The effects of degradation products on the rates of photo-reactions in polystyrene films

Polystyrene films have been exposed to 254 nm radiation under high vacuum at 25°C. The time evolution characteristics of new ultraviolet-absorbing chromophores have been examined; in particular, increased absorptions at 240 nm and 400 nm have been measured. Whilst the initial rates of increase in these absorbances are relatively rapid, they are not maintained, and decline gradually with increasing irradiation. The fall-off in rates has been attributed to an optical filter effect, the products, which have higher extinction coefficients for 254 nm radiation, absorbing some of the incident radiation in direct competition with the polymer. The results were analysed in terms of a model which allowed for an appropriately reduced rate of absorption by the polymer, and a good correlation between experimental data and theoretical prediction was observed. Other factors which could contribute to the apparent inhibition of chromophore formation are discussed, and their effects assessed.