Vacancy Diffusion with Time-Dependent Length Scale: An Insightful New Model for Physical Aging in Polymers

Physical aging models are reviewed and further developed to describe the physical aging within thin (>100 nm) and ultrathin polymer films (<100 nm). The phenomenological models of Kovacs et al. and Struik have the adaptability necessary to follow thin film aging trends while the fundamental models of Curro et al., based on the aging mechanism of vacancy diffusion, provide more physical insight into the aging phenomenon at the nanoscale. It is found that a newly formulated diffusion model which includes time-dependent length scale can describe the glassy-state dynamics that result in physical aging. Lattice contraction is found to be a direct result of vacancy diffusion and is not a separate phenomenon. The new model predicts the change in permeability due to physical aging for ultrathin and thin films. The model's use of a time-dependent length scale holds promise for renewed examination of complex dynamics in glassy polymers such as the role of alpha and beta relaxations and the nature of the glass transition.