Solubility of Poly(tetrafluoroethylene-co-19 mol % hexafluoropropylene) in Supercritical CO2and Halogenated Supercritical Solvents

High-pressure cloud-point data are presented for poly(tetrafluoroethylene- co -19.3 mol % hexafluoropropylene) (FEP 19 ) in CF 4, C 2 F 6, C 3 F 8, C 3 F 6, CClF 3, CO 2, and SF 6 at 118−250 °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 CO 2 at temperatures less than 185 °C due to strong quadrupolar self-interactions relative to cross-interactions between FEP 19 and CO 2 . The location of the cloud-point curves in pressure−temperature space are directly related to the product of the polarizability and molar density, ρ i α i, of the solvent as determined at the cloud-point pressure at a given temperature. The average of ρ i α i is 5.14 × 10 -24 mol ±7% for the SCF solvents considered in this study calculated at 200 °C and it is 5.41 × 10 -24 mol ±7% for all of the solvents except CF 4 and CO 2 at 170 °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 CO 2 at temperatures below 185 °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 CO 2 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.