Synthesis and Gas Separation Performance of Aryl-Substituted [2.2]Paracyclophane Polyimide Membranes

Polyimides incorporating [2.2]paracyclophane have attracted attention for gas separation applications due to their unique face-to-face “deck”-like structures that promote microporosity. In this study, four novel polyimides were synthesized by modifying benzene-substituted [2.2]paracyclophane diamine monomers with fluorine and methyl groups at meta and ortho positions relative to the imide nitrogen. All polymers exhibited excellent thermal stability, good solubility, and robust film-forming capability except for the ortho F-substituted paracyclophane polyimide (PI–PCA-oF), which suffered from low molecular weight. The benzene-extended paracyclophane polyimide (PI–PCA-Ben) exhibited relatively low CO2 permeability (60.52 Barrer) and CO2/CH4 selectivity (32.5) owing to chain flexibility. In contrast, introducing meta substituents, especially methyl groups, enhanced backbone rigidity and disrupted chain packing, resulting in simultaneous gains in permeability and selectivity. Remarkably, the meta-methyl substituted polyimide (PI–PCA-mMe) delivered a CO2 permeability of 237.5 Barrer with a CO2/CH4 selectivity of 53, surpassing the 2008 Robeson upper bound and demonstrating strong potential for biogas upgrading and natural gas purification.