Fluorination as a powerful tool for improving water/salt selectivity of hydrophilic polyethersulfone

Fluorination is an important tool for tuning polymer free volume properties, but the effect of fluorination on the water/salt transport properties of desalting polymers remains uncertain. In this work, a series of novel cysteine-based polyethersulfones (PES-Cys and PES-Cys-2F, PES-Cys-6H and PES-Cys-6F, PES-Cys-8H, and PES-Cys-8F) were designed in pair (4,4'-difluorodiphenyl sulfone versus 3,3',4,4'-tetrafluorodiphenyl sulfone, bisphenol A versus hexafluorobisphenol A, and 4,4'-dihydroxybiphenyl versus decafluorobiphenyl). Upon casting into membranes via the solvent evaporation method, the membranes/polymers were characterized by ¹H NMR, FTIR, XRD, DSC, Zeta potential, and the water/salt transport properties of the membranes was investigated systematically. It has been demonstrated that fluorination brings out greater spatial site resistance, stronger hydrophobicity, and higher electronegativity to polyethersulfone chains, leading to decreased water sorption but increased salt sorption. Fluorinated polyethersulfone displayed higher water diffusion coefficient because the C–F group can shatter water clusters into water molecules, which can diffuse more rapidly. Fluorinated polyethersulfone exhibit superior water/salt selectivity, with the most rigid PES-Cys-8F of the highest fluorine content showing water/salt selectivity over the empirical upper bound line. The enhanced water/salt selectivity is mostly because that fluorination improves water permeability by significantly increasing the water diffusion coefficient, while the electrostatic barrier provided by the electronegative fluorine atoms inhibits salt diffusion. This study provides valuable theoretical guidance for future molecular design of antioxidant desalting polymers.