Impact of Melting Temperature on Poly(Tetrafluoroethylene-co-hexafluoropropylene) Solubility in Supercritical Fluid Solvents

Experimental cloud-point data to 2800 bar and 250 °C are presented for binary mixtures of partially crystalline poly(tetrafluoroethylene-co-19 mol % hexafluoropropylene) (FEP19) and amorphous FEP48 with CF4, CO2, and SF6. It takes less pressure and lower temperatures to dissolve FEP48 compared to FEP19 in CF4 and in SF6. The sharp increase in the FEP48−CO2 cloud-point curve occurs at 20 °C lower temperature than the increase for the FEP19−CO2 system. Since the addition of hexafluoropropylene groups is not expected to significantly change the type of interactions between an average segment of the copolymer with that of the solvent, the increased ease of dissolving FEP48 is associated with the greater free volume expected for this more highly branched amorphous copolymer.