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Elimination of CO₂/N₂ Langmuir Sorption and Promotion of “N₂-Phobicity” within High-Tg Glassy Membranes

Maroon, Christopher R., Townsend, Jacob, Gmernicki, Kevin R., Harrigan, Daniel J., Sundell, Benjamin J., Lawrence, John A., Mahurin, Shannon M., Vogiatzis, Konstantinos D., Long, Brian K.
Macromolecules 2019 v.52 no.4 pp. 1589-1600
carbon dioxide, models, nitrogen, permeability, solubility, sorption
We demonstrate that the CO₂/N₂ gas separation performance of alkoxysilyl-substituted vinyl-added polynorbornenes (VAPNBs) may be significantly enhanced via incorporation of the monomer 5-tris(2-methoxyethoxy)silyl-2-norbornene. As the molar ratio of this monomer is increased, substantial increases in CO₂/N₂ selectivity are realized with minimal decrease in CO₂ permeability. This trend ignores the traditional permeability/selectivity “trade-off” relationship and yields an optimal membrane whose performance reaches the 2008 upper bound for CO₂/N₂ separations. Though the inclusion of 5-tris(2-methoxyethoxy)-silyl-2-norbornene units was initially hypothesized to maximize CO₂ solubility, detailed gas sorption studies reveal that these highly glassy materials essentially lack any Langmuir sorption component and indicate that their improved CO₂/N₂ selectivity is due to decreased N₂ solubility within the matrix. Computational modeling suggests that the source of this apparent “N₂-phobicity” is likely explained through comparative analyses of polymer–polymer and polymer–gas interactions. Lastly, mixed-gas permeation tests are performed to provide a more realistic look at real-world gas separation performance.