Disulfonated Poly(arylene ether sulfone) Random Copolymer Blends Tuned for Rapid Water Permeation via Cation Complexation with Poly(ethylene glycol) Oligomers

Here we present fundamental studies of a new blending strategy for enhancing water permeability in ionomeric reverse osmosis membrane materials. A random disulfonated poly(arylene ether sulfone) copolymer containing 20 mol percent hydrophilic units (BPS-20) in the potassium salt (−SO 3 K) form was blended with hydroxyl-terminated poly(ethylene glycol) oligomers (PEG, M n = 600−2 000) to increase the water permeability of BPS-20. Blending PEG with the copolymer resulted in pseudoimmobilization of the BPS-20 polymer chains because PEG complexes with cations in the sulfonated polymer matrix. Strong ion−dipole interactions between the potassium ions of the BPS-20 sulfonate groups (−SO 3 K) and the PEG oxyethylene (−CH 2 CH 2 O−) groups were observed via NMR spectroscopy. These interactions are similar to those reported between crown ethers and free alkali metal systems. The PEG oligomers were compatible with the copolymer at 30 °C in an aqueous environment. Transparent and ductile BPS-20_PEG blend films exhibited a Fox−Flory-like glass transition temperature depression as the PEG volume fraction increased. This depression depended on both PEG chain length and concentration. Both ion−dipole interactions and high coordination of −CH 2 CH 2 O− with −SO 3 K yielded a defined and interconnected hydrophilic channel structure. The water permeability and free volume of BPS-20_PEG blend films containing 5 or 10 wt % PEG increased relative to BPS-20. The blend films, however, exhibited reduced sodium chloride (NaCl) rejection compared to BPS-20. Addition of PEG did not significantly alter the material’s dry- and hydrated-state mechanical properties. Unlike commercial state-of-the-art polyamide RO membranes, the blend materials do not degrade when exposed to aqueous chlorine (hypochlorite) at pH 4. This comprehensive suite of measurements provides understanding of the molecular and morphological features needed for rational design of next-generation, chlorine-tolerant water purification materials.