Abstract
Poly(norbornene)s and poly(ionic liquid)s are two different classes of attractive materials, which are known for their structural tunability and thermal stabilities, and have been extensively studied as gas separation membranes. The incorporation of ionic liquids (ILs) into the poly(norbornene) through post-polymerization has resulted in unique materials with synergistic properties. However, direct polymerization of norbornene-containing IL monomers as gas separation membranes are limited. To this end, a series of norbornene-containing imidazolium-based mono- and di-cationic ILs (NBM-mIm and NBM-DILs) with different connectivity and spacer lengths were synthesized and characterized spectroscopically. Subsequently, the poly(NBM-mIm) with bistriflimide [Tf2N−] and poly([NBM-DILs][Tf2N]2) comprising homo-, random-, and block- (co)polymers were synthesized via ring-opening metathesis polymerization using the air-stable Grubbs second-generation catalyst. Block copolymers (BCPs), specifically, [NBM-mIM][Tf2N] and [NBM-ImCnmIm] [Tf2N]2 (n = 4 and 6) were synthesized at two different compositions, which generated high molecular weight polymers with decent solubility relative to homo- and random (co)polymers of [NBM-DILs] [Tf2N]2. The prepared BCPs were efficiently analyzed by a host of analytical tools, including 1H-NMR, GPC, and WAXD. The successfully BCPs were cast into thin membranes ranging from 47 to 125 μm and their gas (CO2, N2, CH4, and H2) permeations were measured at 20 °C using a time-lag apparatus. These membranes displayed modest CO2 permeability in a non-linear fashion with respect to composition and a reverse trend in CO2/N2 permselectivity was observed, as a usual trade-off behavior between permeability and permselectivity.
Highlights
Membrane technology is enthusiastically investigated for gas separations due to its energy efficiency relative to well-established technologies utilized for separation of industrial gas streams
The polymeric membranes act as a selective layer for efficient separation of gas mixtures, where the gas molecules are absorbed on the upstream surface, transported through the membrane based on their solubility and kinetic diameter, and desorbed from the membrane through the downstream surface, known as the solution-diffusion (S-D) mechanism [6]
Considering the benefits of poly(IL)s and poly(norbornene)s, we explore new designs based on norbornenyl-containing imidazolium ionic liquids (ILs)
Summary
Membrane technology is enthusiastically investigated for gas separations due to its energy efficiency relative to well-established technologies utilized for separation of industrial gas streams. All the above-mentioned glassy poly(IL) membranes resulted in low permeability and diffusivity, but they showed favorable solubility of CO2 , which aided in gas separation performance These membranes have high selectivity (CO2 /N2 ) when compared to SILMs. In spite of extensive research reports that are available on poly(IL) materials for CO2 separations, most of the material designs are confined to limited types of polymerizable groups (vinyl-, (meth)acrylic-, and stryenyl-) and common IL cations (imidazolium and pyrrolidinium). Among the many types of polymeric materials that have been synthesized and investigated for gas separation membranes, substituted poly(norbornene) have emerged as an intriguing materials class [33–36] This is because of several advantages, including ease of norbornenyl monomers in gram scale, tunability of structural scaffolds and thermal properties, and amenable to a variety of polymerization mechanisms. These developed polymers are suitable for many applications including alkaline fuel cells, catalyst support, solid polymer electrolytes, and ion-exchange membranes
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