Effect of Solvent Composition and Polymer Molecular Weight on Cloud Points of Poly(l-lactide) in Chlorodifluoromethane + 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 ∼0.82 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 ∼0.03 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 disso...