A Phenomenological Model for Enthalpy Recovery in Polystyrene Using Dynamic Mechanical Spectra

This paper studies the effects of annealing time on the specific heat enthalpy of polystyrene above the glass transition temperature. We extend the Tool-Narayanaswamy-Moynihan (TNM) model to describe the endothermic overshoot peaks through the dynamic mechanical spectra. In this work, we accept the viewpoint that the enthalpy recovery behavior of glassy polystyrene (PS) has a common structural relaxation mode with linear viscoelastic behavior. As a consequence, the retardation spectrum evaluated from the dynamic mechanical spectra around the primary T_g peak is used as the recovery function of the endothermic overshoot of specific heat. In addition, the sub-T_g shoulder peak around the T_g peak is found to be related to the structural relaxation estimated from light scattering measurements. The enthalpy recovery of annealed PS is quantitatively described using retardation spectra of the primary T_g, as well as the kinetic process of the sub-T_g relaxation process.