STRUCTURE PROPERTY RELATIONSHIPS IN THE CASE OF THE DEGRADATION OF MASSIVE POLY(ALPHA-HYDROXY ACIDS) IN AQUEOUS-MEDIA .2. DEGRADATION OF LACTIDE-GLYCOLIDE COPOLYMERS - PLA37.5GA25 AND PLA75GA25

Degradation of two lactic-glycolic copolymers, namely PLA37.5GA25 (75% DL-lactide and 25% glycolide in the feed) and PLA75GA25 (75% L-lactide and 25% glycolide) was investigated in vitro using aqueous media to model physiological conditions. Various techniques were used to monitor the effects due to hydrolytic degradation including weighing, SEC (size-exclusion chromatography), potentiometry, cryometry, enzymatic assay, X-ray scattering, H-1-nuclear magnetic resonance and differential scanning calorimetry. It was found that degradation proceeded faster in the centre than at the surface of standard parallelepipedic specimens. This feature had already been found for PLA50 (poly(DL-lactic acid)). The degradation rates of PLA37.5GA25 and PLA75GA25 were compared and it was found that intrinsically amorphous PLA75GA25 crystallized as degradation proceeded, in contrast to PLA37.5GA25. The crystallization of PLA75GA25 was related to the preferential degradation at glycolic units, which led to L-lactic-enriched fragments susceptible to crystallize. No major differences were observed between ageing in iso-osmolar saline and pH 7.4 phosphate buffer. In contrast, in the case of PLA37.5GA25, distilled water favoured surface-centre differentiation, probably because of osmotic exchange related to the absence of ionic strength.