Understanding hydrogen solubility and free volume characteristics in charge-transfer phthalonitrile prepolymer and polyimide blend membranes

The increasing demand for hydrogen production has necessitated the development of H₂-selective membranes. Polyimides are excellent membrane materials for gas separation; however, commercial polyimides generally lack sufficient H₂ selectivity due to their low H₂ affinity. Understanding the relationship between gas transport properties and free volume microstructure is critical to advancing H₂-selective membrane design. Herein, we report a facile material strategy to adjust the free volume characteristics and H₂ separation performance via blending Matrimid (PI) and crosslinkable resorcinol-based phthalonitrile prepolymer (RPN) with electron donor/acceptor properties. The novel RPN30/PI70 membrane exhibits H₂/N₂ and H₂/CO₂ permselectivity of 1637 and 66.4, respectively, with H₂ permeability of 2.7 Barrer in pure gas test, surpassing Robeson upper bounds (2008). The increased H₂ permselectivity of RPN/PI membranes was attributed to the narrowed free volume size and distribution, giving rise to the considerably improved H₂ solubility and selectivity of the blends. Moreover, the H₂ permeability of crosslinked RPN30/PI70 membranes can be further improved via thermal treatment. The H₂/CO₂ mixed-gas test reveals that the H₂ gas separation performance of the RPN30/PI70 membrane is influenced by plasticization effect and competitive sorption. This study demonstrates a new versatile strategy for designing high-performance hydrogen-selective polymeric membranes.