Effect of Phenylamino Groups on Polyimide Membranes Containing Spiro-Bis-Indane Structures for Gas Separation

Incorporating functional groups capable of interacting with gas molecules into the molecular structure is a pivotal strategy for enhancing the performance of polymer-based gas separation membranes. In this study, two diamines featuring spiro-bis-indane moieties and distinct functional groups were separately polymerized with 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) to synthesize two polyimide membranes. The effects of hydroxyl (  OH) groups and phenylamino (  NHPh) groups on gas separation performance were systematically investigated. Hydroxyl groups are known to enhance CO 2 solubility via hydrogen-bond-like interactions while disrupting chain packing to increase free volume, synergistically improving separation efficiency. This work further demonstrates that the basic phenylamino groups selectively adsorb CO 2 through Lewis acid–base interactions, boosting CO 2 solubility and amplifying solubility differences with nonpolar gases. Additionally, the rigid spiro-bis-indane structure collaborates with these functional groups to tailor the microporous network, thereby optimizing gas transport properties. Graphical In this work, two diamine monomers containing two different polar groups (phenylamino group and hydroxyl group, respectively) were synthesized and then polymerized with 6FDA to prepare gas separation membranes. This study verifies the modification method applicable for carbon dioxide/nitrogen separation.