Zero-Oil-Fouling Membrane With High Coverage of Grafted Zwitterionic Polymer for Separation of Oil-in-Water Emulsions.

Current hydrophilic modification strategies improve the antifouling ability of membranes but fail to completely eliminate the fouling of emulsified oil droplets with a wide size distribution. Constructing membranes with superior anti-oil-fouling ability to resist various oil droplets especially at high permeation fluxes is challenging. Here, the fabrication of a zero-oil-fouling membrane by grafting considerably high coverage of zwitterionic polymer and building defect-free hydration defense barrier on the surface is reported. A uniform layer of protocatechuic acid with COOH as abundant as existing in every molecule is stably deposited on the membrane so as to provide sufficient reactive sites and achieve dense grafting of the zwitterionic polymer. The coverage of zwitterionic polymer on the membrane plays a crucial role in promoting the antifouling ability to emulsified oil droplets. The poly(vinylidene fluoride) membrane with 93% coverage of the zwitterionic polymer exhibits zero oil fouling when separating multitudinous oil-in-water emulsions with ≈0% flux decline, ≈100% flux recovery, and a high water flux of ≈800 L m-2 h-1 bar-1 . This membrane outperforms almost all of the reported membranes in terms of the comprehensive antifouling performance. This work provides a feasible route for manufacturing super-antifouling membranes toward oil/water separation application.

Regulation of molecular transport in polymer membranes with voltage-controlled pore size at the angstrom scale.

Polymer membranes have been used extensively for Angstrom-scale separation of solutes and molecules. However, the pore size of most polymer membranes has been considered an intrinsic membrane property that cannot be adjusted in operation by applied stimuli. In this work, we show that the pore size of an electrically conductive polyamide membrane can be modulated by an applied voltage in the presence of electrolyte via a mechanism called electrically induced osmotic swelling. Under applied voltage, the highly charged polyamide layer concentrates counter ions in the polymer network via Donnan equilibrium and creates a sizeable osmotic pressure to enlarge the free volume and the effective pore size. The relation between membrane potential and pore size can be quantitatively described using the extended Flory-Rehner theory with Donnan equilibrium. The ability to regulate pore size via applied voltage enables operando modulation of precise molecular separation in-situ. This study demonstrates the amazing capability of electro-regulation of membrane pore size at the Angstrom scale and unveils an important but previously overlooked mechanism of membrane-water-solute interactions.

Microporous polymer adsorptive membranes with high processing capacity for molecular separation.

Trade-off between permeability and nanometer-level selectivity is an inherent shortcoming of membrane-based separation of molecules, while most highly porous materials with high adsorption capacity lack solution processability and stability for achieving adsorption-based molecule separation. We hereby report a hydrophilic amidoxime modified polymer of intrinsic microporosity (AOPIM-1) as a membrane adsorption material to selectively adsorb and separate small organic molecules from water with ultrahigh processing capacity. The membrane adsorption capacity for Rhodamine B reaches 26.114 g m-2, 10-1000 times higher than previously reported adsorptive membranes. Meanwhile, the membrane achieves >99.9% removal of various nano-sized organic molecules with water flux 2 orders of magnitude higher than typical pressure-driven membranes of similar rejections. This work confirms the feasibility of microporous polymers for membrane adsorption with high capacity, and provides the possibility of adsorptive membranes for molecular separation.

Two-dimensional fractal nanocrystals templating for substantial performance enhancement of polyamide nanofiltration membrane.

In this study, we report the emergence of two-dimensional (2D) branching fractal structures (BFS) in the nanoconfinement between the active and the support layer of a thin-film-composite polyamide (TFC-PA) nanofiltration membrane. These BFS are crystal dendrites of NaCl formed when salts are either added to the piperazine solution during the interfacial polymerization process or introduced to the nascently formed TFC-PA membrane before drying. The NaCl dosing concentration and the curing temperature have an impact on the size of the BFS but not on the fractal dimension (∼1.76). The BFS can be removed from the TFC-PA membranes by simply dissolving the crystal dendrites in deionized water, and the resulting TFC-PA membranes have substantially higher water fluxes (three- to fourfold) without compromised solute rejection. The flux enhancement is believed to be attributable to the distributed reduction in physical binding between the PA active layer and the support layer, caused by the exertion of crystallization pressure when the BFS formed. This reduced physical binding leads to an increase in the effective area for water transport, which, in turn, results in higher water flux. The BFS-templating method, which includes the interesting characteristics of 2D crystal dendrites, represents a facile, low-cost, and highly practical method of enhancing the performance of the TFC-PA nanofiltration membrane without having to alter the existing infrastructure of membrane fabrication.

Pseudo-zwitterions self-assembled from polycation and anion clusters showing exceptional water-cleanable anti-crude-oil-adhesion property.

It is of great importance and practical value to develop a facile and operable surface treatment method of materials with excellent antipollution and antiadhesion property, but still a huge challenge. In this work, a series of pseudo-zwitterions are prepared from electrostatic assembly of cationic polyethyleneimine and anionic phosphonic clusters. These pseudo-zwitterionic assemblies provide a strong hydration through electrostatic interaction with water and in turn create a barrier against oil foulants, leading to a nearly zero crude oil adhesion force. The pseudo-zwitterions-decorated surfaces exhibit exceptional water-cleanable oil-repellent property, even when they are completely dried and without prehydration before fouled by crude oil. While using these pseudo-zwitterions-modified polymeric membranes for separating surfactant stabilized oil-in-water emulsion, less than 10% decline of permeating flux is observed throughout a 2-h continuous separation experiment, showing excellent emulsion separation ability and antipollution performance for high viscous oil.

Constructing Strong Interfacial Interactions under Mild Conditions in MOF-Incorporated Mixed Matrix Membranes for Gas Separation.

Although mixed matrix membranes (MMM) possess remarkably improved gas separation performance compared to traditional polymeric membranes, membrane stability including CO2 plasticization and aging is still a serious issue due to the existence of interfacial defects. In this work, we report an efficient and less destructive route to cross-link the MOFs/polyimide (PI) MMM, where amine group-functionalized MOF (NH2-UiO-66) nanoparticles are thermally cross-linked with a carboxylic acid-functionalized PI (COOH-PI) matrix to form an amide bond at the interface at 150 °C under vacuum condition. Such a chemical cross-linking strategy conducted at a relatively mild condition improves membrane stability greatly while ensuring that the membrane structure is not destroyed. The resulting cross-linked MMM achieves enhanced mechanical strength with higher Young's modulus than a pristine polymer membrane. The CO2 antiplasticization pressure of the MMM after cross-linking is enhanced by 200% from ∼10 to >30 bar and the CO2 permeability of MMM only drops slightly from 995 to 735 Barrer after 450 days. At the same time, the separation performance of H2/CH4 gas pair surpasses the 2008 upper bound and that of CO2/CH4 gas pair nearly approaches the 2008 upper bound. The cross-linking strategy used herein provides a feasible and effective route for improving membrane stability and membrane performance in the MMM system for gas separation.

A Single-Walled Carbon Nanotube/Covalent Organic Framework Nanocomposite Ultrathin Membrane with High Organic Solvent Resistance for Molecule Separation.

Covalent organic framework (COF)-based membranes are burgeoning candidates for separation technologies owing to their well-ordered channel structures. The exponential interest in the stability of the COF membrane on exposure to harsh organic solvents is directed to develop a composite membrane for dye separations in polar aprotic solvents. Here, we reported a nanocomposite membrane composing of a single-walled carbon nanotube (SWCNT)/COF (an imine-based COF) hybrid on a commercial polytetrafluoroethylene (PTFE) substrate, with a thickness of ∼58 nm prepared in a diffusion cell. This membrane displayed high permeability and stability toward nonpolar and aprotic solvents. It exhibited high permeability for lower viscous organic solvents such as hexane (66 L m-2 h-1 bar-1), acetone (60 L m-2 h-1 bar-1), and acetonitrile (59 L m-2 h-1 bar-1) with a desirable dye rejection (92.8% for Brilliant blue in acetone). The long-time operation demonstrated the excellent stability of the nanocomposite membrane. We herein reported a facile and mild method to prepare an ultrathin COF-based nanocomposite membrane with a porous, robust structure coupled with solvent durability capable of efficient dye separation.

Ultrafast Ion Sieving from Honeycomb-like Polyamide Membranes Formed Using Porous Protein Assemblies.

Despite the commercial success of thin film composite polyamide membranes, further improvements to the water permeation of polyamide membranes without degradation in product water quality remain a great challenge. Herein, we report the fabrication of an interfacially polymerized polyamide nanofiltration membrane with a novel 3D honeycomb-like spatial structure, which is formed from a tobacco mosaic virus (TMV) porous protein nanosheet-coated microfiltration membrane support. TMV nanosheets with uniform pores and appropriate hydrophilicity deposited inside the support membrane pores facilitate the construction of a localized water-oil reaction interface with evenly distributed monomers and guide the formation of a defect-free polyamide layer with a spatial structure that copies the geometry of the membrane cavities. Such a 3D morphology possesses ultrahigh specific surface area, leading to unprecedented membrane water permeance as high as 84 L m-2 h-1 bar-1, high MgSO4 rejection of 98%, and monovalent/divalent ion sieving selectivity up to 89.

Zwitterionic Nanohydrogels-Decorated Microporous Membrane with Ultrasensitive Salt Responsiveness for Controlled Water Transport.

Highly sensitive responsiveness is vital for stimuli-responsive membranes. However, it is a great challenge to fabricate stimuli-responsive membranes with ultrahigh gating ratio (the ratio of the salt solution permeating flux to the pure water permeating flux) and high response speed simultaneously. In this work, a salt-responsive membrane with an ultrahigh gating ratio is fabricated via a facile strategy by grafting zwitterionic nanohydrogels onto a poly(acrylic acid)-grafting-poly(vinylidene fluoride) (PAA-g-PVDF) microporous membrane. Due to the synergistic effect of two functional materials, PAA chains and zwitterionic nanohydrogels tethered on PAA chains, this stimuli-responsive membrane exhibits an ultrasensitive salt responsiveness with a gating ratio of up to 8.76 times for Na+ ions, 89.6 times for Mg2+ ions, and 89.3 times for Ca2+ ions. In addition, such zwitterionic nanohydrogels-grafted PAA-g-PVDF (ZNG-g-PVDF) membranes exhibit very rapid responses to stimuli. The permeating flux changes swiftly while altering the feed solution in a continuous filtration process. The excellent salt-responsive characteristics endow such a ZNG-g-PVDF membrane with great potential for applications like drug delivery, water treatment, and sensors.

Ultrathin Polyamide Nanofiltration Membrane Fabricated on Brush-Painted Single-Walled Carbon Nanotube Network Support for Ion Sieving.

Recently, ultrathin polyamide nanofiltration membranes fabricated on nanomaterial-based supports have overcome the limitations of conventional supports and show greatly improved separation performance. However, the feasibility of the nanomaterial-based supports for large-scale fabrication of the ultrathin polyamide membrane is still unclear. Herein, we report a controllable and saleable fabrication technique for a single-walled carbon nanotube (SWCNT) network support via brush painting. The mechanical and chemical stability of the SWCNT network support were carefully examined, and an ultrathin polyamide membrane with thickness of ∼15 nm was successfully fabricated based on such a support. The obtained thin-film composite (TFC) polyamide nanofiltration membranes exhibited extremely high water permeability of ∼40 L m-2 h -1 bar-1, a high Na2SO 4 rejection of 96.5%, and high monovalent/divalent ion permeation selectivity and maintained highly efficient ion sieving throughout 48 h of testing. This work demonstrates a practical route toward the controllable large-scale fabrication of the TFC membrane with an SWCNT network support for ion and molecule sieving. This work is also expected to boost the mass production and practical applications of state-of-the-art membranes composed of one-dimensional and two-dimensional nanomaterials as well as the nanomaterial-supported TFC membranes.

Ultrathin Nanofiltration Membrane from Confined Polymerization within the Nanowire Network for High Efficiency Divalent Cation Removal.

Membranes with high permeance and high rejection for di- and multivalent cation removal are highly desired for efficient brackish water and industrial water treatment. In this work, we report a facile strategy for constructing ultrathin nanofiltration (NF) membranes by in situ cross-linking of amine which is confined in a network film. The network made of single-walled carbon nanotubes (SWCNTs) serves as a framework for poly(ethylene imine) (PEI) to attach and stay, facilitating the formation of a polyamine (PA) layer with high quality and controlled thickness. Benefiting from the ultrathin thickness of the SWCNT network (∼31 nm), an active layer (∼34 nm thick) comes with a high permeance of 27 L m-2 h-1 bar-1 along with a high rejection of 97% to MgCl2, 2-5 times higher than the NF membranes with the same high rejection for MgCl2 reported so far. In addition, the SWCNT-interpenetrated PA structure endows the ultrathin NF membrane with good operational stability. This work demonstrates the capability to control the position, thickness, and even quality of the PA layer by using a confined framework and provides a feasible strategy for the fabrication of highly permeable ultrathin NF membranes with a reinforced active layer.

Ultralarge Single-Layer Porous Protein Nanosheet for Precise Nanosize Separation.

Highly permeable and precisely size-selective membranes are the subject of continuous pursuit for energy-efficient separation of fine chemicals. However, challenges remain in the fabrication of an ultrathin selective layer with homogeneous pores, in particular, with the pore sizes in the 1-10 nm range. We report the design of a free-standing porous nanosheet assembled with a single layer of proteins. Tobacco mosaic virus mutant (TMVm), a cylinder-shaped protein containing an inner pore of 4 nm in diameter, was cross-linked via a Cu2+-catalyzed disulfide-bond-forming reaction along the 2D orientation. By such a design, ultralarge single-layer TMVm nanosheets extending over tens of micrometers in width and with well-defined nanopores were successfully developed. A ∼40 nm thick ultrafiltration membrane laminated by the single-layer TMVm nanosheets through simple vacuum filtration accomplished the precise separation of ∼4 nm sized substances. Meanwhile, the membrane exhibited water permeance up to ∼7000 L m-2 h-1 bar-1, which is an order of magnitude improvement compared with traditional ultrafiltration membranes with a similar rejection profile.