Crystal Structures and Phase Transitions of Vinylidene Fluoride–Vinylene Fluoride Copolymers

A series of vinylidene fluoride–vinylene fluoride (VDF–VLF) copolymers of various VDF contents were newly synthesized. Their characterization was performed on the basis of the thermal analysis and the temperature-dependent measurements of X-ray diffraction, FTIR spectra, and dielectric constants. The copolymers of VDF 50–90 mol % content were found to experience the ferroelectric phase transition between the polar low-temperature (LT) phase (equivalent to PVDF β form) and the nonpolar high-temperature (HT) phase, where the remarkable change of chain conformation occurs between the trans-zigzag form in the LT phase and the statistically irregular trans–gauche form in the HT phase. These behaviors are similar to those reported for a series of vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymers. By comparing the details of the phase transition behaviors between these two kinds of VDF copolymers, such structural factors governing the phase transitions were extracted concretely, as the effective cross-sectional area of zigzag chains, the trans–gauche conformational stability, and the cooperativity of polar VDF monomer sequences. One of the practical advantages of VDF copolymers as ferroelectric polymer materials is the easy formation of the ferroelectric LT phase even when the sample is cooled from the melt. The VDF content to realize this advantage was found in the range of 50–82 mol % for VDF–TrFE copolymers, while the range becomes wider, 50–90 mol %, for VDF–VLF copolymers, allowing us to widen the industrial application of ferroelectric polymer.