FERROELECTRIC BEHAVIOR AND RELATED PHYSICAL-PROPERTIES OF SOME COPOLYMERS OF VINYLIDENE FLUORIDE

Some physical properties related to ferroelectric behavior are presented for two different copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) and with tetrafluoroethyiene (TeFE). The ferroelectric-to-paraelectric transition or the Curie transition in the copolymers is discussed with respect to various physical properties such as thermal, mechanical. and electrical properties. In VDF/TrFE copolymers the Curie transition and the melting temperature are noticed for copolymers with VDF content above 50 mol% by the DSC method. The Curie transition is found for thermal expansion in VDF/TrFE copolymers which shows an abrupt increase near the transition. The copolymer with high VDF content exhibits large thermal hysteresis of the temperature for such an increase in thermal expansion between heating and cooling runs. The thermal expansion under hydrostatic pressure provides data on the bulk modulus which shows a minimum at the Curie transition. The thermal hysteresis of the minimum of bulk modulus is also noted for a VDF rich copolymer. The permittivity in VDF/TrFE copolymers displays an anomalous peak at the Curie transition the temperature of which rises with increasing VDF content. Variation of the permittivity with temperature is expressed by the Curie-Weiss relation in the vicinity of the transition. The copolymers exhibit multiple relaxation processes over a wide range of temperature. The relaxation processes are demonstrated by a contour diagram of the permittivity. Pressure dependence of the peak temperature of permittivity is well conformed with that of the Curie temperature obtained from the thermal expansion. The Clausius-Clapeyron equation holds for the pressure dependence of the Curie transition. In VDF/TeFE copolymers the Curie transition is not so stable as in VDF/TrFE copolymer. shifting to higher temperature by thermal and electrical treatments. The increase in the transition temperature is observed by means of the DTA technique. The permittivity peak in VDF/TeFE copolymer is noted only for copolymers with particular TeFE content. Unlike the case of high VDF content in VDF/TrFE copolymers the polarization reversal process in copolymers with intermediate and high TrFE content is markedly dependent on comonomer composition and preparation conditions of specimens. In copolymers with high TrFE content above 50 mol% two current peaks take place or different electric fields in switching process. This indicates that the polarization reversal occurs in a double step manner, yielding a ferroelectric hysteresis loop of constricted shape. The switching process is considerably susceptible to the influence of temperature, mechanical stress, and electric field. By controlling these factors two current peaks coalesce in a single peak, as observed for the VDF rich copolymers. The polarization reversal in VDF/TeFE copolymers is usually characterized by a single current peak at the coercive field and a rectangular shape of hysteresis loop.