SPECTRAL-ANALYSIS OF OPTICAL-EMISSION DUE TO ISOTHERMAL CHARGE RECOMBINATION IN POLYOLEFINS

Light emission from electrically stressed polymers, so called electrol uminescence, is a subject of great interest because it is associated w ith electrical aging and dielectric breakdown of highly insulating mat erials. Radiative recombination of charge carriers on luminescent cent ers generally is evoked as the main contribution to the emission, but this is difficult to assess in polyolefins since the spectral features associated with this mechanism are not known. The reason is twofold. First, there is a lack of knowledge on the nature of the luminescent c enters in these materials, and second, it is difficult to get waveleng th-resolved electroluminescence spectra due to the low light level. By using an alternative activation method, we have isolated the emission spectrum associated with isothermal charge recombination in polyethyl ene and polypropylene. Electrical charges of both polarities are broug ht to the surface of the samples by contact with a cold plasma powered at a frequency of approximate to 5 kHz. The kinetic and spectral feat ures of the recombination-induced emission are recorded after discharg e switch off. The recombination mechanism is considered on the basis o f the light decay kinetics whereas the chemical nature of the luminesc ent centers is discussed by a comparison with the photoluminescence sp ectrum of the polymer. It is shown that charges recombine by tunneling from traps to the luminescent centers which are unsaturated species. The chromophores involved in photo- and recombination-induced luminesc ence appear to be the same, but not the electronic transitions. Light emitted upon charge recombination has been assigned to transitions fro m the lowest lying tripler states of poly-enone sequences.