Solubility of vinylidene fluoride polymers in supercritical CO2 and halogenated solvents

Abstract

The cloud-point behaviors of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-22 mol % hexafluoropropylene) (VDF-HFP22) are reported at temperatures up to 250 degrees C and pressures up to 3000 bar in supercritical CO2, CHF3, CH2F2, CHClF2, CClF3, CH3CHF2, CH2FCF3, CHF2CF3, and CH3CClF2. The molecular weight of PVDF has a smaller effect on the cloud point than the solvent quality. Cloud-point pressures for both fluoropolymers decrease as the solvent polarizability, polar moment per molar volume, and density increases. However, it is extremely difficult to dissolve either fluoropolymer in CClF3, which has a large polarizability and a small dipole moment. CO2 is an effective solvent because it complexes with the C-F dipole at low temperatures where energetic interactions fix the phase behavior. In addition, polymer architecture has a strong impact on the cloud-point pressure. VDF-HFP22 has lower cloud-point pressures than PVDF in all solvents because it has a larger free volume that promotes facile interactions between the solvent and the polymer segments. Cloud-point data are also reported for amorphous poly(tetrafluoroethylene-co-x mol % 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxale) (TFE-PDDx; x = 65 and 85) in CO2. These data provide an interesting comparison to the PVDF-CO2 and VDF-HFP22-CO2 systems because TFE-PDD65 and TFE-PDD87 have very high glass-transition temperatures of 160 and 240 degrees C, respectively. (C) 2000 John Wiley & Sons, Inc.