9 - Dielectric spectroscopy of ferroelectric polymers

Abstract

The presence of branching chemical defects in PVDF chains leads to broadening of the dielectric relaxation times spectrum for the mobility in the amorphous phase and to the decrease in the relaxation times, qualitatively predicted by the soliton relaxation mechanism, for the mobility in crystals. An increase of the TFE ratio in VDF/TFE copolymers leads to a decrease in the activation and relaxation parameters of the micro-Brownian cooperative mobility in the amorphous phase. According to the SAXS data, this should be associated with a less dense packing of chains in the noted phase. Cold drawing of VDF/TFE copolymers is accompanied by an increase in orientation polarization. Structural data indicate that this should be associated with an increase in the packing density of chains in the interlamellar amorphous regions due to the formation of tie taut chains. It was found that the initial morphology of the isotropic film in the form of small crystals is favorable for cold stretching. The use of Al electrodes leads to the formation of new functional groups on the surface of ferroelectric polymers, which must be taken into account at the interpretation of the dielectric data, especially for thin films. The “anomalies” in the characteristics of the ferroelectric-paraelectric transition may be related to the metastable paraelectric phase which is formed upon low-temperature crystallization. The space charge relaxation processes at high temperatures are sensitive to the thermal prehistory of the sample, which determines the parameters of its supramolecular structure. The relaxation times and activation parameters of this process are determined by the depth of the traps for impurity carriers, which are maximal in case of the presence of polar crystal planes.