Ferroelectric polarization, pyroelectric activity and dielectric relaxation in Form IV poly(vinylidene fluoride)

Abstract

Ferroelectric, pyroelectric and dielectric properties were investigated for Form IV poly(vinylidene fluoride) (PVDF). We fabricated highly crystalline Form II thin films by spin-casting from acetone or methyl ethyl ketone followed by annealing at 155 °C and then converted into Form IV using a high electric field of 500 MV/m. We observed displacement (D) - electric field (E) hysteresis loop and attendant pyroelectric activity that supported ferroelectric polarization reversal in Form IV PVDF. The highly rounded D-E loop with a coercive field of 240 MV/m suggests that reversal of the dipole component perpendicular to TGTG¯ chain requires very high field and progresses based on step-by-step switching of respective molecules. We also observed similar crystalline relaxations in Form II and IV suggesting that fluctuations of parallel dipoles in TGTG¯ chains were not affected by chain packing. Simultaneous observation of polarization reversal and dielectric relaxation associated with two orthogonal directions (a- and c-axis) of crystallites is interpreted using the tilt angle between of the chain axis and sample surface. Microscopic features are discussed in terms of electric-field-induced and thermally-induced changes in TGTG¯ conformational sequences.