Effects of crystal structure on piezoelectric and ferroelectric properties of copoly(vinylidenefluoride-tetrafluoroethylene)

Piezoelectricity and ferroelectricity in 80 mol % vinylidene fluoride/20 mol % tetrafluoroethylene copolymers (VDF-TFE) were measured at various crystallinities. In order to control the crystallinity, the crystallization pressure was varied from 1 bar to 5 kbar. All the samples prepared by this method contain only form I crystals but with various crystallinity and crystal thickness. Especially, a band structure characterizing an extended chain crystal was observed in the sample crystallized under more than 4 kbar. Remanent polarization (Pr) obtained by the D-E hysteresis curve increases with the crystallinity, and the extrapolated Pr of 126 mC/m2 nearly coincides with the value of 140 mC/m2 calculated from the unit cell dimension and dipole moment. The coercive field (Ec) decreases with the crystallinity. This would be explained by the expansion of intermolecular distance and a decrease in the number of folds, which affect the rotation of dipoles under an electric field.