S.A.X.S. investigation of the lamellar morphology in rolled and poled PVF2 thick films

Small angle scattering patterns over the angular range 5-50 mrad are reported for biaxially oriented poly(vinylidene fluoride) specimens (β phase) prepared under different conditions of poling and annealing. This investigation shows that the crystalline lamellae are stacked along the draw axis with a long period of 8.5 nm, but the typical four-point patterns obtained for different incident angles of the X-ray beam show that the lamellae are not perpendicular to the draw axis. Instead, the normals to the lamellar planes make with the draw axis an angle δ which ranges between 20 deg and 33 deg. Therefore, only a few lamellae are parallel to the ferroelectric axis b. Such results provide a new insight on the role of the depolarizing field which breaks the symmetry of the orthorhombic phase and gives rise to an alternative contribution to the mechanism of the piezoelectricity including the possible rotation of the lamellar crystallites under applied field or stress.