Dioxolane-Based Perfluoropolymers with Superior Membrane Gas Separation Properties

Glassy perfluoropolymers have become an exciting materials platform for membrane gas separation as they define the upper bounds for some gas separations, such as He/H₂, He/CH₄, and N₂/CH₄. However, due to the difficulty in synthesis, only a few glassy perfluoropolymers are commercially available, including Teflon AF and Hyflon AD derived from dioxoles and Cytop derived from dihydrofuran. In this study, two perfluoropolymers based on dioxolanes, poly(perfluoro-2-methylene-1,3-dioxolane) (poly(PFMD)) and poly(perfluoro-2-methylene-4-methyl-1,3-dioxolane) (poly(PFMMD)), were synthesized by radical polymerization and characterized thoroughly for physical properties such as glass transition temperature (T_g), d-spacing between polymer chains, and fractional free volume (FFV). The gas permeability and solubility were determined at 35 °C for a series of pure gases in these perfluorodioxolanes and compared with the commercial perfluoropolymers. Poly(PFMD) and poly(PFMMD) exhibit separation properties of He/H₂, He/CH₄, H₂/CH₄, H₂/CO₂, and N₂/CH₄ near or above the upper bounds in Robeson's plots, and superior to the commercial perfluoropolymers, despite their similar T_g and FFV. The underlying reasons for the superior gas separation properties in these dioxolane-based perfluoropolymers are discussed.