Dielectric relaxation in PVDF, P(VDF-TFE) and PVDF-BaTiO/sub 3/ composites

Abstract

Summary form only given. Relaxation processes in ferroelectric polymer-ceramic composites are not a simple superposition of those observed in pure components. In our case, pure PVDF, P(VDF-TFE) copolymer and PVDF-BaTiO/sub 3/ composite were studied by the thermally stimulated depolarization (TSD) method from -160/spl deg/C to +100/spl deg/C and by the AC dielectric spectroscopy method from -60/spl deg/C to +100/spl deg/C at frequencies from 100 Hz to 5 MHz. Polymer films were extruded and stretched, while composite samples were prepared by hot pressing of PVDF powder mixed with BaTiO/sub 3/ particles in equal volume proportion. Three relaxation processes were identified, namely /spl alpha/-relaxation in amorphous phase of PVDF and P(VDF-TFE) related to the glass transition, /spl beta/-relaxation in the glassy state of the polymer and the interfacial or space charge relaxation at 60-100/spl deg/C. A non-Debye TSD /spl alpha/-peak around -45/spl deg/C indicated that there was a broad distribution of relaxation times. The same process gave rise to a peak at the frequency dependence of imaginary part of the dielectric constant during the AC dielectric measurements. The peak in PVDF-BaTiO/sub 3/ composite appeared at -20/spl deg/C and 50 kHz compared to 30 kHz in pure PVDF, showing a weak anti-plasticizing effect of the filler. Our experimental data were fitted to Cole-Davidson and Havriliak-Negami theoretical expressions and corresponding distribution parameters were obtained. The /spl alpha/-relaxation was believed to be caused by micro-Brownian motions of segments of the main polymer chain, while the /spl beta/-relaxation, seen as a broad TSD peak at -135/spl deg/C corresponded most probably to the local mode motions in the glassy state of the polymers. The space charge peak at 60-100/spl deg/C has been explained in the framework of the phenomenological model stipulating existence of charge trapping during the buildup of polarization either in crystallites in case of PVDF and copolymer, or in BaTiO/sub 3/ particles of the composite.