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 po sitron annihilations in polymers. These suggest to us that some report ed ortho-positronium (o-Ps) lifetimes and intensities in these polymer s could be artifacts of the computer-fitting procedure. To evaluate th is hypothesis, we have developed a computer simulation of experimental data, which can then be used to test the accuracy of the fitting prog ram. The input to this simulation consists of the lifetimes and intens ities of any number of positron populations (including para-positroniu m 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. Th e output spectrum is then used as input to the usual fitting program P OSFIT, which determines the best-fitting values of lifetime and intens ity for three positron lifetime components. When the shortest lifetime , tau1, Was fixed at the theoretical value of 120 ps, the values of th e other lifetimes, tau2 and tau3, were found to be very close to the v alues in the simulated input. When the simulated input contained sever al o-Ps lifetime components-tau3,1, tau3,2, tau3,3, etc.-the fitted (a pparent) value of tau3 (tau3,app) Was extremely close to the number-av erage input value (tau3). However, the fitted value for the total inte nsity 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 the se new perceptions, we report investigations of the temperature depend ence and aging behavior of free volume in glass and melt states for si x 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 me thod for comparison of rejuvenated samples with well-aged material.