Stochastic Model for Volume Relaxation in Glass Forming Materials: Local Specific Volume Model
A stochastic model for structural relaxation in glassy materials is developed, where the rate of relaxation in a mesoscopic domain depends upon the state in that meso-domain. Because the meso-domains have nanometer dimensions, fluctuations are included. The model predicts the volume relaxation for arbitrary thermal histories at ambient pressure in the glass transition region, where the local rate of relaxation is given by a single relaxation time that depends on the local density and temperature. Using a single set of parameters, the stochastic model accurately predicts the entire Kovacs poly(vinyl acetate) volume relaxation data set. The meso-domain size was determined to be 2.8 nm, consistent with NMR and other measurements. This model indicates that the origin of the wide relaxation spectra observed experimentally is a single Debye process that experiences significant fluctuations in its state. The stochastic model also naturally predicts the small amount of thermorheological complexity that is observed experimentally near Tg.