(45e) Investigating the Effects of Polymer Fluorine Functionality on Free Volume and CO2 and Fluorocarbon Sorption in Poly(ether imide) Membranes

For several decades, fluoropolymer membranes have been identified as exceptionally high-performing gas separation materials, specifically on account of their high sorption selectivity for small gas pairs and plasticization resistance. These properties, especially resistance to penetrant-induced plasticization, make fluoropolymers extremely attractive candidates as membrane materials for grand challenges to chemical separations, such as carbon capture and fluorocarbon refrigerant recovery. However, there remains a key knowledge gap in the membrane literature: what specific polymer-penetrant interactions do fluorine substituents induce to unlock unique transport properties? Addressing this question would enable an even more intelligent forward-design approach to engineering high-performing fluoropolymer gas separation membranes.

Herein, this work explores the effect of fluorine substituents on sorbed-phase penetrants CO₂, tetrafluoromethane (R-14), difluoromethane (R-32), and tetrafluoroethane (R-134a) in poly(ether imide) (PEI) membranes by leveraging experimental validation along with insight granted by atomistic simulations. Four analogous PEI structures of systematically tuned fluorine content were used to study trends in gas transport behavior. Specifically, we combined pure-gas sorption experiments with dilation experiments in order to analyze trends in CO₂ partial molar volume against polymer fluorine content. Then, using combined molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations, we uncovered site-specific polymer-penetrant interactions which grant the PEIs the aforementioned beneficial gas transport properties. These sorption experiments are repeated with fluorocarbon refrigerants, and trends between penetrant behavior and PEI fluorine content are discussed. After establishing trends between fluorine content and penetrant partial molar volume, we demonstrate how these properties are linked to plasticization resistance by comparing sorption to permeability performance. These permeability experiments involve sweeping the feed gas to high pressure, allowing an in-depth analysis of high-pressure diffusivity in the penetrant, specifically in the context of the partial-immobilization model combined with free volume principles [1]. From these experiments, we present an intuitive and data-backed interpretation on the mechanistic nature of fluorine substituents’ effect on sorption of heavily condensable penetrants and discuss their viability as membrane materials for highly challenging gas separations.

[1] Moon JD, et al. Modeling water diffusion in polybenzimidazole membranes using partial immobilization and free volume theory. Polymer 189 (2020) 122170