Thermal Contraction and Ferroelectric Phase Transition in Vinylidene Fluoride-Trifluoroethylene Copolymers. 1. An Effect of Tensile Stress along the Chain Axis

Thermal contraction has been measured for the uniaxially oriented samples of vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymers with various VDF contents. In the heating process, for example, the bulk sample length was found to contract largely in the temperature region of the phase transition. This thermal behavior, including the dependence on VDF content, was found to intimately relate with the DSC data and the structural data such as infrared absorption and X-ray diffraction, indicating that the dimensional change in the crystalline region occurring in the ferroelectric phase transition reflects directly on the bulk dimensional change observed here. The transition point estimated from the thermal contraction curve was found to shift to the lower temperature side with an increasing tensile stress, about-4 ~-7 °C/MPa. On the basis of the modified Clausius-Clapeyron equation the phenomenon was interpreted reasonably to originate from the shift in the phase-transition temperature of the crystalline region under the tensile stress. The direct measurement of the transition point shift by the X-ray diffraction under stress supported this interpretation experimentally.