Tuning the free volume cavities of polyimide membranes via the construction of pseudo-interpenetrating networks for enhanced gas separation performance

A novel synthetic strategy to fine-tune the cavity size and free volume distribution of polyimide membranes via the formation of homogeneous pseudo-interpenetrating polymer networks (IPN) is explored in this study. The transformation in the free volume characteristics of the pseudo-IPN can be effectively exploited for achieving enhanced gas transport properties. The construction of the pseudo-IPNs entails the in situ polymerization of azido-containing monomers with multireactive sites within rigid polyimide molecular scaffolds. The intrinsic free volume of the host polyimide and the dimensions of the azido-containing monomer predominantly influence the mean cavity size of the semi-IPN. The pseudo-IPNs assembled using fluorinated polyimides and 2, 6-bis(4-azidobenzylidene)-4-methylcyclohexanone (azide) display improved CO₂/ CH₄ and H₂/N₂ separation performance. The alterations in the gas permeability and gas pair permselectivity of the semi-IPNs are adequately mapped to the variation in the free volume distributions characterized by the positron annihilation lifetime spectroscopy. Depending on the functionalities of the host polyimides, chemical cross-links are formed between the azide network and the preformed linear polyimide. The chemical bridges in conjunction with the interpenetrating network restrict the mobility of the polymer chains and suppress CO₂-induced plasticization.