Thickness Dependence of Orientation Factors of Crystal Axes in Poly(vinylidene fluoride/trifluoroethylene) Single Crystalline Films

Poly(vinylidene fluoride/trifluoroethylene)single crystalline (SC) films, which were prepared by uniaxial stretching followed by crystallization in the paraelectric phase, are highly double-orientated, that is, the orthorhombic c-axis, and the axes perpendicular to the (110)/(200) planes of fully extended chain crystals, are preferentially oriented parallel to the stretching axis and normal to the film surface, respectively. The thickness dependence of orientation distributions of these axes in SC films were evaluated using wide-angle X-Ray diffraction. The results revealed that the orientation factors of the c-axis and the axes perpendicular to the (110)/(200) planes increase with decreasing film thickness. It is suggested that the preferential orientation of the axes perpendicular to the (110)/(200) planes to the direction parallel to the film normal, which is developed in the hexagonal phase, is very high near the film surface, but it becomes gradually weaker as the distance from the surface increases. The formation mechanism of the orientational order is discussed, and a model for the preferential orientation distribution of the crystal planes is proposed.