Ferroelectric properties of vinylidene fluoride copolymers

Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylidene fluoride at temperatures from 70 to 140°C. The transition accompanies cooperative dipole motions associated with elementary trans-gauche transformations. Changes in various properties as well as in structures in the vicinity of the Curie point are consistent with an order-disorder transition of first order. Detailed characteristics specifying their ferroelectricity are summarized and discussed in relation to the molecular, crystalline, and higher-order structures inherent to ferroelectric polymers.