Dielectric relaxation in PVDF, P(VDF-TFE) and PVDF-BaTiO3 composites

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-BaTiO₃ composite were studied by the thermally stimulated depolarization (TSD) method from -160°C to +100°C and by the AC dielectric spectroscopy method from -60°C to +100°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 BaTiO₃ particles in equal volume proportion. Three relaxation processes were identified, namely α-relaxation in amorphous phase of PVDF and P(VDF-TFE) related to the glass transition, β-relaxation in the glassy state of the polymer and the interfacial or space charge relaxation at 60-100°C. A non-Debye TSD α-peak around -45°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 ₃ composite appeared at -20°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 α-relaxation was believed to be caused by micro-Brownian motions of segments of the main polymer chain, while the β-relaxation, seen as a broad TSD peak at -135°C corresponded most probably to the local mode motions in the glassy state of the polymers. The space charge peak at 60-100°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 BaTiO₃ particles of the composite