Photoconduction of Polyethylene

Abstract

Electronic conduction in various polymeric insulating materials has been discussed by many investigators.1–10 Since Fowler gave successful explanation to X-ray induced conductivity in plastics on the basis of energy-band theory including electron traps,1 electron traps have been considered to play an important role in electrical conduction, of polymer insulators. Similar experiments by means of γ-rays were made for polyethylene by Yahagi 2 and for polystyrene by Coppage.10 The former showed that electrons dominate over ions as charge carriers and the latter proposed from the results of pulsed radiation that an activation energy of 0.2 eV in the long-lived photo-current of polystyrene is attributed to a thermally activated drift mobility. No temperature dependence for the induced conductivity of polyamide copolymers could be detected between 10 and 60°C,11 while the dark conductivity followed the usual exponential rule with activation energies between 1 and 2 eV. Ebeoglu 12 investigated pulsed radiation effects to establish correlation between degree of crystallinity and decay characteristics of photo-currents using the band theory. Yahagi 3 also studied the electrical conductivity of polyethylene irradiated in air at 77°K by high-energy electron beam of 1.5 MeV to propose that if allyl type radicals produced in polyethylene have positive electron affinity, they may serve as electron traps. Partridge 13 pointed out from thermoluminescence of polyethylene that 0.03, 0.2 and 0.5 eV trap centers are attributed to the crystal regions, the amorphous regions and the surfaces of the crystallites, respectively. Electron-excited conductivities have also been investigated,14–17 which essentially leads to the build-up of the space charge in an insulator.