Effects of Pressure on Amorphous Polymers. III. Thermodynamic Properties of Densified Polymethyl Methacrylate

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Abstract
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 glassyspecific 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.