The piezoelectric properties and ferroelectric transition behaviors of vinylidene fluoride and trifluoroethylene copolymers, P(VDFx−TrFE1−x), were studied as a function of x in the range of x=0.37–0.94. The electromechanical coupling factor kt and piezoelectric constant d31 are strongly dependent on both x and annealing temperature. In the copolymers of 0.65≲x≲0.82, a large kt value of 0.27–0.3 has been found when annealed above ferroelectric-to-paraelectric transition temperature Tc. The large kt is attributable to their high remanent polarization (∼110 mC/m2) which arises from high degrees of crystallinity and preferred orientation of well-grown crystallites. Structural and morphological changes induced by annealing and poling were characterized by SEM, DSC, and x-ray diffraction studies. Phase diagram of the copolymers were obtained by DSC measurements. Tc increases with increase of x and coincides with melting temperture Tm at x≂0.82. In the range of x≳0.82, Tc=Tm.

Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

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Piezoelectricity in βphase poly(vinylidene fluoride) having a ''singlecrystal'' orientationPoly͑vinylidene fluoride͒ ͑PVDF͒ was crystallized under high pressures on the basis of the pressure-temperature ( P-T) phase diagram obtained recently, and piezoelectric and ferroelectric properties of the resulting films were evaluated. Crystallization in the metastable hexagonal phase appearing at high pressure and temperature below the triple point yielded a film comprising extended chain lamellar crystals of a mixture of ␤ and ␥ forms, the latter of which is transformed into ␤ form by poling at an elevated temperature ͑120°C͒. The poled film exhibits strong piezoelectric effect persistent up to the melting temperature ͑205°C͒. The electromechanical coupling factor k t for the thickness extensional piezoelectric effect is 0.27 at 25°C, the largest value ever found in PVDF. The results of studies on morphology, structure, and thermal properties are also described.

The polarization behavior in films of vinylidene fluoride trifluoroethylene copolymer (P(VDF-TrFE), VDF/TrFE molar content ratio of 75/25) of thickness less than 1 µm is reported. Ferroelectricity is maintained even in 0.06 µm-thick film, whose saturated polarization is as large as that for thick films (100 mC/m2). However, the crystallites are much smaller than in thick film, and the coercive field in the thin film is higher (125 MV/m) for the 0.06 µm-thick film) than that of the thick one (40 MV/m). The switching time does not follow the exponential law, but is inversely proportional to the applied field. To analyze these data, we introduce a model in which the thin film contains many defects in its crystalline phase, and the local field in the film is modulated by the defects. The observed switching behavior is consistent with the model.