| The fundamental thermodynamic parameters, enthalpy and volume, are used as the bases to study physical aging or structural recovery of polymeric glasses. It is important to use experimental methods sensitive enough to obtain unambiguous data. In the present study, isothermal microcalorimetry is used to monitor enthalpy recoveries, and density gradient techniques are used to follow density changes. Systematic single temperature jump experiments were used to study enthalpy recoveries in poly(vinyl acetate). The results are comparable to existing volume data in the literature. Both enthalpy and volume recovery show the same qualitative features, nonlinearity and asymmetry. However, quantitative comparison shows that enthalpy recovers faster than volume. The effects of annealing temperature on enthalpy recovery stabilize at low temperature. The enthalpy varies more with increasing temperature step than volume does. Enthalpy and volume recoveries are measured in pressure-densified poly(styrene). The effects of processing parameters, vitrification pressure and cooling rate, are studied. These poly(styrene) samples were prepared by a specially designed high pressure apparatus. Densities of poly(styrene) samples formed at the low vitrification pressures (... |
