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

• Published in: Polymer degradation and stability 2010-07, Vol.95 (7), p.1238-1243
• Main Authors: Yasuda, Nobuhiko, Wang, Yan, Tsukegi, Takayuki, Shirai, Yoshihito, Nishida, Haruo
• Format: Article
• Language: English
• Subjects: Ageing; Applied sciences; Chain scission; Exact sciences and technology; Irradiation; Mark–Houwink–Sakurada parameter; Molecular weight; Photodegradation; Physical properties; Poly( l-lactic acid); Polylactic acid; Polymer industry, paints, wood; Properties and testing; Quantitative analysis; Quantitative evaluation; Racemization; Stability; Technology of polymers
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/j.polymdegradstab.2010.03.034

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.