Effects of pressure on amorphous polymers. III. Thermodynamic properties of densified polymethyl methacrylate

Measurements of the temperature dependence of specific heat are presented for polymethyl methacrylate densified in various pressure-temperature-time cycles. It is found that treatment in the glass transition region produces samples with higher glassy specific heat (0. 38 vs 0. 35 cal/g°C) and lower heat content (difference in enthalpy of 2 to 5 cal/g) than the original material. The change in thermal properties is directly correlated with increased density and increased resistance to annealing. High-pressure treatment in the glassy state is found to produce little change in the thermodynamic properties. Samples vitrified by rapid application of high pressure show complex specific-heat-temperature curves. The results are related to previously reported data on densification and annealing and are discussed in terms of theories of the glassy state and the glass transition.