Ionic Liquid/Sulfonated Polyimide Composite Membranes: Effect of Polyimide Sequence on CO2 Transport Properties

Abstract

The carbon dioxide capture and storage (CCS) plays an important role in reducing greenhouse gases, but its high energy cost is a critical issue. The CO2 separation is recognized as a key process for reducing cost, and a membrane separation method is one of the promising candidates with a low energy cost. Ion gels comprising polymer networks and ionic liquids (IL), which exhibit unique properties such as non-volatility and CO2 absorption selectivity, have attracted attention as environmentally friendly separation membranes. Previous work in our group has shown that sulfonated polyimide (SPI) with an imidazolium cation exhibits good compatibility with an IL. SPI can hold high content of IL (~75 wt%) and form a uniform, flexible, tough (~10 MPa of Young’s modulus) and thin composite membrane.[1-3] This SPI was synthesized by random copolymerization of a sulfonated ionic monomer and a non-ionic monomer. The composite membranes separated into bicontinuous two phases of ionic and non-ionic domains, which contribute to the formation of gas diffusion path and mechanically tough network, respectively. By using multiblock copolymers of SPI, it is expected that the size and connectivity of each domain are improved, which could lead to the increase of the CO2 permeability and mechanical toughness of the membrane. In this study, we fabricated an 1-buty-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim][NTf2])/SPI composite membranes composed of multiblock-type SPI (mb-SPI) and compared their property with those of random-type SPI (r-SPI).75 wt%[C4mim][NTf2]/mb-SPI composite membranes exhibited higher CO2 permeability (372 barrer) than the corresponding r-SPI (235 barrer) while the gas selectivity (CO2/N2) is 26, which is of the same magnitude as that of r-SPI (CO2/N2 is 28). The mb-SPI can hold higher IL content up to 80 wt% than r-SPI, resulting in higher CO2 permeability (480 barrer; CO2/N2 is 27). The higher ionic conductivity also suggested the higher connectivity of ionic domains in mb-SPI system. On the other hand, from tensile tests, it was found that the mb-SPI membrane was less ductile than the r-SPI membrane. The result of dynamic mechanical analysis (DMA) indicates that the difference of internal structure between the membranes composed of mb-SPI and r-SPI is one of the reasons of the poor mechanical property.To fabricate more strong membrane, we used SPI with more rigid polymer backbone. Rigid mb-SPI exhibits high CO2 permeability (450 barrer) and gas selectivity (CO2/N2 is 24), comparable with the conventional one, and the mechanical property was superior to the conventional one. From these results, we concluded that the mb-SPI membrane having high CO2 permeability and toughness was successfully fabricated.