Membranes with high flux, ∼1 nm pore size, and unprecedented protein fouling resistance were prepared by forming selective layers of self-assembling zwitterionic amphiphilic random copolymers on porous supports by a simple coating method. Random copolymers were prepared from the hydrophobic monomer 2,2,2-trifluoroethyl methacrylate (TFEMA) and four zwitterionic monomers (sulfobetaine methacrylate, sulfobetaine 2-vinylpyridine, sulfobutylbetaine 2-vinylpyridine, and 2-methacryloyloxyethyl phosphorylcholine) by free radical polymerization. All copolymers microphase separated to form bicontinuous ∼1.2 nm nanodomains with the zwitterionic domains acting as nanochannels for the permeation of water and solutes. The resultant membranes all had a ∼1 nm size cutoff independent of zwitterion chemistry. There were, however, significant differences in the hydrophilicity, water uptake, water flux, and fouling resistance among membranes prepared with different zwitterionic monomers. Membranes prepared from the copolymer with 2-methacryloyloxyethyl phosphorylcholine were the most hydrophilic and had the highest water permeance, higher than that of commercial membranes of similar pore size. Furthermore, these membranes showed unprecedented fouling resistance, exhibiting no measurable flux decline throughout a 24 h protein fouling experiment. The structure-property relationships gleaned from this survey of different zwitterion structures serves as a guideline to develop new zwitterionic materials for various applications such as membranes, drug delivery, and sensors.

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We report membranes with ultrathin <200 nm zwitterionic copolymer selective layers exhibiting ∼1 nm size cutoff and permeances as high as 50 L/m2·hr·bar. The thin layer is formed by the deposition of random zwitterionic copolymers in trifluoroethanol/ionic liquid mixtures onto a porous support. The resultant membranes have the same low molecular weight cutoff of ∼1000 Da and narrow pore size distribution but fluxes up to 10 times higher than membranes prepared without ionic liquid and 20 times higher than commercial membranes of similar pore size, making them promising for wastewater treatment and pharmaceutical purification.

Membrane technologies are essential for water treatment, bioprocessing and chemical manufacturing. Stimuli-responsive membranes respond to changes in feed conditions (e.g., temperature, pH) or external stimuli (e.g., magnetic field, light) with a change in performance parameters (permeability, selectivity). This enables new functionalities such as tunable performance, self-cleaning and smart-valve behavior. Polymer self-assembly is a crucial tool for manufacturing such membranes using scalable methods, enabling easier commercialization. This review surveys approaches to impart stimuli responsive behavior to membrane filters using polymer self-assembly.

Heterogeneous materials with alternating hydrophobic and hydrophilic domains have challenged the long-standing paradigm that surfaces with higher hydrophilicity are more effective at resisting foulant adsorption. This work probes the underlying mechanisms for the observed phenomenon by exploring the effect of domain size, domain contrast, and foulant properties on adsorption to heterogeneous surfaces. We fabricate chemically heterogeneous surfaces from random zwitterionic copolymers of varied composition that phase-segregate at low weight percent to yield equivalently sized hydrophilic domains. We systematically vary the hydrophobic domain size by changing the weight percent of the zwitterionic monomer and the domain contrast by changing the hydrophobic monomer. We investigate the adsorption of heterogeneous and homogeneous foulants to these surfaces, as well as to control surfaces of uniformly blended hydrophilic and hydrophobic polymers. We find that hydrophilicity is a poor predictor of protein and heterogeneous macromolecular adsorption to chemically heterogeneous surfaces. Instead, adsorption is primarily determined by the local interaction energy between the foulant and surface domains and is inhibited when the length scale of foulant anchoring sites is larger than the length scale of the surface domains. These observations do not hold for chemically homogeneous macromolecules, where the adsorption is strongly correlated with average interaction energy and surface hydrophilicity.