Versatile synthetic platform for elucidating water and ion transport in post-functionalized polymer membranes

Joshua D. Moon

University of Florida

Developing functional membrane materials with solute-tailored selectivity could enable recovery of solutes from non-traditional source waters, such as lithium from wastewater or brines, which is not possible with conventional polymer membranes, such as reverse osmosis or ultrafiltration membranes. Unfortunately, identifying functional groups that could enable such precision separations is hindered by synthetic difficulties of efficiently incorporating diverse chemical moieties into well-controlled polymer structures.

We addressed these challenges by developing a versatile synthetic platform using active ester click chemistry to prepare controlled libraries of functionalized hydrogel membranes. PEGDA copolymer networks were prepared with pentafluorophenyl acrylate co-monomers that can be readily substituted after polymerization with a wide array of ligands, including acidic, basic, and metal-chelating moieties.  This strategy allows us to maintain direct control over network crosslink density, hydrophilicity/hydrophobicity, and functional group grafting density via facile tuning of monomer ratios. Such control further enables us to deconvolute the effect of each of these properties on the transport rates of water, monovalent ions, and divalent ions.

Salt uptake and permeation measurements on a series of membranes with different imidazole ligand densities revealed that ion-imidazole interactions enhance NaCl and LiCl uptake but lack sufficient strength to affect monovalent ion diffusion. Stronger interactions between imidazole ligands and Mg²⁺ and Cu²⁺ cations substantially increase MgCl₂ and CuCl₂ uptake by 2x and 4x, respectively, while slowing salt permeation and diffusion. Pulsed-Field Gradient NMR measurements on membranes grafted with Lewis base ligands further show that microscale water, cation, and anion self-diffusion coefficients are strongly coupled to membrane water content, with ligand functionality having a secondary effect.

This post-functionalization strategy offers a powerful and versatile tool to enable mechanistic insights into both macroscopic and microscopic water and ion transport. Looking forward, such strategies could enable development of next-generation membranes for other applications such as carbon capture, hydrocarbon purification, and wastewater remediation.