TEMPERATURE AND TIME-DEPENDENCE OF FREE-VOLUME IN BISPHENOL-A POLYCARBONATE STUDIED BY POSITRON LIFETIME SPECTROSCOPY

New positron lifetime experiments have been carried out for Bisphenol A polycarbonate. The influence of unavoidable e+ irradiation on the lifetime and intensity of o-Ps annihilation was investigated. Our results obtained using a state of the art lifetime spectrometer (count rate 670 cps with a 30-mu-Ci Na-22 source, time resolution 260 ps fwhm for Na-22) are free from potential artifacts due to irradiation damage. Three lifetime components were computed from each spectrum. The temperature dependence of the o-Ps lifetime (tau-3) and intensity (I3) were measured in the range between 20 and 150-degrees-C for the as-received material and between -30 and +180-degrees-C after annealing above T(g). We find that tau-3 is unaffected by annealing but that I3 is significantly larger for the annealed samples. In addition, the time dependence of tau-3 and I3 due to physical aging in the glassy state was investigated at 30 and 90-degrees-C, respectively. While tau-3 appeared constant for both temperatures, I3 decreased significantly with aging time. Anomalies in the absolute tau and I values derived from such fits and the temperature dependence of these values indicate that a distribution of tau-3 lifetimes must be present in the o-Ps decay. This is consistent with the presence of a distribution of hole sizes which would be expected from current models of free volume in amorphous polymers. Variations in the distribution, e.g. conversion of larger into smaller holes may result in changes of the apparent I3. Adopting a simple free volume definition, a comparison of the thermal expansivity of the vacancy fraction and of the macroscopic value yields a free volume fraction f of 8% at T(g) = 150-degrees-C.