Pressure and Strain Effects

The principal effect of hydrostatic pressure on a substance is an increase in density and a preference of better packed sttuctures, in contrast to tensile strain which mainly causes a chain extension. To describe the change of melting temperature with pressure 

The experimental increase of T with pressure of polystyrene is indicated in Fig. 6.122 by line BE. The liquid D cooled at elevated pressure, freezes at the higher glass transition temperature E than liquid C cooled at atmospheric pressure (T = B). On releasing the pressure below T, the compressed sample F expands to G, not to the level A of a glass cooled at atmospheric pressure. The glass remains pressure densified by the amount AG FF, where F is the volume after isothermal compression from A. This densification corresponds to a volume strain. 

Enthalpy and Volume Relaxation of Poly-(methyl Methacrylate) Penslfied by Pressure 

Enthalpy relative to the sample cooled at atmospheric pressure. 

Relaxation at 343 K at atmospheric pressure after vitrification on cooiing at 310 MN/m at a rate of 5 K/hour. 

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