Solubility of poly(tetrafluoroethylene-co-19 mol% hexafluoropropylene) in supercritical CO2 and halogenated supercritical solvents

High-pressure cloud-point data are presented for poly(tetrafluoroethylene-co-19.3 mol % hexafluoropropylene) (FEP(19)) in CF4, C2F6, C3F8, C3F6, CClF3, CO2, and SF6 at 118-250 degrees C and pressures as high as 2700 bar. Cloud-point curves for a given solvent virtually superpose for FEP(19) concentrations between 2 and 10 wt %. It is not possible to dissolve FEP(19) in CO2 at temperatures less than 185 degrees C due to strong quadrupolar self-interactions relative to cross-interactions between FEP(19) and CO2. The location of the cloud-point curves in pressure-temperature space are directly related to the product of the polarizability and molar density, rho(i) alpha(i) of the solvent as determined at the cloud-point pressure at a given temperature. The average of rho(i) alpha(i) is 5.14 x 10(-24) mol +/-7% for the SCF solvents considered in this study calculated at 200 degrees C and it is 5.41 x 10(-24) mol +/-7% for all of the solvents except CF4 and CO2 at 170 degrees C. This simple correlation provides a means for estimating cloud-point pressures for nonpolar polymers with nonpolar solvents, or for polar solvents at very high temperatures where polar interactions are diminished. Using this correlation, it is not possible to predict when crystallization may occur or when polar interactions will dictate the phase behavior as observed for CO2 at temperatures below 185 degrees C. With one temperature-independent and one temperature-dependent mixture parameter the Sanchez-Lacombe equation of state (SLEOS) is capable of modeling the phase behavior of FEP(19) in the solvents considered in this study except for CO2 which required two temperature-dependent parameters. It is not possible to even qualitatively model the cloud-point behavior if the two mixture parameters are set to zero. Hence, the utility of the SLEOS is limited since cloud-point data are needed to fix the values and the temperature dependence of the mixture parameters.