(338aa) Designing Membranes Using Bottlebrush Poly(1,3-dioxolane) Acetate for CO2/N2 separation

Poly(ethylene oxide) (PEO) based polymers are leading membrane materials for CO₂/N₂ separation with a balanced high CO₂ permeability and CO₂/N₂ selectivity because the ether oxygen groups show affinity towards CO₂. Herein, we synthesize polymers based on 1,3-dioxolane with higher ether oxygen content than PEO and thus improved CO₂/N₂ separation performance. Specifically, macromonomers of poly(1,3-dioxolane) acrylate (DXLA_n) with short DXL branches (n = 4 – 12) were synthesized via cationic ring-opening polymerization of 1,3-dioxolane, and polymers (pDXLA_n) were synthesized by free radical polymerization. The chemical structure of the macromonomers was confirmed using NMR, and the amorphous nature of the polymers was validated using DSC and WAXD. Pure- and mixed-gas CO₂/N₂ separation properties were determined as a function of feed pressure, composition, and temperature, and the effect of PDXLA chain-end group on the gas separation performance was also investigated. Increasing the n value decreases glass transition temperature (T_g) and increases CO₂ permeability. The PDXLA₁₂ exhibits the best separation performance with CO₂ permeability of 250 Barrer and CO₂/N₂ selectivity of 58 at 35 ^oC, above the 2008 Robeson’s upper bound. Moreover, the hydroxyl (-OH) chain-end group in PDXLA_n can be converted to acetyl- (PDXLAc_n) to disrupt the hydrogen bonding, thus increasing polymer chain flexibility. For example, converting PDXLA₈ to PDXLAc₈ increases CO₂ permeability from 165 to 340 Barrer with a slight decrease of CO₂/N₂ selectivity from 57 to 51. This can be attributed to the increase in polymer chain flexibility, indicated by the decreased Tg from -57 to -62°C. The structure/property relationship in this series of amorphous, highly polar polymers with exciting separation performance will be elucidated. Thin-film composite membranes based on PDXLAc₈ were prepared and exhibit pure-gas CO₂ permeance of 1600 GPU and CO₂/N₂ selectivity of 47 at 25°C. We will also discuss the mixed-gas separation properties and the performance when challenged with simulated flue gas and compare them with state-of-the-art membranes for this application.