(152ag) Title: Zwitterionic and Polyampholyte Tethered Electrolyte Active-Layer Membranesdepartment: Chemical

Department: Chemical Engineering, Auburn University

Authors: Mohammad Mehdi Pour, Elnur Jabiyev, Cassandra Porter

Abstract:

Polymers with ionizable pendants, or polyelectrolytes (PEs), are of interest in membranes for both water-salt and ion-ion selectivity via charge-based exclusion. Although polyelectrolyte multilayer membranes (PEMMs) are relatively easily produced through layer-by-layer assembly, the electrostatic stabilization of these membranes can be disrupted, leading to membrane swelling and degradation.

In previous studies, an alternative PE membrane, the tethered electrolyte active-layer membrane (TEAM), was introduced. In TEAMs, charged polymers were covalently grafted-from porous supports for stabilization under different solution conditions. Unlike PEMMs that show increased ion rejection with additional layers, multiblock TEAMs showed diminished salt rejection over single-block TEAMs. Copolymers within multiblock TEAMs likely intercalated, screening charge.

In this research, we investigated how the location of charged groups within the active layer in TEAM membranes can affect the membrane performance. This work gives us a better understanding of how charge distribution in zwitterionic polymers affects ion transport. First, a polyampholyte polymer comprising both positively- and negatively-charged pendants with a random structure was synthesized as brush active layer. Nuclear magnetic resonance (NMR) spectroscopy was used to ensure a completely random distribution of charges. The performance of these types of TEAMs, including permeability and salt rejection, was tested. The highest achieved salt rejection was 50% of 2 mM CaCl₂ solution with a permeability of around 3 Lm^-2h^-1bar^-1, which is less than rejections achieved by single-block TEAMs with pendants of one charge type.

TEAMs that incorporated zwitterionic monomers were subsequently synthesized. The results show that the salt rejection of the membrane increases based on the degree of dissociation of the charged groups. Additionally, salt rejection is dependent on charge distribution and the freedom of charges to approach each other within the membrane and cause screening. The highest salt rejection achieved was by grafting poly(N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine). These TEAMs showed around 89% rejection of 2 mM Na₂SO₄ with a permeability of around 10 Lm^-2h^-1bar^-1. The negative monovalent versus divalent co-ion selectivity of these membranes changed from 7 to 11 as the concentration of SO₄^2- increased in the feed from 25% to 75% of total anions. This fundamental study further elucidates structure-function mechanisms regarding covalently-grafted polyelectrolyte membranes.