Volume and enthalpy recovery of polystyrene

Volume and enthalpy recovery measurements were used to study the physical aging behavior of a polystyrene. Isothermal aging at temperatures near Tg was studied with aging times ranging from several minutes to several days. The data are satisfactorily fit using the Tool–Narayanswamy–Moynihan model of structural recovery. The times required to reach equilibrium are the same for the two properties at temperatures where equilibrium was achieved. Extrapolation to lower temperatures using the model indicates that the times required to reach equilibrium may be different for the two properties at these lower temperatures. The recovery data were plotted as the departure from equilibrium and then were shifted horizontally to superpose at long times. The shift factors for volume and enthalpy recovery data agree with one another above 94°C but they diverge below this temperature. The shift factor data deviates from the WLF or VTFH equation for temperatures below Tg. In addition, we find that the normalized rate of approach is the same for volume and enthalpy recovery at 97°C in the limit of a linear temperature jump, but it differs between the two properties for a nonlinear jump even though the times required to reach equilibrium are the same.