Dynamics of Polarization Growth and Reversal in PVDF Films

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