ORTHO-POSITRONIUM LIFETIME STUDIES OF FREE-VOLUME IN POLYCARBONATES OF DIFFERENT STRUCTURES - INFLUENCE OF HOLE SIZE DISTRIBUTIONS

We have observed certain anomalies in computer fitting of data from positron annihilations in polymers. These suggest to us that some reported ortho-positronium (o-Ps) lifetimes and intensities in these polymers could be artifacts of the computer-fitting procedure. To evaluate this hypothesis, we have developed a computer simulation of experimental data, which can then be used to test the accuracy of the fitting program. The input to this simulation consists of the lifetimes and intensities of any number of positron populations (including para-positronium and free positron decays), plus the spectrometer resolution function, a contribution from annihilation in the positron source, and random background. The simulation uses the computer's random number generator to make the output spectrum resemble an actual experimental curve. The output spectrum is then used as input to the usual fitting program POSFIT, which determines the best-fitting values of lifetime and intensity for three positron lifetime components. When the shortest lifetime, tau1, Was fixed at the theoretical value of 120 ps, the values of the other lifetimes, tau2 and tau3, were found to be very close to the values in the simulated input. When the simulated input contained several o-Ps lifetime components-tau3,1, tau3,2, tau3,3, etc.-the fitted (apparent) value of tau3 (tau3,app) Was extremely close to the number-average input value (tau3). However, the fitted value for the total intensity of these components departed significantly from the total input intensity. The deviations increase drastically when the full width at half-maximum (fwhm) greater-than-or-equal-to 280 ps. Incorporating these new perceptions, we report investigations of the temperature dependence and aging behavior of free volume in glass and melt states for six polycarbonates of different T(g)'s. We have also evaluated chemical effects attributable to e+- and gamma-irradiation. In seeking a way to minimize effects of exposure to radiation, we have developed a new method for comparison of rejuvenated samples with well-aged material.