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-NH₂-OH membrane exhibits a high microporous surface area (272 m² g^-1) and high methanol permeance (22.7 L m^-2 h^-1 bar^-1), whereas the amide-dominated SBI-NH₂ 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-NH₂ and SBI-NH₂-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.