IMPACT OF MELTING TEMPERATURE ON POLY(TETRAFLUOROETHYLENE-CO-HEXAFLUOROPROPYLENE) SOLUBILITY IN SUPERCRITICAL-FLUID SOLVENTS

Experimental cloud-point data to 2800 bar and 250 degrees C are presen ted for binary mixtures of partially crystalline poly(tetrafluoroethyl ene-co-19 mol % hexafluoropropylene) (FEP19) and amorphous FEP48 with CF4, CO2, and SF6. It takes less pressure and lower temperatures to di ssolve FEP48 compared to FEP19 in CF4 and in SF6. The sharp increase i n the FEP48-CO2 cloud-point curve occurs at 20 degrees C lower tempera ture than the increase for the FEP19-CO2 system. Since the addition of hexafluoropropylene groups is not expected to significantly change th e type of interactions between an average segment of the copolymer wit h that of the solvent, the increased ease of dissolving FEP49 is assoc iated with the greater free volume expected for this more highly branc hed amorphous copolymer.