(728c) Block Polymer Hollow Fiber Membranes Functionalized with Nanoconfined Polyelectrolyte Brushes Achieve Sub-Nanometer Selectivity

Manufacturing membranes with well-defined nanostructures and robust performance in a controllable manner is of critical import to meeting the growing demand for highly selective membranes in the water treatment, pharmaceutical, and electronics industries. Due to their high density of nanoscale pores with a uniform size distribution, self-assembled block polymer membranes are an ideal platform for the development of advanced membrane-based applications. In this study, we demonstrate the development of high-performance self-assembled block polymer nanofiltration membranes in the hollow fiber geometry. Specifically, dual-layer hollow fiber membranes were fabricated by combining the dip-coating archetype and the self-assembly and non-solvent induced phase separation (SNIPS) methodology to coat uniform block polymer films onto the shell side of the hollow fiber membrane support. A set of optimized membrane fabrication conditions (e.g., polymer solution concentration, dip-coating parameters, and solvent evaporation time) that generate high-performance hollow fiber nanofiltration membranes were identified by using a combination of electron microscopy analysis and transport experiments to guide the construction of nanoscale architecture of block polymer thin-film at the interface between the hollow fiber support and the self-assembled membrane. In this manner, ultra-thin (as thin as 200 nm) block polymer thin films with ordered nanostructures were produced. These membranes had a pore radius of 2.5 nm and a hydraulic permeability value of 27 L m^-2 h^-1 bar^-1.

The block polymer coating was based on a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) macromolecular precursor, which allowed the separation selectivity of the membrane to be further tailored using a carbodiimide coupling reaction to covalently attach sulfonic acid moieties to the pore walls of the membrane. The inherently charged, nanoconfined polyelectrolyte brush constrict the pore radius down to a value of 1 nm and result in an extremely high solute selectivity by fully fractionating solutes with only a 4 Å difference in radius. The use of the strong polyelectrolyte also generates a robust membrane that operates reliably in complex environments. Specifically, the transport performance of this membrane was examined over a broad range of solution pH (1 ≤ pH ≤13) and ionic strength solutions (1mM ≤ I ≤ 2.5 M). The membrane demonstrated a constant hydraulic permeability 4 L m^-2 h^-1 bar^-1, which is consistent with a polymer brush that lines the pore walls and holds a constant conformation, thus exhibiting a robust separation performance in response to the presence of external stimuli. These dual-layer hollow fiber membranes are promising candidates for fabrication of novel nanofiltration membrane based application devices for a variety of separation needs.