Membranes derived from copolymer materials are a promising platform due to their straightforward fabrication and small yet tunable pore structures. However, most current applications of these membranes are limited to the size-selective filtration of solutes. In this study, to advance the utility of copolymer membranes beyond size-selective filtrations, a poly(acrylonitrile-r-oligo(ethylene glycol) methyl ether methacrylate-r-glycidyl methacrylate) (P(AN-r-OEGMA-r-GMA)) copolymer is used to fabricate membranes that can be chemically modified via straightforward schemes. The P(AN-r-OEGMA-r-GMA) copolymer is cast into asymmetric membranes using a nonsolvent induced phase separation technique. Then, the surface charge of the membrane is modified to tailor its performance for nanofiltration applications. The oxirane groups of the glycidyl methacrylate (GMA) moiety that line the pore walls of the membrane allows for both positively charged and negatively charged moieties to be introduced directly without any prior activation. Notably, the highly size-selective nanostructure of the copolymer materials is retained throughout the functionalization processes. Specifically, amine moieties are attached to the pore walls using the aminolysis of the oxirane groups. The resulting amine-functionalized membrane is positively charged and rejects up to 87% of the salt dissolved in a 10 mM magnesium chloride feed solution. Further modification of the amine-functionalized membrane with 4-sulfophenyl isothiocyanate results in pore walls lined by sulfonic acid moieties. These negatively charged membranes reject up to 90% of a 10 mM sodium sulfate feed solution. Throughout the modification scheme, the membrane permeability remains equal to 1.5 L m(-2) h(-1) bar(-1) and the rejection of neutral solutes (i.e., sucrose and poly(ethylene oxide)) is consistent with the membrane having a single well-defined pore diameter of ∼5 nm. The performance of the membrane as a function of ion valence number, solution pH, and ionic strength is investigated.

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.

The further advancement of membrane separation processes will require the development of more selective membranes. In this study, membranes that take inspiration from the mosaic structure of cell walls and use multiple functionalities of unique chemical design to control solute transport through chemical factors as well as steric factors are detailed. Specifically, a poly­{acrylonitrile-co-[oligo­(ethylene glycol) methyl ether methacrylate]-co-(3-azido-2-hydroxypropyl methacrylate)} copolymer tailor-made for the generation of nanofilters, which possess a high density of well-defined pores that are lined by azido moieties, allowed for the generation of chemically patterned mosaic membranes in a rapid manner through the use of printing devices. By engineering the composition of the reactive ink solutions printed on the membrane to allow for the rapid, one-to-one covalent attachment of alkynyl-functionalized groups to the azido moieties, we generated large areas of patterned membranes in seconds rather than hours as detailed in previous reports. Charge mosaic membranes, in particular, were used as an example of this novel platform. These membranes possess distinct cationic and anionic domains that traverse the membrane thickness, which results in the emergence of negative osmosis. As demonstrated through transport testing, this novel transport mechanism results in the preferential permeation of electrolytes over neutral molecules and solvents. For example, a negative rejection of −15% was observed for an aqueous feed solution containing 0.1 mM potassium chloride. The versatile and precise control over membrane chemistry at the nanoscale provided by the technique suggests that it could be engineered to prepare a variety of highly selective mosaic membranes.

The development of membranes that separate molecules on the basis of chemical factors, rather than physical factors, is one promising approach to meeting the demand for membranes that are more selective. In this study, the design of multifunctional, pH-responsive membranes that selectively pump a target solute is detailed. The membranes consist of two functional components: a gate layer made from an amine-functionalized copolymer and a reactive matrix lined by iminodiacetic acid groups that bind divalent cations reversibly. These two chemistries exhibit concurrent changes in the cation binding affinity and gate permeability in response to the pH value of the surrounding solution such that when the membranes are exposed to an oscillating pH, the combination drives a facilitated transport mechanism that pumps ions. In mixed solute systems, calcium permeated through the membrane four times faster than sucrose in the presence of an oscillating pH even though the solutes possess similar hydrodynamic sizes and permeated through the membrane at the same rate when the pH value was constant. The development of polymeric ion pumps was guided by a model that provided several critical insights. First, the solute binding capacity and thickness of the membrane define the asymptotic limit for enhanced selectivity. Second, the maximum enhancement in selectivity is realized in the limit of infinitely rapid oscillations. The multifunctional membranes discussed here provide a platform for the development of processes that can fractionate molecules of similar size but varying chemistry.

We propose a minimal coarse-grained model capable of capturing the performance and guiding the design of copolymer membranes tailored for ion-separations.