Effect of solvent composition and polymer molecular weight on cloud points of poly(L-lactide) in chlorodifluoromethane plus carbon dioxide

Abstract

Phase behavior data are presented for poly(L-lactide) in supercritical solvent mixtures of chlorodifluoromethane and carbon dioxide. Experimental cloud-point pressures were measured using a high-pressure equilibrium apparatus equipped with a variable-volume view cell and were investigated as functions of temperature, solvent composition, and polymer molecular weight. The carbon dioxide composition in the solvent mixture was varied up to similar to0.82 mass fraction on a polymer-free basis at temperatures to 373.15 K. Poly(L-lactides) of three different molecular weights (2000, 50 000, 100 000) were examined, and the polymer concentration was kept constant at similar to0.03 mass fraction of total in solution. This system exhibited the characteristics of a lower critical solution temperature phase behavior. As the carbon dioxide composition in the mixed solvent increased, the cloud-point pressure at a fixed temperature increased significantly. Addition of carbon dioxide to chlorodifluoromethane caused a lowering of the dissolving power of the mixed solvent due to the decrease of the solvent polarity. The cloud-point pressure increased linearly with a logarithmic increase of the poly(L-lactide) molecular weight.