Preparation and characterization of polyimide membranes modified by a task-specific ionic liquid based on Schiff base...

In order to increase CO₂/N₂ selectivity of polyimide (PI) dense membranes, task-specific ionic liquid (TSIL), 1-aminoethyl-3-buthylimidazolium hexafluorophosphate ([NH₂ebim][PF₆]), has been grafted to polymer chains as large side groups by forming the structure of Schiff base for the first time. The modified membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), dynamic thermomechanical analysis (DMA), and stress-strain testing. The results showed that TSIL had been successfully linked to PI chains by forming "C=N." The modified membranes had more free volume, which was favorable to the improvement of CO₂ permeability. The reduction of spin degree of freedom means the rigidity increment of polymer chains, which indicated that the selectivity of CO₂/N₂ can be enhanced. As a result, CO₂ permeability of the modified membrane (TSIL-0.8 wt%) was increased from 5.28 to 10.2 Barrer, and CO₂/N₂ selectivity was increased from 21.9 to 92.8 at 30 °C and 0.1 MPa. Meanwhile, the effects of different feed pressures (0.1-0.6 MPa) and different operating temperatures (30-60 °C) on CO₂/N₂ transport properties were also investigated, and it was found that the separation performances of the modified membranes had already exceeded Robeson's upper bound.