Mechanical and physical properties of poly(vinylidene fluoride) at high pressures and temperatures

Abstract

Abstract Stress-strain behavior in tension and compression, Pressure-Volume-Temperature (PVT) relations, and the pressure dependency of transitional behavior, by dielectric technique, of Phase I and Phase II ploy(vinylidene fluoride) (PVDF) have been determined. The result obtained are used in analyzing the complex behavior of piezoelectric and pyroelectric properties of the material at high pressures and temperatures. The Young's modulus(E) of Phase II PVDF increases, at room temperature, monotonically but nonlinearly, with increasing pressure as long as the amorphous material is in the rubbery state. It then takes a step-jump at Pg (5 Kbars) and continues to increase with pressure. However, the yield strength in both tension and compression is observed to be linear functions of pressure. PVT measurements clearly show the glass transion temperature (Tg) at −50°C and the crystal transition (Tc) at 22°C at atmospheric pressure. The Tg of both unoriented and oriented Phase I PVDF increase, in most part, linearly with increasing pressure, except at low pressures where the increase is parabolic. The linear compressibility of unoriented Phae I PVDF at room temperature is a nonlinearly decreasing function of pressure, varying from 6.8 × 10−6 cm2/N at atmospheric pressure to 2.7 × 10−6 cm2/N. The linear compressibility of oriented Phase I PVDF in the transverse direction is much greater than that in the machine direction. The difference in the linear compressibilities diminishes with increasing pressure. A comparison between the minimum points in the plots of dp and Py verses temperature and the Tg at various pressures shows that, even though different molecular motions are responsible for these activities, there appears to be a common influence by hydrostatic pressure.