Positron annihilation lifetime spectroscopy was used to investigate strain-induced free volume changes in well-aged Bisphenol A polycarbonate. From our previous studies, it appears that the orthopositronium (0-Ps) lifetime, ~3 , yields an estimate of the mean volume, ( u t ) , of 0-Ps trapping sites ("holes"), whereas the intensity of the 0-Ps decay component, 18, is proportional to the number density Nof holes. Thus the product 13( v f ) is a measure of the fractional free volume, h. A miniature load cell was constructed which enables in situ positron annihilation measurements on polymer samples under well-specified tensile strains. We find that, for well-aged polycarbonate, h increases with strain up to a level of 4%, after which no further change in h occurs. This behavior is consistent with literature observations of strain-enhanced rejuvenation of the mechanical properties of polycarbonate. Upon long-term room-temperature annealing of a polycarbonate specimen held at 3% strain, we find that h gradually decreases to levels below the initial value, suggesting a strain-induced reactivation of physical aging, and hence a closer approach to the equilibrium density.
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