Methoxy groups increase water and decrease salt permeability properties of sulfonated polysulfone desalination membranes

Abstract A promising desalination membrane material, sulfonated polysulfone, was structurally modified to increase its water/salt transport selectivity. The structural modification was achieved by adding methoxy groups to the polymer backbone. This modification simultaneously increased water permeability and decreased salt permeability. These changes in properties are both favorable for desalination applications, and they may result from changes in polymer density and the free volume element size distribution. Relative permittivity and salt sorption data for these charged polysulfones suggest that Donnan exclusion may influence salt sorption properties to a greater extent than electrostatic or dielectric exclusion forces, which is reasonable for charged polymers. The Donnan-Manning model did not quantitatively describe the salt sorption properties of the sulfonated polysulfone materials perhaps due to a combination of low water content and a heterogeneous distribution of fixed charge groups throughout the polymer. Interestingly, addition of methoxy groups to the charged polysulfone backbone more strongly influenced the diffusion properties than the sorption properties, and this observation may be important to guide the engineering of highly selective polymeric materials for desalination applications.