A Two Time Scale Relaxation Model and Its Application to Enthalpy Relaxation of Glassy Polystyrene

The enthalpy relaxation of polystyrene (PS) is studied by assuming a two time scale relaxation process. Kohlrausch–Williams–Watts and exponential decay functions are used to describe the isothermal relaxation behavior of the two time scale processes and three cases are considered. The relaxation time of the fast and slow relaxation processes are calculated using Tool–Narayanaswamy–Moynihan, Adam–Gibbs–Vogel, Free Volume or Macedo model and their corresponding linear forms, respectively. The assumption that the slow relaxation has an exponential character gives better fitting quality to the experimental results than that of assuming the fast relaxation has an exponential character. When Macedo model and its linear form were used, the fitting quality of both normalized heat capacity data and limit fictive temperatures improved greatly. This result shows that combining of two time scale and Macedo models may help to describe the enthalpy relaxation mechanism more properly.