductivity, volume, mass, mechanical properties and ion permeability upon doping makes them efficient and low cost sensing materials. In the first report relating to sensing properties of irradiated polymers, the ammonia sensitivity of Polyvinyl chloride-polyaniline composites exposed to 120 MeV Si ions was observed to increase with increasing fluence. The response time of the irradiated composites was also observed to be

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comparably shorter as compared to the unirradiated composites [143]. Recently, the sensing response of polypyrrole–poly(n-methylpyrrole) composite film towards ammonia was investigated and the effects of 85 MeV O7+ ion irradiation at various fluences from 1× 105 to 1 × 107 ions/cm2 upon the sensing characteristics of these composites were reported [214]. Remarkable improvements in electrical and morphological properties suitable for gas-sensing applications were observed. The irradiated composite film was evaluated for the sensing of various concentrations of ammonia and excellent improvement in terms of sensitivity, lower detection limit and response time was observed, which was attributed to the increase in conductivity and the heavy morphological modification of the composite matrix due to irradiation. The lower detection limit decreased from 1 ppm to 100 ppb, the response time also decreased from 20 to 15 minutes, while the sensitivity increased (from 9.47 to 30.03) after irradiation. Srivastava et al. [215] reported the effect of swift heavy ion (SHI) irradiation on the gas sensing properties of tantalum (Ta)/Polyaniline (PAni) composite thin film based chemiresistor type gas sensor for hydrogen gas sensing application. They fabricated the chemiresistor sensing films by depositing PAni onto finger type Cu-interdigitated electrodes and then spin casting a thin layer of Ta. These films were irradiated with 150 MeV Au12+ ions at different fluences and their hydrogen sensing properties were investigated as shown in the Figure 8. The irradiated composite thin films showed better sensitivity and response as compared to the pristine ones and were also found to have better repeatability than the pristine unirradiated sample [215]. A very recent work reports the facile electrochemical synthesis of poly(N-methyl pyrrole) (PNMP) / single wall carbon nanotubes (SWNTs) composite nanofibrillar matrix for sensing ammonia and the effects of 100 MeV O 7+ ions irradiation upon the sensing parameters. It was observed that the sample irradiated at a fluence of 1 x 1010 ions/cm2 showed the best sensing behavior for 50 ppb to 500 ppm concentration of ammonia with excellent linearity. However, they reported a drift of about 13.47% from the initial values at elevated temperatures. The sensor was also found to be quite stable up to around 70 days [216].

Fig. 8 Response versus time plot for unirradiated and irradiated Ta/PANI composite sensors after hydrogen exposure at room temperature. Reprinted from [215], with permission from Elsevier. 5.2 Batteries and Supercapacitors. Batteries and supercapacitors are energy storage and conversion devices, which satisfy the requirements of high specific power and energy in a complementary way. Both conducting polymers and polymer electrolytes have found applications in batteries and supercapacitors as electrode and electrolyte materials, respectively. By virtue of their high specific capacitance and high dc conductivity in the charged state, electronically conducting polymers are suitable electrode materials for high performance supercapacitors. Swift heavy ion [energy > 1 MeV] irradiation leads to the enhancement or alteration of the properties like

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conductivity, density, chain length and solubility of polymers and as such can be used to engineer the properties of supercapacitors or batteries fabricated using these polymers [57, 138, 217, 218]. Polypyrrole thin films doped with LiClO4 were electrochemically synthesized on ITO coated glass substrate and were irradiated with 160 MeV Ni12+ ions at different fluences and all polymer redox supercapacitors were fabricated with these electrodes in order to investigate the variation in the capacitance of the device. Redox supercapacitors with LiClO4 and LiCF3SO3 doped polypyrrole electrodes and P(VDF-HFP)-PMMA based polymer gel electrolyte were fabricated [217, 218]. It was observed that although the capacitance of the irradiated films decreased, the supercapacitor showed enhanced stability compared to the devices with unirradiated electrodes for a cycle life up to 10,000 cycles as shown in Figure 9(a). Figure 9(b) shows the variation in the charge-discharge studies of the redox supercapacitor fabricated with polypyrrole electrodes. A comparative study made between unirradiated and irradiated supercapacitors with polypyrrole-based electrodes showed that an average capacitance of about 200 F gm−1 and exhibited enhanced electrochemical stability. The Coulombic efficiency of all the supercapacitors was about 90%. Similar phenomena were also observed with polyaniline and poly(3-methylthiophene) electrodes irradiated with 160 MeV Ni12+ ions [217, 218]. The capacitance was less as compared to the unirradiated sample, while the stability and life-time of the device were enhanced. This effect was ascribed to the stabilization of volatile surface groups upon SHI irradiation.

Fig. 9 (a) Stability plot and (b) Charge-discharge plots of redox supercapacitors fabricated with LiClO4 doped unirradiated and irradiated polypyrrole electrodes. Reprinted from from [217, 218], with permission from Elsevier. 5.3. Biomedical and Health Sciences: Ion beams have been found to be very useful when it comes to modifying or preparing novel biomaterials. This has led to immense use of ion irradiation techniques in the field of biomedical engineering and health sciences for improving medical device function, biocompatibility and as a new mutation breeding method. Ion beam based processes such as ion implantation and ion beam assisted deposition have been extensively used for polymeric biomaterials modification, such as in improving the wear resistance of artificial joint components, in improving wettability, anticoagulability, anticalcific behavior of biomedical polymers, and in minimizing biofouling of medical devices [112, 219]. Some of the functional properties of polymeric biomaterials that can be enhanced by ion irradiation are (i) Wettability and (ii) Bacteriostaticity. It has been observed that ion irradiated polymers exhibit 20% enhancement in the polar component of the surface energy leading to an increased wettability in water [219, 220]. One of the most serious problems for polymers in biomedical applications is the adhesion of bacteria to the polymer surface and of their survival rate. The property of bacteriostaticity (restriction of bacterial adhesion and growth) can be conferred on a polymer by three possible ways when using ion beams: (i) ion beam based doping of the surface

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layer of polymer with an bacteriostatic agent (e.g. silver), (ii) ion-beam induced modification in polymer wettability [112] or (iii) change in surface morphology of polymer [221] as the cells adhesion increases when the surface rugosity is similar to the size of bacteria, i.e. in the few micrometers range. Ion beam irradiated surfaces of polyethersulphones and polyurethane were found to exhibit remarkable improvement of the cell adhesion, spreading and proliferation and also showed [113] fluence dependence with a threshold (~1 x 1015 ions/cm2). The use of ion irradiation for the formation of hydrogels from different polymers in solid state and solution as well as their applications as drug delivery systems, implants, injectable systems, stimuli-responsive systems, hybrid-type organs in medical science were reviewed by Rosiak et al. [222]. Cell adhesion is strongly influenced by irradiation induced surface transformation of polymers. Ion beam irradiation has been employed for surface modification of engineering polymers such as polyurethane (PU), polyethersulphone (PES), polystyrene (PS), silicon rubber, expanded polytetrafluoroethyrene (ePTFE), polydimethylsiloxane (PDMS) or poly-hydroxy-methyl-siloxane (PHMS) and applied for biomedical applications [221, 222-224]. Remarkable enhancement in adhesion, proliferation and spreading of various cells such as fibroblast, astrocyte or endothelial cells was observed. It was proposed that ion beam irradiation increased the wettability of the surface of these polymers and as such enhancement in cell-material interaction was observed [221, 222-224]. The modification of the adhesion and spreading of BHK21 fibroblast cells was studied for new biocompatible surfaces obtained by irradiating polyhydroxymethylsiloxane thin films with increasing doses of 5 keV Ar+ ion beams [223]. Dural substitute in restorative cranial surgery is one of the most spectacular examples of the possibilities offered by ion implantation for surface modification of polymeric biomaterials. Ideally, a dural substitute should consist of two different surfaces: a cell adhesive external side and a non-adhesive cerebral side. Typically, these elements are manufactured from e-PTFE, which is highly hydrophobic and of very limited cell adhesion. It was observed that Ne+ irradiation of one side of the e-PTFE resulted in excellent bone attachment and tissue in-growth without altering the properties of the pristine side of the sample [224]. This study proved that ion implantation can allow desirable combination of very different properties in various parts of the same object. Improvement of polydimethylsiloxane guide tube for nerve regeneration treatment by carbon negative-ion implantation was also reported [225]. The irradiated surfaces showed a dose-dependent increase of cytocompatibility, with an observed onset of the effect at about 5x1014 ions/cm2. At this dose, both the enhancement of cell adhesion, for an incubation time of 2 h, and complete cell confluence after an incubation time of 48 h was observed. The observed fluence-dependent trends in cell adhesion and spreading were correlated with the irradiation-induced modifications of the polymer surface composition and the related change in surface energy. Ion implantation at 25 and 100 keV were used for modifying the surface properties of two biomedical polymers viz., chemically functionalized polycaprolactone (PCL) and poly ethylene glycol (PEG). The modulation induced by the different energy dispersion mechanisms of Ar and He allowed satisfactory modifications for both the activation of the surfaces of chemically functional polycaprolactone (PCL) and the stabilization of anti-fouling poly(ethylene glycol) (PEG). The response of the modified surfaces towards biomolecular interaction were demonstrated by the induction of preferential adsorption on irradiated PCL and the inhibited adsorption onto implanted PEG regions for selected oligopeptides and proteins [226]. The adsorption of human serum albumin was studied for poly-hydroxy-methylsiloxane and polyethylene terephthalate surfaces modified by 5 keV Ar+ irradiation. The adsorption kinetics of albumin was investigated as a function of the modifications induced by irradiation of the two polymer surfaces [227].

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Vascular smooth muscle cells derived from the rat aorta were cultured on unmodified and 150 keV F+ ion-implanted polystyrene (PS) at fluences of 5 ×1012 and 5 ×1014 ions/cm2. In 3-day-old cultures, the cells adhered to the modified polystyrene in higher numbers and over larger contact areas as shown in Figure 10. Increased resistance of the cells to trypsin-mediated detachment from the growth support indicated an improved adhesion of cells to the modified polymer at later culture intervals. The cells cultured on ion-modified polymers also were larger and had higher total protein content [228]. Polyacrylic acid was grafted into PVDF using both swift heavy ions and electrons. In order to anticipate the best grafting conditions, acrylic acid swelling of PVDF films was investigated as a function of temperature and monomer concentration. Grafted and functionalized films were characterized using infrared spectroscopy. The PVDF-g-PAA films exhibit different structures depending on the monomer concentration. Immobilization of an amino-terminated molecule and a peptide onto PVDF was achieved using water soluble carbodiimide [229]. The technique of radiation initiated grafting for the modification of polymeric surfaces has been well reviewed by Bhattacharya et al. [230]. Ion irradiated conducting polymers have also been found to have biomedical applications. Polyaniline nanofibers irradiated by 90 MeV O7+ ions have been found to exhibit better antioxidant and haemolysis prevention properties. These properties were found to increase with increasing irradiation fluence. The observed enhancements in the antioxidant and haemolysis prevention properties were attributed to the physico-chemical modifications in PAni nanofibers caused by ion irradiation [145].

Fig. 10 Rat aortic smooth muscle cells in 3-day-old cultures with unmodified (A) and ion-irradiated polystyrene at the dose of (B) 5 ×1012 and (C) 5 ×1014 F+ ions/cm2. Phase contrast, original magnification 300x. Adopted from [228]. 5.4. Nuclear Reactors and Space Applications. Some special types of polymers such as elastomers and a few rubber products have found critical applications in nuclear reactors and nuclear reactor propelled ships. Fission generates energetic particles as well as -radiation. Polymer components in spacecrafts can be subjected to damaging cosmic radiations, and need special consideration. For unavoidable use of polymers in radiation environments as mentioned above, the main goal of research in this field is to minimize radiation damage in these materials and to design and develop superior radiation resistant organic monomers with improved antirads. Designing special monomers to produce radiation resistant polymers requires detailed investigation of the influence of the number of aromatic substitutions and their distance from the polymer backbone chain on radiation stability of the polymers. Predictions of life of elastomers for inflatable seals of Fast Breeder Reactors under fission-related radiations and developing better materials have already been listed in the web [231]. Two polymers namely poly(phenyl acrylate) and poly (phenyl methacrylate) were synthesized and their radiation stability investigated. Poly(phenyl methacrylate) exhibited cross-linking with a G(x) of 0.054, which was much less as compared to 0.37 for poly(ethyl acrylate). Poly(phenyl

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methacrylate) however, showed chain scissioning behavior with G(s) of approximately 0.48 as compared to 1.67 (approx) for poly(methyl methacrylate) [PMMA]. A conjugated polymer poly(Nvinyl carbazole) was found to be two times more effective as a radiation resistant polymer as compared to polystyrene. Co-polymerization of N-vinyl carbazole with acrylates was reported to generate elastomers with promising radiation stability. It was also observed for a 90/10 ethyl acrylate-maleic anhydride copolymer series with various aromatic amine substituents that radiation stability increases with increasing resonance energy of the substituents [232]. The radiation stability of ethylene-propylene terpolymer subjected to radiation degradation under various stabilization states were investigated for applying these polymeric materials in nuclear reactors. Two sorts of elastomers were tested free of stabilizer or in the presence of phenolic antioxidants: IRGANOX 1010 and ETHANOX 330. The additive concentration of 2 % places polymers under high stabilization state. Low dose rate (0.04 and 0.4 kGy/h) was applied for the simulating of real parameters of material ageing. The presence of used antioxidants offered a remarkable resistance to the action of -radiation (137Cs) emphasizing the preservation of either physical and chemical properties, or integrity of neat materials [233]. Superior nuclear shields were constructed from gadolinium filled chloro-fluoro substituted ethylene polymers, particularly filled ethylene-chlorotrifluoroethylene or filled polychlorotrifluoroethylene. The fillers used in conjunction with these polymers comprise gadolinium compounds, preferably compounds such as gadolinium boride, gadolinium oxide, gadolinium aluminate and gadolinium aluminum borate. Radiation resistant oils from alkyl diphenyl ethers and alkyl aromatic polymer mixtures were gelled with conventional aliphatic soaps to make new greases for nuclear power plants [234]. Improved radiation resistance was obtained; even better greases ensued when aromatic gelling agents were used. Certain compounds containing selenium or sulfur were found to be most satisfactory in reducing this effect. These finished oils and greases performed well after radiation doses which destroyed conventional products. As with organic fluids, the radiation resistances of elastomers and plastics have been found to depend upon the base material and on the additives or compounding ingredients used. It was shown in a survey that the common solid polymers such as polyurethanes to be the most resistant of the elastomers while poly(vinyl chlorides) were the best of the flexible plastics [234]. 5.5. Microelectronics: One of the emerging applications of ion beam irradiation of polymers is in the fabrication of microelectronics, micro-machines and ultra-small devices [3, 235]. Irradiation can be employed for lithography of polymer films since radiation induces variations in the solubility of the polymer by virtue of cross-linking or chain-scissioning events. Both “negative” and “positive” resists can be fabricated through ion beam lithography of polymers. The solubility of the negative resists decreases upon exposure to ion beams primarily because of cross-linking, while that of a positive resist increases due to chain-scission. In either case, a suitable solvent mediated etching of the more soluble part leads to a patterned polymer surface. Complex micro-device production based on PMMA has been reported using 2 MeV proton irradiation. A 1 m diameter beam was employed for deep penetration of PMMA resulting in patterns [236]. An advantage of this technique is that there is no requirement of masks and after the etching process the irradiated PMMA, which suffers chain-scission dissolves in the solvent leaving behind beautiful patterned structure having smooth, submicron pores [3]. Ion beam modification of polymers was also used for fabrication altering refractive index and also non-linear optical properties of polymers, in research aimed at patterning of optical and optical devices such as waveguides [105]. 5.6. Nanopatterning: In recent years swift heavy ion irradiated polymers have been used as template for nanopatterning for making different nanostructures. Piraux et al. [237] irradiated polycarbonate (PC) membranes with 120 MeV Ar9+ ions followed by chemical etching. The etching conditions were adapted to produce pore diameters ranging between 10 and 50 nm. They

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successfully grew magnetic multilayered nanowires by electro-deposition into nanometer-sized pores of the template polymer membrane. Skupinski et al. [238] were able to achieve the smallest open pores formed in polyimide (PI) by ion track technology and studied in dry state. High aspect ratio magnetic Ni nanowires were successfully electroplated in an ion track template PI resist on a silicon wafer. Much recent work has been reported in this field. For example, copper and gold nanowires have been synthesized using potentiostatic electrodeposition within confined nanochannels [239, 240] of a porous ion-crafted membrane, and the morphology of the electrodeposited copper nanowires was studied using scanning electron microscopy. These nanowires had uniform diameters of about 100 nm, which corresponded to the pore size of the templates used in that work. There are various reports in literature on electrodeposition in ion track membranes (template method) which was used to create large two-dimensional arrays of wires and tubes of different kinds of metals [239-242]. Gehrkewe et al. [243] demonstrated the fabrication of conducting vertical nanostructures in ta-C together with self-aligned gate electrodes. A multilayer assembly consisting of polycarbonate on the top was irradiated with 1GeV U heavy ions and subsequently exposed to several selective etching processes. Chemical track etching opens nanochannels in the polymer which are self-aligned with the conducting tracks in ta-C because they are produced by the same ions. Through the pores in the polymer template, the Cr and SiN x layers are opened by ion beam sputtering and plasma etching, respectively. The resulting structure consists of nanowires embedded in the insulating carbon matrix with a built-in gate electrode and has potential application as gated field emission cathode. 6. Summary The present paper reviews the effects of ion irradiation in engineering and functional polymers (including electroactive polymers) with a focus on some of the newer analytical tools that have been used for characterizing radiation damage in these types of polymers. It also reviews current use of fast ion damage and ion implantation in different polymers and polymer-composites for biomedical, electronic, nanopatterning, gas sensing and energy storage applications. Radiation damage in polymeric materials is a complex phenomenon and proper understanding of the radiation damage mechanisms in polymers require careful investigation and analysis of many factors. X-ray, electron microscopy and optical investigation of the molecular structure of the polymers is very important in order to understand the damage caused by ion beams in the polymer. Occurrence of phenomena such as chain-scission and cross-linking on irradiation, observed in many polymers, can be guessed to a great extent from proper knowledge of the molecular structure. Use and further development of newer analytical techniques such as positron annihilation lifetime spectroscopy must be encouraged for investigation of radiation damage; this technique is very useful to investigate free volume change and thereby the size of defects in polymers upon irradiation. Along with FTIR spectroscopy, conventionally used to characterize conformational modifications in polymers due to irradiation, micro-Raman spectroscopy can be very useful for characterizing conformational variations in the polymer due to ion irradiation. In electroactive polymers, molecular level details about radiation effects such as benzenoid to quinoid transition can be obtained by RS. Differentiating between ion irradiation induced cross-linking and chainscission effects in polymers can be accomplished by the measurement of the melting point of polymers using DSC, which is a very noble way to characterize radiation damage. Enhancement in melting point of polymers generally indicates cross-linking, whereas chain-scission in polymers should reduce their melting points. Other newer approaches for characterizing radiation damage in polymers include dielectric relaxation spectroscopy, on-line ERDA for determination of track dimension and composition. Quadrupole mass analyzer (QMA) can probe variation of the extent of damage within the tracks. Ion beam induced physico-chemical modifications in polymers have been employed in different applications. Some of the emerging applications of ion irradiated polymers like active layers in polymeric sensors, biomedical and health sciences, “O”-rings, lubricants,

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nuclear shields in nuclear power plants, microelectronics, micromachining and nanolithography have been briefly reviewed to present the latest trend of research in this area. Present bibliography of 243 references is representative but not exhaustive, as ion irradiation effects in polymers and polymer-composites constitute a vast and fast growing area. Acknowledgement One of the authors (UD) thanks Alexander von Humboldt Foundation, Germany, and two host institutes KIT, Karlsruhe, and PPF, Dresden, for partially sponsoring his participation in useful polymer conferences in Germany. Authors thank the Editor for good guidelines and advices. References [1]

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Defect and Diffusion Forum Vol. 341 (2013) pp 51-68 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/DDF.341.51

Swift Heavy Ion Induced Modification in Physical Properties of Poly methylmethacrylate (PMMA)/Nickel (Ni) Nanocomposites N. L. Singh1,a, Chaitali Gavade1,b, and P. K. Khanna2,c 1

Physics Department, M. S. University of Baroda, Vadodara-390002, India 2 Nanochemistry Lab., Department of Applied Chemistry, Defence Institute of Advanced Technology (DIAT), Pune - 411 025, India

a

[email protected] (corresponding author), [email protected], c [email protected]

Keywords: Heavy Ions Irradiation, Nanocomposites, UV-visible, XRD, DSC, Dielectric and Magnetic Properties

Abstract. We have These films were irradiated with 85 MeV C- ions at the fluences of 1 x 1011 and 1 x 1012 ions/cm2. Changes in the optical, structural, dielectric, magnetic and thermal properties of (PMMA)/Ni nanocomposites of different concentrations of nickel nanoparticles (5%, 10%, 15%) due to swift heavy ion irradiation were studied by means of UV–visible spectroscopy, X-ray diffraction, impedance gain phase analyzer, SQUID and differential scanning calorimetry. Optical properties like band gap were estimated for pure polymer and nanocomposite films from their optical absorption spectra in the wavelength range 200-800 nm. It was found that the band gap value shifted to lower energy on doping with metal nanoparticles. Differential scanning calorimetry analysis revealed a decrease in the glass transition temperature upon irradiation, which may be attributed to the scissioning of polymer chain due to ion beam irradiation which is also corroborated with XRD analysis. Surface morphology of the pristine and irradiated films was studied by scanning electron microscopy (SEM). The breakage of chemical bonds resulted in an increase of free radicals, unsaturation etc. as revealed from FTIR analysis. The dielectric properties were observed to enhance with an increase in metal compound concentration as well as with irradiation dose. This may be due to metal/polymer bonding and conversion of polymeric structure into hydrogen-depleted carbon network. Zero-Field-Cooled (ZFC)/Field-Cooled (FC) magnetization and magnetic hysteresis measurements were performed using a superconducting quantum interference device (SQUID) magnetometer from temperatures ranging from 5 K to 300 K, to investigate the magnetic properties of nanocomposites. The changes in topography of surfaces were also observed upon irradiation. 1. Introduction Recently, the unique physical and chemical properties of nanoparticles, due to surface or quantumsize effects, have been the subject of passionate investigations. It is also no exaggeration to say that an innovative breakthrough has been made in this area. The synthesis and functionalization of nanostructured magnetic materials has been an interesting area of study because of its possible applications in a variety of widely diversified areas ranging from information technology to nanobiotechnology [1, 2]. Ferromagnetic nanoparticles such as Fe, Co, and Ni are the focus of growing interest because of both the richness of their physical properties and potential applications like catalysts, high density magnetic recording media, magnetic cooling system, cell separation, enzyme immunoassay, magnetic sensors, electromagnetic device applications, and electromagnetic interference suppression, ferrofluids, and medicine (local drug delivery, magnetic resonance tomography, etc.) [3]. There is a serious obstacle in assembling and maintaining magnetic nanoparticles due to their tendency to agglomerate, which is a deterrent to its different purposes. To achieve in these areas, metallic or oxide nanoparticles have been prepared in the form of ferrofluids or embedded in various “rigid” matrices (polymers, zeolites, and others). Magnetic polymer nanocomposites represent a class of functional materials, where magnetic nanoparticles are

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embedded in polymer matrices. In case of nanocomposites, the interplay between the intrinsic properties and interparticle interactions are crucial to control the overall magnetism [4-7]. There are few reports on this system. E.Muhammad Abdul Jamal et al studied the synthesis and dielectric, magnetic properties of rubber-Ni nanocomposites [8, 9]. S. Shah et al studied the effect of proton irradiation and 120 MeV Ni-ions on the physical properties of PMMA/Ni composites [10, 11]. Singh et al. [12] showed that the Vickers' microhardness of the composites increases by doping filler in PMMA and also with the irradiation by 120 MeV Ni10+ ions. Sweta et al. [13] studied the effect of 100 MeV Si+7 ions beam on optical property and observed that the band gap changed from 4⋅38 to 3⋅60 eV as the irradiation fluence increased with respect to the pristine CdCuS nanocomposite PMMA film. Sharma et al. [14] investigated the change in microstructure of PMMA matrix after irradiation as well as enhancement in luminosity of ZnO/PMMA nanocomposite with swift heavy ions (100 MeV Ni+8 ions) irradiation. Swift heavy ion (SHI) irradiation is a unique post-deposition treatment in view of its ability to modify the polymers properties which are associated with the collisions of penetrating high energy ions with the target atoms and the formation of defects. These defects lead to structural changes in the modified polymer resulting in the formation of new chemical groups, cross-links between macromolecules, degassing of volatiles, as well as oxidation processes upon exposure to air [15]. Swift heavy ion irradiation tends to damage polymers significantly by two ways mainly, electronic excitation and ionization. The nature of defects and the relative radiative sensitivity of different polymers depend on the properties such as their composition and molecular weight and on the charge, mass and energy of the ion and also on the environmental conditions during irradiation [16, 17]. In the present work, we have studied changes in the optical, structural, dielectric, magnetic and thermal properties of PMMA/Ni nanocomposites of different concentrations of nickel nanoparticles (5%, 10%, 15%) due to swift heavy ion irradiation by means of UV–visible spectroscopy, X-ray diffraction, differential scanning calorimetry, impedance gain phase analyzer and SQUID. 2. Experimental details 2.1 Sample Preparation and Irradiation. (PMMA) is characteristically used polymer in biomedical applications [18] and is generally hydrophilic in nature. PMMA was purchased from National Chemical, Gujarat, India of AR grade (95.5%). The nickel nanoparticles were synthesized by the reduction method and of particle size around 10-15 nm reported by Khanna et al. [19]. The composite films (thickness~120µm) of different concentrations of nickel nanoparticles powder (5%, 10%, 15%) in PMMA were prepared by casting method in which Tetrahydro furan (THF) is used as a solvent and stirred and provided ultrasonic bath for half an hour to mix well. These films were irradiated with 85 MeV C- ions at the fluences of 1 x 1011 and 1 x 1012 ions/cm2 at Inter University Accelerator Centre (IUAC), New Delhi, India. 2.2 Characterization. UV- visible spectroscopy was carried out in the range of 200-800 nm for the evaluation of band gap energy by using Perkin Elmer Lambda 25 UV visible spectrometer. The structural studies were carried out by X-ray powder diffractometer (Bruker AXS D8 Advance) with Cu Kα radiation (1.5418 Å) in a wide range of Bragg angles (10◦ ≤ 2θ ≤ 80◦).The thermal analysis was performed with differential scanning calorimetry (S II EXSTAR 6000, DSC 6220) with the heating rate of 5oC min-1. Surface morphology of pristine and irradiated surfaces was studied by means of scanning electron microscopy (JEOL model: JSM 6380LV). FTIR spectra of pristine and irradiated films were recorded in the wave number range 4000–500 cm-1 using ATR-FTIR spectroscopy (Jasco- 4100) with a resolution of 4 cm-1.

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Zero-Field-Cooled (ZFC)/Field-Cooled (FC) magnetization and magnetic hysteresis measurements were performed using a superconducting quantum interference device (SQUID) magnetometer from 5 K to 300 K to investigate the magnetic properties of polymer nanocomposites by Quantum Design (SQUID) at IIT-Roorkee. The dielectric properties of all the samples were measured using an impedance gain/ phase analyzer (Solartron-1260) in the wide frequency range of 100 Hz – 10 MHz. The conductivity of the material was estimated using equation: σ = (2πfCpDt) / A (Ώ-1cm-1)

(1)

Dielectric permittivity of the samples was determined using relation: ε = Cp/Co

(2)

Where Cp is the capacitance measured using an LCR meter, f is the frequency, D is dielectric loss and vacuum capacitance Co = εo A/t, A and t are the cross-sectional area of the electrodes and thickness of the sample, respectively. εo is dielectric permittivity of air = 8.85 x 10-12 F/m. 3. Results and Discussion 3. 1 Optical Properties. The promotion of electrons in the σ, π and n orbitals from ground state to the higher energy states which are described by molecular orbitals due to the absorption of light energy by polymeric samples in the UV and visible regions. Many of the optical transitions which occur due to the presence of impurities have energies in the visible region of the spectrum, consequently the defects are referred to as colour centres. The effect of ion beam interaction with polymers produces damage and leads to the generation of new defects and charge states [20, 21]. A shift in the absorption edge towards longer wavelength is observed for irradiated samples as shown in Fig. 1. This behaviour is generally interpreted as caused by the formation of extended systems of conjugated bonds, i.e. possible formation of carbon clusters. In the investigated range of wavelengths the absorption bands are associated to the π–π* electronic transitions. These types of transitions occur in the unsaturated centers of the molecules, i.e. in compounds containing double or triple bonds and also in aromatics. The excitation of p electron requires smaller energy and hence, transition of this type occurs at longer wavelengths [22].

Fig. 1 The optical absorption spectra of pure PMMA, composites and irradiated samples

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3.1.1 Determinations of band gap. The change in optical properties have been studied by PerkinElmer 25 Lambda UV-Visible spectrophotometer for the pristine and irradiated samples in the frequency range 200-800 cm-1 as illustrated in figure 1. The optical band gap Eg is obtained by Tauc’s equation [25]: ωε(λ) = (ħω-Eg)2

(3)

Where ε(λ) is the optical absorbance and λ is the wavelength. The intersection of the extrapolated spectrum with the abscissa of the plot (ε1/2/λ) versus (1/λ) yields the gap wavelength (λg) from which the energy gap is derived as Eg = (hc/λg). It is noticed that the band gap (energy gap) value shifted to lower energy from 4.58 eV upto 3.35 eV due to doping of nickel nanoparticles and also upon irradiation. This is because of the scissioning of polymer chain and as a result, creation of free radicals, unsaturation etc. and thus a capability of increasing the conductivity of the composites [24]. The number of carbon atoms per cluster (N) can be calculated by following relation [23] in eqn. (4); the values are listed in Table 1. Eg = 34.3/N eV

(4)

Where N is the number of carbon atoms per cluster and Eg is the energy band gap. Table 1 Band gap by direct allowed transitions, number of carbon atoms in pure PMMA and composites films irradiated by 85 MeV C- ion beam. Sample

Band gap in eV

Pure PMMA Pure PMMA (1 x 1011) Pure PMMA(1 x 1012) PMMA+15% Ni PMMA+15% Ni (1 x 1011) PMMA+15% Ni (1 x 1012)

4.58 4.35 4.26 3.82 3.56 3.35

Number of carbon atom (N) per conjugation length ~2 ~2 ~2 ~2 ~3 ~3

3.2 XRD Analysis. The diffraction patterns of the pristine and irradiated polymer composites films are shown in Fig.2. The broad peak at 2 θ = 14.79 in the diffraction pattern of the pristine film indicates that the polymer is amorphous in nature. Because of influence of nickel nanoparticles in PMMA matrix, it shows semi-crystalline diffraction pattern. It is observed from figure 2 that intensity of diffraction peak varies after irradiation. The crystallite size has been calculated before and after irradiation using Scherrer’s formula [26]: b = Kλ/Lcosθ

(5)

Where b is FWHM in radians, λ is the wavelength of X-ray beam (1.5418 Å), L is the crystallite size in Å, K is a constant which varies from 0.89 to 1.39, but for most of the cases it is close to 1. The appearance of sharp peak in composite indicates some degree of crystallinity. It is also observed that the intensity of the peak decreases and to some extent broadened after irradiation; hence offers confirmation of decrease in crystallinity. Since no significant change in the peak position is observed, this reveals that lattice parameters do not change significantly.

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Fig. 2 X-ray diffraction patterns of pristine and irradiated samples The crystallite size of the pristine and irradiated samples is listed in Table 2. Results show that the ion beam irradiation causes large amount of energy deposition in the material which leads to decrease in crystallite size. It may be attributed to splitting of crystalline grains due to absorption of large amount of energy and it also reflects the formation of disorder system. The irradiation may induce chain scissioning, which is also corroborated by DSC analysis, assumed to be responsible for the reduction in crystallinity of the composite [27]. Table 2 Crystallite size of pristine and irradiated samples Sample

2θ

Crystallite size (nm)

Pure PMMA

14.794

10.11

Pure PMMA(1 x 1012)

14.977

9.52

PMMA+15%Ni

14.394

12.86

PMMA+15%Ni (1 x 1012)

14.716

11.84

PMMA+15%Ni

44.597

10.14

PMMA+15%Ni (1 x 1012)

44.583

9.58

3.3 Differential Scanning Calorimetery (DSC). The important property of the polymer is the glass transition temperature (Tg), which is defined as the temperature at which the plastic becomes hard and brittle when cooled rapidly after heating. At the glass transition temperature, the weak

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secondary bonds that stick the polymer chains together are broken, and the macromolecule starts to move. A differential scanning calorimetry (DSC) experiment was performed using a reference material and a predetermined heating (or cooling) rate was imposed to the system with a temperature excursion. If the temperature difference develops between the sample and reference the power is adjusted to remove the diffrence and DSC measures the heating power difference between the sample and reference which was recorded and shown in Fig. 3. The pure PMMA has a Tg value of about 64.60 oC, while tendency of increase of Tg value after insertion of nanoparticles was observed for Ni/PMMA nanocomposites. We found that the value of Tg, for pristine and irradiated samples is observed at about 78.74oC and 76.33oC, respectively, for highest concentration of Ni nanoparticles. After irradiation, it was found that Tg shifted to lower temperature. It reveals that the ion irradiation leads to polymer chain scissioning and subsequently reduction in molecular weight. As a result, the system moved towards the more disordered state, which is also corroborated by XRD results [28].

Fig. 3 DSC thermograms for pure PMMA, nanocomposite pristine and irradiated samples 3.4 Surface Morphology. Figure 4(a–e) shows the SEM images of Ni nanoparticles, pristine, composites and irradiated composite films with magnification of X250. The analysis shows that the filled partilces are distributed randomly in the matrix which display continuous contact between themselves and formed conducting paths. After irradiation, significant changes in surface morphology were observed. The surface roughness is observed to increase with concentration of Ninanoparticles and also upon irradiation.The increase in roughness with Ni-nanoparticles may be attributed to the increase in density and size of metal particles on the surfaces of the films, which is also responsible for decrease in crystallinity of the material as indicated by XRD analysis [11].

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(a)

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(b)

(c)

(d)

(e) Fig. 4 (a-e) SEM images of (a) Nickel nanoparticles, (b) Pure PMMA, (c) Pure PMMA (1 x 1012), (d) PMMA+ 15 % Ni, and (e) PMMA + 15 % Ni (1 x 1012) 3.5 Chemical Response. The nature of chemical modifications can be studied through the characterization of the vibration modes determined by infrared spectroscopy. There is maximum absorption in the wave length range of 2800-3000 cm-1 which corresponds to the C-H stretching vibrations. The band around 3000 cm-1 corresponding to CH2 is not showing any significant change after irradiation, which means CH2 groups are resistant to radiation. The peak around 1716 cm-1 shifted towards lower side, i.e. 1705 cm-1 after irradiation which corresponds to C = O group. OH

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stretching vibrations are also not affected upon irradiation. The other functional groups of the polymer structure are identified from the pristine spectrum as follows, (a) –CH=CH- (trans.): 966 cm-1, (b)-O-C=O, C-O: symmetric and asymmetric stretching vibrations: 1000–1500 cm-1, (c) nonconjugated C-O ester stretching band independent group of PMMA (-COOCH3): 1700 cm-1. The FTIR spectra show the interaction between the macromolecule and the filler particles. As a consequence, polymer composites are showing anomalous nature [29, 30].

Fig. 5 FTIR spectra of pristine and irradited samples 3.6 Magnetic Properties. The temperature (T) and magnetic field (H) variations of the magnetization (M) were measured with a SQUID (superconducting quantum interference device) magnetometer. The temperature variations of M for the zero field- cooled (ZFC) and the fieldcooled (FC) cases were measured from 5-300 K at H=500 Oe applied field. Hysteresis measurements were carried out at 300 K with magnetic field swept from 50 kOe to -50 kOe. Nickel is known to be one of the important magnetic materials. The magnetic measurement of Ni/PMMA nanocomposites having Ni concentration of 15% by weight is discussed here. Figure 6(a, b) shows the M-H curves for the pristine and irradiated ( 1 x 1012 ions/cm2) samples. The magnetic parameters extracted from the measurements are listed as saturation magnetization (MS) 0.085 emu/gm, coercive field (HC) 138 Oe, remnant magnetization (MR) 0.015 emu/gm for pristine and MS ~ 0.1070 emu/gm, HC ~147 Oe, MR ~ 0.020 emu/gm for irradiated samples, respectively. For the sake of comparison, we note that the saturation magnetization and the coercive field for commercial bulk nickel powder at 300 K are about 57.5 emu/g and 100 Oe, respectively. Kumar et al.[31] showed that MS ~ 30.1 emu/gm for nickel nanoparticles in the polystyrene matrix is smaller than that of commercial bulk nickel powder; the magnetization of the nickel nanoparticles in the polystyrene matrix at 1.6 Tesla did not reach full saturation and shows a very weak hysteresis. This deviation is undoubtedly a result of the nanostructure of the sample. The saturation magnetization in nanoparticle system is generally lower than that of the bulk materials and is strongly influenced by the supporting matrix. The increase in values of MS and HC after irradiation may be attributed to the change in exchange and dipolar interactions mediated by the matrix[29].

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Fig. 6(a) M-H loop for pristine sample

Fig. 6(b) M-H loop for irradiated sample Figure 7 shows the comparison of FC-ZFC curves of pristine and irradiated nanocomposite samples. For the zero-field-cooled (ZFC) measurement, the samples were cooled down from room temperature to 5 K in the absence of an external magnetic field and the magnetic data were acquired during the warming run in a constant external field. In the field-cooled (FC) measurements, the samples were initially cooled down to 5 K in the presence of a magnetic field and the FC data were recorded during the warm up cycle in the same magnetic field. FC magnetization decreases continuously with the increase of temperature. Such characteristic behavior of FC magnetization data is attributed to ferromagnetism in material [32]. None of these curves show any characteristic sharp change in magnetization associated with the well established ferromagnetic to super paramagnetic transition in single domain nanoparticles. This indicates that the particles (mostly in clusters) in the polymer matrix are predominantly ferromagnetic ( blocking tempareture for both the samples is > 300 K) [29].

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Fig. 7 FC- ZFC magnetization for pristine and irradited samples 3.5 AC Electrical Conductivity. Fig. 8(a, b, c) shows the frequency dependence of AC electrical conductivity of pristine and irradiated nanocomposites. The increase in conductivity with nickel concentration for pristine samples may be attributed to the conductive phase formed by dispersed metal nano particles in the polymer matrix. It is known that electrical conductivity of such composites depends on the type and concentration of the dispersed materials [33, 34]. An AC field applied to the metal–polymer–metal structure may cause a net polarization which is out of phase at sufficiently high frequency with the field. This may result in an increased AC conductivity which appears at frequencies greater than that at which traps are filled or emptied [35]. It is also observed that conductivity increases on increasing fluence. This increase in conductivity is probably due to the generation of large number of charged and active chemical species, cations, anions, and radicals [36].

Fig. 8(a) Conductivity versus log frequency for pristine samples

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Fig. 8(b) Conductivity versus log frequency for irradiated (1 x 1011) samples

Fig. 8 (c) Conductivity versus log frequency for irradiated (1 x 1012) samples 3.6 Dielectric Properties. The dielectric constant showed the ability of a material to store electric potential energy under the influence of an alternative electric field. The effect of nickel concentrations and irradiation on the dielectric constant under various applied sweep frequencies is shown in Fig. 9(a, b, c). The variation in dielectric constant with frequency indicated the presence of an interfacial polarization in the PMMA matrix. This decreasing dielectric constant with increased frequency ( above 1MHz) is thought to be caused by the slow dielectric relaxation of the matrix and the interface of the composite [37]. At higher frequencies, the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field. Hence dielectric constant decreases with increasing frequency. Also according to the Dissado and Hill theory, in intra-cluster motions, the relaxation of a dipole will produce a ‘chain’ response in its

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neighbouring dipoles and the reaction of the neighbouring dipoles will, in turn, affect the first dipole, so the overall effect will be seen as a single-cluster dipole moment relaxation. This reduces the dielectric constant at these frequencies [26]. The observed behaviour of the fluence dependence of dielectric constant in studied frequency range can be explained by the enhanced free carriers due to irradiation [11].

Fig. 9(a) Dielectric constant versus log frequency for pristine samples

Fig. 9(b) Dielectric constant versus log frequency for irradiated (1 x1011) samples

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Fig. 9(c) Dielectric constant versus log frequency for irradiated (1 x1012) samples 3.7 Dielectric Loss. Fig. 10 (a, b, c) shows the variation of dielectric loss with log frequency. The dielectric loss decreases exponentially and then became less dependent on frequency. As the frequency increased further, the charge accumulation decreases and because of that the dipole polarization effects reduced, and the value of the loss factor declined accordingly [36]. The increase in dielectric loss with increasing filler contents may be attributed to the interfacial polarization mechanism of the heterogeneous system. Further, moderate increase in loss occurs due to irradiation. The growth in dielectric loss and thus increase in conductivity is brought by an increase in the conduction of residual current and the conduction of absorption current [38].

Fig. 10(a) Dielectric loss versus log frequency for pristine samples

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Fig. 10(b) Dielectric loss versus log frequency for irradiated (1 x1011) samples

Fig. 10(c) Dielectric loss versus log frequency for irradiated (1 x1012) samples 4. Summary The observations from UV-visible, DSC, FTIR, XRD, SEM, AC electrical, dielectric magnetization measurement studies are summarized as follows:

and

(i) It was observed from the UV- visible spectroscopy analysis that the band gap value moved to the lower energy (from 4.58 eV upto 3.35 eV) on doping with nickel nanoparticles, as well as upon irradiation.

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(ii) An XRD analysis reveals that the crystallite size of the composites decreased after ion beam irradiation which is also corroborated by the DSC and FTIR analysis due to the chain scissioning upon irradiation. (iii)SEM images reveal that the surface roughness increases with increasing concentration of nanoparticles and also upon irradiation. (iv)The ion beam irradiation of polymer nanocomposites leads to chain scission and cross linking, which leads to increase the free radicals, unsaturation etc. As a result of that there are changes in magnetic and dielectric properties. (v) The magnetic properties enhanced after irradiation which may be attributed to the exchange dipolar interaction of particles in the matrix and generation of free radicals. (vi)AC electrical and dielectric properties of PMMA/Ni nanocomposites were studied over a wide range of frequency as a function of filler concentration. Both the dielectric constant and the electrical conductivity of the composites increased with the increase of Ni content. These phenomena could be interpreted from interfacial polarization of heterogeneous system. Acknowledgement Authors are thankful to the Inter University Accelerator Centre (IUAC), New Delhi, for providing irradiation facility and financial assistance and PKK thanks his student P More for experimental assistance. UGC-DRS laboratory, Physics Department, M. S. University of Baroda is gratefully acknowledged for providing characterization facilities. One of the authors CG is really thankful to Dr. Sanjeev Kumar for his help in the the SQUID measurement. References [1] S. P. Gubin, Yu. A. Koksharov, Preparation, Structure, and Properties of Magnetic Materials Based on Co-Containing Nanoparticles, Inorganic Materials 38 (11) (2002) 1085–1099. [2] D. Bahadur, J. Giri, Bibhuti B. Nayak, T. Sriharsha, P. Pradhan, N. K. Prasad, K. C. Barick, R. D. Ambashta, Processing, properties and some novel applications of magnetic nanoparticles, Pramana journal of physics 65 (4) (2005) 663-679. [3] Shanghua Li, Jian Qin, Andrea Fornara, Muhammet Toprak, Mamoun Muhammed, Do Kyung Kim,Synthesis and magnetic properties of bulk transparent PMMA/Fe-oxide nanocomposites,Nanotechnology 20 (2009) 185607 (6pp). [4] S. Shekhar, E. P. Sajitha, V. Prasad, S. V. Subramanyam, High coercivity below percolation threshold in polymer nanocomposite, Journal of Applied Physics 104 (2008) 083910 (4 ages). [5]

Veronica Sáez and Timothy J. Mason, Sonoelectrochemical Synthesis of Nanoparticles, Molecules 14 (2009) 4284-4299.

[6] J. Alam, U. Riaz, S. Ahmad, Effect of Ferrofluid Concentration on Electrical and Magnetic Properties of the Fe3O4/PANI Nanocomposites, Journal of Magnetism and Magnetic Materials 314 (2007) 93–99.

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[7] F. C. Fonseca, G. F. Goya, R. F. Jardim, R. Muccillo, N. L. V. Carren˜o, E. Longo, and E. R. Leite, Superparamagnetism and magnetic properties of Ni nanoparticles embedded in SiO2, Physical Review B 66 (2002) 104406 (5 pages). [8] E. Muhammad Abdul Jamal, P. A. Joy, Philip Kurian, M. R. Anantharaman, Synthesis of nickel-rubber nanocomposites and evaluation of their dielectric properties, Materials Science and Engineering B 156 (2009) 24-31. [9] E. Muhammad Abdul Jamal, P. A. Joy, P. Mohanan, Philip Kurian, M. R. Anantharaman,Effect of nickel nano-fillers on the dielectric and magnetic properties of composites based on rubber in the X-band (Published online first, Applied Physics A doi10.1007/s00339-009-5284-1) [10] Sejal Shah, N.L. Singh, Anjum Qureshi, Dolly Singh, K.P. Singh,V. Shrinet, A. Tripathi, Dielectric and structural modification of proton beam irradiated polymer composite, Nuclear Instruments and Methods in Physics Research B 266 (2008) 1768–1774 [11] N. L. Singh, S. Shah, A. Qureshi, A Tripathi, F Singh, D. K. Avasthi, P. M. Raole, Effect of ion beam irradiation on metal particle doped polymer composites, Bull. Mater. Sci.34(1) (2011) 81–88. [12] N. L. Singh, S. Shah, A. Qureshi, F. Singh, D. K. Avasthi, V. Ganesan, Swift heavy ion induced modification in dielectric and microhardness properties of polymer composites, Polymer Degradation and Stability 93(6) (2008) 1088-1093. [13] S. Agrawal, S. Srivastava, Sumit Kumar, S. S. Sharma, B. Tripathi, M. Singh, Y. K. Vijay, Swift heavy ion irradiation effect on Cu-doped CdS nanocrystals embedded in PMMA, Bull. Mater. Science 32(6) (2009) 569–573. [14] S. Sharma, R. Vyas, S. Shrivastava, Y. K. Vijay, Effect of swift heavy ion irradiation on photoluminescence properties of ZnO/PMMA nanocomposite films, Physica B 406 (2011) 3232-3233. [15] K. Singh Samra, S. Thakur, L. Singh, Structural, thermal and optical behavior of 84 MeV oxygen and 120 MeV silicon ions irradiated PES, Nuclear Instruments and Methods in Physics Research B 269 (2011) 550–554. [16] R. Kumar, S. Asad Ali , A. K. Mahur , H. S. Virk , F. Singh , S. A. Khan ,D. K. Avasthi, R. Prasad, Study of optical band gap and carbonaceous clusters in swift heavy ion irradiated polymers with UV–Vis spectroscopy, Nuclear Instruments and Methods in Physics Research B 266 (2008) 1788–1792. [17] N. Bajwa, K. Dharamvir, V. K. Jindal, A. Ingale, D. K. Avasthi, R. Kumar, A. Tripathi, Swift Heavy Ion Induced Modification Studies of C60 Thin Films, Journal Of Applied Physics 94 (2003) 326-333. [18] Ana Bettencourt and Antonio J. Almeida, Poly(methyl methacrylate) particulate carriers in drug delivery, Journal of Microencapsulation, 2012, 1–15. [19] P.K. Khanna, Priyesh V. More, Jagdish P. Jawalkar, B.G. Bharate, Effect of reducing agent on the synthesis of nickel nanoparticles, Materials Letters 63 (2009) 1384–1386.

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[20] A. Semwal, A. Negi, R. G. Sonkawade, J. M. S. Rana, R. C. Ramola, Effect of swift heavy ion irradiation on optical and structural properties of polysulphones polymer films, Indian J of pure and App Phy 48 (2010) 496-499. [21] H S Virk, P S Chandi And A K Srivastava, Physical and chemical response of 70 MeV carbon ion irradiated Kapton- H polymer, Bull. Mater. Sci. 24(5) (2001) 529–534. [22] L. Singh, K. Singh Samra, Opto-structural characterization of proton (3 MeV) irradiated polycarbonate and polystyrene, Radiation Physics and Chemistry 77 (2008) 252–258. [23] A. Das, S. Dhara and A. Patnaik, Fractal nature of p-bonded nanocrystalline clusters: A N+ beam induced phenomenon in poly (2,6-dimethyl-1,4-phenylene Oxide), Physical Rev B 59 (1999) 11069. [24] Anjum Qureshi , Dolly Singh , N.L. Singh, S. Ataoglu , Arif N. Gulluoglu , Ambuj Tripathi, D.K. Avasthi, Effect of irradiation by 140 Mev Ag 11+ ions on the optical and electrical properties of polypropylene/TiO2 composite, Nuclear Instruments and Methods in Physics Research B 267 (2009) 3456-3460. [25] A. K. Srivastava, H. S. Virk, Modifiaction of optical response of Polyvinyl Aceate induced by 250 keV D+ion bombardment, J. Polym. Mater. 17 (2000) 325-328. [26] N.L. Singh, Sejal Shah , Anjum Qureshi , K.P. Singh , V. Shrinet , P.K. Kulriya , A. Tripathi, Radiation induced modification of Organometallic compound dispersed polymer composites, Radiation Effects & Defects in Solids 163(2) (2008) 169-177. [27] R. Kumar, U. De, R. Prasad, Physical and chemical response of 70 MeV carbon ion irradiated polyether sulphone polymer, Nucl. Instrum. Methods B 248 (2006) 279-283. [28] N. L. Singh, D. Singh, A. Qureshi, Swift heavy ion-induced modification of the physical properties of polymethyl methacrylate/carbon black composites, Radiation Effects & Defects in Solids 66(8-9) (2011) 640–647. [29] R. Vijaya Kumar, Yu. Koltypin, O. Palchik, A. Gedanken, Preparation and characterization of nickel–polystyrene nanocomposite by ultrasound irradiation, Journal of Applied Polymer Science 86 (2002) 160–165. [30] Chaitali Gavade, N. L. Singh, D. K. Avasthi , Alok Banerjee, Effect of SHI on dielectric and magnetic properties of metal oxide/PMMA nanocomposites, Nuclear Instruments and Methods in Physics Research B 268 (2010) 3127-3131. [31] Paramjit Singh, Rajesh Kumar, H. S. Virk, Rajendra Prasad, Modification of optical, chemical and structural response of polymethyl methacrylate polymer by 70 MeV carbon ion irradiation, Ind. J. Pure and Appl. Phys. 48 (2010) 321-325. [32] S. Manna, A. K. Deb, J. Jagannath, and S. K. De, Synthesis and Room Temperature Ferromagnetism in Fe Doped NiO Nanorods, J. Phys. Chem. C 112 (29) (2008) 1065910662. [33] Anjum Qureshi, N. L. Singh, Sejal Shah, F. Singh, D. K. Avasthi, Ion Beam Modification of Polymethyl methacrylate (PMMA) Polymer Matrix Filled with Organometallic Complex, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 45 (2008) 265-270.

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[34] Anjum Qureshi, N. L. Singh, A.K. Rakshit, F. Singh, D.K. Avasthi, Swift heavy ion induced modification in polyimide films, Surface & Coatings Technology 201 (2007) 8308–8311. [35] Amit L. Sharma, Alok Srivastava, Ion beam induced modifications in nitroso substituted polyaniline: Spectral and electrical studies, Current Applied Physics 7 (2007) 650–654. [36] G. Sui, S. Jana, W.H. Zhong, M.A. Fuqua, C.A. Ulven, Dielectric properties and conductivity of carbon nanofiber/semi-crystalline polymer composites, Acta Materialia 56 (2008) 2381– 2388. [37] T. Phukan, D.Kanjilal, T. D. Goswami, Dielectric response of irradiated PADC polymer track detector, Nucl. Instr. And Meth. B 234 (2005) 520–524. [38] P. Balaji Bhargav, B. A. Sarada, A. K. Sharma And V.V. R. N. Rao, Electrical Conduction and Dielectric Relaxation Phenomena of PVA Based Polymer Electrolyte Films, Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 47 (2010) 131–137.

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Ion Beam Induced Modifications in Conducting Polymers R. C. Ramolaa and Subhash Chandrab Department of Physics, H.N.B. Garhwal University, Badshahi Thaul Campus Tehri Garhwal – 249 199, India a [email protected] (corresponding author), [email protected] Keywords: Conducting Polymers, Polyaniline (PANI), Polypyrrole (Ppy), SHI, XRD, UV-Visible, SEM, FTIR

Abstract. High energy ion beam induced modifications in polymeric materials is of great interest from the point of view of characterization and development of various structures and filters. Due to potential use of conducting polymers in light weight rechargeable batteries, magnetic storage media, optical computers, molecular electronics, biological and thermal sensors, the impact of swift heavy ions for the changes in electrical, structural and optical properties of polymers is desirable. The high energy ion beam irradiation of polymer is a sensitive technique to enhance its electrical conductivity, structural, mechanical and optical properties. Recent progress in the radiation effects of ion beams on conducting polymers are reviewed briefly. Our recent work on the radiation effects of ion beams on conductive polymers is described. The electrical, structural and optical properties of irradiated films were analyzed using V-I, X-Ray diffraction (XRD), scanning electron microscopy (SEM), UV-Visible spectroscopy and Fourier transform infrared spectroscopy methods. 1.

Introduction

Polymers are generally known for their insulating property. The first polymer (polyacetylene), capable of conducting electricity, was reportedly prepared by accident by Shirakawa et al. [1]. The subsequent discovery that the polymer would undergo an increase in conductivity of 12 orders of magnitude by oxidative doping quickly reverberated around the polymer and electrochemistry communities. Shirakawa et al. [2] along with a group of young students started research in the field of conducting polymers and the ability to dope these polymers over the full range from insulator to metal. This was particularly exciting because it created a new field of research and a number of opportunities on the boundary between chemistry and condensed-matter physics [2-4]. As the commonly known polymers in general are saturated and so insulators, these were viewed as uninteresting from the point of view of electronic materials. In conjugated polymers the electronic configuration is fundamentally different, where the chemical bonding leads to one unpaired electron (the π electron) per carbon atom. Moreover, π bonding, in which the carbon orbitals are in the sp 2 configuration and the orbitals of successive carbon atoms along the backbone overlap, leads to electron delocalization along the backbone of the polymer. This electronic delocalization provides the highway for charge mobility along the backbone of the polymer chain. Therefore, the electronic structure in conducting polymers is determined by the chain symmetry, i.e. the number and kind of atoms within the repeated unit, with the result that such polymers can exhibit semiconducting or even metallic properties. Electronically conducting polymers are extensively conjugated in nature and therefore it is believed that they possess a spatially delocalized band-like electronic structure. These bands stem from the splitting of interacting molecular orbitals of the constituent monomer units in a manner reminiscent of the band structure of solid-state semiconductors. It is generally agreed that the mechanism of conductivity in these polymers is based on the motion of charged defects within the conjugated framework. The charge carriers, either positive (p-type) or negative (n-type), are the products of oxidation or reduction of the polymer. The simplest possible form of conducting polymer is of course the arch type polyacetylene (CH)x. Polyacetylene itself is very unstable for any practical

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value but its structure constitutes the core of all conjugated polymers. Because of its structural and electronic simplicity, polyacetylene has played a critical role in the elucidation of the theoretical aspects of conducting polymers. Little [5] had proposed that properly substituted polyacetylene molecule would exhibit superconductivity at room temperature. Hatano et al. [6] are the first to report the electrical conductivity of the order of 10-5 S/cm for trans- polyacetylene sample. After late seventies, a large number of polymers have been added to the list of conducting polymers; such as polypyrrole [7], polythiophene [8], polyparaphenylene [9], polyphenylene sulphide [10], polyaniline [11], polyphenylene vinylene [12] etc. Conjugated polymers are organic semiconductors, which with respect to electronic energy levels hardly differ from inorganic semiconductors. Both have their electrons organized in bands rather than in discrete levels and both have their ground state energy bands either completely filled or completely empty [13-14]. The band structure of a conjugated polymer originates from the interaction of the π-orbitals of the repeating units throughout the chain. Addition of every new thiophene unit causes hybridization of the energy levels yielding more and more levels until a point is reached at which there are bands rather than discrete levels. Interaction between the π-electrons of neighbouring molecules leads to a three-dimensional band structure. Analogous to semiconductors, the highest occupied band (which originates from the HOMO of a single thiophene unit) is called the valence band, while the lowest unoccupied band (originating from the LUMO of a single thiophene unit) is called the conduction band. The difference in energy, E g, between these levels is called the band gap. Since π-conjugated polymers allow virtually endless manipulation of their chemical structure, control of the band gap of these semiconductors is a research issue of ongoing interest. This “band gap engineering” may give the polymer its desired electrical and optical properties, and the reduction of band gap to approximately zero is expected to afford an intrinsically conducting polymer [15-17]. Conducting polymers can be classified into different types on the basis of conduction mechanism that renders electrical conductivity to polymers. Some commonly used conducting polymers are i) Conducting polymer composites, ii) Organometallic polymeric conductors, iii) Polymeric charge transfer complexes, and iv) Inherently conducting polymers. This paper deals with the inherently conducting polymers (Polyaniline and Polypyrrole). Research in the field of inherently conducting polymer (conjugated polymers) started after Shirakawa’s group [2], who found drastic increase in the electrical conductivity of polyacetylene when exposed to iodine. The conjugated polymers are studied as the intrinsically conductive polymers. The electronic properties of conjugated polymers are due to the presence of π-electrons, the wave functions of which are delocalized over long portions of polymer chain when the molecular structure of the backbone is planar [18-19]. Hence it is necessary that there are no torsion angles at the bonds, which would decrease the delocalization of the π-electron system. Low band gap, low excitations, semiconducting behavior, net charge carrier mobilities in the conducting state are large enough and because of this high electrical conductivity differentiate the conjugated polymers from conventional polymers. The aim of present study is to synthesize polyaniline and polypyrrole conducting polymers by chemical and electrochemical methods and to study the modifications induced by swift heavy ions (SHI). This will not only increase the accessibility of these polymers, but will also help to gain the understanding of various parameters that influence the electrical, structural, chemical and optical properties of conducting polymers. The irradiated polymers were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, V-I measurements and scanning electron microscopy (SEM). This paper presents the review of our work on conducting polymers.

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Synthesis of Conducting Polymers

The synthesis of conducting polymers involves the oxidation of the monomer viz. aniline and pyrrole with oxidizing agents such as Ammoniurm Persulphate (APS) or Para-toluene sulphonic acid (PTS) in acidic medium. Solvents used for the polymerization reaction is distilled water or NMP-water (1:2) mixture. Detailed procedures for the synthesis are given below: 2.1 Chemical Synthesis of Polyaniline Film. Polyaniline was synthesized chemically by doping of hydrochloric acid (HCl) and modified by incorporating different concentration of ammonium persulphate into aniline monomer. Synthesis of HCl doped PANI was carried out in the N-Methyl 2-Pyrrolidone (NMP). Sample Preparation. The chemical synthesis of polyaniline was done at aqueous medium in presence of HCl using Ammonium persulphate (APS) as the initiator. In one liter beaker, 1 M of HCl added to 1 M of aniline and the solution stirred for half an hour. 2 M of APS was dissolved in 100 ml of distilled water and added slowly by drop wise to the aniline and HCl solution for half an hour. The reaction was carried out at ice temperature for 2-3 hours. The solution was then filtered for further use. Purification of the Polymer. After the reaction was over, the reaction mixture was poured in to beaker containing 500 ml distilled water, stirred for half an hour and the precipitated polymer was filtered by conventional method. The polymer was washed with distilled water several times till the filtrate obtained was colorless and neutral in nature. The polymer samples obtained in powder form were dried first at room temperature for few hours and then in an oven at 60 ºC for 4-5 hours. The dried polymer powder was then treated with alcohol for further purification in Soxhlet assembly for 12 hours. The dried polymer was preserved in desiccators and dried at 40 oC in order to obtain a brownish green colour powder. The resulting polyaniline is the leuco-emaraldine salt. Undoping of the Polymers. Prior to synthesis of polyaniline film, the polyaniline powder was treated with liquor ammonia for the undoping. The powder was mixed with 25% of ammonia solution and 75% of distilled water and reaction goes upto 12 hours. The obtained polymer was filtered and washed thoroughly with distilled water to remove ammonia. A brown colour powder was then formed in undoped form. Preparation of Polyaniline Film. The resulting brown colour powder was then treated with various weight percentage of N-methyl 2-Pyrrolidone (NMP). The standard concentration was found 2.5% (weight percentage) PANI powder to 100 ml of NMP. Thus 2.5% PANI powder was mixed with 100 ml of NMP slowly and reaction goes upto 24 hours. The resulting solution was filtered and left in vacuum oven for 24 hours. A smooth film of polyaniline was then obtained in the undoped form. Doping of Polyaniline Film. Redoping of the prepared polyaniline films was done with HCl of concentrations 1.0 M and 2.0 M as dopants. The solutions were prepared in the aqueous medium and the polymer films were doped for several hours. The film of polyaniline thus obtained in the doped form and was of green in colour. The resistance of the prepared film is measured in the kiloohm (kΩ) range. The thickness of the films was measured by  source and found to be ~30 μm [20]. The thickness of PANI film was selected thin enough to allow the ions to completely pass through it.

2.2 Electrochemical Synthesis of Polypyrrole Film. The simplest electrochemical cell has two electrodes, an anode and a cathode, in solution separated by an ionically conductive electrolyte. The electrolyte is most commonly a salt in solution. A voltage may be applied between electrodes to

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drive a chemical reaction (electrolytic cell). In electrochemical experiments, we are often only interested in the potential at one electrode, the working electrode (WE). The electrochemical deposition of polypyrrole was carried out in a single compartment cell with two electrodes system. The experimental setup is shown in Fig. 1.

Fig. 1 Schematic diagram of electrochemical set up for Ppy thin films preparation The other electrode completing the current pathway in solution is called the counter electrode (CE). The working electrode was indium tin oxide (ITO) coated glass substrate while the counter electrode was a platinum plate. Under normal conditions, the electrolyte was pyrrole monomer along with Para-toluene Sulphonic acid (PTS) as an oxidizing agent in appropriate concentrations, whereas the electrolyte medium was distilled water. The electro-polymerization was carried out at a constant anodic potential of +0.8 V DC and the thickness of the film was controlled by the deposition time and the electric charge passed during film growth. The samples were then washed with distilled water and dried before being used for SHI experiments. The thickness of the films was controlled by varying the deposition time [21]. The thickness in this work was selected so as to be thin enough to allow the ions to completely pass through it 3.

Irradiation of Polymers by Swift Heavy Ions (SHI)

In the present work, thin films of polyaniline and polpyrrole were irradiated with 45 MeV Li, 100 MeV O and 120 MeV Ni ions. The Li, O and Ni ions were selected to create points or extended type of point defects and to create the columnar defects. In fact, the defect morphology depends upon threshold value of the particular material to be irradiated [22]. The variations in electronic (dE/dx)e and nuclear (dE/dx)n energy loss with energy of the incident 45 MeV Lithium (Li), 100 MeV Oxygen (O) and 120 MeV Nickel (Ni) ions beam and depth in polyaniline are shown in Figs. 2-4, respectively. The corresponding plots for Polypyrrole are shown in Figs. 5-7. The electronic energy loss, (dE/dx)e, is calculated from the SRIM code programme [23] for polyaniline and polypyrrole for 45 MeV Li, 100 MeV O and 120 MeV Ni ions. The electronic energy loss for Li, O and Ni ions in polyaniline is 63.56, 44.01E01 and 48.8E02 eV/nm and for polypyrrole it is 61.38, 42.5E01 and 47.05E02 eV/nm.

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The thickness of PANI film was selected thin enough (~ 30 μm) to allow the ions to completely pass through it. The range of ions in the polymer films was calculated by the stopping range of ions in matter (SRIM) programme [23]. The range for 45 MeV lithium, 100 MeV oxygen and 120 nickel ions beam in polyaniline films are given in Table 1.

Fig. 2 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polyaniline for 45 MeV Lithium ions.

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Fig. 3 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polyaniline for 100 MeV Oxygen ions.

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Fig. 4 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polyaniline for 120 MeV Nickel ions.

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Table 1 SRIM calculated Se, Sn values and range of Li+3, O+8 and Ni+9 ions for polyaniline Ion Species and Energy Li+3, 45 MeV O+8, 100 MeV Ni+9, 120 MeV

Se (eV/Å) 5.910E+00 4.100E+01 4.567E+02

Sn (eV/Å) 3.403E-03 2.288E-02 6.390E-01

Range (μm) 433 156 34

Fig. 5 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polypyrrole for 45 MeV Lithium ions.

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Fig. 6 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polypyrrole for 100 MeV Oxygen ions.

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Fig. 7 Variation of (dE/dx)e and (dE/dx)n with energy and depth in Polypyrrole for 120 MeV Nickel ions.

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The thickness of polypyrrole films was around 25 µm and the range of 45 MeV lithium, 100 MeV oxygen and 120 nickel ions beam for Ppy films are shown in Table 2. Table 2 SRIM calculated Se, Sn values and range of Li+3, O+8 and Ni+9 ions for polypyrrole Ion Species & Energy Li+3, 45 MeV O+8, 100 MeV Ni+9, 120 MeV

Se (eV/Å) 6.13 E+00 4.25 E+01 4.70 E+02

Sn (eV/Å) 3.49 E-03 2.35 E-02 6.57 E-01

Range (μm) 417 151 32

The films of polynailine and polypyrrole of size 1cm2 were irradiated in material science beam line under high vacuum (5x10-6 torr) using 45 MeV lithium, 100 MeV oxygen and 120 MeV nickel ions with ion fluences ranging from 3x1010 to 9 x 1012 ions/cm2 and beam current of 1 pnA for lithium and oxygen beams and 0.5 pnA for nickel beam, available from 15 UD Pelletron at Inter University Accelerator Centre, New Delhi. X-Ray diffraction (XRD) of the polyaniline films were carried out by a Bruker AXS D8, X-Ray diffractometer with Cu-K radiation (1.54 Å) for a wide range of Bragg’s angle 2θ (15