Elucidating the Role of Fluorine Content on Gas Sorption Properties of Fluorinated Polyimides

Abstract

Hydrocarbon and perfluorinated polymers display distinct thermodynamic partitioning characteristics. These differences enable perfluoropolymers to outperform hydrocarbon polymers for many membrane-based gas separations, but the mechanism in which fluorine affects gas sorption and sorption selectivity in polymers is still not well understood. To bridge the existing gap in our fundamental understanding of sorption in hydrocarbon and perfluorinated polymers, this study investigates gas sorption across a range of temperatures, pressures, and gas species for four polyimides containing varying fluorine content. Observed improvements in sorption selectivity for the highly fluorinated polymers were analyzed through the dual-mode model and were found to result primarily from increased Henry sorption selectivity. Additionally, analysis of the energetics of sorption revealed a greater enthalpic penalty for Henry sorption in highly fluorinated polymers. Finally, consistent with the anomalous solubility behavior observed for hydrocarbon−perfluorocarbon liquid mixtures, our results indicate that fluorination appears to affect bulk penetrant−polymer mixing through unfavorable mixing interactions.

Sorption-enhanced mixed-gas transport in amine functionalized polymers of intrinsic microporosity (PIMs)

“…This trend is consistent with the decrease in sorption expected for glassy polymers with increasing pressure in the framework of the dualmode sorption model when no significant plasticizationinduced variations in diffusion are expected 36 . As pressure increases and Langmuir modes saturate, the sorption coefficient, and thus permeability, decreases due to the higher energetic penalty required to sorb into the Henry's law mode 70,71 . The CO 2 /CH 4 mixed-gas permselectivity slightly increased with increasing content of CO 2 , demonstrating how competition (i.e., CH 4 exclusion from the polymer matrix) is proportional to the molar ratio of CO 2 over CH 4 in the mixture.…”

“…41 As shown in Fig. S5a and b , describes sorption ∞ of the first gas molecule into the polymer matrix and is dependent on temperature 41,70 . By taking the limit of Eq.…”

Surface Modification of Matrimid® 5218 Polyimide Membrane with Fluorine-Containing Diamines for Efficient Gas Separation

“…On the other hand, FDA1-MAT showed a higher d-spacing than that of DA1-MAT despite DA1 and FDA1 having the same molecular structure except for the fluorine-substituted groups. Such differences can be attributed to the larger size of fluorine compared with hydrogen, leading to a larger free volume size with less efficient chain packing, resulting in enhanced permeability ( Table 2 ) [ 32 , 34 , 35 ]. Moreover, the density of the membranes presents an inverted order from the XRD analyses (i.e., MAT < FDA1-MAT < DA1-MAT < DA0-MAT) ( Table 4 ), confirming the d-spacing data since a membrane consisting of a more densely packed structure normally exhibits a higher film density [ 3 , 7 ].…”

“…There are huge size differences between hydrogen and fluorine atoms, as indicated by their covalent radii (0.3 Å vs. 0.64 Å) and Van der Waals radii (1.10 Å vs. 1.35 Å), which are responsible for the relatively longer C–F bond compared to the C–H bond [ 33 ]. Thus, the bulky fluorine groups are more prone to disrupting chain packing in polymers compared to their hydrogen counterparts [ 34 ]. In addition, the highly polar C–F bonds may boost the dipole–dipole interactions between the gas molecules and the polymer chains, which affects the solubility selectivity of the membranes [ 32 , 34 , 35 ].…”

“…Thus, the bulky fluorine groups are more prone to disrupting chain packing in polymers compared to their hydrogen counterparts [ 34 ]. In addition, the highly polar C–F bonds may boost the dipole–dipole interactions between the gas molecules and the polymer chains, which affects the solubility selectivity of the membranes [ 32 , 34 , 35 ]. Smith et al investigated the effects of aliphatic and aromatic fluorine groups in 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based polyimides [ 35 ].…”