A tunable mixture solvent for poly(?-caprolactone): Acetone + CO2

We report on a tunable, versatile solvent system for poly(?-caprolactone) (PCL). It is shown that acetone + carbon dioxide mixtures are efficient solvents for this biodegradable polymer. Phase behavior and volumetric properties of PCL + acetone + CO₂ mixtures were determined in a variable-volume view cell. Effect of temperature (323-398 K), pressure (0.1-50 MPa), polymer concentration (1-20 wt%), polymer molecular weight (14k and 65k) and carbon dioxide concentration (0-60 wt%) on the liquid-liquid (L-L) phase boundaries and the densities was explored. Complete miscibility of mixtures with polymer concentrations up to 20 wt% could be achieved in the fluid mixtures containing up to 50 wt% carbon dioxide at modest pressures (5-40 MPa). The solutions all showed LCST-type phase behavior. Comparisons with literature data on the miscibility pressures in other solvent mixtures such as dimethyl ether + carbon dioxide or chlorodifluoromethane + carbon dioxide show that complete miscibilities of PCL in acetone + carbon dioxide mixtures are achievable at much lower pressures.The mixture densities were in the range 0.58-1.20 g/cm³. Mixtures with carbon dioxide content more than 20 wt% showed higher sensitivity and larger change in density with pressure. Densities of the polymer solutions were found to increase significantly with PCL concentration. The densities of solutions with different polymer molecular weights were close to each other, with the lower molecular weight polymer samples showing slightly higher densities.A unique contribution of the present paper is the comparison of compressibility and expansivity of the solutions with the corresponding properties of the solvents. Analysis of the data shows that the compressibilities of PCL solutions are lower than that of the acetone + carbon dioxide solvent mixture at temperatures lower than 373 K. At around 373 K, compressibilities become equal to each other and a switchover is observed at higher temperatures. The difference in the isothermal compressibility of the polymer solution and the solvent decreases with pressure, but reaches a plateau value for pressures greater than 25 MPa. Compared to their solvent mixture, the polymer solutions display a higher isobaric expansivity at the same pressure.