Enzymatic degradation of poly(l-lactide) treated with supercritical carbon dioxide

To investigate the factors that affect the physical properties of poly (l-lactide) [poly (L-LA)] processed with supercritical carbon dioxide (scCO2), the present work assessed the degradability of poly (L-LA), as well as its thermal and mechanical properties before and after processing. The thermal properties of three types of poly (L-LA) (H100, H440 and REVODE), each having different properties, were examined. Poly (L-LA) films were treated with scCO2 using a high pressure reaction apparatus at 40 °C and 14 MPa for 3 h. The treated samples subsequently underwent enzymatic degradation tests using proteinase K. The poly (L-LA)s processed with scCO2 degraded more slowly compared to polymers not treated with scCO2 during the early stages of degradation. Scanning electron microscopy images of the degraded, scCO2-processed poly (L-LA)s indicated close spacing of the cavities generated by degradation. The melting point (Tm) values of all poly (L-LA)s increased with scCO2 processing, which influenced the degradability. Although the degradation of processed poly (L-LA) was slower than that of unprocessed poly (L-LA) in the early stages, the degradability of the treated H440 and REVODE samples was identical to that of specimens without scCO2 processing after 160 h. In addition, the poly (L-LA) treated with scCO2 was found to be degraded by proteinase K at a constant rate. The relationship between degradability and crystallinity was examined, and untreated H100 was observed to rapidly degrade in contact with proteinase K. The crystallinity indicators Xc DSC and Xc XRD increased after scCO2 processing, such that the degradability of the treated sample was reduced. In addition, both the Xc DSC and Xc XRD values of untreated and treated H100 increased with degradation. Although the degradation curve of the REVODE was similar to that of the H440, the changes in the crystallinity of untreated REVODE were different from the results observed for the H100 and H440. XRD data showed that the diffraction peaks of the untreated H100 and the poly (L-LA)s treated with scCO2, which were more highly crystalline, shifted to smaller angles as the enzymatic degradation progressed. Examination of the mechanical properties indicated increases in tensile strength and elastic modulus and decreases in elongation after scCO2 processing, suggesting that the polymer chains were moved closer together. In conclusion, scCO2 processing appears to uniformly contract polymer chains in both the amorphous and crystalline regions.