Solubility of vinylidene fluoride polymers in supercritical CO2 and halogenated solvents

The cloud-point behaviors of poly(vinylidene fluoride) (PVDF) and poly(viny lidene fluoride-co-22 mol % hexafluoropropylene) (VDF-HFP22) are reported a t temperatures up to 250 degrees C and pressures up to 3000 bar in supercri tical CO2, CHF3, CH2F2, CHClF2, CClF3, CH3CHF2, CH2FCF3, CHF2CF3, and CH3CC lF2. The molecular weight of PVDF has a smaller effect on the cloud point t han the solvent quality. Cloud-point pressures for both fluoropolymers decr ease as the solvent polarizability, polar moment per molar volume, and dens ity increases. However, it is extremely difficult to dissolve either fluoro polymer in CClF3, which has a large polarizability and a small dipole momen t. CO2 is an effective solvent because it complexes with the C-F dipole at low temperatures where energetic interactions fix the phase behavior. In ad dition, polymer architecture has a strong impact on the cloud-point pressur e. VDF-HFP22 has lower cloud-point pressures than PVDF in all solvents beca use it has a larger free volume that promotes facile interactions between t he solvent and the polymer segments. Cloud-point data are also reported for amorphous poly(tetrafluoroethylene-co-x mol % 2,2-bistrifluoromethyl-4,5-d ifluoro-1,3-dioxale) (TFE-PDDx; x = 65 and 85) in CO2. These data provide a n 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.