A solvent-resistant thin-film composite nanofibrous membrane: high permeance, high rejection in virtue of high porosity and low tortuosity.

Covalent organic frameworks (COFs) are expected to provide exceptional permselectivity in molecular separations because of their stable, uniform, molecular-sized pore channels. To realize the superior performances of COF-based membranes, it remains strongly desired for the rapid and controllable growth of COFs on substrates under mild conditions. Herein, we report on the layer-by-layer (LbL) strategy for the rapid synthesis of imine-linked COFs on porous polymeric substrates in ethanol at room temperature. This strategy exploits the alternative availability of each COF monomer to endow a self-limiting nature to the reaction, thus resulting in conformal growth of COFs along the pore wall of the substrate. The grown COFs with thicknesses tunable by LbL cycles reduce the effective pore sizes, while the inherent ∼2 nm channels in COFs allow additional water to permeate. Thus-produced membranes exhibit significantly enhanced selectivity (>99% rejection to dyes) and unprecedentedly high water permeances, which are ∼3− 20 times higher than other membranes with similar rejections. This work potentially presents a new and general methodology to prepare imine-based COF membranes for molecular separations.

Improving the stability and photocatalytic performance of GO/PI composite membranes by a one-step cross-linking method.

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Smart voltage-gated nanofiltration membranes have enormous potential for on-demand and precise separation of similar molecules, which is an essential element of sustainable water purification and resource recovery. However, the existing voltage-gated membranes are hampered by limited selectivity, stability, and scalability due to electroactive monomer dimerization. Here, for the first time, the host–guest recognition properties of cucurbit[7]­uril (CB[7]) are used to protect the viologen derivatives and promote their assembly into the membrane by interfacial polymerization. Viologen functions as a voltage switch, whereas CB[7] complexation prevents its dimerization and improves its redox stability. The inhibited diffusion of the CB[7]-viologen complex enables the precise patterning of the surface structure. The resultant voltage-gated membrane displays 80% improved rejection performance, excellent recovery accuracy for similar molecules, and anti-fouling properties. This work not only provides an innovative strategy for the preparation of voltage-gated smart nanofiltration membranes but also opens up new avenues for ion-selective transmission in water treatment, bionic ion channels, and energy conversion.

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BACKGROUNDAt present, most commercial nanofiltration (NF) membranes are negatively charged. An effective way to prepare a positively charged NF membrane is to cross‐link polyimide (PI) substrate with polyethyleneimine (PEI). However, the lack of an efficient process for selecting proper cross‐linking conditions is an important issue. Therefore, the major objective of this work is to develop an efficient method to cross‐link only the top surface of the membranes.RESULTSThe optimum cross‐linking condition was found to be at a cross‐linking temperature of 70 °C, cross‐linking time of 80 minutes and stirring speed of 500 rpm. Comparing the membrane before and after cross‐linking, the molecular weight cut‐off changed from 17520 Da to 458 Da and the membrane surface changed from positive from negative. The cross‐linked membrane showed high rejection (>95%) for basic dyes with different molecular weights and relatively high antifouling properties to these dyes (flux recovery ratio > 92%, total flux decline ratio < 26.7%, irreversible flux decline ratio < 7.3%).CONCLUSIONSStirring speed was found to greatly influence the membrane performance. It was found that there exists an optimum stirring speed at which the cross‐linker distributes evenly on the membrane surface for the most effective cross‐linking to take place. The newly developed surface cross‐linking method also may have the potential to modify other types of membrane surfaces. © 2018 Society of Chemical Industry