Influence of Diamine Structure on Chain Packing and H2/CH4 Transport in BTDA-Based Polyimide Membranes

Developing high-performance membranes for H2/CH4 separation is a critical imperative for the advancement of the hydrogen economy. However, conventional polymeric membranes often suffer from the intrinsic trade-off between permeability and selectivity, as well as the technical dilemma between chain rigidity and processability. In this study, we systematically investigated polyimides based on 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) containing three structurally distinct diamines to elucidate the effects of intermolecular hydrogen bonding, steric bulk, backbone rigidity, and flexibility on H2/CH4 transport. By systematically incorporating a hydrogen-bonding diamine (DABA) alongside bulky (DAPI) and flexible (MDA) units, we established a precise structure–property relationship governing subnanometric gas transport. Experimental characterizations confirmed that DABA-driven hydrogen-bonding networks create an interaction-rich matrix with significantly contracted interchain distances (d-spacing 4.58 Å) and exceptional thermal stability (exceeding 520 °C). Gas permeation measurements showed that B-DABA exhibited the highest H2/CH4 ideal selectivity of 72.81 among the membranes investigated in this study, primarily owing to its strongly suppressed CH4 permeability. Atomistic molecular dynamics simulations indicated that free-volume characteristics and polymer-chain mobility can jointly influence gas-transport behavior. While bulky DAPI units increase free volume, their rigid indane structures restrict polymer-chain mobility, whereas the flexible MDA-containing matrix exhibits greater segmental mobility. These results highlight the combined importance of intermolecular interactions, free-volume characteristics, and molecular mobility in determining H2/CH4 transport in BTDA-based polyimides.