Positron annihilation lifetime studies of free volume changes in polycarbonate under static tensile deformation

Abstract

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.

Positron Annihilation Lifetime Studies of Free Volume in Heterogeneous Polymer Systems

“…(12.17), ∆f/f 0 ≈ 47%, which is in excellent agreement with the corresponding maximum change in ∆f/f 0 in Figure 12 Ruan et al [1992]. )…”

“…Finally, we note that a comparison has been reported of free-volume measurements by PALS versus bulk specific volume values for a polycarbonate specimen subjected to tensile deformation [Ruan et al, 1992]. It was observed that the o-Ps intensity remains essentially constant, while the lifetime τ 3 increases significantly, from about 2.0 to 2.05 ns, as the strain increased up to 4%, and then levels off.…”

“…PALS studies have been performed on amorphous [Ruan et al, 1992] and semicrystalline [Monge et al, 2004] polymers subjected to tensile deformation. Ruan et al [1992] compared free-volume measurements by PALS versus bulk specific volume data for a polycarbonate specimen subjected to tensile deformation.…”

Stress-induced densification of glassy polymers in the subyield region

“…The random energy model (17) was employed in the analysis of physical aging in disordered media. 3,11,34 Among other expressions suggested to fit experimental data, we mention the exponential function, 5,11 and the stretched Gaussian function.…”

“…This conclusion is confirmed fairly well by observations for polycarbonate in the subyield region. 17 We assume that any rearranging region contains only one immobile spherical void, and the radii of all holes coincide. The latter hypothesis contradicts some recent observations 14,15,18 that reveal that volumes of voids are randomly distributed.…”