Degradation of poly(L-lactic acid) by γ-irradiation

Poly(L-lactic acid) (PLA), was irradiated with gamma -rays in air and in vacuum at 25 degreesC. Melting point T-m, glass transition temperature T-g and number-average molecular weight (M) over bar (a) decreased with increasing irradiation dose. At low doses, T-m, T-g and (M) over bar (n) decreased sharply and then the rate became slower. Up to 200 kGy, PLA was predominantly degraded by random chain-scission. Above 200 kGy, recombination reactions or partial crosslinking may occur in addition to chain-scission. The degree of chain-scission G(s) of irradiated PLA was calculated to be 1.97 in air and 0.83 in vacuum. The decrease in (M) over bar (n), T-m and T-g of the sample in air was faster than in vacuum because of oxidative chain-scission. Biodegradation of PLA was retarded with increasing dose due to the introduction of crosslinking during irradiation. ESR spectroscopy showed five kinds of radicals assigned to three kinds of ESR spectra. (C) 2001 Elsevier Science Ltd. All rights reserved.