Mechanical and electromechanical properties of vinylidene fluoride terpolymers

Electromechanical, mechanical, and volumetric measurements on vinylidene fluoride terpolymers with trifluoroethylene and trichlorofluoroethylene are reported. One purpose of the investigation was to assess the contribution of an electric field-induced crystal phase transformation to the large electrostrictive coefficient (43 nm(2) V-2 at 1 Hz, parallel to the field). Such a crystal transformation is evidenced by the change in the Curie transition enthalpy after application of an electric field. Using the volume change at the Curie transition, along with X-ray diffraction data, we quantify the amount of the ferroelectric and paraelectric crystalline forms. From this, we conclude that, for electric fields up to 10 MV/m, the strain resulting from the field-induced crystal phase change is too small to account for the large electrostriction observed in the terpolymer. The transition from the a to the beta-crystalline form is also induced by hydrostatic pressure, with the beta-crystals formed at elevated pressures stable at ambient pressure. The Curie transition in the terpolymer, similar to the behavior of the copolymer, is accompanied by changes in both the shear and bulk moduli. The Curie transition exhibits significant hysteresis during thermal cycling, implying that both the electrostriction and the mechanical properties will be sensitive to processing.