Positron annihilation lifetime studies of polymers.

摘要

The ability to extract distributions of orthopositronium (o-Ps) lifetimes in polymers from PALS spectra, using the computer program CONTIN, is tested with simulated spectra. While the simulations indicate that it is technically possible to extract o-Ps lifetime distributions, many experimental factors must be accounted for which may make it beyond the scope of most experiments. A model for calculating fractional free volumes without the use of the o-Ps intensities is explored. Calibrating the average hole volumes measured from o-Ps lifetimes with the bulk specific volume allows for the determination of an occupied volume, V_occ, and the number of hole sites per gram, N′. The method gives fractional free volumes which are in reasonable agreement with the values calculate from the Simha-Somcynsky theory. Positron Annihilation Lifetime Spectroscopy (PALS) was utilized to study High-Vinyl Polybutadiene (HVBD)/cis-Polyisoprene (CPI) miscible blends over a wide range of temperatures, covering the glass transition regions, and the full composition range. The composition dependence of the glass transition temperature, T_g was found to be described by a free volume model in which the high T_g component “freezes-in” at a fractional free volume intermediate that of the blend components at their respective T_gs. Syndiotactic Polystyrene is studied by PALS as a function of crystallinity. It was found that the intensity of o-Ps annihilation was independent of crystallinity, indicating that o-Ps does not annihilate in the crystal phase. The formation of large channels in the crystal phase was proposed to explain the behavior of the intensity. This agrees with the gas diffusion properties of the same polymer. Polyethylene terephthalate was also studied by PALS as a function of crystallinity. While a similar behavior of o-Ps intensity was observed, the idea of channel formation is inconsistent with gas diffusion and density measurements. Therefore, it was proposed that a three phase model best describes the intensity behavior, where a rigid amorphous phase, distinct from the regular mobile amorphous phase, is present. The three phase model also explains gas diffusion measurements.