Nonlinear modification of vogel-fulcher-tamman (VFT) model and its application in enthalpy relaxation of glassy polystyrene

Abstract When relaxation kinetic around glass transition is studied, Vogel-Fulcher-Tamman (VFT) law usually fails because the structure dependence of relaxation time has not been taken into consideration. VFT law is modified nonlinearly in two classes of method here: phenomenological modification and combination with Arrhenius form. The modified equations have been applied to the enthalpy relaxation data of polystyrene (PS) after cooled at different rates. Best fit qualities are usually obtained when phenomenological modified VFT equations are used but the best fit values of D are unreasonable. When modified VFT equations combining VFT form with Arrhenius form are used, the fitting qualities are better than the widely used Adam-Gibbs-Vogel (AGV) model, and more reasonable values of best fit D were obtained. This indicates that the modified equation could describe the relaxation time of polymer in a wide range of temperature both above and across the glass transition.