Phase behavior and density of polysulfone in binary fluid mixtures of tetrahydrofuran and carbon dioxide under high pressure: Miscibility windows

Abstract

ABSTRACT:Mixtures of tetrahydrofuran (THF) and carbon dioxide (CO 2 ) were identified as new solvent systems for polysulfone. The miscibility and density of polysulfone in binary fluid mixtures of THF and CO 2 were investigated from 300 to 425 K at pressures up to 70 MPa. The influence of the CO 2 and polysulfone concentrations was studied, with the concentrations of the other two components kept constant. At a 4.5 wt % polymer concentration, the demixing pressures in a 10 wt % CO 2 and 90 wt % THF mixture increased with temperature (310 -425 K) from 15 to 40 MPa. With increasing CO 2 concentration (from ca. 10 to 14 wt %), a significant increase (from 15 to 70 MPa at 310 K) was observed in the demixing pressures. Furthermore, with an increasing amount of CO 2 , the nature of the phase boundary shifted from lower critical solution temperature behavior to upper critical solution temperature behavior. The influence of the polymer concentration was studied in the 0 -5 wt % range at two CO 2 levels, with solvent compositions of 10 wt % CO 2 and 90 wt % THF and 13 wt % CO 2 and 87 wt % THF. The system with a higher level of CO 2 (13 wt %) showed highly unusual phase behavior: on pressure-composition and temperature-composition diagrams, the system displayed two distinct regions of miscibility. In the system with 10 wt % CO 2 , the distinct regions of miscibility that were observed in the system with 13 wt % CO 2 partially overlapped and led to a W-shape phase boundary. The densities of the polymer solutions were measured from the one-phase region through the demixing point into the two-phase region at a constant temperature. No significant change in density was found around the phase boundary; this indicated that the coexisting phases had similar densities, as is often the case with liquid-liquid phase separation in polymer solutions under high pressure.