Zwitterion-Functionalized Membranes via Post-Self-Assembly Modification of Cross-Linked Block Terpolymers: Decoupling Morphology Formation from Functionalization

We report a versatile strategy for fabricating zwitterion-functionalized membranes via postmodification of cross-linked poly(isoprene)-block-poly(styrene)-block-poly(glycidyl methacrylate) (xISG) triblock terpolymers. This approach decouples morphology formation from chemical functionalization, enabling systematic evaluation of domain-selective water uptake and salt transport behavior across nanostructured membranes with distinct morphologies and tunable compositions. xISG terpolymers self-assemble into well-defined lamellar, hexagonal, or gyroid nanostructures. These nanostructures are preserved through solid-state cross-linking and subsequent gel-state functionalization using a secondary amine-functionalized sulfobetaine zwitterion. The resulting membranes exhibit tunable water swelling (6.4–31 wt %), nanoscale domain spacing, and, most importantly, retain their original morphologies upon selective water uptake, as confirmed by spectroscopy and X-ray scattering analyses. Salt permeation measurements of NaCl, KCl, and LiCl demonstrate that permeability generally increases with water uptake and correlates directly with the hydration mass ratio relative to the hydrophilic zwitterion-modified G-block (λ). Thus, salt permeability is correlated with the λ of the hydrated ion- and water-permeating nanodomain and seemingly independent of the nanomorphology within the ranges of λ obtained here. Additionally, water uptake increases with ionic strength due to the antipolyelectrolyte effect, the extent of which depends on the overall cross-linker fraction, while permeability directly follows the trend of salinity-dependent hydration. Strong ion-specific selectivity is not observed between the studied membranes, which we attribute to the high λ of over 100% in the hydrophilic domain, despite small overall volume fractions of water in the membranes. This morphology-preserving platform provides a powerful tool to isolate structure–composition–property relationships in functional membranes and offers a generalizable design framework for future development and fundamental studies of ion-selective, antifouling, or energy-relevant separation materials.