Quantitative evaluation of photodegradation and racemization of poly( l-lactic acid) under UV-C irradiation

To obtain details of poly( l-lactic acid) (PLLA) photodegradation behavior, PLLA films were irradiated by UV-C light ( λ = 253.7 nm) to directly excite carbonyl groups, resulting in a rapid decrease in the molecular weight accompanying a gradual decrease in the optical purity of monomeric units in the chains. The racemization during the photodegradation was first detected as a result of the chain scission by irradiation. From quantitative analyses of the molecular weight and the monomeric unit composition, it was found that the chain scission ratio and the d-lactate unit ratio increased in parallel during the irradiation, suggesting that approximately one d-lactate unit formed for every chain scission. From a mechanistic consideration, the racemization equilibrium was proposed to occur at both carboxyl and hydroxyl chain ends.