Study on enthalpy relaxation of polystyrene by assuming the existence of an intermediate aging plateau

The enthalpy relaxation of polystyrene (PS) is studied by assuming the existence of an intermediate aging plateau between the glass and the extrapolation of the liquid. The intermediate aging plateau is characterized by Tgd and δ which represent the temperature below which the thermodynamic property deviates from the extrapolation of the liquid and the extent of the deviation. Both Tool-Narayanaswamy-Moynihan (TNM) and Adam-Gibbs-Vogel (AGV) equations are used to calculate the relaxation time. Tgd and δ are obtained through fitting the heat capacity data in the heating of PS and the results for the TNM and AGV models are mutually consistent with each other. The result suggests the existence of such an intermediate aging plateau and the thermodynamic property of which deviates greatly from the extrapolation of the liquid. It is also indicated that both Tgd and δ are influenced by molecular weight.