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

The in vitro hydrolytic degradation study was carried out at 37 °C 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 microstructural stereoregularity 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 (KMw) of 0.060 and 0.047 days⁻¹, respectively, the highest of the study. On the other hand, high Tgs (>37 °C) 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 (KMw = 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 KMw (0.030 days⁻¹) that was slightly lower than those of the crystallisable copolymers with l-LA contents between 69 and 74%. WA started at shorter times due to its low Tg (at 6 °C), 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-ɛ-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 Tgs. To illustrate this, rac-LA-co-CL 85:15 used in a previous work presented a KMw of 0.066 days⁻¹ and a Tg value of 29 °C whereas the equivalent PLVL of this study has a KMw of 0.047 days⁻¹ and a Tg of 37 °C.