Free Volume Manipulation and In Situ Oxidative Crosslinking of Amine-Functionalized Microporous Polymer Membranes

Membranes for gas separations are limited by the trade-off relationship between permeability and selectivity. In this study, we demonstrate an in situ thermal oxidative crosslinking strategy for amine-functionalized polymers using tert-butoxycarbonyl (tBOC) groups to improve separation performance. The use of labile tBOC groups offers two major benefits for inducing thermal oxidative crosslinks: (1) they trigger free radical chain reactions at more moderate temperatures, preventing polymer backbone degradation pathways that otherwise occur at elevated temperatures, and (2) they enable free volume manipulation (FVM) conditions that yield increased free volume and narrower free volume element size distribution. This thermal oxidative crosslinking strategy is demonstrated using an amine-functionalized polymer of intrinsic microporosity (PIM-NH2). The resulting crosslinked polymer yielded up to a 22-fold increase in H2/CH4 selectivity while retaining 96% of H2 permeability from pristine PIM-NH2 films. By combining thermal oxidative crosslinks and FVM, we demonstrate an effective approach to overcome the traditional permeability–selectivity trade-off and offer a greater resistance to major performance stability issues like plasticization and physical aging, making membranes better suited for industrial applications.