Piezoelectricity and pyroelectricity in vinylidene fluoride/trifluoroethylene copolymers

Piezoelectric and pyroelectric properties have been investigated in the 52/48, 65/35, and 73/27 mol % copolymers of vinylidene fluoride and trifluoroethylene. The transverse and longitudinal piezoelectric constants as well as the elastic constant were determined for undrawn samples as a function of the remanent polarization Pr created by a room-temperature poling. The elastic constant shows a significant increase with increasing Pr. Thus, the d constant (polarization/stress) tends to saturate at high Pr. On an assumption of elastic isotropy, true e31 and e33 (polarization/strain) were calculated. It is found that e33 is proportional to Pr with a coefficient of −1.2 in three copolymers while e31 is almost zero. The results indicate that the piezoelectric activity in undrawn copolymers primarily arises from macroscopic dimensional changes while molecular dipoles are rigid. The pyroelectric constant p3 and the thermal expansion coefficient α3 were also measured in mechanically-free samples. By the use of e31, e33, and α3, the contribution from the secondary effect was evaluated and found to be smaller than 1/4 of the total activity. The primary effect due probably to disordering of crystalline dipoles seems to play a major role in the pyroelectricity of the copolymers.