Physical aging in poly(vinyl acetate). 2. Relative rates of volume and enthalpy relaxation

The isothermal volume contractions obtained on physically aging a sample of poly(vinyl acetate) at three temperatures below the glass transition were measured dilatometrically. These were compared with enthalpy aging data for the same polymer sample on the basis of characteristic relaxation times by fitting both types of relaxation to the Kolrausch-Williams-Watt (KWW) expression. It was found that the ratios of enthalpy and volume relaxation times depend on the temperature and that there is no simple correspondence between them. Further comparison of the isothermal aging using a time-dependent characteristic relaxation time indicated that conclusions on rates of aging must be considered within the context of the model chosen for data analysis. The possibility of predicting the rate of volume aging from enthalpy data was explored using an expression derived from the equation of state of Simha and Somcynsky, but the correlation between the actual behavior and that predicted was disappointing. An order parameter description of relaxation is discussed with reference to the equation of state.