Improvement of piezoelectric properties of poly(vinylidene fluoride) and its copolymers by crystallization under high pressures

Abstract

We describe the results of our work performed to improve piezoelectric properties of poly (vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) and with tetrafluoroethylene (TeFE) by means of crystallization under high pressures. In addition, a short review is given on the related work reported in literature. Analysis of VDF concentration dependences of Curie temperature, melting point, and electromechanical coupling factor (k_t ) of P (VDF/TrFE) revealed that, for the improvement, PVDF and VDF-rich P (VDF/TrFE) have to be crystallized into extended chain crystals in the hexagonal phase which exists only at high pressures. On the basis of the pressure-temperature phase diagrams obtained by differential thermal analysis, we studied high-pressure crystallization of these polymers, and evaluated piezoelectric properties of the resulting materials. A PVDF film, initially melted at 270°C under low pressure and then crystallized in the hexagonal phase by increasing pressure up to 500 MPa, exhibited a k_t value of 0.27, and retained its piezoelectric, activity up to the melting temperature (205°C). This result is favorable for the work for further improvement (k_t is expected to be 0.37 as an ultimate value in PVDF). An application of these films to ultrasonic transducers usable at high temperatures is demonstrated.