Non equilibrium annealing behavior of poly(vinyl acetate)

The water absorbed by poly(vinyl acetate), PVAc, at 23°C was found in two states. The first, which can account for up to 4 weight percent, was bound to the polymer. The second was in a freezable or clustered form. The latter type of water had no effect on PVAc's glass temperature, whereas, the former kind plasticized T_g. In annealing studies, the enthalpic and dielectric response of PVAc when held at a fixed temperature increment, ΔT, below T_g, was observed to be independent of the amount of bound water. The time dependence of the shift in the dielectric relaxation spectrum and the recovery of the enthalpy towards its equilibrium value as PVAc approached its equilibrium glassy state from a lower temperature as compared to a higher temperature was initially slower. This delayed response to expansion was of the order of the polymer's average relaxation time at the lower temperature. A model was proposed to explain this asymmetric behavior based upon changes in the polymer's free volume as well as its occupied volume.