Effects of polyimide sequence and monomer structures on CO2 permeation and mechanical properties of sulfonated polyimide/ionic liquid composite membranes

The effects of polymer structure on CO2 separation and mechanical properties of ion gel membranes composed of ionic liquids (ILs) and sulfonated polyimides (SPIs) were investigated. SPIs with different sequential distributions of ionic groups (multiblock and random) were synthesized. The multiblock copolymer exhibited higher IL uptake (∼80 wt.%) than the random copolymer, resulting in higher CO2 permeability (∼480 Barrer). The multiblock copolymer exhibited a clearer phase-separated structure than the random copolymer. However, the strain at the break of the multiblock copolymer was lower owing to the brittleness of the non-ionic phase. To improve the mechanical properties, an SPI containing fluorinated groups as a non-ionic part was also synthesized. Compared with the random SPI ion gel membranes, the multiblock SPI ion gel membranes containing fluorinated groups exhibited good CO2 permeability (∼500 Barrer) and simultaneously ductile properties with higher strain at the break due to the plasticization of the non-ionic phase, enabling a thin and tough membrane with excellent CO2 separation properties. These results indicate that the polyimide sequence in addition to the chemical structure of monomers affects CO2 permeation and mechanical properties of the sulfonated polyimide/ionic liquid composite membranes.