Plasma-treated polymer of intrinsic microporosity membranes for enhanced CO₂/CH₄ separation

Natural gas upgrading demands advanced CO₂/CH₄ separation membranes for CO₂ removal. Polymer of intrinsic microporosity (PIM-1) represents a novel polymeric molecular sieve featuring exceptional permeability and processability, but its moderate CO₂/CH₄ selectivity remains challenging. Plasma treatment, a well-applied technique for surface modification of dense polymers, has been adopted to PIM with abundant interchain micropores for the first time. The physical alteration of plasma treatment in He gas atmosphere, primarily the chain scission, has significant impacts on CO₂/CH₄ separation performance of PIM-1 membrane, by means of regulating its hierarchical microporous structure. Wide angle X ray scattering (WAXS) results suggest that the relatively large micropores are more prone to shrinkage and collapse than relatively small ones, which effectively enhances the selectivity. Moreover, owing to near-surface localization effect, the plasma modification is constrained within a skin layer onto a bulk intact layer. This asymmetric membrane structure allows the balance of enhancing CO₂/CH₄ selectivity and sustaining high CO₂ permeability. By optimizing plasma input power and duration, the selectivity of plasma-treated PIM-1 membrane increased by 1.65 times, with a high permeability of over 4000 Barrer. After 2-month aging, the permeability was still maintained 60 % of its initial value. This suggests plasma treatment as a promising post-modification method for PIM CO₂/CH₄ separation membranes.