Advancing Surface-Initiated Brush Active-Layer Membranes: Green Synthesis, Bottlebrush Architectures, and Selective Ion Separations for Lithium Recovery

Abstract

Chemical separations consume approximately 15% of total U.S. energy, yet conventional separation technologies lack the molecular-level selectivity needed for critical applications such as selective ion transport and recovery of valuable minerals like lithium. Thin-film composite (TFC) membranes, comprising a selective thin active layer atop a porous support, dominate desalination and water treatment markets due to their superior permeability, durability, and scalability. However, conventional TFC membranes integrate the active layer through traditional methods like interfacial polymerization and casting, which lacks molecular-level control. This limits their selectivity between ions of similar charge and size and restricts their broader application in chemical separations. The recent advent of tethered electrolyte active-layer membranes (TEAMs) introduced a new fabrication approach for TFC membranes, incorporating the thin active layer by growing polyelectrolyte polymer brushes directly from the porous support via surface-initiated polymerization, forming the first brush active-layer membranes (BAMs) as TFCs. This covalent grafting strategy enabled molecular-level control over active layer composition and thickness, producing ultrathin, defect-free layers with permanent adhesion to the support and exceptional ion-ion selectivity. Despite their promise, early TEAMs were constrained by complex synthesis procedures, insufficient charge density at elevated salt concentrations, and limited charge diversity. This dissertation addresses these limitations through four interconnected projects that systematically advance the TEAM platform.

First, surface-initiated free radical polymerization (SI-FRP) was demonstrated as a greener and more accessible alternative to SI-ATRP, enabling one-pot aqueous TEAM synthesis completed in approximately two hours. SI-FRP-produced TEAMs achieved over 95% divalent salt rejection and monovalent/divalent ion selectivity of approximately 8-fold for cations and 9-fold for anions at 2 mM feed concentration.

Second, bottlebrush polymer architectures were introduced into TEAMs for the first time, increasing surface charge density approximately four-fold and extending reliable salt rejection from 2 mM to 200 mM, with 18-fold Na⁺/Ca²⁺ selectivity and 15-fold Li⁺/Mg²⁺ selectivity.

Third, pH-controlled SI-ATRP was demonstrated for direct grafting of negatively charged poly(sodium 4-vinylbenzenesulfonate) brush layers without post-polymerization modification, achieving 7-fold Cl⁻/SO₄²⁻ selectivity at 200 mM, the first report of significant monovalent anion selectivity in polyanion TEAMs at high salt concentrations.

Fourth, charge type and crown ether incorporation were systematically investigated for lithium/magnesium separation, with positively charged PMACC TEAMs achieving up to 120-fold Li⁺/Mg²⁺ selectivity. Future work will focus on in-situ hyperbranching polymerization via SI-ATRP as a simplified route to enhanced surface charge density, scale-up through photoATRP-enabled roll-to-roll manufacturing, and process optimization and hybrid separation strategies to improve lithium recovery from real brine matrices, collectively positioning brush-active TFC membranes as a transformative platform for critical minerals recovery and sustainable chemical separations.