Comparison of abiotic and biotic degradation of PDLLA, PCL and partially miscible PDLLA/PCL blend

Important differences between the biotic and abiotic hydrolysis of neat poly(dl-lactide) (PDLLA), poly(ε-caprolactone) (PCL) and a partially miscible PDLLA/PCL blend were found in this work. In abiotic conditions, the degradation process of all the studied specimens mainly proceeded by a bulk degradation mechanism and the diffusion rate of degradation products into the medium was relatively slow after 18 weeks. The hydrophobic and semi-crystalline nature of PCL matrix made this polymer very stable against abiotic hydrolysis, whereas the degradation of PDLLA proceeded much faster due to its amorphous structure. In the first stages of degradation, PDLLA/PCL blend showed that its rich continuous PDLLA phase catalyzed the hydrolysis of the PCL phase, while the low content of PCL molecules partially delayed hydrolysis of PDLLA matrix. In compost, higher levels of degradation were found for all the studied systems during the first 12 weeks of degradation, indicating an important effect of enzymes from microbes in compost on the degradation of PDLLA and PCL. Neat PDLLA and PDLLA/PCL blend seemed to degrade mainly by a bulk degradation mechanism, at least during the first 12 weeks, such as previously observed in abiotic conditions, whereas PCL was preferentially degraded by a surface mechanism due to a very strong catalytic effect of microorganisms on its degradation. Contrary to the abiotic degradation, it was observed that the presence of a low content of PCL molecules in the rich-PDLLA blend did not interfere with the degradation trend of PDLLA in compost, and that the continuous PDLLA phase could partially suppress the biotic degradation of PCL molecules. In general, abiotic and biotic hydrolysis of PDLLA/PCL blends seems to be a complex phenomenon depending not only on the mixing ratio of both polymers but also on their crystallinity, miscibility level of polymer phases and the preferential degradation mechanism of each polymer component. If blends of PDLLA/PCL present a similar level of miscibility, it would be possible to estimate if the presence of the PDLLA or PCL phase could have a general catalytic or delaying role in the blend during the initial degradation stages, depending on which environment they are subjected to and the content of each polymer phase in the blend.