Two Relaxation Mechanisms for Rejuvenation of Stable Polystyrene Glass.

We report on the rejuvenation of thin films of polystyrene (PS) as they are heated from stable glassy states, prepared either through vapor deposition or physical aging. For films with thicknesses ≳ 200 nm and ≲ 50 nm, the rejuvenation of vapor-deposited stable PS glass films is quantitatively described with zero free parameters by simulations using parameters previously obtained from isothermal rejuvenation measurements in the same material. For films with thickness ∼ 140 nm, the behavior of the vapor-deposited films becomes more complicated and exhibits significant deviations from model predictions. After rejuvenation and physical aging, such films are then described by a single free parameter. The results of these studies suggest a new distinct mechanism that can result in rejuvenation of glassy PS, and hence two distinct relaxation processes that can couple to the material density.