Thermally stimulated discharge currents in solution-grown thin polyvinyl butyral films

Abstract

Thermally stimulated discharge (TSD) currents of polyvinyl butyral (PVB) thin films grown on glass substrates by isothermal immersion technique have been studied as a function of the polarization field, (750 to 25000 V/cm) and iodine doping concentration (0.5 to 0.9 g/100 ml of PVB solution). Polarized PVB films exhibit two glow discharge peaks, β - at 350 °K and α - at 430 °K corresponding to activation energies of 0.19 and 0.53 eV, respectively. It has been observed that the β-peak disappears at low polarizing fields (≈750 V/cm). The α-peak is attributed to the depolarization of the aligned dipoles connected to the main chain whereas the β-peak arises due to the local motion or twisting of the side groups connected to main chain. On doping with iodine, both unpolarized and polarized films exhibit TSD currents. This is attributed to the formation of charge transfer complexes between iodine and PVB chains where the dipoles may be aligned due to electrostatic interaction between the charge complexes. The disappearance of the β-peak at high field (≈10 kV/cm) in sharp contrast to the increasing and shifting to lower temperatures of the α-peak show that the complexes are formed with the side groups of the main chain. These conclusions regarding the charge complex formation have been further confirmed by UV and infrared optical absorption data on pure and doped PVB films.