The piezoelectric resonance method was adopted in order to measure piezoelectric and elastic constants of polyvinylidene fluoride films over the temperature range from −40 to 60 °C and at pressure up to 3 kbar. We have shown that the glass transition temperature increases with pressure, dTg/dP being approximately equal to 17 °C/kbar at atmospheric pressure. We have demonstrated that the electromechanical coupling factor K32 is not independent of temperature. Previous measurements, carried out at atmospheric pressure, which indicated the invariance of K32 with temperature, corresponded to a particular case of more general thermodynamic behavior.
Atactic polystyrene has been densified under hydrostatic pressure, up to 2 kbar, at about 180°C. Dielectric measurements were made on densified and nondensified polystyrene samples by using the thermally stimulated depolarization current (TSDC) and the absorption and resorption current methods. The dielectric loss is modified by the densification. This effect can be explained in terms of the free‐volume reduction and the internal stress induced in the material during densification.
The energy state of densified polymeric glasses is examined by using the Lennard–Jones model. We question the proposal that densified glasses are closer to equilibrium than glasses formed at the same cooling rate at atmospheric pressure. The origin of an internal stress quenched into the sample during densification treatment is shown. Some dielectric results on polyethylene terephthalate, polyvinyl chloride, and polystyrene are discussed in terms of the free volume decrease or the quenched energy. Our conclusion is that the densification treatment induces two concurrent phenomena: (1) a decrease of fractional free volume, which is preponderant on the energy state of the material when the densification pressure is small; (2) an internal stress which increases the internal energy and develops quickly for high pressures.
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