(268i) Mobility and Recovery of Pressure-Densified and Pressure-Expanded Polystyrene Glass

The ability to model the behavior of pressure-densified glass is important not only for fundamental understanding, but also for describing materials made through the injection molding process. Here, we experimentally investigate the structural recovery of pressure-densified glasses and, to the best of our knowledge, for the first time, pressure-expanded glasses using pressurizable dilatometry. Both glasses, which are made by changing the pressure deep in the glassy state, show early devitrification on heating, indicating that these glasses have more mobility than the conventional glass made under isobaric conditions. In addition to novel experiments, we test the ability of the Kovacs-Aklonis-Hutchinson-Ramos (KAHR) model of structural recovery to predict the behavior of these glasses. The results suggest two limitations of the KAHR model: i) the structural recovery is assumed to depend on the instantaneous liquid state, and ii) the same relaxation kinetics are assumed for the temperature and pressure perturbations. Modification of the KAHR model, allowing the departure from equilibrium to initially depend on the liquid state that the glass came from and to evolve towards the state that the glass is going to, improves the ability of the model to predict the early devitrification for the pressure densified glass. Another modification of the KAHR model, allowing the temperature and pressure perturbations to relax independently of one another, well describes the observed relaxation behavior and also captures the increased thermal expansion coefficient of pressure-densified glass lines during heating and 'memory'-like aging behavior.