(83f) Versatile Platform for Synthesizing Polymer Membrane Libraries Using Functional Networks

Energy-efficient treatment of non-traditional source waters, such as produced water and agricultural runoff, requires development of next generation membrane technologies possessing solute-tailored selectivity. Such membranes could enable targeted removal of harmful contaminants such as perfluoroalkyl substances (PFAS) or enrichment of valuable solutes such as lithium that are currently unachievable with conventional membrane materials. Conventional materials used in water treatment such as ion-exchange membranes (IEMs) generally exhibit limited selectivity for monovalent and divalent counterions and for neutral or fluorinated solutes. Underlying this challenge is the limited set of chemical functionalities typically found in commercial membranes and the difficulty of expanding this design space due to synthetic limitations. In particular, conventional desalination and water treatment polymer membranes frequently employ hydrophilic, covalently crosslinked polymer networks containing a limited set of neutral and charged functional moieties, such as quaternary ammonium groups for anion exchange or sulfonate or carbonate groups for cation exchange. Incorporating new solute-selective functional groups into a membrane typically requires tedious redesign of monomer and polymer chemistry, which hinders development of vital structure-property relationships.

To address this challenge, we have developed a versatile synthetic platform that is capable of generating a library of functional materials from the same polymer platform using active ester chemistry. Polyethylene glycol diacrylate (PEGDA) copolymer networks were prepared with pentafluorophenyl active ester comonomers that can be easily substituted after crosslinking with a wide variety of primary amines containing neutral, basic, acidic, charged, and solute-chelating moieties. This network platform further enables direct control over crosslinking density, water uptake, and functional group grafting density via tuning of monomer ratios, which is crucial for elucidating membrane structure-property relationships. The diversity of functional groups allows for tailored uptake of basic and acidic organic dyes and metal chloride salts. Ion sorption and permeation measurements for imidazole-containing PEGDA networks with controlled crosslinking and imidazole grafting densities illustrate the ability to separate the effects of ligand functionality on ion transport from competing effects of water uptake and crosslinking density. Furthermore, ligand quaternization allows preparation of ion exchange membranes containing cationic functional groups such imidazolium, pyridinium, and trimethylammonium moieties. Developing a membrane library from these materials with tailored water contents and fixed charge densities enables systematic investigation of cation and anion transport in well-controlled ion exchange networks and could also facilitate a deeper understanding of the role of ligand identity on the absorbance of difficult to remove compounds such as perfluoroalkyl substances.