National Conference on Application Oriented Materials (NCAOM 2006), SRM University, Kattankulathur, Tamil Nadu, India, Volume: Apr 21-22, 2006 Proceedings of the National Conference on Application Oriented Materials (NCAOM 2006), At Department of Physics, SRM University, Kattankulathur, Tamil Nadu, India, Volume: Apr 21-22, 2006 SRM Institute of Science & Technology; page133-134. MICROSTRUCTURAL CHANGES IN POLYVINYLALCOHOL- POLY(VINYLPYRROLIDONE) BLEND CAUSED BY GAMMA IRRADIATION M R Ranganath and Blaise Lobo Department of Physics, Karnatak Science College, Dharwad 580 001, Karnataka, India e-mail: blaise.lobo@gmail.com Abstract. Positron Lifetime Technique (PLT) of Positron Annihilation Spectroscopy (PAS) was used to investigate the micro-structural changes in polyvinylalcohol - poly(vinylpyrrolidone) blend (PVA-PVP) films subjected to gamma irradiation dose upto 21 Mrad. The o-Ps intensity I 3 drops sharply on initial irradiation, followed by a partial rise and subsequent saturation. The o-Ps lifetime  3 shows an initial drop, followed by saturation, and shows significant rise only at high dose of 21 Mrad. The drop in I 3 on initial irradiation is interpreted as due to inhibition of Ps formation due to macro-radical formation. The initial drop in  3 is explained as due to pick-off annihilation of positron (in o-Ps) with the electrons generated during scission and macro-radical formation processes due to interaction of gamma radiation with the polymeric material, subsequently resulting in saturation effect due to a sort of equilibrium between the two processes. At very high doses, molecular degradation and rearrangement of chain fragments can be supposed to be responsible for increase in  3 (at 21 Mrad). 1. Introduction. The polyvinylalcohol - polyvinylpyrrolidone (PVA-PVP) blend has shown significant promise as a bio- polymer /1,2/ and so a study of its radiation modification and radiation degradation assumes importance /3/. Irradiation of polymers with gamma rays can cause cross-linking or degradation. It is necessary to understand the changes undergone in the sample, at the micro-structural level for different doses of gamma irradiation. This can enable one to tune microscopic properties in order to achieve the desired properties in the polymeric material. A sensitive probe is however required. Positron is used as a nano-probe for the non- destructive study of micro-structural changes in polymers. In polymeric materials, positrons form a bound state with one of the electrons of the material , during the last stages of the thermalization of positron in the material (spur model) /4/. There is a preferential localization of positronium (Ps) in moving open spaces called free volume holes. PLT has been used for a study of micro-structural changes in polymeric materials due to doping, sorption, irradiation etc.,/5-7/. PLT technique of PAS was utilized to monitor variation of longest lifetime parameters  3 and I 3 w.r.t gamma irradiation dose. The ortho-positronium (o-Ps) lifetime is a measure of free volume hole size whereas its intensity is a measure of number density of free volume holes. The presence of macro-radicals in gamma-irradiated polymer complicates this interpretation due to the processes of Ps quenching and inhibition of Ps formation respectively. 2. Experimental. Polyvinylalcohol (PVA) was procured in granule form from s.d fine chem., Mumbai, India, and polyvinylpyrrolidone (PVP) was procured in powder form. Two gram of PVA and two gram of PVP were dissolved in 100 ml of distilled water, and films were obtained on a glass substrate, by solution casting method. The films, after removal from the substrate, were irradiated to different doses of gamma energy, upto 21 Mrad. PLT measurements were made using a fast-fast coincidence lifetime spectrometer with 40% energy gating and time resolution 330ps /8/. A 10 micro-curie sodium-22 source, with source component 10% was used for PLT measurements, with polymer films in sandwich configuration. Three fit analysis was performed using the program PATFIT-88 /9/. 3. Results and Discussion. Figure (1) shows the variation of o-Ps intensity I 3 w.r.t gamma irradiation dose. I 3 drops sharply on initial irradiation, followed by a partial rise & subsequent saturation. The o-Ps lifetime  3 shows an initial drop, followed by saturation, and shows significant rise only at high dose of 21 Mrad (See figure 2). The drop in I 3 on initial irradiation is interpreted as due to inhibition of Ps formation due to macro- radical formation. The positron tends to form a bound state with the macro-radical, and this process competes with the Ps formation process. The initial drop in  3 is explained as due to pick-off annihilation of positron (in o-Ps) with the electrons generated during scission and macro-radical formation processes due to interaction of gamma radiation with the polymeric material. The processes explained above dominate over the expected increase in free volume size and number density of free volume holes due to scission