Dynamics of polarization growth and reversal in PVDF films

Abstract

We present measurements of the time development of the dielectric displacement and the remanent polaroization in PVDF for poling times ranging from 1 ~s to 1000 s and poling fields between 0.8 and 2.0 MV /cm. For longer times (0.1 to 1000 s) we determined also the time dependence of the polarization distribution across the film thickness. After applying a steep rectangular HV pulse, the sample is shorted to .aero voltage. The remanent polarization under short-circuit conditions is compared with the maximum dielectric displacement under the external poling field. We observed a significant time delay of the build-up of the remanent polarization as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal we observed for shorter poling times of up to 200 IlS a <flipping back' of the polarization. Under these conditions, a large part of the polarization is reversed under the field , but after removal of the field, most of the polarization returns to the original direction. The results can be understood by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites. This, we think is responsible for the extremely high stability of the permanent polarization in PVOF.461

Pyroelectric study of polarization switching in Langmuir-Blodgett films of poly(vinylidene fluoride trifluoroethylene)

“…Therefore, the switching properties of individual domains vary, causing the strong dependence of the reversed polarization on the pulse duration. 37,49,50 The ferroelectric copolymer LB films show a similarly broad phase transition 8 and duration dependence of the switched polarization, 13 indicating the presence of ferroelectric units with varied properties. The units, though, may be not the same as in spun films, because of completely different crystallization conditions.…”

“…The existence of the cr was shown experimentally, e.g., in triglycine sulfate ͑TGS͒, 36 and the PVDF family polymers. 2, 37 At least two kinds of ferroelectric units, representing different levels of the inhomogeneity, may be identified in ferroelectric materials. These are, first, ferroelectric domains in single crystals, and, second, crystallites in polycrystalline materials, which may be in a single-domain state or subdivided into several domains.…”

“…The unstable contribution to the pyroelectric response may come also from the charge injected from electrodes and trapped at crystallite boundaries. 18,37,49,[69][70][71] Reversible electric-field conversion of paraelectric phase into ferroelectric may also contribute to the decay, especially prominent at elevated temperatures.…”

Polarization of thick polyvinylidene fluoride/trifluoroethylene copolymer films

“…22 It has been shown previously that non-uniform poling of thin P͑VDF-TrFE͒ film can occur, with the polarization starting at the anode, and the mechanism has been discussed in detail. 8,10,11,[23][24][25] In this paper we study the poling of thick copolymer films at elevated temperature. The resonance frequencies of these films have been measured.…”

“…[6][7][8][9][10][11][12] Although studies on the poling of thick copolymer films are rare, the principles governing the polarization mechanisms in thin and thick films are expected to be similar. Various methods have been developed to measure the polarization distribution within a polymer film.…”