Structure-Property Relationships in Polymer Membranes for Desalination Applications

Low cost and highly energy efficient membrane-based separation techniques are poised to address global water needs. These techniques use polymer membranes to control the rates of water and ion transport. For membranes to be effective for desalination, they must have excellent water/ion permeability selectivity, thus allowing water molecules through the polymer while blocking ions. The water and ion permeability properties of non-porous polymer membranes have been traditionally described in terms of sorption (thermodynamic) and diffusion (kinetics) factors via the solution-diffusion model, so high water/ion permeability selectivity can be achieved by preparing membranes that suppress either ion sorption or diffusion, or both, to a greater extent compared to water sorption and diffusion. Such membranes will have properties that favor desalination applications. One strategy to achieve high permeability selectivity is to prepare polymers that preferentially slow ion diffusion relative to water diffusion, which can be done by preparing polymers that have rigid and glassy backbones. Another way to enhance membrane selectivity properties is to prepare polymers that suppress ion sorption by evenly distributing the chemical functional groups throughout the polymer matrix, thus giving rise to high permeability selectivity in the polymers. Using these structure-property relationships, a promising desalination membrane material, sulfonated polysulfone, that has a combination of excellent mechanical and chemical stability and relatively good desalination performance compared to commercially available polyamide membranes, can be further modified in an attempt to control and engineer the water and salt transport and selectivity properties. Ultimately, structure-property understanding is important to guide the design of chemically and mechanically stable and highly selective future generation desalination membranes to purify water efficiently and effectively.