Polymer Backbone Dynamics Influence Water/Ion Transport Selectivity in Membranes for Water Purification

Water scarcity is a severe challenge facing both developing and industrialized countries. Over the past 20 years, the desalination market has been grown significantly, and polymer membrane-based water purification techniques dominate the market. More efficient and low-cost membrane-based water purification can be achieved by improving water/salt selectivity, membrane fouling resistance, etc., but these improvements rely on deep understanding of the relationships between polymer structure and water/ion transport properties.
This work discusses the effect of polymer backbone dynamics on water and salt permeability, sorption and diffusion properties. Two homogeneous, uncharged copolymers, poly(HEA-co-EA) and poly(HEMA-co-MMA) were chosen based on prerequisite material characteristics to control the influence of water uptake and polymer chemistry. Backbone dynamics were varied by using an acrylic poly(HEA-co-EA) backbone and a methacrylic poly(HEMA-co-MMA) backbone. Low water uptake (< 0.2 g(water)/g(dry polymer)) materials, similar in water content to those used in commercial desalination membranes, were considered because the effect of backbone dynamics on transport properties was expected to be significant in low water content polymers as opposed to high water content hydrogels or ion exchange membranes.
Experimental data indicate that increasing polymer backbone rigidity results in an increase in the water/salt permselectivity and diffusion selectivity of these uncharged polymers. Ion sorption (thermodynamic) properties of the polymers appear to be unaffected by the change in backbone dynamics. Additionally, the polymer with the more rigid backbone (poly(HEMA-co-MMA)) exhibits greater size selectivity (quantified by ion permeability and diffusion selectivity) compared to the less rigid backbone material (poly(HEA-co-EA)).