COLOR CENTER ANNEALING IN GAMMA-IRRADIATED POLYSTYRENE, UNDER VACUUM AND AIR ATMOSPHERES

A primary effect of low doses of ionizing radiation on polystyrene is the production of absorption centers in the UV and Visible regions of the spectrum. There are two types of color centers: 'annealable'' and 'permanent''. Annealable centers are most likely due to the formation of cyclohexadienyl, benzyl and possibly other structurally related radicals in the polymer. These species have not only strong UV absorption bands (largely obscured by the parent polystyrene absorption) but also weak visible absorptions above 420nm. Post-irradiation annealing has been associated with diffusion of O2 into the solid polymer sample. We now find that annealing can also occur in the absence of oxygen. These two annealing mechanisms can be understood in terms of colored free radicals, trapped within the irradiated solid matrix, which form colorless products upon reaction with O2 or reaction with other (nearby) radicals. The rate of ambient temperature annealing in vacuum is drastically slower than in air. Annealing rates increase significantly with increasing temperature, particularly in vacuum where annealing times under ambient conditions are on the order of months or years, compared with several days at 80-degrees-C. In all cases, samples which were irradiated in vacuum and subsequently exposed to air, developed sharp annealing fronts. Front movement rates were found to be dose-dependent with an apparent saturation effect occurring at the high end of the dose range. Analyses of the front movements allowed estimates to be made of the oxygen consumed during the annealing process.