A field-cycle-induced high-dielectric phase in ferroelectric copolymer

We report phase transitions from ferroelectric to paraelectric states that are induced in poly(vinylidene fluoride-trifluoroethylene) by electric field cycles. The transitions are systematically characterized electrically and structurally by using ferroelectric capacitors. The top electrodes of the capacitors can be peeled off from the surfaces of ferroelectric layers; this enables us to obtain clear x-ray diffraction patterns with adequate strength. The diffraction patterns clearly indicate the emergence of the stable phase, in which copolymer molecules are aligned with their b-axis perpendicular to the substrate. The dipoles become nonswitchable without changing the molecular structure. The new field-cycle-induced phase, which has high permittivity in the out-of-plane direction due to anisotropic molecular structures, is identified as the main mechanism of polarization fatigue.