Impact of backbone architecture on the solubility of fluorocopolymers in supercritical CO2 and halogenated supercritical solvents: Comparison of poly(vinylidene fluoride-co-22 mol % hexafluoropropylene) and poly(tetrafluoroethylene-co-19 mol % hexafluoropropylene)

Cloud-point data to 270 degrees C and 2800 bar are presented for poly(vinylidene fluoride-co-22.0 mol % hexafluoropropylene) (Fluorel) in supercritical fluid (SCF) C3F6, CClF3, CHF3, and CO2. The impact of backbone architecture on intermolecular interactions and fluorocopolymer solubility is determined by comparing the phase behavior of Fluorel to that of poly(tetrafluoroethylene-co-19.3 mol % hexafluoropropylene) (FEP19) in the same SCF solvents. Good solvents for nonpolar FEP19 (SF6 and C3F8) did not dissolve polar Fluorel even at temperatures in excess of 260 degrees C. Although the small dipole moments of C3F6 and CClF3 interact favorably with the polar, vinylidene fluoride segments of Fluorel, the cloud-point curves in these two solvents exhibit a sharp rise in cloud-point pressures as the temperature is decreased. Apparently, dipolar interactions between vinylidene fluoride segments are much stronger than Fluorel-C3F6 or Fluorel-CClF3 cross-interactions as the temperature is lowered. In contrast, quadrupolar CO2 and dipolar CHF3 are good solvents for Fluorel due to favorable polar, vinylidene fluoride segment-SCF solvent cross-interactions. However, at or near room temperature, CO2 is a much better solvent for Fluorel than is CHF3, which is speculated to be a result of specific interactions between CO2 and the vinylidene fluoride segments.