Block Polymer Membranes Functionalized with Nanoconfined Polyelectrolyte Brushes Achieve Sub-Nanometer Selectivity

The well-defined nanostructure of membranes manufactured from self-assembled block polymers enables highly selective separations; however, recent efforts to push the pore size of block polymer-based membranes to the lower end of the size spectrum have only been moderately successful for a variety of reasons. For instance, the conformational changes of the stimuli-responsive functional groups lining the pore walls of some block polymer membranes result in varied pore sizes that limit their operational range. Here, we overcome this challenge through the directed design of the third moiety of an A-B-C triblock polymer. The use of this macromolecular design paradigm allows for the preparation of a 500 nm thick polyisoprene-b-polystyrene-b-poly­(2-acrylamido-ethane-1,1-disfulonic acid) (PI-PS-PADSA) coating atop a hollow fiber membrane support. This nanoporous test bed, which exhibits an average pore radius of 1 nm, demonstrates an extremely high solute selectivity by fully gating solutes that have only an 8 Å size difference, a separation that is based solely on a sieving mechanism. Furthermore, the nanoscale structural characteristics of the solvated PADSA pore walls are elucidated by quantifying the rejection of neutral solutes and calculating the hydraulic permeability values in solutions of high ionic strength (1 mM ≤ I ≤ 3 M) and over a broad range of solution pH (1 ≤ pH ≤ 13). These key results provide a solid foundation for defining structure–property–performance relationships in the emerging area of nanoporous triblock polymer thin films. Moreover, the successful demonstration of the test bed separation device offers a tangible means by which to manufacture next-generation nanofiltration membranes that require a robust performance profile over a dynamic range of conditions.