Thermal and mechanical properties of a ferroelectric copolymer P(VDF/TrFE)

Abstract

Ferroelectric copolymer samples of Poly(vinylidene/trifluoroethylene) [P(VDF/TrFE)] of two VDF mole fractions were prepared using the hot-press method. Samples of different crystallinities were made by controlled heat treatment. Thermal and mechanical properties of the samples were investigated as a function of temperature. These properties exhibit a drastic change near the ferroelectric phase transition temperature. The results also show that both kinds of properties were dependent on the degree of crystallinity. Thermal expansivity and specific heat of the copolymer samples generally increase with temperature, peaking at the Curie temperature. On the other hand, the density decreases with temperature and drops abruptly at around the Curie temperature. The thermal diffusivities were determined by the laser flash radiometry method. The results show that they decrease with temperature and drop rapidly during ferroelectric phase transition. As a result, the thermal conductivities increase with temperature, exhibiting a peak at the Curie temperature, and then drop to similar values as those obtained before the ferroelectric phase transition. The mechanical properties of the copolymer as revealed by the elastic moduli were determined by the ultrasonic method. The elastic stiffness constants calculated from the longitudinal and transverse ultrasonic velocities show a decreasing trend with increasing temperature and drop with a great magnitude at the phase transition. Since the Young's, shear and bulk moduli are all related to the stiffness constants, they decrease in a similar manner with temperature. The measured properties display the thermal hysteresis effect to differing extents. Samples with higher VDF content have a more profound shift to lower temperature at the phase transition during the cooling measurements. This result is consistent with the investigations found in the literature.