Spectral analysis of optical emission due to isothermal charge recombination in polyolefins

Light emission from electrically stressed polymers, so called electroluminescence, is a subject of great interest because it is associated with electrical aging and dielectric breakdown of highly insulating materials. Radiative recombination of charge carriers on luminescent centers 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 centers in these materials, and second, it is difficult to get wavelength-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 polyethylene and polypropylene. Electrical charges of both polarities are brought to the surface of the samples by contact with a cold plasma powered at a frequency of ≈5 kHz. The kinetic and spectral features of the recombination-induced emission are recorded after discharge switch off. The recombination mechanism is considered on the basis of the light decay kinetics whereas the chemical nature of the luminescent centers is discussed by a comparison with the photoluminescence spectrum 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 luminescence appear to be the same, but not the electronic transitions. Light emitted upon charge recombination has been assigned to transitions from the lowest lying triplet states of poly-enone sequences