Structural and Dielectric Investigation on the Nature of the Transition in a Copolymer of Vinylidene Fluoride and Trifluoroethylene (52/48 Mol %)

摘要

The effect of temperature on the structure and dielectric properties of a 52/48 mol % copolymer of vinylidene fluoride and trifluoroethylene has been investigated at temperatures up to 140 C. Undrawn or unpoled specimens contain an intimate mixture of two disordered crystalline phases, both of which undergo a large increase in d-spacing at 65-70 C, with eventual transformation to a single phase in which the chains assume a disordered 3/1-helical conformation above 90 C. The 70 C transition is accompanied by a dielectric anomaly. High electric fields applied at temperatures below 70 C induce a phase change to a single, well-ordered all-trans conformation, leading to remanent polarization with piezoelectric and pyroelectric coefficients comparable to those of poly(vinylidene fluoride). The changes in crystal phase and dipole orientation upon poling result in a reduction of the dielectric constant at room temperature, a shift of the dielectric anomaly to about 80 C, stability of the all-trans crystal phase to somewhat higher temperatures, and a discrete change in d-spacing to that of the disordered 3/1-helical conformation at the transition region. The loss of polarization in poled specimens at this ferroelectric-to-paraelectric transition is attributable primarily to the molecular change from the polar all-trans conformation to its non-polar, disordered 3/1-helical counterpart, as well as to the onset of rotational dipolar motions leading to the dielectric anomaly. (Author)