Elimination of CO₂/N₂ Langmuir Sorption and Promotion of “N₂-Phobicity” within High-<i>T</i>_g Glassy Membranes

Abstract

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.