Gas-Permeable Carbon Molecular Sieve Membranes Fabricated from a Norbornene-Functionalized Polyimide–Polyhedral Oligomeric Silsesquioxane Composite

Polyhedral oligomeric silsesquioxanes (POSSs) are a promising family of regularly structured silsesquioxanes with resilient cage-like configurations and exterior edges that can be functionalized with various organic groups. In this study, POSS was functionalized with a polyimide–phenylene (PI–Ph) unit, which yielded POSS–PI–Ph to fabricate carbonized-POSS membranes (denoted as carbon-POSS) via inert pyrolysis. Replacing some PI–Ph units with norbornene (NB) increased the amount of residual carbon formed in the carbon-POSS structure. X-ray photoelectron spectroscopy and ²⁹Si nuclear magnetic resonance analysis revealed that residual sp²-hybridized carbon atoms were connected to the POSS cage after pyrolysis at 700 °C. Analysis of single-gas permeation at 200 °C with the carbon-POSS membranes pyrolyzed at different temperatures (200–800 °C) indicated that the permeance of all investigated gases (He, H₂, CO₂, N₂, CH₄, CF₄, and SF₆) increased with increasing membrane pyrolysis temperature. Notably, the early onset of the decline in permeance of large-molecule gases such as CF₄ and SF₆ at 600 °C ensured a high N₂ permeance and ideal N₂/SF₆ selectivity of 10^–6 mol m^–2 s^–1 Pa^–1 and 100, respectively. Overall, this study demonstrates the feasibility of preparing high-performance carbon-POSS-derived membranes by optimizing the NB functionality and the POSS content of the hybrid copolymer precursor.