(565b) Bottlebrush Polyelectrolyte Tethered Electrolyte Active-Layer Membranes for Enhanced Ion Selectivity

This study presents the development of novel bottlebrush polyelectrolyte tethered electrolyte active-layer membranes (TEAMs) with significantly enhanced salt rejection and ion selectivity than prior produced TEAMs, even at higher salt concentrations. The separation performance of conventional single-block TEAMs suffered when using saline feeds above 2 mM concentration due to charge screening effects. To address this limitation, we engineered a unique bottlebrush architecture by first grafting poly(2-hydroxyethyl methacrylate) (PHEMA) from cellulose substrates via surface-initiated atom transfer radical polymerization (SI-ATRP), followed by esterification of PHEMA hydroxyl groups with α-bromoisobutyryl bromide to create initiation sites for subsequent side-chain polymerization.

Two polymers were explored: (1) positively charged poly(vinylbenzyl trimethylammonium chloride) (PVBMAC) and (2) negatively charged poly(sodium 4-vinylbenzenesulfonate) (PSVBS). Both positive and negative bottlebrush TEAMs demonstrated significantly improved salt rejection at higher concentrations (up to 200 mM) compared to conventional single-block TEAMs. The positively charged PVBMAC bottlebrush membranes exhibited exceptional rejection of divalent cations, with almost 100% rejection of CaCl₂ at 2 mM and maintaining ~80% rejection at 200 mM, along with ~60% NaCl rejection at the same concentration. Similarly, the negatively charged PSVBS bottlebrush membranes showed comparable performance with Na₂SO₄, rejecting ~100% at 2 mM and ~60% at 200 mM salt concentrations.

Importantly, both membrane types maintained pronounced ion selectivity at high salt concentrations: PVBMAC bottlebrush membranes achieved up to 18-fold selectivity for monovalent over divalent cations at 2 mM (compared to 9-fold for conventional TEAMs at only 2 mM), maintaining 8-fold selectivity even at 20 mM. PSVBS bottlebrush membranes showed similar selectivity patterns for monovalent anions over divalent anions. When testing PVBMAC with mixed chloride and sulfate solutions, anion monovalent/divalent selectivity was also observed despite having positively charged functional groups. This high selectivity is attributed to the dense arrangement of charged functional groups in the bottlebrush architecture, creating more effective electroactive barriers that maintain functionality even at elevated salt concentrations.

Salt rejection studies revealed that bottlebrush TEAMs operate through a combination of charge-based separation and size exclusion, the latter being negligible in conventional TEAMs. This combination brings their performance closer to reverse osmosis membranes while maintaining valuable ion selectivity. Interestingly, hydraulic pressure (tested from 30-110 bar) had minimal effect on salt rejection, unlike conventional RO membranes, suggesting novel transport mechanisms unique to the bottlebrush architecture.

These novel bottlebrush TEAMs represent a significant advancement in membrane separation technology, offering a versatile platform for ion-specific separations across a broad range of salt concentrations with applications in water treatment, resource recovery, and selective separation processes.