Volume and enthalpy relaxation rate in glassy materials

Volume and enthalpy relaxation in glassy materials subjected to a temperature jump Delta T = T-0 - T can conveniently be compared on the basis of the fictive relaxation rate R-f = -(dT(f)/d log t)(i). It has been shown that within the current phenomenological model, involving the stretched exponential relaxation function and the reduced time integral, the R-f can be described by a simple equation R-f = 2.303/[e/Delta T beta + sigma]. A remarkable feature of this equation is that it separates the contribution of nonexponentiality (beta) and nonlinearity (sigma). The nonlinearity contribution corresponds to structural changes during the relaxation process. It can be expressed for the Narayanaswamy-Moynihan (NM) model as sigma = (1 - x)Delta h*/RTg2 and for the Adam-Gibbs (AG) model as sigma = BT2/T(T-g - T-2)(2). This equation for R-f(Delta T) predicts an increasing fictive relaxation rate with the magnitude of the temperature jump and it has been tested by using reported NM and AG parameters and experimental volume and enthalpy relaxation data reported for various glassy materials, such as As2S3, As2Se3, polystyrene, poly(vinyl acetate), polycarbonate, poly(methyl methacrylate), poly(vinyl chloride), epoxy, etc. The R-f data for volume and enthalpy relaxation are very similar (within the limit of experimental errors) for all materials examined. The prediction of the R-f(Delta T) for the NM and AG models agrees well with experimental data at moderate temperature departures (Delta T less than or equal to 10 K). Discrepancies observed at higher temperature departures Delta T > 10 K are discussed.