Ferroelectric behavior in the high temperature paraelectric phase in a poly(vinylidene fluoride-co-trifluoroethylene) random copolymer

In this report, ferroelectric behaviors were investigated as a function of temperature and poling frequency for a uniaxially stretched poly(vinylidene fluoride-co-trifluoroethylene) 50/50 (mol./mol.) film. A variety of ferroelectric behaviors, including normal ferroelectric, antiferroelectric-like, and paraelectric behaviors, were observed by varying temperature, poling frequency, and poling electric field. Especially, the ferroelectric (FE) to paraelectric (PE) transition and ferroelectric behaviors of the high temperature PE phase were studied in detail by electric displacement-electric field loop measurements. At a high poling frequency (e.g., 1000 Hz) and 100 °C (above the Curie temperature at 64 °C), a paraelectric behavior was obtained due to the nucleation of electric field-induced FE nanodomains inside the PE phase matrix. These FE nanodomains were highly reversible and they quickly depolarized upon removal of the poling field. At an intermediate poling frequency (e.g., 10 Hz) and 100 °C, an antiferroelectric-like behavior was observed, which could be attributed to the competition between depolarization and polarization fields upon reverse poling. Finally, at a low poling frequency (e.g., 1 Hz) and 100 °C, a normal ferroelectric behavior with rectangular hysteresis loops was seen because the small, reversible FE domains had enough time to grow into large irreversible ones. The presence of electric field-induced FE domains in the PE matrix was proved by field dependent Fourier transform infrared study. On the basis of this study, understanding of the paraelectric behavior in polar crystalline polymers will help us design new materials to meet the requirements for high energy density and low loss dielectric applications.