This letter reports the ferroelectric and electromechanical properties of a class of ferroelectric polymer, poly(vinylidene-fluoride–trifluoroethylene–chlorotrifluoroethylene) terpolymer, which exhibits a slim polarization hysteresis loop and a high electrostrictive strain at room temperature. The dielectric and polarization behaviors of this terpolymer are typical of the ferroelectric relaxor. The x-ray and Fourier transform infrared results reveal that the random incorporation of bulky chlorotrifluoroethylene (CTFE) ter-monomers into polymer chains causes disordering of the ferroelectric phase. Furthermore, CTFE also acts as random defect fields which randomize the inter- and intrachain polar coupling, resulting in the observed ferroelectric relaxor behavior.

The microstructural changes in high-energy electron-irradiated poly(vinylidene fluoridetrifluoroethylene) 68/32 mol % copolymer have been studied by X-ray diffraction, FTIR spectroscopy, and differential scanning calorimetry. The macroscopic polarization response in these materials was investigated by examining the dielectric and polarization behavior in a broad temperature and frequency range. It was found that besides reducing crystallinity in the copolymer film, irradiation produces significant changes in the ferroelectric-to-paraelectric phase transition behavior. The irradiation leads to a reduction in the polar domain size to below a critical size (a few nanometers), resulting in the instability of the macroscopic ferroelectric state and transforming the structure of the crystalline region in the copolymer from a polar all-trans ferroelectric to a nonpolar state represented by a trans-gauche conformation. However, a reentrant polarization hysteresis was observed in the copolymers irradiated with higher doses (>75 Mrad). Therefore, there is an optimized dose that generates a copolymer with a nonpolar structure but relatively high crystallinity whose electromechanical performance is the best. In the copolymers in this optimum dose range, FT-IR data revealed that there is not much change in the molecular conformation with temperature, even as the temperature passes through the dielectric peak, indicating that there is no symmetry breaking in both the macroscale and local level. Although the lattice spacing of the crystalline region along the molecular chain direction discontinuously changes between two special cases, the interchain spacing continuously changes with the irradiation dose, reflecting a strong intrachain coupling between the nonpolar and polar regions. On the other hand, the X-ray data reveal that the crystalline size perpendicular to the polymer chain does not change with irradiation until at doses exceeding 85 Mrad.

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Science Foundation under Grants CHE78-04258 and CHE81-10541. K. Hagen is grateful to the Norwegian Research Council for Science and the Humanities for partial support and a travel grant.