The polymorphism of poly(vinylidene fluoride). II. The effect of hydrostatic pressure

Abstract

The crystallization and melting of poly(vinylidene fluoride) as a function of hydrostatic pressure was investigated. Poly(vinylidene fluoride) can be crystallized in at least two crystalline modifications at pressures of 1 atm (phase I and phase II). The crystallization of poly(vinylidene fluoride) under hydrostatic pressure over a range of conditions yields a new crystalline phase, designated phase III. The new phase melts at 185°C, approximately 25C° higher than phase I or phase II. A noticeable similarity in the melting behavior of phase II and phase III as a function of pressure was observed. Over a range of pressures (up to 3000 atm) the rate of change of the melting temperature with pressure for phases II and III has a single constant value within experimental error. The specific volume of melting (ΔV_m) and the entropy of melting (ΔS_m) of the two phases varied with pressure in a similar manner. The higher values of ΔV_m and ΔS_m observed for phase III apparently reflect the difference in density between the two crystalline phases.