Mobility in Pressure-Densified and Pressure-Expanded Polystyrene Glass: Tests of the KAHR Model

The mobility of pressure-densified and pressure-expanded polystyrene glass is explored using a custom-built pressurizable dilatometer. Various pressure-volume-temperature (PVT) histories are performed, including pressure up- and down-jumps in the glassy state followed by temperature ramps with (or without) the succeeding isothermal aging, as well as conventional isobaric temperature scans and isothermal pressure scans. The isobaric cooling and heating scans and the isothermal pressure scans, as well as the pressure-dependent glass transition temperature (T_g), are quantitively described by the Kovacs-Aklonis-Hutchinson-Ramos (KAHR) model of structural recovery. On the other hand, the behavior of pressure-densified and pressure-expanded glass is not captured by the conventional KAHR model. The model predictions improve for changes to the equation for the relaxation time and its dependence on the fictive temperature and pressure. The physics "missing" from the conventional KAHR model is further probed by examining the isothermal aging of the pressure-densified and pressure-expanded glasses. In these aging experiments, a memory-like effect is observed that allows rigorous testing of the modified model.