(14e) Chain Scission Reaction in Hydrolytic Degradation of Polylactic Acid

The in vitro degradation behavior of polylactic acid (PLA) has been examined in terms of degree of degradation during an incubation period of up to 15 days. High performance liquid chromatograph and gel permeation chromatography (HPLC-GPC) were employed to characterize their degradation profiles. The degradation was carried out in a sealed system at 80°C. After degradation the amount of the degradation species were measured on HPLC and the molecular weight of degradation species were determined using GPC according to polystyrene standards and polylactic acid standards. The extent of random scission reaction was evaluated by the ratio of the number of degraded polymers to the number of repeating units in initial PLA chains, and extend of end scission reaction was evaluated by the ratio of the number of monomers degraded to the number of repeating units in initial PLA chains.

A significant decrease in molecular weight was observed in the first three days due to random scission reaction. After three days, the degradation is mainly governed by end scission reaction and the molecular weight drops slowly. The results confirmed hydrolysis of ester linkage as the primary mechanism of degradation of PLA. The gradient degradation and auto-catalysis affect of lactic acid in the degradation of PLA matrix were also studied. Although a great number of condensation polymers undergo random scission reaction, different hydrolytic degradation rates for internal and chain-end bonds have been reported. The different rates were attributed to differences in the electronic environment near the reaction sites. Efforts are underway in ascertaining which chain-end (carboxyl, hydroxyl or both) of PLA degrades faster than the internal bonds.