Specifics of structural transformations in poly(vinylidene fluoride)-based ferroelectric polymers in high electric fields

Abstract

The behavior of polymorphic transformations in the solid phase under the action of strong electric fields in crystallizable ferroelectric polymers based on PVDF was analyzed. The occurrence of two transitions, α → αp and α → β, was revealed by means of IR spectroscopy in homopolymer films crystallized as a mixture of the nonpolar α phase and the polar β phase. If such films were crystallized into the ferroelectric β phase, the action of the field reduced to the processes of reorientation of chain segments in the all-trans (planar zigzag) conformation. Reversible and irreversible changes in the crystallinity are possible in this case. A shift in the frequency of some skeletal-vibration bands indicates a change in the mechanical stress on atomic bonds. For vinylidene fluoride copolymers in polar crystals, the field can initiate molecular rearrangements, which lead to an increase in the interchain density of chain packing in the lattice. It was assumed that the delayed kinetics of field-induced solid-state transformations is controlled by two factors. On the one hand, the nucleation of the new phase follows the fluctuation mechanism of appearance of conformation defects of the kink-link type in the initial crystal. An important role is played in such processes by the dynamics of amorphous-phase tie chains bordering the crystals, when a decrease in their activation energy in micro-Brownian motion processes increases the probability of a conformational defect appearing in the crystal. On the other hand, the role of space charge (including the charge due to carrier injection from the electrode metal) in the formation of a local electric field is substantial. Allowance for both factors may provide a qualitative explanation of the specifics of the kinetics of these structural transformations in an electric field.