The high-pressure crystallization behaviours and piezoelectricity of extended chain lamellar crystals of vinylidene fluoride trifluoroethylene copolymers with high molar content of vinylidene fluoride

Abstract

In order to improve piezoelectric properties, high-pressure crystallization behaviours of copolymers of vinylidene fluoride (VDF) and trifluoroethylene with high molar content of VDF (86 and 94 mol%) were studied by high-pressure d.t.a., and high-pressure crystallized films were characterized by d.s.c., X-ray diffraction, and piezoelectric resonance. The pressure-phase diagrams showed that these polymers have the hexagonal (paraelectric) phase at high pressures. The films crystallized in this phase result in extended β-form crystals (β-ECCs) composed of planar zig-zag chains. Since their hexagonal phase appears in a lower temperature range as compared to the hexagonal phase of PVDF, well-developed β-ECC films were obtained easily without serious thermal degradation. The electromechanical coupling factor (kt) of β-ECC film for thickness extensional mode is 0.19 for 86 mol% copolymer, and 0.15 for 94 mol% VDF copolymer. These copolymers retain stable piezoelectric activity up to the melting temperature (165 and 186°C for 86 and 94 mol% VDF copolymers, respectively). © 1997 Elsevier Science Ltd.