Development of Structure/Property Correlations for Water and Ion Transport in Polymers

The development of advanced polymer membranes for water purification, in applications such as desalination via reverse osmosis, forward osmosis, nanofiltration, membrane-assisted capacitive deionization, or electrodialysis, and in energy generation applications such as forward osmosis and reverse electrodialysis, relies on an intimate understanding of the fundamental relationship between polymer structure and ion and water transport properties.  Much of this understanding does not currently exist.  This presentation will describe efforts to characterize ion and water sorption, diffusion and permeation in several systematic series of polymers.  We find strong correlations between free volume in hydrated polymers, as characterized via water uptake or results from positron annihilation lifetime spectroscopy studies, and ion and water diffusion and permeation in polymers.  We also find that there is a distinct tradeoff between water permeability (and diffusivity) and water/ion permeability (and diffusivity) selectivity.  Efforts to independently control membrane charge and water uptake can yield polymers with improved transport and separation properties.