Photodegradation of St-MMA copolymers: Correlation of degradation, styrenecontent,and photophysics

A series of styrene(St)-methyl methacrylate(MMA) copolymers with compositio ns ranging from 0 to 100% St were synthesized and exposed to UV light for v arious periods of time. The changes in yellowing (yellowness index-YI) and absorbance of exposed films were evaluated as a function of photolysis time and St content. Both the changes in YI and absorbance at 400 nm were found to increase dramatically for polymer films with styrene (St) content great er than about 50%. A direct correlation between increases in the YI and abs orbance at 400 nm with mean styrene sequence length was found with a maximu m change being obtained for copolymers with mean sequence length of approxi mately 5 St units or greater. A small absorbance shoulder at 270-290 nm was observed for copolymers with St content less than 30% after relatively sho rt irradiation times (e.g. 2 hours). Excimer formation of copolymers in dil ute solution was found to increase with an increase in St content and St se quence length; in films, a plateau in the relative excimer emission was att ained with a St content of 60%, corresponding to a mean St sequence length of 2.