الفهرس 
CHAPTER 1: INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Positron annihilation lifetime (PAL) and Doppler broadening of annihilation radiation (DBAR) techniques have been applied in the present work to study the microstructure changes of the polymers under investigation. A conventional fast-fast coincidence system is used in measuring the PAL spectra. On the other hand, -spectrometer using HPGe detector is employed for the DBAR measurements. The analysis of the PAL data has been carried out by finite term and continuous method while the DBAR data have been analyzed in terms of S- and W-parameters. The PAL and AC conductivity were measured for polyoxymethylene (POM) copolymer in the temperature range from 5 to 90 oC. At a certain temperature (20 oC), the variation of ortho-positronium (o-Ps) lifetime, 3 shows a distinct increase in its slope. This temperature is considered as the glass transition temperature, Tg which is consistent with the obtained value from DSC. The hole volume shows a small linear increase with temperature below Tg and a steeper increase above Tg. Based on the free volume theory of Cohen-Turnbull, the ionic conductivity is correlated with the mean hole volume (Vh). A linear relation between ln (T0.5) and 1/Vh was observed and the critical hole volume (V*) required for an elementary jump of an ion was estimated. At low temperatures, the distributions of the free volume holes are the same. As the temperature is raised the distribution broadens toward large holes.
The size of the free volume holes and its distribution in polyhydroxybutyrate (PHB) biopolymer have been studied using positron annihilation lifetime spectroscopy (PALS). The measurements were performed as a function of the temperature from - 30 to 90 oC. The temperature dependence of the free volume hole shows a glass transition temperature, Tg at 5 oC. The free volume behavior of PHB shows a small linear increase with temperature below Tg and a steeper increase above Tg. The thermal expansion coefficients of the free volume hole were determined to be 4.94x10-3 and 13.8x10-3 /oC below and above Tg, respectively. The free volume hole distribution in PHB shows a narrow distribution below Tg. With increasing temperature, the maximum of the distributions of the free volume holes shifts to higher values. Meanwhile, widening of the distribution is smaller in lower temperature ranges and more pronounced at higher temperatures, especially above Tg. Also the validity of using an electric field in studying the positronium formation mechanism is tested experimentally. The behavior of the o-Ps intensity in the presence of the electric field is explained and discussed in the frame of the spur model.