Compensation and neutralization of depolarizing field in ferroelectric polymers by trapped charges

We show in this paper that favourable conditions for trapping exist not only at boundaries of polarized crystallites, but also in macroscopic transition zones, by which polarized parts are separated from unpolarized ones. Depending on poling conditions, the zones are located either near surfaces, or inside the volume. We found that the charge density which is necessary to compensate the depolarizing field does not depend on thickness of either the transition zone, or the polarized volume. Assuming that compensating charges are trapped at boundaries of crystallites and considering that FP has a brick-like structure, we found that the charge density necessary to provide for a complete compensation must be about 3×10²⁶ m^-3 , which is 10⁸ times higher than the total charge available in the sample. It means that the compensating charges cannot be trapped at the boundaries of crystallites, but they reside, most probably, in the transition zones. We calculated that the field acting at a dipole inside a crystallite was zero in this case. At the same time, the second component of polarization appears under action of rather high field applied to the amorphous phase of the FP, although it is not as large and stable as the ferroelectric polarization in crystallites. We calculated that the charge density of about 3×10 ^-6 C/m² can be obtain by separation of the intrinsic charges, but this value is much lower than that necessary for compensation of the depolarizing field and estimated as 10^-1 C/m². It means that additional charges are needed which can be supplied either by injection, or by thermal stimulation during the poling. Considerable decrease of apparent conductivity in FPs observed by us at different poling conditions, was in good agreement with our model calculations