(519g) Elucidating the Effect of Chlorination Versus Fluorination on the Transport Properties of Polymer Membranes for Gas, Vapor and Organic Solvent Separation

Due to their peculiar transport properties, as well as their larger plasticization and physical aging resistance compared to conventional glassy polymers, perfluoropolymers garnered increasing attention in membrane science [1]. While their swelling resistance has been ascribed to the lower sorption capacity of traditional swelling agents, such as hydrocarbons, relative to hydrocarbon-based polymers, their superior physical aging resistance has been attributed to the presence of pendant rings directly attached to the polymer backbone, which hampers polymer chains to collapse [2,3]. Recent studies of gas transport in glassy polyimides exhibiting systematically varied degree of fluorination explored the effect of penetrant-perfluorinated moieties interactions, including CO₂, CH₄, O₂ and N₂, on gas solubility [4]. The multitude of results published since the early 1990s thus far indicates that while the role of fluorination on small molecule transport in polymer membranes has been the subject of a vast body of research, the role of other halogens, such as chlorine, on the transport properties of glassy polymers did not receive enough attention and, for this reason, it is still poorly understood. Some studies from Lin et al. indicate that while fluorine acts mostly on solubility, selectivity, physical aging rate and swelling propensity, chlorine acts essentially on the size-sieving ability, that is, on gas diffusivity [3]. The molecular origin of this phenomenon, however, is still unknown.

To fill these knowledge gaps, we propose a fundamental investigation of small molecule transport in a recently synthesized copolymer of perfluoro(2-methylene-4-methyl-1,3-dioxolane, PFMMD) and chlorotrifluoro ethylene (CTFE), namely poly (PFMMD-co-CTFE) [5]. The solubility and diffusivity of three model vapors, water, methanol (polar organic penetrant) and n-pentane (non-polar organic penetrant) were measured at multiple temperatures and activities, and compared to previously reported sorption and transport data in perfluoropolymers, such as Teflon^® AF and Hyflon^® AD. Interestingly, due to the larger polarity of the C-Cl bond compared to the C-F bond, poly (PFMMD-co-CTFE) exhibits a much larger hydrophilicity relative to chlorine-free perfluoropolymers, as well as much better swelling resistance in the presence of methanol, which makes poly (PFMMD-co-CTFE) an ideal candidate for the separation of organic solutions via organic solvent nanofiltration and reverse osmosis. The analysis of transport data, as well as heats of sorption and diffusion, provided relevant fundamental insights regarding the role of polymer-penetrant interactions, clustering and swelling, as well as the mechanism of small molecule transport in this class of materials, which helps individuate possible applications in membrane separations.

To put our result in a broader perspective, water, methanol and hydrocarbon transport in poly (PFMMD-co-CTFE) was compared to the transport of the same molecules in chlorinated organic liquids.

References

[1] Y. Okamoto et al., Perfluorodioxolane Polymers for Gas Separation Membrane Applications, Membranes 2020, 10, 394

[2] Y. Li, et al., Volumetric Properties and Sorption Behavior of Perfluoropolymers with Dioxolane Pendant Rings, Ind. Eng. Chem. Res. 2020, 59, 5276

[3] M. Yavari et al., The role of halogens in polychlorotrifluoroethylene (PCTFE) in membrane gas separations, J. Membr. Sci. 2018, 548, 380

[4] A. Wu et al., Non-equilibrium Lattice Fluid Modeling of Gas Sorption for Fluorinated Polyimides, Macromolecules 2020, 50, 6620

[5] M. Fang et al., High-performance perfluorodioxolane copolymer membranes for gas separation with tailored selectivity enhancement, J. Mater. Chem. A 2018, 6, 652