(313c) Enhancing Selectivity through Fluorination

For polymer membranes, upper bound theory is defined exclusively for hydrocarbon-based polymers. These polymers have nearly identical sorption selectivities across multiple families of materials, which has encouraged the membrane community to focus predominantly on improving diffusion selectivities. In contrast, perfluoropolymers have completely different sorption selectivities, which results in significant improvements in performance for certain separations, such as N₂/CH₄, He/H₂, He/N₂, He/CH₄, and He/CO₂. To account for these differences, this presentation will re-define the polymer upper bound based on perfluoropolymers and demonstrate that using these materials is beneficial for enhancing membrane selectivity. Interestingly, the mechanism behind these selectivity enhancements is poorly understood today. To elucidate the role of fluorination on sorption enhancements, a systematic series of non-fluorinated, partially fluorinated, and perfluorinated polyimides were synthesized and characterized for sorption over multiple temperatures, pressures, and with various gases. Fluorination has a significant effect on penetrant–polymer mixing through unfavorable mixing interactions, even with glassy polymers, resulting in systematic improvements in sorption selectivity with increasing fluorine content. This presentation will highlight the potential benefits of using fluorination to enhance membrane selectivity.