Piezoelectricity and related properties of vinylidene fluoride and trifluoroethylene copolymers

The piezoelectric properties and ferroelectric transition behaviors of vinylidene fluoride and trifluoroethylene copolymers, P(VDFx−TrFE1−x), were studied as a function of x in the range of x=0.37–0.94. The electromechanical coupling factor kt and piezoelectric constant d31 are strongly dependent on both x and annealing temperature. In the copolymers of 0.65≲x≲0.82, a large kt value of 0.27–0.3 has been found when annealed above ferroelectric-to-paraelectric transition temperature Tc. The large kt is attributable to their high remanent polarization (∼110 mC/m2) which arises from high degrees of crystallinity and preferred orientation of well-grown crystallites. Structural and morphological changes induced by annealing and poling were characterized by SEM, DSC, and x-ray diffraction studies. Phase diagram of the copolymers were obtained by DSC measurements. Tc increases with increase of x and coincides with melting temperture Tm at x≂0.82. In the range of x≳0.82, Tc=Tm.