(520b) Tethered Electrolyte Active-Layer Membranes: From Single-Block to Zwitterionic, from Atom-Transfer Radical to Free-Radical Polymerization

Cast membranes typically can only achieve thicknesses at a minimum of several micrometers, while interfacial polymerization that results in thinner layers is limited to a few polymer types that rapidly react at organic-aqueous interfaces. It is predicted that ultrathin selective layers are necessary to achieve the highest selectivity with materials that exploit host-guest chemistry. Recently developed brush-active layer membranes, or BAMs, are grown from bottom up to produce ultrathin selective layers. BAMs have been formed through well-controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on ultrafiltration cellulose supports. One type of BAM membrane, the tethered electrolyte active-layer membrane (TEAM), was developed as an alternative to polyelectrolyte multilayer membranes (PEMMs). Previously, TEAMs comprising a covalently bonded single negative or positive block rejected salt more than multiblock TEAMs, which is counter to behaviors of PEMMs where more layers enhance salt rejection. Copolymers within multiblock TEAMs likely intercalated, screening charge.

In this work, we further investigated the idea of membrane self charge-screening by producing zwitterionic and randomly copolymerized ampholytic TEAMs. Ampholytic TEAMs with randomly co-polymerized negatively and positively charged pendants only achieved as high as 50% rejection of divalent co-ions. Meanwhile, zwitterionic membranes with pendants each having both one positive and one negative group rejected up to 90% of divalent ions. These outcomes suggest that the freedom of ionized groups to move toward oppositely charged functional groups determines the level of internal charge-screening. We also considered the scalability of these polyelectrolyte membranes, developing greener single block TEAMs produced in water through surface-initiated free radical polymerization. Positively charged green TEAMs rejected up to 83% NaCl, 97% CaCl₂, and 16% Na₂SO₄ with 4.3 Lm^-2h^-1bar^-1 pure water permeability. Negatively charged green TEAMs rejected 78% NaCl, 95% Na₂SO₄, and 7% CaCl₂ with 8.2 Lm^-2h^-1bar^-1 pure water permeability. For green TEAMs and zwitterionic TEAMs, monovalent over divalent ion selectivity using mixed salt solutions was shown to exceed that of ideal selectivity calculated from single salt rejections. Furthermore, monovalent selectivity increased with an increase in the number of counterions present, which is consistent with previous observations for SI-ATRP-produced single-block TEAMs. These results suggest that ion selectivity is not only dependent on the membrane properties but also the specific ions present in the feed. Control over membrane structure will be important for ion-ion selectivity with unique, complex feeds, such as produced water and geothermal brines. While the BAM and TEAM platforms help to elucidate structure-function properties for ion-selective membranes, they also provide routes for achieving specific structure and ultra thinness toward these goals of solute-solute selectivity.