Enthalpy relaxation in polyvinyl acetate

The enthalpy relaxation of polyvinyl acetate (PVAc) annealed for times up to 2000 h or so at 30 °C, some 10 K below Tg for a cooling rate of 10 K min−1, has been studied by differential scanning calorimetry (DSC). The data have been analysed by the peak-shift method, and a value for the nonlinearity parameter x=0.44±0.02 has been obtained. From DSC scans at 10 K min−1 following cooling at various rates through the transition region, the apparent activation energy and non-exponentiality parameter have been evaluated, respectively, as Δh∗=805 kJ mol−1 and 0.456<β<0.6. These relaxation parameter values are compared with others reported in the literature, and any discrepancies are discussed in terms of differences in experimental procedure and/or data analysis. Furthermore, the equilibrium enthalpic state is examined in some detail, and in particular it is argued that the equilibrium enthalpy loss at this annealing temperature of 30 °C is consistent with an extrapolation of the equilibrium enthalpy from above Tg, albeit with a slight curvature in the enthalpy–temperature equilibrium line. This is in contrast to results obtained by some other workers, where isothermal relaxation data are analysed using the Cowie–Ferguson equation and yield equilibrium enthalpy losses significantly less than those anticipated from an extrapolation. These discrepancies are discussed in terms of the nonlinearity of the relaxation process.