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Effect of composition and nanostructure on CO₂/N₂ transport properties of supported alkyl-imidazolium block copolymer membranes

Nguyen, Phuc Tien, Wiesenauer, Erin F., Gin, Douglas L., Noble, Richard D.
Journal of membrane science 2013 v.430 pp. 312-320
ambient temperature, artificial membranes, carbon dioxide, composite polymers, diffusivity, ionic liquids, mechanical properties, nanomaterials, permeability, polymerization, separation
Polymerized room-temperature ionic liquids (poly(RTIL)s) have garnered attention as new and interesting membrane materials for CO₂/light gas separations because they combine the high CO₂ affinity and thermal and chemical stability of RTILs, with the physical and mechanical properties of polymeric materials. Our group recently synthesized a new type of block copolymer (BCP) combining an imidazolium-based poly(RTIL) and an alkyl non-ionic polymer. These alkyl-b-ionic BCPs phase-separate into ordered nanostructures. Prior work investigating gas transport through phase-separated BCPs is very limited, and none has included RTIL-based BCP systems. However it has been shown that nanoscale phase-separation could facilitate gas transport via nanostructure orientation control or phase connectivity improvement. We have successfully made defect-free, thin-film composite membranes with these novel alkyl-imidazolium BCPs as a 3–20μm thick top layer, and determined their CO₂/N₂ separation properties via single-gas permeability measurements and selectivity calculations. These new BCP materials were found to have distinct advantages over the analogous physical blends of the parent homopolymers with respect to membrane fabrication. The composition of the BCP top layer, which is directly connected to the type of nanostructure formed, was found to have a significant effect on CO₂ permeability (i.e., it can increase CO₂ permeability by two orders of magnitude up to an observed value of 9300barrer). This improvement is mainly due to a large increase in the diffusion coefficient in the ordered nanostructures compared to amorphous BCP materials.