Changes in the morphology of poly(l‐lactide‐ran‐δ‐valerolactone) following supercritical carbon dioxide processing. Issue 4 (22nd April 2019) - Tsutsumi, Chikara
Abstract: Biodegradable polymers can be employed as controlled‐release materials when processed with supercritical carbon dioxide (scCO2 ), and l ‐lactide (L‐LA) random copolymers are useful in such applications. In the present work, poly(l ‐lactide‐ ran ‐δ‐valerolactone) (poly(L‐LA‐ ran ‐VL)) copolymers, which do not melt during scCO2 processing above 80°C, were synthesized. The thermal properties, crystallinity, and polymer chain structures of these materials were evaluated before and after processing at temperatures from 60°C to 120°C and 14 MPa. An efficient one‐pot process (termed azeotropic‐combined one‐pot polymerization or ACOP) produced poly(L‐LA‐ ran ‐VL) or poly(L‐LA) in good yields and high molecular weights, and poly(L‐LA) with an M n of 8.20 × 10 4 was obtained without the use of an inert gas. Poly(L‐LA‐ ran ‐VL) composed a 73/27 ratio of L‐LA and VL had a relatively high molecular weight despite its lower L‐LA content, and its T m and Δ H m were decreased only slightly compared to that of poly(L‐LA). Both Δ H m and crystallinity were found to increase with increases in the scCO2 processing temperature, especially at 120°C. Although the degree of crystallinity increased with increasing L‐LA content, the Δ H m of the 83/17 specimen (which had the lowest M n ) was higher than that of the 91/9 sample. Before processing, the 73/27 copolymer had the highest haze value while the 91/9 specimen had the lowest, and there was a linear relationship between the haze and crystallinity of each copolymer after processing at all temperatures. The infrared spectrum of the 73/27 copolymer prior to scCO2 processing exhibited a maximum absorbance at 1756 cm −1, and this peak was shifted to higher wavenumber with increases in the processing temperature. Thus, infrared spectroscopy could be used to assess increases in crystallinity following scCO2 processing. Abstract : In this study, the relationship between the haze value and the crystallinity of the 91/9 copolymer is investigated at temperatures from 60°C to 120°C at 14 MPa. It is found that the crystallinity and the haze value are affected by both the composition ratios and processing temperatures. These results suggest that calibration curves can be constructed to predict percent crystallinity from the haze value.