Depolarization behaviour of solution-grown thin polyvinylchloride films

Abstract

Thermally stimulated discharge (TSD) current behaviour of polyvinylchloride (PVC) films grown by the isothermal immersion technique has been studied as a function of film thickness (1000 to 7500 Å ), polarizing field (3000 to 10000 V/cm) and iodine doping concentration (0.5 to 0.9 per 100 cm³ solution of PVC). Polarized PVC films exhibit TSD with both peak current and total charge being linearly proportional to the film thickness and the polarizing field. These observations suggest dipolar behaviour of PVC. The TSD peak above room temperature corresponds to an activation energy of about 0.67 eV and a relaxation time of about 1.4 × 10⁴s at 300° K which is attributed to the molecular motion of dipoles. On doping with iodine, the unpolarized films show TSD which decrease rapidly to zero with increasing polarizing field and increasing temperature. Further, the TSD peak current and total stored charge increase with increasing iodine concentration. The activation energy for molecular motion decreases down to 0.17 eV and then increases with increasing iodine concentration above 0.9 g/100cm³ of solvent. These results and also the results of EPR studies on iodine doped PVC films indicate that iodine is either located interstitially or trapped between the folded chains, and forms charge complexes. The charge complexes may be formed with iodine accepting an electron from a monomer or transferring an electron from one monomer to another. The electrostatic interactions between the charge complexes can explain the observed polarization behaviour of iodine doped PVC films.