Gas transport and free volume in hexafluoropropylene polymers

Amorphous perfluorinated polymers reveal interesting gas permeation and sorption properties. However, the number of the polymers of this class described in literature is limited. In this work, synthesis and physicochemical properties of two novel perfluorinated polymers: poly(hexafluoropropylene) (PHFP) and the copolymer of hexafluoropropylene (90 wt.%) and perfluoromethylvinyl ether (10 wt.%) (copoly-HFP–PFMVE) are reported. PHFP and copoly-HFP–PFMVE are completely amorphous polymers with Tg of 150 and 142 °C, respectively. PHFP exhibits a relatively high gas permeability (P(O2) = 124 Barrer) and so the gas permeation properties are intermediate between those of AF1600 and Hyflon copolymers. As is the case for other perfluorinated polymers, the data points of PHFP and copoly-HFP–PFMVE are located above upper bounds (1991) of Robeson diagrams for He/H2 and He/CH4 pairs. Enhanced selectivity of separation of these pairs as well as for N2/CH4 separation can be explained by reduced permeability and solubility of hydrogen containing molecules in perfluorinated polymers. Positron annihilation lifetime spectroscopy showed monomodal size distribution of free volume in these polymers with the concentration of microcavities of about 6–7 × 1020 cm−1 and the corresponding average size of 392 and 364 Å3 in PHFP and copoly-HFP–PFMVE, respectively. Solubility and diffusivity selectivity in the two novel polymers are compared to those reported for other perfluorinated polymers.