Spirobisindane‐Based Microporous Nanofilms With Tunable Network for Precise Molecular Sieving

Manipulating polymer network architecture is critical to advancing organic solvent nanofiltration (OSN) membranes. Here we report a pH‐regulated interfacial polymerization strategy that employs rigid spirobisindane (SBI) monomers bearing hydroxyl and amine functionalities. Regulating phenolic hydroxyl participation enables the same dual‐functional SBI monomer to form either an amide‐dominated or a mixed amide/ester‐linked network, while a hydroxyl‐functional SBI monomer provides an ester‐dominated reference. Spectroscopic characterization, gas sorption analysis, and molecular simulations collectively reveal how network chemistry and packing regulate free‐volume organization and pore‐size distributions, thereby controlling solvent transport and molecular sieving. The mixed amide/ester‐linked SBI‐NH 2 ‐OH membrane exhibits a high microporous surface area (272 m 2 g −1 ) and high methanol permeance (22.7 L m −2 h −1 bar −1 ), whereas the amide‐dominated SBI‐NH 2 membrane provides tight molecular sieving with narrow pore size distributions. In contrast, the ester‐dominated SBI‐OH membrane exhibits larger average interchain spacing and a broader pore‐size distribution, accompanied by reduced selectivity. Notably, the SBI‐derived nanofilms display good pressure stability and solvent resistance. Moreover, successful larger‐area fabrication of both SBI‐NH 2 and SBI‐NH 2 ‐OH membranes demonstrates the compatibility of the fabrication strategy with scale‐up. Taken together, these findings demonstrate pH‐regulated functional‐group participation as a molecular design strategy that translates chemical reactivity into network architecture and transport behavior, offering a rational pathway toward scalable OSN membranes for energy‐efficient separations.