Thermal Contraction and Ferroelectric Phase Transition in Vinylidene Fluoride-Trifluoroethylene Copolymers. 2. An Effect of Tensile Stress Applied in the Direction Perpendicular to the Chain Axis

Anisotropic thermal expansion under externally applied tensile stress has been measured for uniaxially oriented ferroelectric vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers with VDF 65% molar content. In the heating (cooling) process, the sample length along the drawing direction was found to contract (expand) largely in the vicinity of the structural phase transition temperature, while the dimension in the direction perpendicular to the draw axis has been found to expand (contract) in the same temperature region. The unoriented film exhibited a thermal expansion behavior intermediate between the dimensional changes in the parallel and perpendicular directions. A comparison of these thermomechanical curves with the data of X-ray, infrared and Raman, DSC, etc., revealed that the bulk dimensional changes of the sample reflect directly the structural phase transition occurring in the crystalline region in both directions. The transition temperature was found to shift to the lower side with an increasing applied stress: ΔT/Δσ- = -1.0 K/MPa (perpendicular) and -4.6 K/MPa (parallel). These results were interpreted reasonably based on a modified thermodynamical Clausius-Clapeyron equation.