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.