Volume recovery near the glass transition temperature in poly(vinyl acetate): predictions of a coupling model

Time-dependent volume changes following an increasing temperature jump are known to exhibit an anomaly as the equilibrium volume is approached. Small differences in initial temperature result in large differences in the rate of change of volume deviation delta near delta = 10 exp --4 . Kovacs' tau sub eff equivalent to -- delta (d delta /dt) plots exhibit this as a "gap" between the curves, which has not previously been explained. It is shown that the coupling model of relaxation is able to produce the gap. The tau sub eff curves, including the size of the gap, are accurately reproduced. The size of the gap is used to predict the Kohlrausch exponent n for relaxation at equilibrium, and this is found to agree with shear measurements of Plazek. Furthermore, the shapes of the tau sub eff plots of used to predict the delta dependence of the shift factor. The predicted shift factor agrees with the Doolittle equation obtained by Kovacs, Stratton, and Ferry at 35 deg C and crosses over to the Doolittle equation of Williams and Ferry at 40 deg C. Finally, a single primitive molecular relaxation time predicted by the coupling model is shown to account for the terminal dispersion, the softening dispersion, and the volume recovery in poly(vinyl acetate). 43 ref.--AA