Salt Transport Structure/Property Relationships in Polymer Membranes for Water Purification and Power Generation

Providing sustainable supplies of water and energy is a critical global challenge. Polymer membranes dominate desalination and could be crucial to power generation applications. These desalination and power generation applications include reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), electrodialysis (ED), membrane capacitive deionization (MCDI), capacitive reverse electrodialysis (CRED), and reverse electrodialysis (RED). Improved membranes with tailored water and salt transport properties are required to extend and optimize these technologies. Water and salt transport structure/property relationships must be used to provide the fundamental framework for optimizing polymer materials for water and salt transport property-critical applications.

A tradeoff relationship between water/salt selectivity (i.e., the polymer's ability to separate water and salt) and water permeability has been reported where polymers that have high water/salt selectivity tend to have low water permeability and vice versa. This tradeoff relationship is consistent with free volume theory suggesting that attempts to prepare materials with favorable combinations of both high selectivity and high water permeability should rely on approaches that do more than simply vary the polymer's free volume. One strategy is to functionalize an otherwise hydrophobic hydrocarbon polymer with fixed charge groups that can ionize upon exposure to water. Sulfonated hydrocarbon polymers are an example of such an approach. These polymers offer an additional advantage from a desalination perspective because they have been reported to resist degradation by oxidizers such as the chlorine-based chemicals that are widely used in water treatment applications.

The presence of fixed charge (e.g., sulfonate) groups in a polymer influences that polymer's salt transport properties. Salt permeability, sorption, and diffusion properties are sensitive to the uncharged or charged (e.g., sulfonated) nature of the polymer. Poly(ethylene glycol) (PEG) hydrogels, sulfonated poly(arylene ether sulfone) random copolymers, and sulfonated styrenic pentablock copolymers were studied to probe the influence of polymer charge, water content, polymer structure, and micro- as well as nano-scale structural heterogeneity on salt transport properties. Salt sorption can be modeled using a combination of Donnan exclusion theory and salt partitioning to further describe the influence of fixed charge concentration, water content, and micro- as well as nano-scale polymer structure heterogeneity on salt sorption and transport properties.