Investigation of the degradation behaviour of poly(ethylene glycol-co-D,L-lactide) copolymer

The aim was to investigate the degradation behaviour of poly(ethylene glycol-co-D,L-lactide) (PEG-D,L-PLA) multiblock copolymer, in bulk and as microspheres, in aqueous medium. The degradation behaviour of PLA homopolymers in bulk and microspheres was evaluated as comparison. Microsphere preparation was performed by the double emulsion solvent evaporation method. Physicalechemical characterization of the raw polymers and the microspheres was performed by nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry (MDSC). Polymer molecular weight, before and after incubation in aqueous environment, was evaluated by GPC; water uptake and mass loss were determined gravimetrically. The presence of PEG segments inside PLA chains gave a characteristic spongy structure to the microspheres. A significant increase in polymer Tg values was found for the microsphere formulations compared to polymer in bulk. After 63 days of incubation in the aqueous environment, the PEG-D,L-PLA microspheres achieved an average Mw reduction of 47% compared to 20% for PLA microspheres. The corresponding Mw decrease of the polymers in bulk was significantly higher: 72% and 41% for PEG-D,L-PLA and PLA, respectively. The data show how the degradation behaviour of polymer in bulk in an aqueous environment is significantly different from the behaviour of the corresponding microspheres. These results highlight the importance of performing a thorough physicalechemical characterization on microsphere formulations.