Limited polysulfone solubility in supercritical dimethyl ether with THF and DMF cosolvents

Reported in this short communication are the conditions needed to dissolve up to 1.5 wt.% polysulfone (PSU) (Mw=35 000; Mn=16 000) in supercritical fluid (SCF) solvents plus cosolvents. Initial experiments showed that PSU does not dissolve in neat CO2, propane, butane, dimethyl ether (DME), chlorodifluoromethane, or difluoroethane to temperatures as high as 200 °C and pressures of 2100 bar. However, PSU solubility is observed for DME with the addition of 24–65 wt.% tetrahydrofuran (THF) or N,N-dimethyl formamide (DMF), two polar liquid solvents that readily dissolve PSU at room conditions even though PSU melts at ∼188 °C. DMF is a better cosolvent for PSU since DMF has a much higher dipole moment than THF, it has a higher molar density at room temperature, and it has a higher critical temperature which implies that DMF has a higher cohesive energy density than THF. With ∼24 wt.% cosolvent added to DME, the single-phase region extends to 40 °C lower temperatures with DMF compared with THF. Likewise, the cloud-point pressures for 0.15 wt.% PSU in DME with 56 wt.% DMF are ∼400 bar lower than those for a solution with approximately the same overall concentrations but with THF. The solution densities for PSU in DME+THF range from 0.90 to 0.65 g/cm3, depending on THF concentration and the solution densities for PSU in DME+DMF range from 0.53 to 0.66 g/cm3, also depending on DMF concentration. Compared with THF solutions, lower DMF solution densities are observed since it takes lower pressures to obtain a single-phase when DMF is used as a cosolvent. The experimental data demonstrate that DME should be considered an anti-solvent to knock PSU out of solution, since significant amounts of cosolvent are needed to dissolve PSU in DME.