(701c) Polyamide Brush Active-Layer Membranes for Fundamental Understanding of Structure-Function Relationships in Thin-Film Composite Reverse Osmosis Membranes

With freshwater scarcity on the rise, alternative sources like seawater are increasingly vital. Seawater desalination, leveraging the ocean’s vast reserves, offers a promising solution. However, conventional thermal methods—though effective—are costly, energy-intensive, and environmentally detrimental due to their dependence on fossil fuels.

Membrane-based desalination, especially reverse osmosis (RO), is favored for its lower energy use and operational ease. Commercial TFC polyamide (PA) membranes can reject up to 99.98% of salt, though some applications—like ultrapure water production—demand even greater selectivity. Improving these membranes hinges on understanding the still-mysterious factors behind the PA layer’s exceptional performance.

Examining the structure of polyamide reveals certain functional groups, such as aromatic rings and carboxyl groups, that may contribute to the performance of TFC-PA membranes. Higher carboxyl group density has been linked to improved salt rejection, not merely due to electrostatic repulsion but likely through polymer shrinkage that reduces pore size when surface charges are neutralized. At the same time, aromatic groups are believed to enhance membrane rigidity and facilitate the formation of well-ordered micropores via pi-pi stacking interactions. In this study, we aim to replicate the structure of polyamide to develop a new class of PA membranes that maintain the same separation mechanism while allowing the incorporation and controlled tuning of various functional groups to better investigate polyamide chemistry.

To probe these structure-performance relationships, we utilize surface-initiated atom transfer radical polymerization (SI-ATRP) to fabricate polyamide brush active-layer membranes (PA-BAMs) as a baseline to enable us furthered investigation on effects of different functional groups. Unlike conventional interfacial polymerization with m-phenylenediamine (MPD) and trimesoyl chloride (TMC), our method offers greater monomer flexibility and post-synthesis tunability for charge, crosslinking, and hydrophobicity. Our initial PA-BAMs were synthesized by grafting 2-aminoethyl methacrylate hydrochloride onto cellulose supports and crosslinking with MPD. These membranes exhibited 65% Na₂SO₄, 82% CaCl₂, and 54% NaCl rejection at 2 mM concentrations, with water permeability of 4 Lm⁻² h⁻¹ bar⁻¹ in a 2-bar dead-end cell.

Our next objective involves developing denser bottle-brush architectures for high-pressure operation. We will grow vertical brushes of hydroxyethyl methacrylate (HEMA) via SI-ATRP and subsequently graft horizontal 2-aminoethyl methacrylate chains, which will then be crosslinked using MPD. These configurations are expected to provide better performance under RO operating pressures. Ongoing work includes evaluating salt and neutral solute rejection, and permeability of bottle-brush PA-BAMs. Characterization techniques such as FTIR, water contact angel, and pore size analysis will confirm brush morphology and chemical composition. By correlating chemical structure and membrane performance, this study aims to uncover the fundamental principles governing TFC-PA membranes and to guide the rational design of next-generation RO membranes.

In summary, this work deepens our understanding of how functional groups and polymer design affect membrane performance, guiding the development of more durable, efficient, and tunable membranes for desalination and advanced separations to help address global water challenges.