In vitro degradation of poly(lactide/δ-valerolactone) copolymers

摘要

The in vitro hydrolytic degradation study was carried out at 37 ℃ on lactide-co-δ-valerolactone (PLVL) biopolyesters and demonstrated that the water uptake was favoured by the resistance of polymers to crystallize, the presence of large amorphous domains and the decrease in microstructurai stereoregu-larity of the polymer chains. Those copolymers that displayed higher levels of water absorption (WA ＞30% at day 42) experienced the fastest degradation rates. Thus, two amorphous rac-lactide-co-δ-VL, with a δ-VL content of 25% and 15% and short l-lactide and d-lactide segments, showed degradation rates (K_(Mw)) of 0.060 and 0.047 days~(-1), respectively, the highest of the study. On the other hand, high T_gs (＞37 ℃) and a greater capacity to crystallize (with l-LA-unit average sequence lengths ＞3.82), hinder water diffusion inside the polymer. For example, an 82:18 l-LA-co-δ-VL copolymer (K_(Mw) = 0.017 days~(-1)) did not lose weight until day 98 when it began to absorb water. Conversely, amorphous PLVL 53 (53.4% of l-LA and 46.6% of δ-VL) exhibited a K_(Mw) (0.030 days~(-1)) that was slightly lower than those of the crys-tallisable copolymers with l-LA contents between 69 and 74%. WA started at shorter times due to its low T_g (at 6 ℃), however, its more hydrophobic character and lower amount of ester groups slowed its hydrolysis rate down. It is also worth stating that the PLVLs degraded at a slower rate than the lactide-co-ε-caprolactone (PLCLs) of the same composition. This can be explained by the fact that PLVLs, despite presenting a higher proportion of hydrolyzable esters than PLCLs, have more packed amorphous regions related to higher T_gs. To illustrate this, rac-LA-co-CL 85:15 used in a previous work presented a K_(Mw) of 0.066 days~(-1) and a T_g value of 29 ℃ whereas the equivalent PLVL of this study has a K_(Mw) of 0.047 days~(-1) and a T_g of 37 ℃.