Spectral changes of CR-39 induced by irradiation and heat treatment

Abstract

The structure of CR-39 was investigated under the effect of either gamma radiation or thermal treatment, by following the induced changes in its spectrum in the visible and ultraviolet region. Samples were irradiated with gamma doses of up to 100 kGy in air while heat treatment was carried out by holding the samples at temperatures of up to 180 °C in an air oven for 1 h. The spectra of the samples before and after gamma exposure and thermal treatment were analysed by the direct-allowed transitions suggested by Mott and Davis. The calculated optical energy gap ( E opt) values and the induced changes in absorption spectra of the irradiated samples are interpreted in terms of a degradation process at these doses. On the other hand, the temperature coefficient of the gap is evaluated as 1 × 10 −3 eV K −1 from the obtained E opt values for the heat-treated samples. The induced changes in optical spectra suggest crosslinking, by a free radical mechanism, at lower temperatures and bond rupture at oxygen-containing sites of the main chain at higher temperatures. An abrupt change in the UV spectrum of the sample heated at 180 °C is attributed to a phase transition in the amorphous structure of the polymer. In both cases of treatment, a colour centre band is induced at about 340 and 364 nm for the irradiated and heat-treated samples, respectively.