Modified Adam-Gibbs models based on free volume concept and their application in the enthalpy relaxation of glassy polystyrene

Vogel-Fulcher-Tammann (VFT) equation based on free volume concept is used extensively to describe the temperature dependence of relaxation time of polymer above the glass transition temperature Tg, but failed in fitting the enthalpy relaxation data across the glass transition region. Under the line of the Adam-Gibbs theory on temperature dependence of cooperative relaxation property, two equations based on the free-volume theory of glass transition, free-volume (FV) and modified free-volume (FVM) equations, were proposed and applied to fit the enthalpy relaxation data of PS. Compared with the VFT equation, the fitting quality of FV equation is improved greatly and similar to that of the extensively used Tools-Narayanaswamy-Moynihan (TNM) and Adam-Gibbs (AG) models. The fitting quality of FVM equation is improved further and better than that of TNM and AG models, as well as Macedo model with the same number of fitting parameters, especially on the prediction of the limit fictive temperatures.