Effects of chain microstructures and derived crystallization capability on hydrolytic degradation of poly(l-lactide/ε-caprolactone) copolymers

Hydrolytic degradation of bioabsorbable (co)polyesters is affected by a great number of factors, such as chemical composition, hydrophilicity, pH of the medium, morphology of the sample, initial distribution of molecular weights, etc. In this study it is demonstrated the importance of the amorphous/crystalline character, controlled by the repeat unit sequence distribution in chain microstructure of crystallizable lactide building block copolymers. Three statistical poly(l-lactide/ε-caprolactone) copolymers were degraded in phosphate buffered saline (PBS) at 37 °C for a period up to 14 weeks. PLCL74b and PLCL74r, presenting similar copolymer composition (∼74% of lactide) but a different randomness character (R = 0.46 vs. 0.96) reflected in their lactide sequence length distribution (lLA) (8.16 vs. 4.01), displayed a completely dissimilar behavior during the course of degradation. The blocky PLCL74b showed a high crystallization capability, reaching a value of 51.0 Jg−1 of melting enthalpy (ΔHm) at the end of the study, whereas random PLCL74r presented a ΔHm = 33.7 Jg−1. On the contrary, PLCL62r, having a similar randomness character to PLCL74r, a ∼62% of lactide content and the lowest lLA (2.55), showed a ΔHm = 10.9 Jg−1. As a consequence, PLCL74b exhibited the slowest degradation rate (half degradation time (t1/2) = 31.5 days), while PLCL62r was the less resistant to hydrolytic degradation with a t1/2 = 18.2 days. This is due to the larger hindrance the water finds to penetrate the crystalline domains and consequently, to the higher resistance of crystalline domains to hydrolytic degradation. Apart from slowing down the degradation, the development of crystalline domains caused deterioration in the mechanical properties of the studied copolymers, which make them unworkable as the degradation process progressed.