Elucidating the Role of Fluorine on Gas Transport Through Fluorinated Polymer Membranes

Abstract

Fully fluorinated polymers (i.e., perfluoropolymers) are a unique class of materials that have shown exceptional separation performance due to their anomalous thermodynamic partitioning compared to typical hydrocarbon polymers. The goal of this work is to elucidate the role of fluorine on gas permeability, diffusion, and sorption through the systematic synthesis and characterization of hydrocarbon, partially fluorinated, and fully fluorinated polymer structures, with a particular focus on the development of structure–property relationships and connecting the behavior of hydrocarbon and fully fluorinated polymers. The effect of the higher sorption selectivity displayed by perfluoropolymers on separation performance was demonstrated through a refinement of upper bound theory. Inclusion of aliphatic fluorine groups resulted in higher diffusion due to increased interchain spacing caused by the larger size of fluorine, while inclusion of aromatic fluorine groups resulted in significantly higher diffusion but also lower diffusion selectivity due to weakened interchain interactions as well as increased interchain spacing. Through the lens of the dual-mode sorption model, increased polymer fluorination affected only the Henry sorption mode through increased amounts of unfavorable equilibrium mixing interactions while the sorption in the Langmuir mode was relatively unchanged. Within the scope of the non-equilibrium lattice fluid model, increased fluorine content resulted in larger unfavorable deviations from ideal mixing, particularly for CH4. Increased enthalpic selectivity with fluorine content was also observed, driving the increase in infinite dilution sorption selectivity. Additionally, an updated group contribution method for estimating fractional free volume in polymers was developed to streamline calculation for any polymer structure.