A variable energy positron annihilation lifetime spectroscopy study of physical aging in thin glassy polymer films

The influence of physical aging on the profile of free volume characteristics in thin polysulfone (PSF) films was investigated using variable energy positron annihilation lifetime spectroscopy. The PSF films exhibited decreasing o-Ps lifetime during physical aging, while o-Ps intensity remained constant. The o-Ps lifetime was reduced at lower implantation energies, indicating smaller free volume elements near the film surface (i.e., in the top ∼50 nm). These near-surface regions of the films age dramatically faster than bulk PSF. The accelerated aging is consistent with the notion of enhanced mobility near the film surface, which allows polymer near the surface to reach a lower free volume state more quickly than the bulk. No influence of the silicon wafer support on aging behavior was detected. Additionally, the impact of CO 2 conditioning on physical aging was briefly examined. The results from these studies were compared to aging behavior of ultrathin PSF films tracked by gas permeability measurements, and favorable agreement was found. [Display omitted]