Phase behavior of Poly(L-lactide) in supercritical mixtures of carbon dioxide and chlorodifluoromethane

Phase behavior data are presented for poly(L-lactide) (L-PLA: MW = 2000 g . mol(-1)), a biodegradable polymer, in supercritical solvent mixtures of carbon dioxide (CO2) and chlorodifluoromethane (HCFC-22). Experimental cloud point curves, which were the phase boundaries between single and liquid-liquid phases, were measured using a variable-volume view cell apparatus for various CO2 compositions up to about 82 mass % (on a polymer-free basis) and for temperatures up to about 393.15 K. The location of the phase transition boundaries between liquid (L) and liquid-vapor (LV) and between liquid-liquid (LL) and liquid-liquid-vapor (LLV) was also investigated as a function of solvent composition for the L-PLA dissolved in mixed solvents of CO2 and HCFC-22. The cloud point curves exhibited a lower critical solution temperature phase behavior. As the CO2 content in the solvent mixture increased, the cloud point pressure at a fixed temperature increased significantly. Addition of CO2 to HCFC-22 caused a lowering of dissolving power of the mixed solvent. A lower critical end point (LCEP) was estimated from the cloud point curve and the L-to-LV and LL-to-LLV phase transition curves. The LCEPs were shifted to lower temperatures as the CO2 content in the mixed solvent increased.