PUBLICATIONS

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2019

Table-Salt Enabled Interface-Confined Synthesis of Covalent Organic Framework (COF) Nanosheets

Xiansong Shi, Dongwei Ma, Fang Xu, Zhe Zhang, Yong Wang*, Chem. Sci. 2019

Two-dimensional covalent organic frameworks (COFs) are gaining tremendous interests for their potential applications in a diversity of fields. However, synthesis of COF nanosheets (CONs) usually suffers from tedious exfoliation processes and low yields. Herein, we present an exfoliation-free and scalable strategy to prepare few-layered CONs based on interface-confined synthesis, in which cheap and recyclable table salts (NaCl) are used as the sacrificial substrates. Salt particles are introduced into the reaction system, creating billions of solid-liquid interfaces. Oligomers formed upon the reaction between monomers are immediately adsorbed on salt surfaces, and the following polymerization leading to crystalline CONs is exclusively confined to salt surfaces. Salts can be easily removed by water washing, producing CONs with the thickness down to a few nanometers and lateral sizes up to hundreds of micrometers depending on the size of salt particles and the concentration of monomers. Four different kinds of CONs, both imine-linked and boron-containing, are synthesized from this generic method. As a demonstration, we prepare highly permeable and selective membranes using resultant CONs as building blocks. Thanks to the defect-free stacking of CONs with thin thicknesses and large lateral sizes on porous substrates, the membranes precisely separate similarly sized dyes while allow ultrafast water permeation. This interface-confined strategy opens a new platform for the controllable and scalable synthesis of COF nanosheets, which is essential for the burgeoning real-world applications of COFs in various fields.

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Transport Mechanism of Water Molecules Passing Through Polyamide/COF Mixed Matrix Membranes

Yang Song, Mingjie Wei*, Fang Xu, Yong Wang*, Phys. Chem. Chem. Phys. 2019, 21, 26591-26597

Mixed matrix membranes (MMMs) have gained significant attention due to their high water permeability without the cost of salt rejection. However, the mechanism of water transport through MMMs is still controversial. Herein, a type of two-dimensional covalent organic framework (COF), TpPa-1, was selected as a blending additive for mixing with polyamide (PA) due to its good compatibility with polymeric matrices. We performed nonequilibrium molecular dynamics simulations to investigate the transport properties of the water molecules passing through MMMs. The permeance of MMMs was obviously enhanced compared to that of the pure PA membrane. However, this enhancement in permeance did not originate from the intrinsic pores of COFs. The PA atoms confined in the pores had extremely low diffusivity and thus, the water molecules were trapped inside the COF pores. Outside the COF pores, there was an interfacial region between COFs and PA, in which the water molecules aggregated to form a high-density region. In this region, the average flow velocities of the water molecules were much higher than those in pure PA or PA outside the mixed region of MMMs. The water transport mechanism discovered in this work offers an alternative and more likely explanation for the high permeance observed in PA/COF MMMs, and this understanding can be helpful to design more efficient MMMs.

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Block Copolymer Ultrafiltration Membranes by Spray Coating Coupled with Selective Swelling

Dongwei Ma, Jiemei Zhou, Zhaogen Wang, Yong Wang*, J. Membr. Sci. 2019, 117656

Block copolymers (BCPs) have been demonstrated to be promising precursors in the fabrication of ultrafiltration membranes with well-defined porosities. To maximize the separation performance, BCPs are usually used in the forms of thin-film composite membranes with nanoporous BCP separation layers on the top of macroporous substrates. However, it remains a challenge to prepare such composite structures in an efficient and controllable way. Herein, we demonstrate that spray coating of BCP solutions on macroporous substrates followed by selective swelling is a facile and highly controllable strategy to produce BCP composite membranes. First, the BCP solution was spray-coated on a macroporous nylon support. By changing polymer concentrations as well as various spray conditions, a defect-free and uniform dense BCP layer was obtained. The thickness of the BCP layer can be controlled within the scope of a few micrometers simply by altering the step width of spray coating. Neither pre-filling of the macroporous supports to prevent solution permeation nor post-treatment to strengthen the adhesion of the two layers is required during the spray coating process. The BCP-coated support was then soaked in hot ethanol to create nanoporosity in the BCP layer following the mechanism of selective swelling-induced pore generation. Thus-produced composite membranes exhibit excellent ultrafiltration performances. Meanwhile, the pore sizes, surface hydrophilicity and consequently separation performances of the membranes can be conveniently tuned by changing swelling conditions. We expect this spraying coating coupled with selective swelling to produce BCP-based composite membranes at large scale for industrial production.

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Ion Rejection in Covalent Organic Frameworks: Revealing the Overlooked Effect of In-pore Transport

Fang Xu, Mingjie Wei*, Xin Zhang, Yong Wang*, ACS Appl. Mater. Interfaces 2019, 11, 45246-45255

Covalent organic frameworks (COFs), possessing highly ordered and intrinsic nanopores with high density and tunable sizes, are expected to find important applications in ion separations and desalination. Design of COF membranes with outstanding permselectivity requires understandings on the ion rejection behaviors of COF multilayers. However, the ion rejection mechanism of COF multilayers remains to be elucidated because it may significantly differ from that of conventional polyamide membranes. Herein, we use nonequilibrium molecular dynamics simulations to investigate the ion transport through multilayers of TpHZ, which is a stable, imine-linked COF. Surprisingly, we find that the rejection to NaCl is determined by its rejection to Na+ rather than to Cl- although hydrated Cl- is bigger than hydrated Na+. Inside the channels of the TpHZ multilayers, Na+ ions transport evidently slower than water molecules, implying that the in-pore transport effect instead of the commonly thought pore-entrance sieving effect governs ion rejection. The in-pore transport effect of Na+ is mainly due to the hydration of Na+ with pore wall and stronger capability of the hydrated Na+ ions to form hydrogen bonds with pore wall, both of which are primarily originated from the polarity of the COF. This work reveals the significant role of in-pore transport in ion rejection which is overlooked in commonly used polyamide desalination membranes, and develops a universal equation capable of describing ion rejections of all membranes especially those contain structures of nanopores or nanochannels by considering both the effects of pore-entrance sieving and in-pore transport.

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Enabling Covalent Organic Framework Nanofilms for Molecular Separation: Perforated Polymer-Assisted Transfer

Ankang Xiao, Zhe Zhang, Xiansong Shi, Yong Wang*, ACS Appl. Mater. Interfaces 2019, 11, 44783-44791

Covalent organic frameworks (COFs) with ordered arrays of sub-2-nm regular pores are drawing increasing attentions in membrane separation, and it remains highly desired for effective and controllable strategies to fabricate COF-based membranes. Herein, we demonstrate a perforated polymer-assisted transfer strategy enabling COF nanofilms for molecular separation. Solvothermal synthesis is used for the confined growth of TpPa, a stable, imine-linked COF, on the smooth surfaces of silicon substrates. Continuous, crystalline COF nanofilms are obtained, and their thicknesses are tunable in the range from a few tens nm to several hundred nm depending on monomer concentrations and reaction time. A block copolymer layer is coated on the COF nanofilms, which is then perforated to produce interconnected mesopores by the mechanism of selective swelling-induced pore generation. The perforated polymer coating functions as a protective but permeable layer enabling the easy transfer of the COF nanofilm onto porous substrates. Thus, we obtain a new type of composite membranes with the microporous COF nanofilm as the selective layer, sandwiched between the macroporous substrate and the mesoporous protective layer. The composite membranes exhibit good separation performances with water permeance up to ~51 L m-2 h-1 bar-1 and high rejection rates to various dyes. This work demonstrates a new method to prepare COF-based membranes for molecular separation, and the invented perforated polymer-assisted transfer technology is expected to find applications in transferring other ultrathin materials to demanded substrates.

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Nanofluidic Behaviors of Water and Ions in Covalent Triazine Framework (CTF) Multilayers

Mingjie Wei, Wei Zhou, Fang Xu, Yong Wang*, Small 2019, 1903879

Covalent triazine frameworks (CTFs) hosting arrays of highly ordered sub-2-nm pores are expected to exhibit unusual nanofluidic behaviors, which may enable important applications such as desalination. Herein, non-equilibrium molecular dynamics simulations are applied to investigate transport of water and ions inside two typical CTFs, CTF-1 and CTF-2, having the intrinsic pore of 1.2 nm and 1.5 nm, respectively. Their monolayers exhibit extremely high water permeance but weak ion rejection. CTF multilayers are then investigated. Transport resistances composed of interior and interfacial contribution are correlated with stacking numbers of CTF monolayers to develop equations of predicting water permeance. It is revealed that both the stacking fashion and the number of CTF monolayers to form multilayers significantly influence permeation and ion rejection. Staggered multilayers exhibit much higher ion rejection than eclipsed ones. Staggered CTF-2 multilayers completely reject ions because the interlayer paths between two adjacent staggered monolayers allow only water molecules to pass through. Importantly, it is predicted from our equations that, few-layered staggered CTF-2 multilayers, which can be relatively easily produced by experimental methods, exhibit 100% NaCl rejections and up-to-100 times higher permeance than commercial reverse osmosis membranes, implying their great potential as building blocks to prepare next-generation desalination membranes.

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Ultra-Permeable Polyamide Membranes Harvested by Covalent Organic Framework Nanofiber Scaffolds: A Two-in-One Strategy

Zhe Zhang, Xiansong Shi, Rui Wang, Ankang Xiao, Yong Wang*, Chem. Sci. 2019, 10, 9077-9083

Polyamide-based thin-film composite nanofiltration membranes are widely used for potable water decontamination, brackish water desalination and wastewater reutilization. However, enhancing water permeance of the polyamide layer within the thin-film composite nanofiltration membrane still remains a great challenge, because of the relatively large thickness and high transport resistance of the polyamide layer produced from the intractable interfacial polymerization. Herein, we reported a two-in-one strategy to prepare ultra-permeable nanofiltration membranes via covalent organic framework (COF) nanofiber scaffolds mediated interfacial polymerization. The highly porous and hydrophilic COF nanofiber scaffolds enhanced the controlled release of amine monomers, leading to ultrathin polyamide layers. Also, the relative smooth COF nanofiber scaffolds can be spontaneously evolved into rugged and uneven architectures during interfacial polymerization, providing rough substrates for enlarging actual areas of polyamide layers. Thereby, the increased areas of polyamide layers were employed as additional water permeable domains. Arising from the synergetic effect of the ultrathin and increased water permeation domains, the produced membranes exhibit exceptional nanofiltration performance with a water permeance up to 31.1 L m-2 h-1 bar-1 and a Na2SO4 rejection rate of about 95%, outperforming most of other nanofiltration membranes. This highly accessible technique opens a new avenue for design and engineering of ultrapermeable thin-film composite nanofiltration membranes, highlighting its great potential in providing clean water.

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CO2-Responsive Graphene Oxide Nanofiltration Membranes for Switchable Rejection to Cations and Anions

Congcong Yin, Liangliang Dong, Zhaogen Wang, Mingqing Chen*, Yong Wang*, Yue Zhao*, J. Membr. Sci. 2019, 592, 117374

Inspired by biological membranes with charge-gated channels that allow the transport of water and ions but restrict the undesired substances, various regulated nanofiltration (NF) membranes containing charge-gated channels have been developed for water purification. However, most of these membranes are mono-charged (positively or negatively) that show high rejection only to the corresponding co-ions but low rejection to the counter-ions, which limits their wider applications. Herein, we report NF membranes with gas-tunable charge-gated channels and high rejection to different salts on demand. The membranes are prepared by assembling a CO2-responsive polymer, poly(N,N-dimethylaminoethyl methacrylate), on the surface of graphene oxide (GO). As a result of CO2-induced reversal of the charge sign, the membranes exhibit “self-regulation” of their surface charges. Upon bubbling CO2 into water, the membranes are positively charged, which shows high rejection to MgCl2. After bubbling Ar to remove CO2, the membranes gradually transform to negatively charged, and high rejection to Na2SO4 is obtained. This study demonstrates the gas-tunable charge-gated ion transport behavior of the CO2-responsive NF membranes, and opens up new avenues for the design of on-demand NF membranes.

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Carbonization of Gradient Phenolics Filled in Macroporous Substrates for High-Flux Tight Membranes: Toward Ultrafiltration of Polypeptides

Qianqian Lan, Yong Wang*, J. Membr. Sci. 2019, 590, 117309

Tight ultrafiltration (UF) membranes with high permeances have gained growing attention in the separation of biomacromolecules. Herein, we report the preparation of gradient carbonaceous structures filled in macroporous polyvinylidene fluoride (PVDF) substrates and their efficient separation of polypeptides. Gradient phenolics composed of nanoparticles with sizes increasing from top side to bottom side are firstly synthesized inside PVDF substrates. When soaked in concentrated H2SO4, phenolic nanoparticles are partially carbonized and foamed to form micropores while the PVDF substrate maintains intact. Thus-produced carbonaceous membranes are mechanical robust and exhibit narrowed pore sizes and enhanced hydrophilicity. Their permselectivity can be adjusted simply by changing the dosages of phenolic precursors filling into the PVDF substrates. Low dosages lead to remarkably high water permeances up to 1721 L/(m2 h bar). At higher dosages, the membrane is tightened to show a molecular-weight-cut-off (MWCO) down to 3.7 kg/mol and high permeances several times larger than other membranes with similar MWCOs. We demonstrate that such a tight UF membrane efficiently separates polypeptides with molecular weights of only a few kg/mol (angiotensin Ⅱ and thymosin β4). Our work opens a new avenue for the synthesis of high-flux tight UF membranes with narrowed pores, enabling the precise separation and fractionation of biomacromolecules.

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Single-Step Coating of Polyethylenimine on Gradient Nanoporous Phenolics for Tight Membranes with Ultrahigh Permeance

Leiming Guo, Yazhi Yang, Yong Wang*, J. Membr. Sci. 2019, 587, 117172

Ultrafiltration membranes with relatively small pores are highly expected in various separation cases. As a flexible manner to efficiently tighten the pores, post modification provides a robust platform for preparing the desired membranes. However, the endeavor to pursue high permselectivity of the membranes has been made by paying close attention to the modification layers while the effects of the substrate membranes are generally overlooked. In this work, tight membranes with ultrahigh permeance were produced by coating polyethylenimine (PEI) on gradient nanoporous phenolic substrates. Cationic PEI was tightly deposited on the negatively charged phenolics via electrostatic deposition, thus reducing the effective pore sizes to ∼2.6 nm through a single-step coating. The gradient porous nanostructures still remained after PEI coating in the phenolic membranes. The coated membranes exhibited ∼4–45 times higher permeance than counterparts with similar rejections. As the cationic PEI was distributed on the membrane surface and pore wall after coating, the positively charged membranes thus produced were able to efficiently separate cationic dyes from water.

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Unidirectional Diffusion Synthesis of Covalent Organic Frameworks (COFs) on Polymeric Substrates for Dye Separation

Rui Wang, Xiansong Shi, Zhe Zhang, Ankang Xiao, Shi-Peng Sun, Zhaoliang Cui, Yong Wang*, J. Membr. Sci. 2019, 586, 274-280

Covalent organic frameworks (COFs) are regarded as important candidates for the preparation of advanced membranes with remarkable performances on account of their uniform and tailorable channels and good chemical stability. The development of COFs-based membranes is burgeoning, and there remains lacking of facile and mild methods to prepare to these membranes. Herein, we report on the unidirectional diffusion synthesis (UDS) of TpPa (an imine-based COFs) on commercial polyvinylidene fluoride (PVDF) microfiltration substrates for the preparation of composite COF-based membranes at room temperature. The p-phenylenediamine (Pa) aqueous solution and 1,3,5-triformylphloroglucinol (Tp) n-hexane solution were separately charged into the two sides of a diffusion cell. With the suitable solution pairs and molar ratio of two precursors, only Pa molecules can pass through the macropores of PVDF to react with Tp molecules at the phase interface formed by this solution pairs, leading to the in situ synthesis of TpPa layer on the top side of PVDF substrates. The membrane fabricated with a synthesized duration of 24 h exhibits a superior selectivity for dyes (>90%) and appreciable water permeance (60 L m−2 h−1·bar−1). The permeance of THE resultant membrane is ∼2-20 times higher than other reported membranes with comparable dye rejections. The UDS offers a facile and mild process to directly grow COFs on porous polymeric substrates to construct composite membranes capable of efficient separations of dye molecules.

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(Book Chapter) Selective Swelling of Block Copolymers for Porous Nanostructures

Leiming Guo, Zhaogen Wang, Yong Wang*, Block Copolymers 2019, 45-118

We previously aimed at 1D nanofibers and 2D thin films to illustrate the course of selective swelling in constructing porous nanostructures in BCPs. However, the swelling strategy has been extended to the preparation of 3D nanoporous bulk materials afterwards, in which bicontinuous and homoporous structures have been obtained. The swelling-induced pore generation enables the ease of fabrication of porous materials to meet the needs of membrane separation, lithography, drug-delivery, anti-reflectivity, solar cells and other applications, and shows a bright future. Here, we will uncover the mechanism, kinetic control of structures, swelling conditions and applications of the swelling-induced pore generation to provide a detailed yet deep understanding of this pore-generation strategy.

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Gradient Nanoporous Phenolics as Substrates for High-Flux Nanofiltration Membranes by Layer-by-Layer Assembly of Polyelectrolytes

Yazhi Yang, Qianqian Lan, Yong Wang*, Chinese J. Chem. Eng. 2019

Thin film composite (TFC) membranes represent a highly promising platform for efficient nanofiltration (NF) processes. However, the improvement in permeance is impeded by the substrates with low permeances. Herein, highly permeable gradient phenolic membranes with tight selectivity are used as substrates to prepare TFC membranes with high permeances by the layer-by-layer assembly method. The negatively charged phenolic substrates are alternately assembled with polycation polyethylenimine (PEI) and polyanion poly(acrylic acid) (PAA) as a result of electrostatic interactions, forming thin and compact PEI/PAA layers tightly attached to the substrate surface. Benefiting from the high permeances and tight surface pores of the gradient nanoporous structures of the substrates, the produced PEI/PAA membranes exhibit a permeance up to 50.6 L ∙ m−2 ∙ h−1 ∙ bar−1, which is ~ 2–10 times higher than that of other membranes with similar rejections. The PEI/PAA membranes are capable of retaining > 96.1% of negatively charged dyes following the mechanism of electrostatic repulsion. We demonstrate that the membranes can also separate positively and neutrally charged dyes from water via other mechanisms. This work opens a new avenue for the design and preparation of high-flux NF membranes, which is also applicable to enhance the permeance of other TFC membranes.

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Fast Desalination by Multilayered Covalent Organic Framework (COF) Nanosheets

Wei Zhou, Mingjie Wei*, Xin Zhang, Fang Xu, Yong Wang*, ACS Appl. Mater. Interfaces 2019, 11, 16847-16854

Covalent organic frameworks (COFs) are penetrated with uniform and ordered nanopores, implying their great potential in molecular/ion separations. As an imine-linked, stable COF, TpPa-1 is receiving tremendous interest for molecular sieving membranes. Theoretically, atomically thin TpPa-1 monolayers exhibit extremely high water permeance but unfortunately no rejection to ions because of its large pore sizes (~1.58 nm). COF monolayers tend to stack to form laminated multilayers, but how this stacking influence water transport and ion rejections remain unknown. Herein, we investigate the transport behavior of water and salt ions through multilayered TpPa-1 COFs by nonequilibrium molecular dynamics simulations. By analyzing both the interfacial and interior resistance for water transport we reveal that with rising stacking numbers COF multilayers exhibit increasing ion rejections at the expense of water permeance. More importantly, stacking in the offset eclipsed fashion significantly reduces the equivalent pore size of COF multilayers to 0.89 nm and ion rejection is correspondingly increased. Remarkably, 25 COF monolayers stacked in this fashion give 100% MgCl2 rejection while the water permeance remain 1 to 2 orders of magnitude higher than that of commercial nanofiltration membranes. This work demonstrates the rational design of fast membranes for desalination by tailoring stacking number and fashion of COF monolayers.

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Selective Swelling of Polysulfone/Poly(ethylene Glycol) Block Copolymer towards Fouling-Resistant Ultrafiltration Membranes

Hao Yang, Jiemei Zhou, Zhaogen Wang, Xiansong Shi, Yong Wang*, Chinese J. Chem. Eng. 2019

Fouling resistance of ultrafiltration (UF) membranes is critical for their long-term usages in terms of stable performance, so convenient approaches to prepare fouling-resistant membranes are always anticipated. Herein, we demonstrate the facile fabrication of antifouling polysulfone-block-poly(ethylene glycol) (PSF-b-PEG, SFEG) composite membranes. SFEG layer was coated onto macroporous supports and cavitated by immerging them in acetone/n-propanol following the mechanism of selective swelling induced pore generation. Thus-produced SFEG membranes possessed high permeance and excellent mechanical strength. Meanwhile, the structures and separation performances of the SFEG layers can be continuously tuned through simply changing swelling durations. More importantly, the hydrophilic PEG chains were spontaneously enriched onto the pore walls through swelling treatment, endowing intrinsic antifouling property to the SFEG membranes. Bovine serum albumin (BSA)/humic acid (HA) fouling tests proved the prominent fouling resistance of SFEG membranes, and the fouling resistance is expected to be long-standing because of the firm connection between PEG chains and PSF matrix by covalent bonding.

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How Pore Hydrophilicity Influences Water Permeability?

Fang Xu, Mingjie Wei, Xin Zhang, Yang Song, Wei Zhou, Yong Wang*, Research 2019, 2581241

Membrane separation is playing increasingly important role in providing clean water. Simulations predict that membrane pores with strong hydrophobicity produce ultrahigh water permeability as a result of low friction. However, experiments demonstrate that hydrophilic pores favor higher permeability. Herein we simulate water molecules transporting through interlayers of twodimensional nanosheets with various hydrophilicities using nonequilibrium molecular dynamics. We reveal that there is a threshold pressure drop (ΔPT), exceeding which stable water permeability appears. Strongly hydrophobic pores exhibit extremely high ΔPT, prohibiting the achievement of ultrahigh water permeability under the experimentally accessible pressures. Under pressures <ΔPT, water flows in hydrophobic pores in a running-stop mode because of alternative wetting and nonwetting, thus leading to significantly reduced permeability. We discover that hydrophilic modification to one surface of the nanosheet can remarkably reduce ΔPT by > 99%, indicating a promising strategy to experimentally realize ultrafast membranes.

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Tailoring TiO2 Membranes for Nanofiltration and Tight Ultrafiltration by Leveraging Molecular Layer Deposition and Crystallization

Shanshan Wu, Zhaogen Wang, Sen Xiong, Yong Wang*, J. Membr. Sci. 2019, 578, 149-155

As an advanced gas phase deposition technique, molecular layer deposition (MLD) has recently emerged as a precisely controllable method to fabricate organic-inorganic hybrid materials such as metalcones and hybrid films. Porous structures can be prepared by thermally removing the organic constituents of the metalcones deposited by MLD. Herein, we revealed that there is a crystallization-induced volume shrinkage during thermal treatment of metalcone films. By taking advantage of both the chemical degradation of organic moieties of metalcone films and the crystallization-induced volume shrinkage, we built microporous TiO2 layers on porous ceramic substrates and obtained nanofiltration and tight ultrafiltration membranes with tunable molecular-weight-cut-off (MWCO) in the range between 630 Da and 3050 Da. Firstly, 50 MLD cycles of titanicone (TiEG) were deposited on the near-surface of plate ceramic ultrafiltration membranes to achieve a compact layer, which was then subjected to calcination at different temperatures to remove the organic constituents. The bare membrane with a water permeance of ∼130 L m2 h1 bar1 and MWCO of ∼63 kDa was upgraded from ultrafiltration to nanofiltration with ∼7 L m2 h1 bar1 permeance and ∼630 Da MWCO after MLD and calcination at 250 °C. Moreover, as the microporous TiO2 was transformed from amorphous to crystalline with rising calcination temperatures, there was higher volume shrinkage and larger effective pore sizes. Consequently, both water permeance and MWCOs of the membranes were increased. For instance, the membranes after calcination at 400 °C showed a tight ultrafiltration performance of ∼30 L m2 h1 bar1 permeance and ∼3050 Da MWCO. This work provides a simple and effective way to fabricate ceramic membranes with tunable performances between ultrafiltration, tight ultrafiltration, and nanofiltration.

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Nanoporous Block Copolymer Membranes Immobilized with Gold Nanoparticles for Continuous Flow Catalysis

Jiemei Zhou, Chenxu Zhang, Yong Wang*, Polym. Chem. 2019, 10, 1642-1649

Nanoporous polymers with functionalizable surfaces are desired in the preparation of reactors for continuous flow catalysis. Here a facile approach for the fabrication of flow reactors is developed by immobilizing Au nanoparticles (AuNPs) to nanoporous membranes derived from block copolymers (BCPs). We synthesized BCP of poly(2-dimethylaminoethyl methacrylate) and polystyrene (PDMAEMA-b-PS) where the PDMAEMA minority blocks contain tertiary amine groups so as to anchor the Au precursors. Simply soaking the BCP in hot ethanol generated interconnected nanopores with PDMAEMA chains enriched on pore walls following the mechanism of selective swelling induced pore generation. The nanoporous BCP membranes were used as supports to immobilize AuNPs via in situ reduction of Au precursors, producing catalytic reactors with the amount of AuNPs conveniently tunable by changing the concentration of Au precursors. The nanoporous BCPs provided tortuous channels to host more AuNPs and to increase the contacting chance between reactants and AuNPs in flow reactions. Thus-produced AuNP-anchored BCP membranes were used as flow reactors to catalytically transform nitrophenol to aminophenol and a catalytic efficiency up to 100% was reached. We also demonstrated that the membranes were highly efficient in catalytic degradation of organic dyes.

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Two-Dimensional Superstructures Filled into Polysulfone Membranes for Highly Improved Ultrafiltration: The Case of Cuprous Iodide Nanosheets

Jian Zhou, Dan Sun, Lei Wang, Leiming Guo, Wei Chen, Fengjiao Yu, Yong Wang*, Yang Yang*, J. Membr. Sci. 2019, 576, 142-149

Mixed-matrix membranes blended with nanosized inorganic fillers have been widely used in ultrafiltration (UF) for large molecule separation. In this work, we provide a feasible route to construct mixed-matrix polysulfone (PSf) UF membranes via the nonsolvent-induced phase inversion process, utilizing sheet-like copper iodide superstructures (s-CuI) as fillers. The size of s-CuI in lateral dimension reaches micrometer scale, but the nanosheets can be well embedded in the polymer matrix with their 2D basal planes parallel to the membrane surface. On account of the stacking straight-through channel structure of s-CuI and its large dimension spanning across multiple membrane pores within the layer, the PSf membrane blended with 0.8 wt% content of s-CuI displays optimal UF performance with a pure water permeation (PWP) of 1473 L/(m2 h bar) and a bovine serum albumin (BSA) rejection of 92%. In contrast, the pure PSf membrane only presents a PWP of 556 L/(m2 h bar) and a BSA rejection of 58.2%. Besides, the enhancement of mechanical stability for the PSf/s-CuI 0.8% membrane is significant and the leakage of the s-CuI filler is negligible. Though the antifouling performance of the PSf/s-CuI 0.8% membrane against the protein BSA is limited, its flux is always 2–3 times higher than that of the pure PSf membrane in each step during a sequential test of fouling behavior. This study indicates that it is highly promising to employ designable superstructures as functional fillers to construct novel mixed-matrix membranes with good interfacial compatibility and superior performance.

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Growing Covalent Organic Frameworks on Porous Substrates for Molecule-Sieving Membranes with Pores Tunable from Ultra- to Nanofiltration

Xiansong Shi, Ankang Xiao, Chenxu Zhang, Yong Wang*, J. Membr. Sci. 2019, 576, 116-122

Crystalline imine-based covalent organic frameworks (COFs) have attracted great interest as next-generation water treatment materials owning to their numerous merits. The construction of COF layers with tunable pore sizes ranging from meso- to micropores, which can significantly broaden the membrane selectivity from ultra- (UF) to nanofiltration (NF), has remained unexplored. Herein, we demonstrate the controllable solvothermal synthesis of COF layers with adjustable pore sizes on anodic aluminum oxide (AAO) substrates. With rising synthesis duration the modulation on pore sizes of AAO supports can be achieved till the formation of continuous COF layers, leading to a gradually enhanced selectivity. Importantly, the synthesized microporous COFs shrink the pore sizes of AAO substrates and provide additional nanochannels for water transporting, eventuating in a ~2–9 times higher permeance than other reported UF membranes with comparable rejections. Once forming continuous COF layers, more rigid selectivities based on COF inherent nanochannels (~1.83 nm) can be realized with capacities of removing dyes from water or organic solvent. This work provides a facile methodology to construct COF-based membranes with broadly tunable separation performance, recommending the membranes for removing targeted molecules including proteins, nanoparticles, and dyes.

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Gradient Nanoporous Phenolics Filled in Macroporous Substrates for Highly Permeable Ultrafiltration

Qianqian Lan, Yazhi Yang, Leiming Guo, Yong Wang*, J. Membr. Sci. 2019, 576, 123-130

Highly permeable and cost-affordable membranes are highly desired in separation processes. Herein, such membranes are produced by designing gradient phenolics inside the macropores of polyvinylidene fluoride (PVDF) substrates. Solutions consisting of resol, poly(ethylene oxide)/poly(propylene oxide) copolymer and ZnCl2 are infiltrated into the macropores of PVDF. Phenolic nanoparticles and nanopores with increasing sizes are formed across the membrane thickness after fast solvent evaporation, ZnCl2-facilited resol polymerization and acid soaking. The prepared gradient phenolic@PVDF membranes exhibit a permeance up to 1315 L/(m2·h·bar), which is several times higher than that of other membranes with similar rejections to bovine serum albumin (91%) and lysozyme (71%). Moreover, the membranes are robust enough to withstand a pressure of at least 12.0 bar due to the interlaced structures of phenolics and PVDF. We demonstrate that thus-produced membranes can efficiently separate and concentrate gold nanoparticles with sizes down to 5 nm. This work suggests a new strategy for rational design of gradient porosities in macroporous substrates to produce robust membranes with high permeances and tight rejections.

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Synthesis of Poly(2-dimethylaminoethyl Methacrylate)-block-Poly(styrene-alt-N-phenylmaleimide) and its Thermo-Tolerant Nanoporous Films Prepared by Selective Swelling

Jiemei Zhou, Chenxu Zhang, Cailong Shen, Yong Wang*, Polymer 2019, 164, 126-133

Block copolymers (BCPs) are interesting precursors to nanoporous films with well-defined and tunable porosities. So far, BCPs with polystyrene (PS) as the majority block are predominantly used, hindering the using of produced nanoporous films at elevated temperatures. Herein, we propose to use BCPs with poly (styrene-alt-Nphenylmaleimide) (P(S-alt-NMI)) asthe majority blockto developthermallystable nanoporous films considering that this alternative copolymer has a much higher glass transition temperature than PS. We synthesized BCPs of poly (2-dimethylaminoethyl methacrylate) (PDMAEMA) and P(S-alt-NMI) (PDMAEMA-b-P(S-alt-NMI)) with four different block ratios by reversible addition-fragmentation chain transfer (RAFT) polymerization. The swelling behaviors of these BCPs in various solvents were investigated, and we found that nanoporosities will be generated in the BCP films following the mechanism of selective swelling induced pore generation when single solvents or solvent pairs having strong affinity to PDMAEMA and moderate affinity to P(S-alt-NMI) were used as the swelling agents. The pore sizes and porosities of thus-produced nanoporous films can be flexibly tuned by changing the compositions of the BCPs and the swelling conditions. Because of the excellent thermal stability of P(S-alt-NMI) blocks the nanoporous BCP films exhibited excellent thermal tolerance and they maintained intact at a temperature as high as 158°C while the PS-based BCPs started to deform at the temperature of 75°C. This thermal tolerance is very much desired in many applications, for example, high-temperature disinfection for medical purposes.

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Atomic Layer Deposition of TiO2 on Carbon-Nanotube Membranes for Enhanced Capacitive Deionization

Jianhua Feng, Sen Xiong, Yong Wang*, Sep. Purif. Technol. 2019, 213, 70-77

Capacitive deionization (CDI) is an energy-efficient and environment-benign process to produce fresh water. Carbon nanotubes (CNTs) are promising building blocks in constructing high-performance CDI electrodes. Nevertheless, the strong hydrophobicity of CNTs significantly impedes their applications in aqueous environment. Herein, hydrophilic CNT-based membrane electrodes are obtained via atomic layer deposition (ALD). We demonstrate, for the first time, that ALD is an efficient and flexible means of enhancing the CDI performance of free-standing CNT membrane electrodes by depositing TiO2 nanoparticles on the surface of CNTs. The CNT membranes display preferable electrosorption behavior after moderate ALD cycle numbers and stable reusability in the desalination process.

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Design of Gradient Nanopores in Phenolics for Ultrafast Water Permeation

Leiming Guo, Yazhi Yang, Fang Xu, Qianqian Lan, Mingjie Wei, Yong Wang*, Chem. Sci. 2019, 10, 2093-2100

Membrane technology is playing a pivotal role in providing potable water to our thirsty planet. However, it remains strongly demanding for highly permeable and durable membranes with affordable costs. Such membranes are synthesized herein by designing gradient nanopores in low-cost phenolics. The gradient nanopores are achieved by spontaneous assembly of phenolic nanoparticles with gradually enlarged sizes. These particles nucleate and grow as a result of ZnCl2-accelerated thermopolymerization of resol in the progressive downward gelating polymer. Subsequent removal of the gelated polymer and ZnCl2 exposes the gradient nanopore. The gradient nanopores renders the phenolic structures unprecedented permselectivity when used in membrane separations, totally rejecting fine particulates down to 5 nm dispersed in water or aggressive solvents while allowing water to permeate up to two orders of magnitude faster than other membranes with similar rejections. Our work opens up an avenue for the rational design and affordable synthesis of ultrafast membranes.

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Atomic Layer Deposition of Hybrid Metal Oxides on Carbon Nanotube Membranes for Photodegradation of Dyes

Jianhua Feng, Sen Xiong*, Yong Wang*, Compos. Commun. 2019, 12, 39-46

Synthetic dye pollution is a worldwide problem and quick remedies are urgently needed. Photocatalysis is a promising method to solve this problem and carbon nanotubes (CNTs) are promising components in producing high-performance composite photocatalysts. Nevertheless, the strong hydrophobicity dramatically impedes its application in aqueous environments. In this study, hydrophilic CNT-hybrid metal oxides (ZnO and TiO2) membranes were prepared by atomic layer deposition (ALD). We demonstrate that ALD is an efficient and flexible method to enhance the photocatalytic activity of CNT-based membranes, especially the membranes used in wastewater treatments. The hybrid hexagonal wurtzite ZnO and anatase TiO2 on CNTs after calcination could effectively enhance electron transfer and reduce photo-generated electron-holes recombination. The membranes exhibit preferable photocatalytic activity and stable reusability in dye degradation. This strategy of “ALD on CNTs” is expected to create other CNT-based membranes with additional functionalities and has bright prospect for wastewater treatments.

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2018

耐压型聚砜两亲嵌段共聚物超滤膜的制备与表征

陈威,王召根*,汪勇*,膜科学与技术 2018,38,27-33

聚砜类分离膜广泛应用于超滤过程并作为支撑层用于纳滤膜及反渗透膜的制备,然而由于材料自身的疏水性较强,聚砜膜通常要经过亲水化改性来提升渗透和抗污染性能。以两亲嵌段共聚物聚砜-b-聚乙二醇(PSF-b-PEG)为原料,通过相转化方法可无需后期改性制备具有指状孔结构的自发永久亲水性超滤膜,但指状孔的存在使其耐压性能受到限制,难以用作为纳滤或反渗透膜的支撑层。本文采用PSF-b-PEG为制膜材料,通过调节固含量与非溶剂诱导相分离过程中的溶剂/非溶剂体系,将超滤膜断面的指状孔转变为更为耐压的海绵状孔,同时在膜内保留亲水链段以保证膜的渗透及抗污染性能。研究结果表明,当以N,N-二甲基甲酰胺(DMF)为溶剂,去离子水为凝固浴时,可直接制备出具有海绵状孔结构的PSF-b-PEG超滤膜。在25%的固含量下,膜的纯水渗透性为160 L·m-2·h-1·bar-1,对牛血清白蛋白(BSA)截留率达到90%。压密测试表明,相较于指状孔,海绵状孔超滤膜可完全耐受1 MPa的压力,展示了更优的耐压性能。

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Layer-by-Layer Synthesis of Covalent Organic Frameworks on Porous Substrates for Fast Molecular Separations

Xiansong Shi, Rui Wang, Ankang Xiao, Tianzhi Jia, Shipeng Sun, Yong Wang*, ACS Appl. Nano Mater. 2018, 1, 6320-6326

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.

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Highly Permeable Membranes Enabled by Film Formation of Block Copolymers on Water Surface

Nina Yan, Zhaogen Wang*, Yong Wang*, J. Membr. Sci. 2018, 568, 40-46

Separation membranes derived from block copolymers (BCPs) have attracted significant interest due to their well-defined pores and functional surfaces. However, it still remains a challenge to prepare ultrathin BCP composite membranes in an efficient way. Here we report a facile approach for the fabrication of ultrathin BCP composite membranes by forming thin films on water surface. The BCP layer as thin as ~ 17 nm can be prepared by spreading polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) solutions on water surface followed by controlled evaporation. The thin layer can be readily collected and composited with macroporous supports. A thickness of ~ 260 nm of the BCP layer is necessary to guarantee the integrity. Interconnected nanoporous structures are then obtained in the BCP layer by the process of selective swelling induced pore generation. On account of the ultrathin separation layer, the composite membranes exhibit excellent permselectivity, better than many other ultrafiltration membranes. Moreover, the separation performances can be facilely regulated by tuning the swelling durations. This method is featured as cost-effective and convenient, and is expected to provide a higher chance for the production of BCP membranes at large scale.

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Retarded Evaporation-Induced Synthesis of Lamellar Block Copolymer Supramolecules for Solvatochromic Sensing

Leiming Guo, Yong Wang*, Sens. Actuators B: Chem. 2018, 277, 172-178

The responses of conventional solvatochromic sensors to stimuli are usually performed by generating changes in absorption and emission bands based on the polarity of solvents. However, there still remains a high risk of the overlapped bands for the solvents with similar polarities, resulting in a tough realization of the visual colorimetric differentiation. Here, we develop an innovative solvatochromic sensor in the form of film by self-assembly of block copolymer supramolecules. The produced sensor consists of alternating layers of two phases, in which one is polystyrene (PS) and the other is the combination of poly (4-vinyl pyridine) (P4 V P) and polydiacetylene (PDA). Owing to the fact that PDA is embedded into P4 V P, the inherent protection of PDA by block copolymers is thus guaranteed in such lamellar structures. The specific organic solvents with a strong affinity to PS could merely penetrate into the supramolecular lamellae followed by interacting with thus-exposed PDA, leading to the realization of solvatochromism. By incorporating the heating-induced blue-to-red color transition (thermochromism) with solvatochromism of the sensor, a straightforward technique to realize the nonvisualization of an imprint followed by its revisualization is developed.

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Interfacial Polymerization of Covalent Organic Frameworks (COFs) on Polymeric Substrates for Molecular Separations

Rui Wang, Xiansong Shi, Ankang Xiao, Wei Zhou, Yong Wang*, J. Membr. Sci. 2018, 566, 197-204

Covalent organic frameworks (COFs) represent a new family of porous polymers with highly ordered two or three-dimensional channels. Although numerous studies have been focused on the design and synthesis of COF in the form of powders, the development of COF-based separation membranes is still hampered by the challenges of COF particles agglomeration and harsh synthetic conditions. In this work, interfacial polymerization (IP) directly performed on polymeric substrates as employed in the traditional IP process of polyamide (PA) membranes is developed for the synthesis of COF-based membranes. With the moderate reaction rate between monomer pairs in corresponding aqueous and organic solutions, a conformal growth of COF crystallites directly composited with the polysulfone (PSF) ultrafiltration substrates can be realized within 1 min. The synthesis parameters including reaction time and precursor concentrations are optimized, and thus-synthesized COF/PSF membrane presents a stable rejection to dye (Congo red) of 99.5% with a high water permeance of up to 50 L·m−2·h−1·bar−1, which is 2–10 times higher than that of many other membranes with similar rejection. This convenient IP process is expected to facilitate the up-scaling and real-world applications of COF-based membranes.

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Perpendicular Alignment and Selective Swelling-Induced Generation of Homopores of Polystyrene-b-Poly(2-vinylpyridine)-b-Poly(ethylene Oxide) Triblock Terpolymer

Leiming Guo, Zhaogen Wang, Yong Wang*, Macromolecules 2018, 51, 6248-6256

Arrays of perpendicularly ordered nanopores with pore walls decorated by two or more functional polymer chains are of great importance in various applications. However, such porous structures have been rarely reported so far. Herein, lamellar-forming triblock terpolymer of polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) is blended with PS homopolymers to fabricate a PS matrix embedded with perpendicular PEO@P2VP cylinders via solvent annealing. By further adjusting the annealing humidity and temperature, the perpendicular PEO@P2VP cylinders are capable of running through the entire films. The hydrophilic PEO@P2VP domains could be converted into cylindrical homopores by ethanol swelling, where the functional P2VP and PEO chains are distributed on the pore walls. Moreover, water is able to swell PEO and also produces homopores in such films. The pore sizes of the films are readily tuned in a relatively large range depending on the swelling temperatures and durations. The developed ordered nanostructures are expected to find important applications in the range from template synthesis to membrane separation and to nanofluidics.

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Water Flow through Interlayer Channels of 2D Materials with Various Hydrophilicities

Fang Xu, Yang Song, Mingjie Wei*, Yong Wang*, J. Phys. Chem. C 2018, 122, 15772-15779

Water transport through laminated membranes composed of two-dimensional (2D) materials has gained considerable attention because of their great potential in filtration and separation applications. However, water transport between interlayers formed by 2D materials cannot be comprehensively described by traditional transport theory because different 2D materials have varying hydrophilicity, which strongly affects water transport. Herein, we build interlayer channels formed by 2D sheets with various hydrophilicities and investigate the pressure-driven water transport via nonequilibrium molecular dynamics simulations. The influence of channel hydrophilicity on water transport is dominant, especially at the nanoscale. To model the water transport phenomena through channels with various hydrophilicities, we define a new slip length to derive the appropriate equation. By two methods of calculating the slip length based on the simulation results, we validate our derived equation, which predicts the water flux in interlayer channels with a large range of hydrophilicities from relatively hydrophobic (large slip lengths) to extremely hydrophilic (negative slip lengths). This work deepens the understanding of water transport through interlayer channels and assists in the design of 2D-material membranes for water treatment.

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Resistance of Water Transport in Carbon Nanotube Membranes

Xin Zhang, Wei Zhou, Fang Xu, Mingjie Wei*, Yong Wang*, Nanoscale 2018, 10, 13242-13249

Carbon nanotube (CNT) membranes have long been considered as the next-generation membranes due to superfast water transport inside the tubes. However, large pressure loss occurs at the pore mouth and consequently water transport through the whole tubes is significantly retarded. To find out the reason behind, we conduct systematic non-equilibrium molecular dynamics (NEMD) simulations on water transport through CNT membranes with various tube diameters and lengths. The whole transport resistance is contributed by the interfacial and interior part, and the interfacial contribution is playing the dominating role for short tubes and only can be ignored when the tube length reaches the scale of several micrometers. Regard to the origin of the interfacial resistance, the hydrogen bonding rearrangement (HBR) effect accounts for at least 45%, and the rest is attributed to the geometrical or steric crowding of water molecules near the pore mouth. To reduce the dominant interfacial resistance, we change the shape of pore mouth from plate to hourglass by mimicing the aquaporin water channels. The interfacial resistance is thus decreased by > 27%. It is also found that the reduction is originated from the optimized HBR rather than the subdued steric crowding of water molecules near the pore mouth.

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Selective Swelling of Block Copolymer Ultrafiltration Membranes for Enhanced Water Permeability and Fouling Resistance

Dinglei Zhong, Zhaogen Wang, Qianqian Lan, Yong Wang*, J. Membr. Sci. 2018, 558, 106-112

High permeability is one of the most important pursuits of separation membranes. In this work, high-performance ultrafiltration membranes are prepared by synergetically coupling nonsolvent-induced phase separation (NIPS) and selective swelling of a block copolymer, polysulfone-block-poly (ethylene glycol) (PSF-b-PEG). NIPS is used to prepare PSF-b-PEG membranes with a thin skin layer and a fingerlike sublayer. Subsequent selective swelling generates mesopores in the skin layer and enriches PEG blocks on the surface. Compared to the membranes without swelling, the swelling-treated membranes exhibit simultaneously upgraded permeability, hydrophilicity, and fouling resistance. For instance, the permeability of the membrane swollen in 50% acetic acid at 65 °C for 1 h is doubled compared to the pristine one while the rejection is only modestly reduced. Fouling resistance of the swelling-treated membranes was also improved, which was ascribed to the enrichment of PEG on the membrane surface. We demonstrate that the performances of the membranes can be tuned in a relatively wide range by tailoring the swelling conditions, such as swelling reagents, durations and temperatures. Because of the extreme simplicity and high efficiency, this selective swelling strategy is expected to be applicable in tuning the surface properties and improving performances of many other membranes.

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Mesoporous Phenolics Filled in Macroporous Membranes for Tunable Tight-Ultrafiltration

Qianqian Lan, Zhaogen Wang, Yong Wang*, Chem. Eng. Sci. 2018, 187, 98-106

Separation of colloidal nanoparticulates requires tight ultrafiltration (TUF) membranes. Phenolic polymers with well-defined sub-10 nm mesopores are promising candidates for such membranes, but suffer from intrinsic fragility. Herein, robust mesoporous phenolic membranes with flexibly adjustable TUF functions are realized through a new pore-filling strategy. Phenolic prepolymers with tunable fluidity are spontaneously filled into macroporous substrates with precisely controllable filling depths. Subsequent thermopolymerization fully cures the prepolymer and acid soaking removes the templating pluronic copolymers, producing composite TUF membranes with mesoporous phenolics embedded in flexible substrates. The membranes exhibit widely adjustable molecular-weight-cut-offs (MWCOs) (2.6 to 41 kg/mol) depending on the filling depth of phenolics while maintain otherwise unattainable high permeabilities. We further investigate the separation of CdTe and carbon quantum dots dispersed in water and toluene by these membranes, and they show excellent concentration and fractionation effect. This work enables efficient separation of ultra-small colloids by robust TUF membranes.

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(Cover) Polymeric Nanospheres with Tunable Sizes, Water Dispersibility and Thermostability from Heating-Enabled Micellization of Polysulfone-block-Polyethylene Glycol

Zexian Zhang, Jie Li, Zhaogen Wang, Changcheng He*, Yong Wang*, J. Polym. Sci. B: Polym. Phys. 2018, 56, 769-777

Polymeric nanospheres with uniform sizes, functional surfaces, and high mechanical strength and thermostability are attracting wide interest in different applications. Here, a new kind of polysulfone micellar spheres with PEGylated surfaces is prepared via directly heating the solution of an amphiphilic block copolymer, polysulfone-b-polyethylene glycol (PSF-b-PEG). The sizes of the micelles are uniform and tunable between ∼42 and ∼443 nm. TEM characterizations show that the micelles are core-shell structures with PEG as the corona and PSF as the core. PEG endows the micelles with dispersibility in water and good biocompatibility, while PSF provides the mechanical strength and thermostability. The effects of PEG contents, polymer solution concentrations, solvent types, and heating temperatures are systematically investigated. Furthermore, heat resistance tests show that the micelles are stable at 150–180 °C. These PSF-b-PEG micellar spheres are expected to be applied in demanding environmental conditions such as heating involved surface modification process.

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Selective Swelling of Electrospun Block Copolymers: From Perforated Nanofibers to High Flux and Responsive Ultrafiltration Membranes

Xiansong Shi, Zhi Xu, Chaobo Huang, Yong Wang*, Zhanfeng Cui, Macromolecules 2018, 51, 2283-2292

This work is devoted to the development of high-flux ultrafiltration membranes using electrospun nanofibers of amphiphilic block copolymers (BCPs) of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) as building blocks. When soaked in hot ethanol, the solid as-spun BCP fibers are progressively transformed into three-dimensionally perforated fibers with increasing porosities with rising degrees of swelling, which ended up with the equilibrated morphology of spherical micelles. The BCP nanofibers are collected on macroporous substrates and subjected to heating to convert loosely stacked fibers to dense and continuous films. Subsequent swelling in hot ethanol leads to robust composite membranes with nanoporous BCP selective layers tightly adhered to the substrates. Filtration performances of the composite membranes can be conveniently modulated by electrospinning durations. The water permeabilities are as high as 6100 L m–2 h–1 bar–1, which is ∼10–35 times higher than that of commercial membranes with similar rejections. Moreover, with the surface enrichment of P2VP chains the membranes exhibit a strikingly sharp pH-dependent water permeability switchable in the largest amplitude ever reported for multiple cycles. Electrospun fibers can be promising building materials to produce a wide range of membranes with 3D interconnected nanoporosities which also show great potential in separation and biomedical applications.

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Atomic Layer Deposition Fabricating of Ceramic Nanofiltration Membranes for Efficient Separation of Dyes from Water

He Chen, Shanshan Wu, Xiaojuan Jia, Sen Xiong, Yong Wang*, AIChE J. 2018, 64, 2670-2678

To meet the strong need for highly efficient and controllable manufacturing methods to ceramic nanofiltration (NF) membranes, we use atomic layer deposition (ALD) to prepare NF membranes by tightening ultrafiltration (UF) membranes. We confine the ALD deposition of TiO2 to the near-surface region of substrate UF membranes. The pores (∼ 5 nm) in the selective layers are progressively reduced, thus transforming the original UF membranes to NF ones at acceptable expense of permeability. A molecular-weight-cut-off (MWCO) down to 890 Da is obtained after merely 40 cycles and the water permeability remains as high as 32 L·m−2·h−1·bar−1. MWCOs can be flexibly tuned by altering ALD cycles, which is lowered down to 410 Da with 60 ALD cycles. The deposited membranes are used to remove dyes from water and they show significantly enhanced rejection to negatively charged dyes.

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One-Step Synthesis of Carbon-Hybridized ZnO on Polymeric Foams by Atomic Layer Deposition for Efficient Absorption of Oils from Water

Sen Xiong, Yang Yang, Zhaoxiang Zhong, Yong Wang*, Ind. Eng. Chem. Res. 2018, 57, 1269-1276

Reusable absorbents with high absorption capacity, good selectivity, and low cost are highly demanded for efficient removal of oil spills and organic leakages from water. We discover that ZnO produced by atomic layer deposition (ALD) is hybridized with carbon moieties, and consequently exhibits an appreciable hydrophobicity. By taking this advantage, we develop a “dry” process to produce high-performance oil absorbents. Originally amphiphilic polymeric foams are directly turned into strongly hydrophobic oil absorbents simply by ALD of ZnO and exhibit excellent absorption capacity, selectivity and reusability. Such absorption capacities and reusability are better than most foam-based oil absorbents. Surprisingly, we find that ALD-deposited ZnO can reverse the wettability of polymeric foams depending on the foam porosities. This ALD-enabled surface functionalization is expected to find important applications in the synthesis of many other interfacial materials.

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Atomic Layer Deposition of Metal Oxides on Carbon Nanotube Fabrics for Robust, Hydrophilic Ultrafiltration Membranes

Jianhua Feng, Sen Xiong, Zhaogen Wang, Zhaoliang Cui, Shi-Peng Sun, Yong Wang*, J. Membr. Sci. 2018, 550, 246-253

Carbon nanotubes (CNTs) are important building blocks to produce high-performance membranes. However, the strong hydrophobicity significantly hinders their applications in aqueous systems. Herein, we demonstrate, for the first time, that atomic layer deposition (ALD) is an efficient and flexible method to upgrade the permselectivity of CNT-based membranes. We ALD-deposit ZnO on fabrics of multi-walled carbon nanotubes. ZnO grows on the CNT surface as nanoparticulates initially and then forms conformal layers wrapping the CNTs. The originally hydrophobic surface of CNTs is progressively turned to be highly hydrophilic with rising ALD cycles. The deposition of ZnO on CNTs significantly promotes the surface wettability of the CNT membranes on one hand, and endows the membrane an enhanced mechanical stability on the other. The CNT membranes exhibit simultaneously upgraded water permeability and retention in the ultrafiltration category after ALD for moderate cycle numbers. With ever increased ALD cycles, the retention is further improved while the water permeability is decreased due to the competing effect of the increased hydrophilicity and narrowed pores. The strategy of “ALD on CNT substrates” is expected to produce other robust membranes with additional functionalities dependent on the materials to be deposited by ALD.

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Atomic Layer Deposition of Al2O3 on Porous Polypropylene Hollow Fibers for Enhanced Membrane Performances

Xiaojuan Jia, Zexian Low, He Chen, Sen Xiong, Yong Wang*, Chinese J. Chem. Eng. 2018, 26, 695-700

Porous polypropylene hollow fiber (PPHF) membranes are widely used in liquid purification. However, the hydrophobicity of polypropylene (PP) has limited its applications in water treatment. Herein, we demonstrate that, for the first time, atomic layer deposition (ALD) is an effective strategy to conveniently upgrade the filtration performances of PPHF membranes. The chemical and morphological changes of the deposited PPHF membranes are characterized by spectral, compositional, microscopic characterizations and protein adsorption measurements. Al2O3 is distributed along the cross section of the PP hollow fibers, with decreasing concentration from the outer surface to the inner surface. The pore size of the outer surface can be easily turned by altering the ALD cycles. Interestingly, the hollow fibers become much more ductile after deposition as their elongation at break is increased more than six times after deposition with 100 cycles. The deposited membranes show simultaneously enhanced water permeance and retention after deposition with moderate ALD cycle numbers. For instance, after 50 ALD cycles a 17% increase in water permeance and one-fold increase in BSA rejection are observed. Moreover, the PP membranes exhibit improved fouling-resistance after ALD deposition.

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2017

Nanofriction of Graphene/Ionic Liquid-Infused Block Copolymer Homoporous Membranes

Rong An*, Pengpeng Fan, Nina Yan, Qingmin Ji, Goel Sunkulp, Yong Wang*, Langmuir 2017, 33, 11590-11602

We have infused graphene/ionic liquid into block copolymer homoporous membranes (HOMEs) which have highly ordered uniform cylindrical nanopores to, form compact, dense and continuous Gr/ILs (graphene/ionic liquid) lubricating layers at interfaces, enabling a reduction in the friction coefficient. Raman and XPS analyses, confirmed the parallel alignment of the cation of ILs on graphene by the π-π stacking interaction of the imidazolium ring with the graphene layer. This alignment loosens the lattice spacing of Gr in Gr/ILs, leading to a larger lattice spacing of 0.36 nm in Gr of Gr/ILs hybrids than the pristine Gr (0.33 nm). The loose graphene layers which is caused by the coexistence of graphene and ILs, would make the sliding easier, and favor the lubrication. An increase in the friction coefficient was observed on ILs-infused block copolymer HOMEs, as compared to Gr/ILs-infused ones, due to the absence of Gr and the unstably formed ILs film. Gr/ILs-infused block copolymer HOMEs also exhibit much smaller residual indentation depth and peak indentation depth in comparison with ILs-infused ones. This indicates the existence of stably supported Gr/ILs hybrid liquid films aids the reduction of the friction coefficient by preventing the thinning of the lubricant layer and exposure of the underlying block copolymer HOMEs.

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Selective Swelling Blends of Block Copolymers for Nanoporous Membranes with Enhanced Permeability and Robustness

Zhaogen Wang, Rui Liu, Qianqian Lan, Yong Wang*, J. Polym. Sci. B: Polym. Phys. 2017, 55, 1617-1625

Block copolymers (BCPs) are important precursors to produce membranes with well-defined porosities. However, it remains challenging to prepare robust and affordable BCP-based membranes. In this work, cheap commodity styrene-butadiene-styrene (SBS) elastic triblock copolymers are mixed with polystyrene-block-poly (2-vinylpyridine) (SV) block copolymers in solutions, leading to macroscopically stable blend films upon casting because of the compatibilizer effect of PS existing in both copolymers. By soaking the blend films in ethanol, the microdomains of poly(2-vinylpyridine) are selectively swollen and cavitated upon drying, resulting in a hierarchical structure with perforated SV phases interwoven with the SBS phases. The blend membranes with 30% SBS exhibit improved water permeability and mechanical robustness due to the presence of elastic SBS compared to neat SV membranes; meanwhile, the rejections of the blend membranes remain largely unchanged. Moreover, the blend membranes exhibit a pH-responsive function, and homoporous SV regions are obtained by pre-aligning the SV phases.

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Antifouling Ultrafiltration Membranes by Selective Swelling of Polystyrene/Poly(ethylene Oxide) Block Copolymers

Hao Yang, Zhaogen Wang, Qianqian Lan, Yong Wang*, J. Membr. Sci. 2017, 542, 226-232

Ultrafiltration (UF) membranes with high permeability and good fouling resistance hold great promise in water treatment and many industry sectors. Herein, antifouling UF composite membranes with mesoporous films of polystyrene (PS)/poly (ethylene oxide) (PEO) block copolymer as the selective layers were produced by the process of selective-swelling-induced pore generation. We spin-coated thin layers of block copolymer onto macroporous supports, and activated the copolymer layers by soaking them in hot ethanol to induce the selective swelling of the PEO microdomains. The porous structures of the block copolymer selective layers can be continuously modulated simply by varying the swelling durations, resulting in UF membranes with water permeabilities adjustable in the range of ~ 100–600 L m-2 h-1 bar-1. Interestingly, the hydrophilic PEO blocks were enriched on the pore walls after swelling, rendering an inherent fouling resistance to the membranes. Protein fouling tests demonstrated that the membrane exhibited exceptional fouling resistance with a recover ratio in water flux of nearly 100%. This fouling resistance is expected to be long-standing as the PEO chains are covalently bonded to the membrane matrix.

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Nanoporous Polysulfones with In Situ PEGylated Surfaces by A Simple Swelling Strategy Using Paired Solvents

Zhaogen Wang, Rui Liu, Hao Yang, Yong Wang*, Chem. Commun. 2017, 53, 9105-9108

We report the formation of three-dimensionally interconnected nanoporosities with in situ PEGylated pore walls simply by swelling the block copolymers of polysulfone and poly(ethylene glycol) in paired solvents. The produced nanoporous polymers are expected to find important applications in membranes, batteries, chromatography, and haemodialysis, etc.

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Substrate Matters: The Influences of Substrate Layers on the Performances of Thin-Film Composite Reverse Osmosis Membranes

Jie Li, Mingjie Wei*, Yong Wang*, Chinese J. Chem. Eng. 2017, 25, 1676-1684

Thin-film composite (TFC) reverse osmosis (RO) membranes are playing the dominating role in desalination. Tremendous efforts have been put in the studies on the polyamide selective layers. However, the effect of the substrate layers is far less concerned. In this review, we summarize the works that considering the impacts of the substrates, including pore sizes, surface hydrophilicity, on the processes of interfacial polymerization and consequently on the morphologies of the active layers and on final RO performances of the composite membranes. All the works indicate that the pore sizes and surface hydrophilicity of the substrate evidently influence the RO performances of the composites membranes. Unfortunately, we find that the observations and understandings on the substrate effect are frequently varied from case to case because of the lack of substrates with uniform pores and surface chemistries. We suggest using track-etched membranes or anodized alumina membranes having relatively uniform pores and functionalizable pore walls as model substrates to elucidate the substrate effect. Moreover, we argue that homoporous membranes derived from block copolymers have the potential to be used as substrates for the large-scale production of high-performances TFC RO membranes.

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Advanced SERS Sensor Based on Capillarity-Assisted Preconcentration through Gold Nanoparticle-Decorated Porous Nanorods

Longjian Xue*, Wei Xie, Leonie Driessen, Katrin F. Domke, Yong Wang*, Sebastian Schlücker, Stanislav N. Gorb, Martin Steinhart*, Small 2017, 13, 1603947

A preconcentrating surface-enhanced Raman scattering (SERS) sensor for the analysis of liquid-soaked tissue, tiny liquid droplets and thin liquid films without the necessity to collect the analyte is reported. The SERS sensor is based on a block-copolymer membrane containing a spongy-continuous pore system. The sensor's upper side is an array of porous nanorods having tips functionalized with Au nanoparticles. Capillarity in combination with directional evaporation drives the analyte solution in contact with the flat yet nanoporous underside of the SERS sensor through the continuous nanopore system toward the nanorod tips where non-volatile components of the analyte solution precipitate at the Au nanoparticles. The nanorod architecture increases the sensor surface in the detection volume and facilitates analyte preconcentration driven by directional solvent evaporation. The model analyte 5,5′-dithiobis(2-nitrobenzoic acid) can be detected in a 1 × 10−3m solution ≈300 ms after the sensor is brought into contact with the solution. Moreover, a sensitivity of 0.1 ppm for the detection of the dissolved model analyte is achieved.

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Atomic-Layer-Deposition-Enabled Thin-Film Composite Membranes of Polyimide Supported on Nanoporous Anodized Alumina

Hanghua Wang, Mingjie Wei, Zhaoxiang Zhong, Yong Wang*, J. Membr. Sci. 2017, 535, 56-62

Polyimide is deposited on the surface of nanoporous anodized alumina by atomic layer deposition (ALD) using pyromellitic dianhydride and ethylenediamine as the two precursors. Such ALD reactions directly produce polyimide rather than the intermediate polyamic acid. The precursor exposure duration plays an important role in determining the growth rate of PI and consequently the reduction of effective pore sizes. ALD with short precursor exposure confines the deposition predominantly around the pore openings of the alumina substrates, producing an asymmetric structure in the form of thin-film composites. This structure efficiently reduces the pore sizes of alumina, and as a result the retention of the membrane is significantly improved. A moderate cycle number of 50 remarkably increases the rejection of the membrane from nearly none to 82% at an acceptable expense of a reduction of 63% in water permeability. Deposition with long precursor exposure results in uniform coating into deep pores of the membranes whereas the effect in tailoring pore sizes and filtration performances are not as pronounced as deposition with short precursor exposure. This work is expected to be adopted to fabricate highly permeable and selective membranes of different materials by leveraging the deposition behavior of ALD on appropriate substrates.

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Highly Permeable Nanoporous Block Copolymer Membranes by Machinecasting on Nonwoven Supports: An Upscalable Route

Xiansong Shi, Zhaogen Wang, Yong Wang*, J. Membr. Sci. 2017, 533, 201-209

Block copolymer (BCP) membranes are distinguished for their well-defined porosities, tunable pore geometries, and functionable pore walls. However, it remains challenging to produce robust BCP membranes by affordable, convenient methods. Herein, we demonstrate a facile and easily upscalable approach to produce highly permeable BCP membranes in large areas. The membranes possess a bi-layered composite structure with nanoporous polystyrene-block-poly(2-vinylpyrdine) BCP layers directly supported on macroporous nonwoven substrates. The BCP layers are machine-cast on the water-prefilled nonwoven, and interconnected nanoporosities are created in the BCP layers by ethanol swelling. The nanoporous BCP layers exhibit a thickness of ~10 µm and are tightly adhered to the nonwoven. Changes in the swelling temperatures and durations modulate both pore sizes and surface hydrophilicity of the BCP layers, and consequently the permselectivity of the membranes. By increasing swelling duration from 15 min to 12 h, the permeability of the membrane swollen at 65 °C can be increased from ~100 to ~850 L m−2 h−1 bar−1 with the retention to 15-nm gold nanoparticles reduced from ~93% to ~54%. Moreover, we demonstrate that the composite membrane can efficiently fractionate nanoparticles and narrow down their size distribution from ~3–20 nm to ~3–10 nm.

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Tight Ultrafiltration Membranes of Mesoporous Phenolic Resin Filled in Macroporous Substrates

Qianqian Lan, Nina Yan, Yong Wang*, J. Membr. Sci. 2017, 533, 96-102

Mesoporous polymers derived from supramolecules of phenolic resins (PRs) and block copolymers (BCPs) containing highly uniform pores with sizes down to a few nanometers, are expected to deliver promising membrane separation performances. Here we report on the preparation of mesoporous phenolic membranes exhibiting tight ultrafiltration properties through a pore-filling strategy. Solutions of PR/BCP supramolecules are filled into the macropores of polyvinylidene fluoride (PVDF) microfiltration membranes (substrates), followed by thermopolymerization to solidify the solution in the pores. Subsequently, the filled PVDF substrates are treated in hot H2SO4 to remove the BCP components, thus producing mesopores in the PR framework. The produced composite membranes are mechanical robust and ductile as mesoporous phenolic resins are tightly embedded in the pores of the PVDF matrix. The mesoscale porosity in the PR phases endow the composite membrane a tight ultrafiltration performance with the molecular-weight-cut-off (MWCO) down to 2350 Da. The separation properties can be tuned simply by adjusting the dosage of supramolecule solutions used to fill the macroporous substrates. Considering the tight and uniform pore sizes and the robustness of phenolic resins, this strategy opens a new avenue to ultrafiltration membranes with low MWCOs or even nanofiltration membranes.

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Ceramic Tubular Nanofiltration Membranes with Tunable Performances by Atomic Layer Deposition and Calcination

He Chen, Xiaojuan Jia, Mingjie Wei, Yong Wang*, J. Membr. Sci. 2017, 528, 95-102

Ceramic nanofiltration (NF) membranes are of particular significance for molecular separations under harsh conditions. However, they are usually manufactured by the sol-gel process which frequently suffers from low efficiency and poor control in the membrane properties. Herein we demonstrate an efficient and more controllable strategy to produce ceramic tubular NF membranes based on atomic layer deposition (ALD). Tubular ceramic membranes with pore size of ~5 nm are used as the substrates, on which titanium alkoxide (titanicone) is ALD-deposited. Subsequent calcination in air degrades the organic moieties in titanicone, thus converting the dense layer of titanicone into a microporous layer of TiO2. This microporous TiO2 layer serves as a thin separation layer delivering the NF size-sieving function. The thickness of the TiO2 layer can be readily tuned by changing the ALD cycle numbers, and correspondingly the original substrate membranes are progressively tightened with rising ALD cycles, and the membrane with 300 cycles exhibits a molecular-weight-cut-off (MWCO) of ~680 Da and water permeability of 30 L m−2 h−1 bar−1. Such a water permeability is higher than many other ceramic tubular membranes with similar MWCOs because of the ultrathin nature of the microporous TiO2 layer established by titanicone deposition and calcination.

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Water Flow Inside Polyamide Reverse Osmosis Membranes: A Non-Equilibrium Molecular Dynamics Study

Yang Song, Fang Xu, Mingjie Wei*, Yong Wang*, J. Phys. Chem. B 2017, 121, 1715-1722

Water flow inside polyamide (PA) reverse osmosis (RO) membranes is studied by non-equilibrium molecular dynamics (NEMD) simulations in this work. The PA RO membrane is constructed with the all-atom model and the density and average pore size obtained thereby are in consistent with the latest experimental results. To obtain the time-independent water flux, a steady state NEMD method is used under various pressure drops. The water flux in our simulations, which is calculated under higher pressure drops, is in a linear relation with the pressure drops. Hence, the water flux in lower pressure drops can be reliably estimated, which could be compared with the experimental results. The molecular details of water flowing inside the membrane are considered. The radial distribution function and residence time of water around various groups of polyamide are introduced to analyze the water velocities around these groups, and we find that water molecules flow faster around benzene rings than around carboxyl or amino groups in the membrane, which implies that the main resistance of mass transport of water molecules comes from the carboxyl or amino groups inside the membranes. This finding is in good consistence with experimental results and suggests that less free carboxyl or amino groups should be generated inside RO membranes to enhance water permeance.

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Advanced Ultrafiltration Membranes by Leveraging Microphase Separation in Macrophase Separation of Amphiphilic Polysulfone Block Copolymers

Wei Chen, Mingjie Wei, Yong Wang*, J. Membr. Sci. 2017, 525, 342-348

Water-soluble polymers are generally required in the process of nonsolvent-induced phase separation (NIPS) as additives or modifiers to enhance the hydrophilicity and permeability of ultrafiltration membranes. In this work, we demonstrate that amphiphilic block copolymers, polysulfone-block-poly (ethyleneglycol) (PSf-b-PEG), dissolved alone in solvents without any additives lead to highly permeable, fouling-resistant membranes via the NIPS process. PEG blocks deliver dual functions in the membranes. Selective enrichment of PEG blocks on the membrane surface as a result of surface segregation enhances the hydrophilicity and consequently fouling resistance of the membranes. Moreover, microscale phase separation of the block copolymers drives the formation of interconnected PEG microdomains distributed throughout the bulk membrane as confirmed by the transmission electron microscopy analysis on stained membrane slices. PEG microdomains serve as water channels facilitating water transport through the membrane. As a result, thus produced membranes exhibit excellent permeability a few times higher than other PSf-based ultrafiltration membranes with similar retentions. For instance, a membrane having the molecular weight cut-off of 70 kDa gives a water permeability as high as 450 m−2 h−1 bar−1. Furthermore, the retentions of the PSf-b-PEG membranes can be tuned in a relatively wide range simply by adjusting the copolymer concentration in the casting solutions. Using amphiphilic block copolymers alone as the base materials for the preparation of ultrafiltration membranes by NIPS not only simplifies membrane manufacturing process but also opens a new avenue to prepare advanced membranes with upgraded permeability and fouling resistance.

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Depositing Lignin on Membrane Surfaces for Simultaneously Upgraded Reverse Osmosis Performances: An Upscalable Route

Feng Zhang, Yaping Wu, Weixing Li, Weihong Xing*, Yong Wang*, AIChE J. 2017, 63, 2221-2231

Reverse osmosis (RO) have been widely used to produce clean water and there is a strong need to enhance their permeability at no sacrifice of their other performances, e.g. selectivity. We deposit low-cost biopolymer, lignin, onto the surface of RO membranes by a simple filtration method. Lignin is deposited to the membrane surface via both hydrogen bonding and π-π interaction. Lignin deposition reduces the surface roughness of the membrane and enhances its negatively charging, while the surface hydrophilicity is maintained. Surprisingly, water permeation, salt rejection, and fouling resistance of the lignin-deposited membranes are simultaneously improved. More importantly, we demonstrate that this deposition method can be easily extended to modify commercial RO membrane modules, indicating the excellent upscalability of this method. We use the lignin-deposited membranes to treat real effluents of dyeing and papermaking and they perform much better than the virgin, unmodified membranes.

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Direct Silanization of Polyurethane Foams for Efficient Selective Absorption of Oil from Water

Sen Xiong, Zhaoxiang Zhong, Yong Wang*, AIChE J. 2017, 63, 2232-2240

Absorption is an effective method to collect oil spills and solvent leakages from water. However, the currently used oil absorbents are still suffering from high cost, tedious preparation, and low recyclability. In this work, we report an extremely simple and low-cost strategy to produce oil absorbents by directly coupling alkoxysilane onto the surface of polyurethane foams. Such direct silanization renders the initially amphiphilic foams a strong hydrophobicity and consequently a water-repelling and oil-absorptive functionality. The silanized foams exhibit highest absorption capacities as well as best recyclability among all polyurethane-based oil absorbents. More practically, the silanized PU foams can be used to recover crude oil spills with an absorption capacity of higher than 75 times of their own weight, and maintain 90% of the initial absorption capacity after 8 times reusage. Interestingly, we invent portable oil suckers for continuous oil absorption from water by filling vacuum cleaners with the silanized foams.

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Facile Synthesis of Bimodal Nanoporous Carbons by Templating Selective Swelling-Induced Mesoporous Block Copolymers

Leiming Guo, Xiaoting Wang, Yong Wang*, Chem. Eng. J. 2017, 313, 1295-1301

There  is  a  strong  need  for  simple  and  efficient  strategies  to  produce nanoporous  carbons  with  hierarchical  porosities  for a  number  of applications  such  as  supercapacitors.  Here  we  demonstrate  the  ease preparation  of  nanoporous  carbons  with  both  microscale  and  mesoscale porosities by infiltrating oligomeric phenol-formaldehyde resols into mesoporous block  copolymer  (BCP)  templates  followed  by  thermopolymerization  and carbonization.  The  mesoporous  polymeric  templates  are  produced  by  an extremely  simple  and  highly  flexible  selective  swelling  method  which  involves only  the  immersion  of  the  BCP  film  into  hot  ethanol  for  a  few  hours  and  the subsequent  air  drying  of  the  BCP  films.  The  produced  carbons  exhibit mesoporous  structures  with  thin  pore  walls  containing  micropores,  implying  a hierarchical porosity of the porous carbons.Such hierarchically porous carbons work  as  supercapacitors  exhibiting  a  capacitance  as  high  as  173 F/g at  1  A/g. Additionally, an excellent high-rate and steady capacitance of 165 F/g at 5 A/g is maintained  after  10000  cycles. The  excellent  performances  are  originated from  the  fast  ion  transport  pathways  and  the  low  resistance  of  charge diffusion  enabled  by  the  hierarchical  structures. This  work  suggests  a  new path  to  synthesize  nanoporous  carbons  with  hierarchical  porosities which  are expected to find more applications including but not limited to supercapacitors.

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2016

Nondestructive Creation of Ordered Nanopores by Selective Swelling of Block Copolymers: Toward Homoporous Membranes

Yong Wang*, Acc. Chem. Res. 2016, 49, 1401-1408

Pores regulate the entry and exit of substances based on the differences in physical sizes or chemical affinities. Pore uniformity, ordering, and the homogeneity of the surface chemistry of the pore walls are vital for maximizing the performance of a porous material because any scattering in these parameters weakens the capability of pores to discriminate foreign substances. Most strategies for the creation of homogeneous pores are destructive, and sacrificial components in the precursor materials must be selectively removed to generate porosities. The incorporation and subsequent removal of the sacrificial components frequently make the pore-making process complicated and inefficient and impose greater uncertainty in the control of the pore homogeneity.
Block copolymers (BCPs) have been demonstrated to be promising precursors in the fabrication of highly ordered nanoporous structures. Unfortunately, BCP-derived porosities are also predominantly dependent on destructive pore-making processes (e.g., etching or extraction). To address this problem, we have developed a swelling-based nondestructive strategy. In this swelling process, one simply needs to immerse BCP materials in a solvent selective for the minority blocks for hours. After removing the BCPs from the solvent followed by air drying, pores are generated throughout the BCP materials in the positions where the minority blocks initially dwell. This Account discusses our recent discoveries, new insights, and emerging applications of this burgeoning pore-making method with a focus on the development of ordered porosities in bulk BCP materials.
The initial morphology and orientation of the minority phases in BCPs determine the pore orientation and geometry in the produced porous materials. For nonaligned BCPs, three-dimensionally interconnected pores with sizes scattering in the 10−50 nm range are produced after swelling. There is a morphology evolution of BCP materials from the initial nonporous structure to the increasingly opened nanoporous intermediates, to interconnected networks of micellar nanofibers, and finally to isolated micellar spheres with increasing degrees of swelling. When the BCP films are aligned perpendicularly or in-plane, selective swelling results in uniform “standing” (perpendicular orientation) and “sleeping” (in-plane orientation) pores, respectively. Pore sizes can be tuned by changing molecular weights of the BCPs and swelling conditions without the loss of pore uniformity. Due to the nondestructive nature of this swelling process, nothing in the BCPs is lost during the pore-forming procedure, and consequently the formed pores can be progressively closed also by selective swelling. Such reversible pore opening/closing can be repeated many times, enabling the application of these materials in drug delivery and intelligent antireflective coatings. The monodispersed pore sizes, straight pore profile, and hydrophilic pore walls particularly favor the application of the porous BCPs in separations as homoporous membranes (HOMEs) exhibiting high selectivity, permeability, and inherent stimulus responsiveness.

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The Influence of Surface Topography and Surface Chemistry on the Anti-Adhesive Performance of Nanoporous Monoliths

Anna Eichler-Volf, Longjian Xue*, Gregor Dornberg, He Chen, Alexander Kovalev, Dirk Enke, Yong Wang*, Elena V. Gorb, Stanislav N. Gorb*, Martin Steinhart*, ACS Appl. Mater. Interfaces 2016, 8, 22593–22604

We designed spongy monoliths allowing liquid delivery to their surfaces through continuous nanopore systems (mean pore diameter ∼40 nm). These nanoporous monoliths were flat or patterned with microspherical structures a few tens of microns in diameter, and their surfaces consisted of aprotic polymer or of TiO2 coatings. Liquid may reduce adhesion forces FAd; possible reasons include screening of solid−solid interactions and poroelastic effects. Softening-induceddeformation of flat polymeric monoliths upon contact formation in the presence of liquids enhanced the work of separation WSe.On flat TiO2-coated monoliths, WSe was smaller under wet conditions than under dry conditions, possibly because of liquid-induced screening of solid−solid interactions. Under dry conditions, WSe is larger on flat TiO2-coated monoliths than on flat monoliths with a polymeric surface. However, under wet conditions, liquid-induced softening results in larger WSe on flat monoliths with a polymeric surface than on flat monoliths with an oxidic surface. Monolithic microsphere arrays show antiadhesive properties; FAd and WSe are reduced by at least 1 order of magnitude as compared to flat nanoporous counterparts. On nanoporous monolithic microsphere arrays, capillarity (WSe is larger under wet than under dry conditions) and solid−solid interactions (WSe is larger on oxide than on polymer) dominate contact mechanics. Thus, the microsphere topography reduces the impact of softening-induced surface deformation and screening of solid-solid interactions associated with liquid supply. Overall, simple modifications of surface topography and chemistry combined with delivery of liquid to the contact interface allow adjusting WSe and FAdover at least 1 order of magnitude. Adhesion management with spongy monoliths exploiting deployment (or drainage) of interfacial liquids as well as induction or prevention of liquid-induced softening of the monoliths may pave the way for the design of artificial surfaces with tailored contact mechanics. Moreover, the results reported here may contribute to better understanding of the contact mechanics of biological surfaces.

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Atomic Layer Deposition on Block Copolymer Membranes with Gyroidal Nanopores Toward Periodically Nanostructured Vapor Sensors: Nanotubes versus Nanorods

Leiming Guo, Zhaoxiang Zhong, Yong Wang*, Adv. Mater. Inter. 2016, 3, 1600017

3D gyroidal networks of ZnO nanorods or nanotubes are synthesized by replicating block copolymer (BCP) templates with gyroidal nanopores via atomic layer deposition (ALD). The generation of 3D ZnO nanorods or nanotubes depends on the thicknesses of deposited ZnO layers, which can be easily and precisely controlled by adjusting ALD cycles. The as-obtained ZnO nanostructures have porosities as high as 77% for gyroidal nanorods and 86% for gyroidal nanotubes due to their unique interconnected structures and rough surfaces. Both gyroidal ZnO nanostructures are used as vapor sensors to detect ethanol with different concentrations. Gyroidal ZnO nanotubes show higher ethanol sensitivity than nanorods because of the inherently interconnected nanotubes and larger porosity. Gyroidal ZnO nanotubes can also act as formaldehyde sensors with the sensitivity as high as 42 to 50 ppm formaldehyde, which is much higher than previously reported sensors based on ZnO nanorods or nanotubes.

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Nanoporous Films with Superior Resistance to Protein Adsorption by Selective Swelling of Polystyrene-block-Poly(ethylene Oxide)

Hao Yang, Leiming Guo, Zhaogen Wang, Nina Yan, Yong Wang*, Ind. Eng. Chem. Res. 2016, 55, 8133–8140

Poly(ethylene oxide) (PEO) is well-known for its excellent resistance to the nonspecific adsorption of proteins. However, it remains challenging to fabricate surfaces with uniform and robust coverage of PEO chains in a simple and efficient way. Simply by treating the block copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in ethanol, we obtain nanoporous polymeric films with superior resistance to protein adsorption. The pores are nondestructively formed via the selective swelling-induced pore generation mechanism in which PEO microdomains embedded in PS matrix are selectively swollen in ethanol and collapsed in the subsequent air drying. Pore sizes and porosities can be typically tuned in the ranges ∼20–50 nm and ∼20–70%, respectively, by changing swelling durations and temperatures. Interestingly, the PEO chains are simultaneously migrated on the film surface and pore walls in the selective swelling-induced pore forming process. Atomic force microscopy examinations reveal that PEO chains are solvated when the film is wetted with water, closing the pores in the film. Thus, produced SEO films adsorb much less proteins than PS films without PEO on the surface, and also demonstrate a better fouling resistance than many reported PEO-covered surfaces prepared by other methods.

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Dye Adsorption on Zinc Oxide Nanoparticulates Atomic-Layer-Deposited on Polytetrafluoroethylene Membranes

Sen Xiong, Liang Kong, Zhaoxiang Zhong, Yong Wang*, AIChE J. 2016, 62, 3982-3991

In this work, zinc oxide (ZnO) is deposited onto porous polytetrafluoroethylene (PTFE) membranes via atomic layer deposition (ALD) and thus produced ZnO-deposited PTFE membranes are used as adsorbents for the removal of dyes from aqueous solutions. We first examine the evolution of morphology, wettability, and crystallization of PTFE membranes with ZnO ALD. The presence of ZnO nanoparticulates significantly promotes the diffusion and contact of the aqueous solutions within the membrane on one hand, and endows the membrane a strong capability to adsorb dyes from the solutions on the other. Kinetic studies reveal that the adsorption of rhodamine B and acid orange 7 by the ZnO-deposited PTFE membranes can be described by the pseudo-second-order and pseudo-first-order model, respectively, and the adsorption of both dyes follows the Langmuir isotherm. The used membranes can be easily regenerated by rinsing with ethanol and reused for multiple times without loss of removal efficiency.

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大尺寸陶瓷膜原子层沉积过程的 CFD 模拟

朱明,汪勇*, 化工学报 2016, 67, 3720-3729

陶瓷膜具有耐高温、耐酸碱、强度高等优点,在液体分离领域得到了广泛应用。对陶瓷膜进行表面改性,可进一步提升其性能,但基于表面化学反应的改性方法工艺过程复杂,难于控制。原子层沉积(Atomic Layer Deposition, ALD)是基于表面自限制化学反应过程的气固相薄膜沉积技术,可以在纳米尺度精确调控孔道结构,特别适用于多孔分离膜的改性和功能化。目前尚无适用于大尺寸陶瓷膜的ALD设备,需要对ALD过程进行专门的优化设计。通过CFD模型对1m长的单通道陶瓷膜的ALD过程进行了研究,在数学模型中考虑了两种气体源交替进入腔体中所引发不同的表面反应,并考虑了脉冲边界的影响。模拟计算结果与实验比较平均相对误差为1.69%。在数值模拟的基础上,提出了双向交替旋转脉冲的ALD模式,为陶瓷膜的 ALD沉积改性的装备设计和过程优化提供了理论依据。

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Heat Transfer of Nanofluidics in Hydrophilic Pores: Insights from Molecular Dynamics Simulations

Mingjie Wei, Yang Song, Yong Wang*, Chinese J. Chem. Eng. 2016, 24, 1117-1121

Nanofluidics  in  hydrophilic  nanopores  is  a  common  issue  in  many  natural  and  industrial  processes. Among all, the mass transport of nanofluidics is most concerned. Besides that, the heat transfer of  a  fluid flow in nano or micro channels is always considered with adding nanoparticles into the flow, so as to enhance the heat transfer by convection between the fluid and the surface. However, for some applications with around 1 nanometer channels  such  as  nano  filtration  or  erosion  of  rocks,  there  should  be  no  nanoparticles  included.  Hence,  it  is necessary  to  figure  out  the  heat  transfer  mechanism  in  the  single  phase  nanofluidics.  Via  non-equilibrium molecular dynamics simulations, we revealed the heat transfer inside nanofluidics and  the  one between  fluid and walls  by setting simulation into extremely harsh condition.  It was found that the heat was conducted by molecular motion  without  temperature  gradient  in  the  area  of  low  viscous  heat,  while  it  was  transferred  to  the  walls  by increasing the temperature of  fluids.  If the condition back to normal, it was  found viscous heat of nanofluidics could be easily removed by the temperature gradient between fluid and walls being less than 1 K.

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In Situ Cavitation of Phenolic Supramolecules with PEO−PPO−PEO Block Copolymers by Friedel−Crafts Alkylation toward Ordered Nanoporous Polymers

Xikang Huo, Qianqian Lan, Yong Wang*, Ind. Eng. Chem. Res. 2016, 55, 6398−6404

The release of hydrogen halides in Friedel−Crafts alkylation has long been considered as troublesome and detrimental. Here we demonstrate that the released HCl in alkylation is beneficial and highly demanded to remove poly-(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) templates from the phenolic supramolecules for the preparation of ordered nanoporous phenolic polymers as additional treatment for template removal is no longer necessary. Friedel−Crafts alkylation using bifunctional alkylation agent, 1,4-bis(chloromethyl)benzene, leads to the hyper-cross-linking of phenolic polymers, thus generating micropores in the skeleton of the polymers. HCl in situ generated as the byproduct during alkylation facilely extracts the copolymer because of its enhanced diffusion through the micropores. The prepared phenolic polymers exhibit high surface areas and well-ordered porosities in a hexagonal or gyriodal structure. The in situ cavitation by alkylation takes place at much milder conditions and has higher efficiency than other template-removing methods.

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Stretched Homoporous Composite Membranes with Elliptic Nanopores for External-Energy-Free Ultrafiltration

Leiming Guo, Lei Wang, Yong Wang*, Chem. Commun. 2016, 52, 6899-6902

Extremely permeable ultrafiltrationmembranes with elongated elliptical pores are fabricated by stretching composite membranes with homoporous selective layers and macroporous supports. With simultaneously increased porosities both for the selective layers and supports and the thinned selective layers, the stretched membranes exhibit multifold-enhanced permeability with little sacrifice in selectivities. Thus the produced membranes enable, for the first time, gravity-driven ultrafiltration and discrimination of similarly sized nanoparticles with diameters down to 30 nm.

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Enhanced Performances of Polypropylene Membranes by Molecular Layer Deposition of Polyimide

Sen Xiong, Ting Sheng, Liang Kong, Zhaoxiang Zhong, Jun Huang, Yong Wang*, Chinese J. Chem. Eng. 2016, 24, 843-849

Molecular Layer Deposition (MLD) for the deposition of polyimide (PI) at low temperature of 110 °C has been firstly introduced into the field of membrane separation. With the optimized MLD deposition parameters, such low deposition temperature has successfully expanded the application of MLD for the surface modification of polymeric materials. Globular PI particulates grow on both the free surfaces as well as the pore walls of the polypropylene (PP) membranes as isolated islands during progressive precursor exposures. The PI-deposited PP membranes exhibit synergistically improved performances in various aspects. Evidently improved surface hydrophilicity and permeation performance (30%) have been achieved via the MLD deposition of polyimide films. The overall separation efficiency maintained higher than 85% even after 250 cycles of MLD deposition. More importantly, the thermal stability has been improved and the integrity of the porous structure for PI-deposited PP membranes has been well preserved even after harsh treatment, which ensures its potential application in industries.

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Efficient Perovskite Solar Cells Based on Novel Three-Dimensional TiO2 Network Architectures

Hao Lu, Kaimo Deng, Nina Yan, Yulong Ma, Bangkai Gu, Yong Wang*, Liang Li*, Sci. Bull. 2016, 61, 778-786

Mesoscopic lead halide perovskite solar cells typically use TiO2 nanoparticle films as the scaffolds for electron-transport pathway and perovskite deposition. Here, we demonstrate that swelling-induced mesoporous block copolymers can be templates for producing threedimensional TiO2networks by combining the atomic layer deposition technique. Thickness adjustable TiO2network is an excellent alternative scaffold material for efficient perovskite solar cells. Our best performing cells using such a 270 nm thick template have achieved a high efficiency of 12.5 % with pristine poly-3-hexylthiophene as a hole transport material. The high performance is attributed to the direct transport pathway and high absorption of scaffolds, small leakage current and largely reduced recombination rate at interfaces. The results show that TiO2 network architecture is a promising scaffold for mesoscopic perovskite solar cells.

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Selective Swelling Induced Pore Generation of Amphiphilic Block Copolymers: The Role of Swelling Agents

Nina Yan, Yong Wang*, J. Polym. Sci. B: Polym. Phys. 2016, 54, 926-933

Swelling of block copolymers by selective solvents has emerged as an extremely simple and efficient process to produce nanoporous materials with well-controlled porosities. However, the role of the swelling agents in this pore-making process remains to be elucidated. Here we investigate the evolution of morphology, thickness, and surface chemistry of thin films of polystyrene-block-poly (2-vinyl pyridine) (PS-b-P2VP) soaked in a series of alcohols with changing carbon atoms and hydroxyl groups in their molecules. It is found that, in addition to a strong affinity to the dispersed P2VP microdomains, the swelling agents should also have a moderate swelling effect to PS to allow appropriate plastic deformation of the PS matrix. Monohydric alcohols with longer aliphatic chains exhibit stronger ability to induce the pore formation and a remarkable increase in film thickness is associated with the pore formation. High-carbon alcohols including n-propanol, n-butanol, and n-hexanol produce cylindrical micelles upon prolonged exposure for their strong affinity toward the PS matrix. In contrast, methanol and polyhydric alcohols including glycol and glycerol show very limited effect to swell the copolymer films as their affinity to the PS matrix is low; however, they also evidently induce the surface segregation of P2VP blocks.

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Highly Permeable and Robust Responsive Nanoporous Membranes by Selective Swelling of Triblock Terpolymers with a Rubbery Block

Zhaogen Wang, Yong Wang*, Macromolecules 2016, 49, 182−191

The selective swelling of amphiphilic block copolymers has been demonstrated to be extremely facile and efficient in producing nanoporous membranes. However, all previous works are limited to diblock copolymers composed of two glassy blocks, suffering from inherent mechanical weakness. Here we elucidate the selective swelling-induced pore generation of triblock terpolymers with a rubbery polyisoprene (PIP) block, polyisoprene-block-polystyrene-block-poly(2-vinylpyridine) (PIP-b-PS-b-P2VP). A short exposure to ethanol turns the initially dense films to nanoporous membranes with well-defined interconnected porosity. We fabricate composite membranes with the nanoporous terpolymer thin films as the selective layers deposited on macroporous substrates. Using PS-b-P2VP diblock copolymer without a rubbery third block for comparison, we identify the role of the rubbery PIP blocks in determining the mechanical properties as well as the swelling behaviors of the terpolymer. The rubbery PIP blocks enhance the mechanical robustness of the nanoporous membranes as revealed by nanoindentation tests on one hand and evidently accelerate the swelling process because of their softening effect to the PS matrix on the other hand, thus leading to 2–3-fold improved permeability. Moreover, the membranes exhibit a fast stimuli-responsive function as well as enhanced hydrophilicity because of the preferential aggregation of P2VP chains on the pore walls.

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Homoporous Membranes with Tailored Pores by Soaking Block Copolymer/Homopolymer Blends in Selective Solvents: Dissolution versus Swelling

Mingjie Wei, Wei Sun, Xiansong Shi, Zhaogen Wang, Yong Wang*, Macromolecules 2016, 49, 215−223

Extraction homopolymers premixed in aligned films of block copolymers by rinsing with selective solvents has long been used for the preparation of membranes with uniform straight pores (homoporous membranes). It is frequently assumed that only the dissolution of homopolymers contributes to the pore formation. However, in this work, we demonstrate that the effect of swelling plays a significant role in determining the pore sizes. We prepare blended films of block copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) and P2VP homopolymers with low molecular weight and anneal the films to perpendicularly align the P2VP microdomains. Rinsing the aligned films in ethanol results in homoporous membranes, and the pore sizes can be tuned by the dosages of P2VP homopolymers. Interestingly, the pore sizes can also be effectively tailored by changing the rinsing temperatures and/or durations because of the significant contribution of the selective swelling of P2VP blocks under strong rinsing conditions in addition to the contribution of the dissolution of P2VP homopolymers. We identify the portion of the contribution from dissolution and from swelling and demonstrate that the pore sizes can be flexibly tuned within a wider range at no expense of pore ordering and uniformity by balancing the effect of dissolution and swelling.

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Upgrading Polysulfone Ultrafiltration Membranes by Blending with Amphiphilic Block Copolymers: Beyond Surface Segregation

Yuqing Chen, Mingjie Wei, Yong Wang*, J. Membr. Sci. 2016, 505, 53-60

Surface segregation of amphiphilic copolymers in the phase inversion process has long been used to improve membrane hydrophilicity. Typically, the copolymer is sparsely dosed into the casting solutions as additives. Herein we substantially increase the dosages of amphiphilic copolymers, and obtain blend ultrafiltration membranes with synergetically upgraded performances because of extra permeability-enhancing effect of the copolymer in addition to the surface segregation effect. We blend amphiphilic block copolymer, polysulfone-block-polyethylene glycol (PSf-b-PEG), with polysulfone base polymer at various percentages up to 70%. There is no compatible issue between the PSf and PSf-b-PEG as they are miscible at any blend ratio. Infrared spectroscopy and X-ray photoelectron spectroscopy confirm the surface segregation of PEG blocks. Moreover, PSf-b-PEG evidently influences the phase separation process by slowing down the precipitation rate of the polymer solutions, thus producing membranes with thicker skin layers. Interestingly, increasing copolymer percentages result in more water-permeable PEG microdomains in the blend membranes and consequently enhanced water permeance. The blend membranes exhibit simultaneously upgraded permeance, hydrophilicity, fouling resistance, and also performance stability. The highest permeance reaches nearly 400 L/(m2 h bar) at a copolymer percentage of 40%, which is much higher than those of PSf membranes prepared in other works. By comparing with PEG homopolymer, we identify that the superior performances are originated also from the additional water permeability through PEG microdomains in addition to the effect of surface segregation of PSf-b-PEG copolymers.

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均孔膜

汪勇*, 邢卫红, 徐南平, 化工学报 2016, 67, 27-40

为进一步提高膜分离过程的精度,同步提升选择性和渗透性,分离膜孔径均一化是必然途径,均孔膜因此应运而生。首先讨论了均孔膜概念及其结构特点。均孔膜是孔径均一、孔道形状一致且垂直贯穿整个分离层的分离膜。然后介绍了制备均孔结构的不同方法,探讨了这些方法的优势和固有缺陷。利用嵌段共聚物微相分离的特性,可获取规整排列的、孔径在10~50 nm范围内连续可调的均孔结构,具有方法简便、无须特殊设备、易于放大制备等突出优势,是最有可能实现规模化生产的均孔膜制备方法。着重介绍了两亲嵌段共聚物选择性溶胀成孔方法的原理及其在孔径和孔型调节(圆柱孔、槽形孔)、自发永久亲水和制备过程绿色少污染等方面的特点。最后,讨论了嵌段共聚物基均孔膜发展的瓶颈,并指出应加强在孔径小于10 nm的均孔膜、孔型调变和应用领域等方面的研究。认为均孔膜不仅是一种新结构的分离膜,更代表着膜分离的发展方向。

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2015

Monolithic Membranes with Designable Pore Geometries and Sizes via Retarded Evaporation of Block Copolymer Supramolecules

Leiming Guo, Yong Wang*, Macromolecules 2015, 48, 8471−8479

We develop a retarded evaporation approach for the alignment of cylinder-forming block copolymer supramolecular monoliths, 3-n-pentadecylphenol (PDP) hydrogen-bonded polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP). A variety of highly ordered, aligned morphologies are produced by varying the dosages of PDP in the supramolecules. Treatment of the aligned supramolecular monoliths in hot ethanol leads to the dissolution of PDP and the selective swelling of P4VP, yielding enlargeable ordered mesopores along the original P4VP/PDP domains. Particularly, from supramolecular monoliths aligned in the morphology of perpendicular cylinders and gyroids, we obtain highly ordered monolithic membranes containing enlarged straight pores and bicontinuous pores, respectively. The straight and gyroidal pores were filled with phenol–formaldehyde resol and further carbonized to produce well-defined carbon nanostructures including nanofibers and reversed gyroids, demonstrating the pore accessibility and the promising templating functionality of the resulted monolithic membranes.

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Atomic-Layer-Deposition-Enabled Nonwoven Membranes with Hierarchical ZnO Nanostructures for Switchable Water/Oil Separations

Sen Xiong, Liang Kong, Jun Huang, Xiaoqiang Chen, Yong Wang*, J. Membr. Sci. 2015, 493, 478-485

Functional membranes with separation ability for water/oil mixtures based on contrasting wetting behavior have recently received significant attention in the application field of water contamination treatment. Herein, we demonstrated a surface functionalization method for cost-efficient nonwoven fabrics with no selectivity of water/oil mixture. This nonwoven membrane was fabricated by hydrothermal growth of ZnO hierarchical nanorods on the surface of atomic-layer-deposited ZnO ultrathin layer. Due to the ZnO nanostructure with roughened morphology and the amphiphilic nature of the nonwoven fabrics, the functionalized nonwoven membranes were endowed with both underwater oleophobicity and under oil hydrophobicity. Separation of any phase in the mixture of water and oil can be achieved simply by pre-wetting the nonwoven membrane surface with the heavier component. Driven by gravity solely, the as-prepared nonwoven membranes can efficiently and selectively separate water/oil mixtures with exceptional stability. These functionalized nonwoven membranes may find potential application against complex water/oil separation systems.

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Highly Permeable and Robust Membranes Assembled from Block-copolymer-functionalized Carbon Nanotubes

Xueping Yao, Jie Li, Zhaogen Wang, Liang Kong, Yong Wang*, J. Membr. Sci. 2015, 493, 224-231

The fabrication of homogenous integrated layers of assembled carbon nanotubes (CNTs) on porous substrates for advanced membranes via vacuum filtration remains a significant challenge due to the aggregation of CNTs. Here we report the composite membranes with both high flux and good mechanical robustness produced from functionalized CNTs through direct coating with amphiphilic block copolymer. Chemically crosslinked CNT layers are tightly adhered to the substrate, rendering a strong mechanical stability to the composite membranes. The thickness of the separation layer as well as the permselectivity of the membranes can be readily tuned simply by changing the deposition amount of CNTs. The membranes with a moderate nanofiber coverage are capable of complete removal of 10-nm-particles while offer a remarkable pure water flux higher than 5.7×103L/(m2hbar), suggesting their high efficiency in the purification of liquids contaminated by tiny colloids or particulates.

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Unusual Air Filters with Ultrahigh Efficiency and Antibacterial Functionality Enabled by ZnO Nanorods

Zhaoxiang Zhong, Zhe Xu, Ting Sheng, Jianfeng Yao, Weihong Xing *, Yong Wang*, ACS Appl. Mater. Interfaces 2015, 7, 21538-21544

Porous membranes/filters that can remove airborne fine particulates, for example, PM2.5, with high efficiency at low energy consumption are of significant interest. Herein, we report on the fabrication of a new class of unusual superior air filters with ultrahigh efficiency and an interesting antibacterial functionality. We use atomic layer deposition (ALD) to uniformly seed ZnO on the surface of expanded polytetrafluoroethylene (ePTFE) matrix, and then synthesize well-aligned ZnO nanorods with tunable widths and lengths from the seeds under hydrothermal conditions. The presence of ZnO nanorods reduces the effective pore sizes of the ePTFE filters at little expense of energy consumption. As a consequence, the filters exhibit exceptional dust removal efficiencies greater than 99.9999% with much lower energy consumption than conventional filters. Significantly, the presence of ZnO nanorods strongly inhibits the propagation of both Gram positive and negative bacteria on the filters. Therefore, the functionalized filters can potentially overcome the inherent limitation in the trade-off effect and imply their superiority for controlling indoor air quality.

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Interconnected Mesoporous Carbon Sheet for Supercapacitors from Low-cost Resources

Xiaoting Wang, Hao Ma, Hebao Zhang, Moxin Yu, Xiaojun He*, Yong Wang*, Mater. Lett. 2015, 158, 237-240

Interconnected mesoporous carbon sheets (MCSs) were prepared from cheap coal tar pitch with low-cost CaCO3nanoparticles and cetyltrimethyl ammonium bromide (CTAB) as porogens for supercapacitors. The results show that the surface area of MCSs can be tuned in the range of 87–167 m2g-1. The interconnected MCS2–1–1 made at 2:1:1 of CaCO3/coal tar pitch/CTAB mass ratio retains a capacitance of 87 F g-1 at 0.1 A g-1 after 100 cycles in ionic liquid electrolyte with high retention of 93.1%, showing excellent cycle stability. The energy density of MCS2–1–1 capacitor only drops from 64.18 Wh kg-1 to 22.19 Wh kg-1 with the average power density increasing from 115 W kg-1 to 4267 W kg-1, demonstrating good rate performance. This work suggests a low-cost way to synthesize high performance MCS for energy storage materials.

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Enhancing the Hydrophilicity and Water Permeability of Polypropylene Membranes by Nitric Acid Activation and Metal Oxide Deposition

He Chen, Liang Kong, Yong Wang*, J. Membr. Sci. 2015, 487, 109-116

Activation by nitric acid oxidation has been combined with atomic layer deposition (ALD) to modify microporous polypropylene (PP) membranes. Nitric acid oxidation with prewetting of t-butanol generates active groups on the inert surface of PP membranes. Metal oxides including Al2O3 and TiO2 are subsequently ALD-deposited on the activated membrane. Nitric acid oxidation is a mild approach to activate PP membranes. A short immersion in nitric acid for 10 min is able to generate oxygen/nitrogencontaining active species on the membrane surface, which leads to an evident increase (38%) in the membrane permeability. The mechanical stability and the integrity of the porous structure are well preserved after activation. Surface elemental analyses and microscopy observations confirm the successful deposition of metal oxides thin layers onto the activated membranes. In the deposition of either Al2O3 or TiO2, the hydrophilicity of the deposited membranes is continuously improved with increasing ALD cycles, resulting in further increase in water flux. After 100 ALD cycles, the flux is doubled compared to that of the bare membrane. Al2O3 is more efficient in enhancing the hyclrophilicity and permeation than TiO2 under identical deposition conditions because of the faster growth of Al2O3 on the activated PP surface.

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Surface Functionalization of Carbon Nanotubes by Direct Encapsulation with Varying Dosages of Amphiphilic Block Copolymers

Xueping Yao, Jie Li, Liang Kong, Yong Wang*, Nanotechnology 2015, 26, 325601

Encapsulation of carbon nanotubes (CNTs) by amphiphilic block copolymers is an efficient way to stabilize CNTs in solvents. However, the appropriate dosages of copolymers and the assembled structures are difficult to predict and control because of the insufficient understanding on the encapsulation process. We encapsulate multiwalled CNTs with polystyrene-block-poly (4-vinyl pyridine) (PS-b-P4VP) by directly mixing them in acetic acid under sonication. The copolymer forms a lamellar structure along the surface of CNTs with the PS blocks anchoring on the tube wall and the P4VP blocks exposed to the outside. The encapsulated CNTs achieve good dispersibility in polar solvents over long periods. To increase our understanding of the encapsulation process we investigate the assembled structures and stability of copolymer/CNTs mixtures with changing mass ratios. Stable dispersions are obtained at high mass ratios between the copolymer and CNTs, i.e. 2 or 3, with the presence of free spherical micelles. Transmission electron microscopy and thermal gravimetric analysis determine that the threshold for the complete coverage of CNTs by the copolymer occurs at the mass ratio of 1.5. The coated copolymer layer activates the surface of CNTs, enabling further functionalization of CNTs. For instance, atomic layer deposition of TiO2 produces conformal thin layers on the encapsulated CNTs while isolated TiO2 bumps are produced on the pristine, inert CNTs.

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(Inside Cover) Reversible Switch Between the Nanoporous and the Nonporous State of Amphiphilic Block Copolymer Films Regulated by Selective Swelling

Nina Yan, Yong Wang*, Soft Matter 2015, 11, 6927-6937

Switchable nanoporous films, which can repeatedly alternate their porosities, are of great interest in a diversity of fields. Currently these intelligent materials are mostly based on polyelectrolytes and their porosities can change only in relatively narrow ranges, typically under wet conditions, severely limiting their applications. Here we develop a new system, which is capable of reversibly switching between a highly porous state and a nonporous state dozens of times regulated simply by exposure to selective solvents. In this system nanopores are created or reversibly eliminated in films of a block copolymer, polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP), by exposing the films to P2VP-selective or PS-selective solvents, respectively. The mechanism of the switch is based on the selective swelling of the constituent blocks in corresponding solvents, which is a nondestructive and easily controllable process enabling the repeatable and ample switch between the open and the closed state. Systematic microscopic and ellipsometric characterization methods are performed to elucidate the pore-closing course induced by nonsolvents and the cycling between the pore-open and the pore-closed state up to 20 times. The affinity of the solvent for PS blocks is found to play a dominating role in determining the pore-closing process and the porosities of the pore-open films increase with the cycling numbers as a result of loose packing conditions of the polymer chains. We finally demonstrate the potential applications of these films as intelligent antireflection coatings and drug carriers.

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Crosslinking of Polyimide Atomic-layer-deposited on Polyethersulfone Membranes for Synergistically Enhanced Performances

Ting Sheng, Liang Kong, Yong Wang*, J. Membr. Sci. 2015, 486, 161-168

Polyimide (PI) has been uniformly deposited and chemically crosslinked to form a hydrophilic and robust coating layer wrapping the skeleton of the macroporous polyethersulfone (PES) substrate membranes. PI is coated along all the exposed surfaces including the free surfaces and also the pore walls of the PES membranes by atomic layer deposition. The PI-deposited PES membranes are immersed in the excessive methanolic solution of ethylenediamine to initiate the crosslinking of PI chains. The crosslinked PES membranes exhibit synergistically enhanced performances in multiple aspects. They obtain enhanced hydrophilicity due to the formation of polar amide bonds. Control experiments on smooth PI films deposited on nonporous substrates reveal that crosslinking progressively increases the thickness of the PI films. Consequently, we are able to well control and tune the pore sizes by changing the crosslinking durations. The selectivity is enhanced by larger amplitude than the reduction of permeability as a result of reduced pore size and enhanced hydrophilicity after crosslinking. In addition, crosslinking also efficiently improves the thermal and mechanical stability and corrosion resistance of the PES membranes.

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Surface Attachment of Gold Nanoparticles Guided by Block Copolymer Micellar Films and Its Application in Silicon Etching

Mingjie Wei, Yong Wang*, Materials 2015, 8, 3793-3805

Patterning metallic nanoparticles on substrate surfaces is important in a number of applications. However, it remains challenging to fabricate such patterned nanoparticles with easily controlled structural parameters, including particle sizes and densities, from simple methods. We report on a new route to directly pattern pre-formed gold nanoparticles with different diameters on block copolymer micellar monolayers coated on silicon substrates. Due to the synergetic effect of complexation and electrostatic interactions between the micellar cores and the gold particles, incubating the copolymer-coated silicon in a gold nanoparticles suspension leads to a monolayer of gold particles attached on the coated silicon. The intermediate micellar film was then removed using oxygen plasma treatment, allowing the direct contact of the gold particles with the Si substrate. We further demonstrate that the gold nanoparticles can serve as catalysts for the localized etching of the silicon substrate, resulting in nanoporous Si with a top layer of straight pores.

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Filtration-based Synthesis of Micelle-derived Composite Membranes for High-flux Ultrafiltration

Xueping Yao, Leiming Guo, Xiaoqiang Chen, Jun Huang, Martin Steinhart*, Yong Wang*, ACS Appl. Mater. Interfaces 2015, 7, 6974-6981

Ideal membrane configurations for efficient separation at high flux rates consist of thin size-selective layers connected to macroporous supports for mechanical stabilization. We show that micelle-derived (MD) composite membranes combine efficient separation of similarly sized proteins and water flux 5–10 times higher than that of commercial membranes with similar retentions. MD composite membranes were obtained by filtration of solutions of amphiphilic block copolymer (BCP) micelles through commercially available macroporous supports covered by sacrificial nanostrand fabrics followed by annealing and removal of the nanostrand fabrics. Swelling-induced pore generation in the BCP films thus covering the macroporous supports yielded ∼210 nm thin nanoporous size-selective BCP layers with porosities in the 40% range tightly connected to the macroporous supports. Permselectivity and flux rates of the size-selective BCP layers were adjusted by the BCP mass deposited per membrane area and by proper selection of swelling times. The preparation methodology described here may pave the way for a modular assembly system allowing the design of tailored separation membranes.

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Isoporous Membranes with Gradient Porosity by Selective Swelling of UV-crosslinked Block Copolymers

Zhaogen Wang, Leiming Guo, Yong Wang*, J. Membr. Sci. 2015, 476, 449-456

Selective swelling of amphiphilic block copolymers (BCPs) is an effective and nondestructive pore making strategy. Here we coupled swelling-induced pore generation with UV crosslinking to fabricate BCP isoporous membranes with gradient porosity. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) solutions were coated onto macroporous supporting membranes to achieve composite Films, which were then annealed in solvent vapor for the perpendicular alignment of the P2VP phases near the surface of the coating BCP layer. After swelling of BCP in hot ethanol and drying, isopores of similar to 8 nm formed at the surface of BCP layer following the selective swelling-induced pore-formation mechanism. Then UV exposure and subsequent secondary swelling at stronger condition oldie membranes were conducted to enlarge the inner pores while maintaining the surface structures. With balanced UV crosslinking and secondary swelling, the finally obtained membranes showed ordered perpendicular pores at the outmost layer and gradient porosity with enlarged interconnected pores inside the BCP layer. Due to the gradient structures, the membranes exhibited much higher flux while the surface structures and retention remained essentially unchanged. Moreover, compared to the membranes without UV treatment, the membranes showed better performances in discriminating polyethylene glycol molecules with different molecular weights and still kept a sensitive pH-responsive property.

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2014

Extremely Efficient and Recyclable Absorbents for Oily Pollutants Enabled by Ultrathin-Layered Functionalization

Qianqian Wang, Hanghua Wang, Sen Xiong, Rizhi Chen, Yong Wang*, ACS Appl. Mater. Interfaces 2014, 6, 18816-18823

Oils and organic solvents that leak into water bodies must be promptly removed to avoid ecological disasters, for example, by selective absorption using oleophilic absorbents. However, it remains a challenge for the low-cost synthesis of efficient and recyclable absorbents for oily pollutants. By surface functionalization to inexpensive polyurethane (PU) foams, we synthesize oil absorbents exhibiting the highest absorption capacity and the best recyclability among all polymeric absorbents. The synthesis is enabled by atomic layer deposition of ∼5 nm-thick Al2O3 transition layer onto the skeleton surface of PU foams, followed by coupling a single-molecule layer of silanes to the Al2O3 layer. The sub-10 nm functionalization layer provides the PU foam an outstanding water-repelling and oil-absorbing functionality without compromising its high porosity and elasticity. The functionalized foam is able to quickly absorb oily pollutants spread on water surfaces or precipitated in water with a capacity more than 100 times its own weight. This ultrathin-layer-functionalization method is also applicable to renewable porous biomaterials, providing a sustainable solution for oil spills. Moreover, we propose devices than can continuously operate to efficiently collect oil spills from water surfaces based on the functionalized PU foam developed in this work.

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Turning Low-Cost Filter Papers to Highly Efficient Membranes for Oil/Water Separation by Atomic-Layer-Deposition-Enabled Hydrophobization

Liang Kong, Qianqian Wang, Sen Xiong, Yong Wang*, Ind. Eng. Chem. Res. 2014, 53, 16516-16522

It remains a great challenge for the simple and affordable production of membranes for oil/water separation. We prepare low-cost but highly efficient paper-based membranes for oil/water separation through hydrophobic modification to filter papers. The simple modification contains only two steps: a thin layer of aluminum oxide is first coated on the surface of the filter paper by atomic layer deposition, and silane molecules are subsequently coupled on the precoated aluminum oxide layer via their reaction with hydroxyl groups on the surface. Both the alumina layer and the silanization layer are very thin with a total thickness less than 10 nm. The modified filter paper is endowed with strong hydrophobicity and oleophilicity, therefore exhibits strongly retarded permeation to water and enhanced permeation to nonpolar oils. The modified filter paper is demonstrated to show excellent separation efficiencies greater than 90% in the separation of various types of oils and organic solvents from their mixtures with water. The paper-based membranes prepared in this work are distinguished among others for their low-cost substrates and simple modification route. This modification method is expected to be easily extended to hydrophobize a diversity of other substrates.

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Atomic Layer Deposition of Polyimide on Microporous Polyethersulfone Membranes for Enhanced and Tunable Performances

Ting Sheng, He Chen, Sen Xiong, Xiaoqiang Chen, Yong Wang*, AIChE J. 2014, 60, 3614-3622

Atomic layer deposition (ALD) of polyimide (PI) is explored to tune the separation properties of microporous polyethersulfone (PES) membranes and also to improve their mechanic and thermal stability. Conformal and uniform thin layers of PI are deposited along the pore wall throughout the entire PES membrane instead of forming a top layer merely on the membrane surface. With increasing ALD cycles, the pore size of the PES membrane is progressively reduced, leading to increased retention. The permeation is correspondingly decreased but its drop is less pronounced than the increase of retention. For example, the retention to 23-nm silica nanospheres is significantly increased from nearly zero to 60% after 3000 ALD cycles, whereas the water flux is moderately decreased by 54%. Moreover, ALD of PI evidently enhances the mechanical strength and thermal resistance of the PES membrane as PI tightly wraps the skeleton of the membrane.

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(Inside Cover) Nanoslitting of Phase-separated Block Copolymers by Solvent Swelling for Membranes with Ultrahigh Flux and Sharp Selectivity

Leiming Guo, Yong Wang*, Chem. Commun. 2014, 50, 12022-12025

Long and narrow slit-shaped pores are predicted to have enhanced permeation and selectivity. We report, for the first time, the efficient production of membranes containing densely packed slitted pores with a pore width of <15 nm from block copolymers (BCPs). Such membranes exhibit sharp selectivity and ultrahigh water flux.

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Plasma Activation and Atomic Layer Deposition of TiO2 on Polypropylene Membranes for Improved Performances of Lithiumion Batteries

He Chen, Qian Lin, Qiang Xu, Yang Yang, Zongping Shao, Yong Wang*, J. Membr. Sci. 2014, 458, 217-224

Atomic layer deposition (ALD) of TiO2 was applied on porous polypropylene (PP) membranes which were used as separators in lithium-ion batteries (LIBs) composed of Li4Ti5O12 (LTO) anode/Li cathode. Without plasma activation on the bare PP membrane, the initial deposition of TiO2 was based on the subsurface nucleation mechanism, which prevented the formation of a conformal hydrophilic TiO2 layer at low ALD cycles. The improvement of wettability of the PP membrane to the electrolyte could only be achieved at high ALD cycles up to 500. However, the severe narrowing of membrane pores counterbalanced the wetting enhancement, which hardly improved the performance of the LIBs. Plasma pretreatment was efficient to generate active groups on the highly chemically inert surface of polypropylene membranes, thus ultrathin TiO2 films could be conformally deposited by ALD on the membrane surface based on the layer-by-layer mechanism at cycles as low as 20. Such a conformal ultrathin layer of TiO2 was confirmed to concurrently overcome both the thermal shrinkage and poor wettability of the PP membranes. Beneficial from the improved wettability at no expense of pore size, the electrochemical performances of LIBs such as specific discharge capacities at different discharge rates were upgraded.

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Surface-active Isoporous Membranes Nondestructively Derived from Perpendicularly Aligned Block Copolymers for Size-selective Separation

Wei Sun, Zhaogen Wang, Xueping Yao, Leiming Guo, Xiaoqiang Chen, Yong Wang*, J. Membr. Sci. 2014, 466, 229-237

Isoporous membranes which have narrow pore size distributions and straight pore profiles promise sharp selectivity at no expense of permeability. However, it remains a challenge to produce isoporous membranes in an affordable and efficient way. In this work, we report on the facile preparation of isoporous membranes based on the nondestructive creation of straight pores in block copolymers of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP). Thin films of PS-b-P2VP are annealed to induce the perpendicular orientation of the P2VP cylinders, and then transferred to macroporous supporting membranes to form mechanically robust composite structures. The copolymer films are treated in hot ethanol to convert the P2VP cylinders into straight pores following the selective swelling-induced pore generation mechanism. The perpendicular alignment of the P2VP cylinders in the PS matrix facilitates the swelling-induced pore generation process, making the membrane highly permeable. Because of its ultrathin thickness, high porosity, and strong hydrophilicity, the membrane exhibits a water flux as high as 1686 L m−2 h−1 bar−1, which is about 10 times higher than that of commerical membranes and also much higher than that of other isoporous membranes with similar effective pore sizes. Furthermore, the membrane has an inherent reversible pH-responsive functionality because of the enrichment of protonizable P2VP chains on the pore wall and membrane surface.

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2013

Hydrophilization of Porous Polypropylene Membranes by Atomic Layer Deposition of TiO2 for Simultaneously Improved Permeability and Selectivity

Qiang Xu, Jun Yang, Jiawei Dai, Yang Yang, Xiaoqiang Chen, Yong Wang*, J. Membr. Sci. 2013, 448, 215-222

The uses of porous polypropylene (PP) membranes are limited in water-based separations because of their strong hydrophobicity. To improve the separation performances of polypropylene membranes by hydrophilization, we deposited TiO2 on their pore surface using the atomic layer deposition strategy with and without a pretreatment to the membrane by plasma. The direct deposition without plasma pretreatment led to slightly enhanced hydrophilicity because TiO2 was slowly deposited on the membrane as discrete particles due to the lack of active groups on the bare polypropylene surface. In contrast, after a short exposure to plasma generated in air, oxygen-containing active groups were formed on the membrane and subsequent atomic layer deposition yielded conformal thin layer of TiO2 on the pore walls. The deposited membranes showed remarkably enhanced hydrophilicity and higher deposition cycles led to stronger hydrophilicity. As a consequence of the enhanced hydrophilicity, the permeability and retention of the deposited membrane were simultaneously improved with big amplitudes. For example, an increase in pure water flux of ~60% and a more than doubled retention ratio were obtained by deposition of TiO2 for 150 cycles on the plasma-activated polypropylene membrane. Furthermore, the TiO2-deposited membranes showed improved resistance to protein fouling compared to unmodified membranes also because of the enhanced hydrophilicity. Such a hydrophilization strategy of plasma pretreatment and subsequent atomic layer deposition of metal oxides is expected to be also effective in the upgrading of performances of other membranes with inert surfaces.

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Membranes with Highly Ordered Straight Nanopores by Selective Swelling of Fast Perpendicularly Aligned Block Copolymers

Jun Yin, Xueping Yao, Jiun-You Liou, Wei Sun, Ya-Sen Sun, Yong Wang*, ACS Nano 2013, 7(11), 9961-9974

Membranes with uniform, straight nanopores have important applications in diverse fields, but their application is limited by the lack of efficient producing methods with high controllability. In this work, we reported on an extremely simple and efficient strategy to produce such well-defined membranes. We demonstrated that neutral solvents were capable of annealing amphiphilic block copolymer (BCP) films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) with thicknesses up to 600 nm to the perpendicular orientation within 1 min. Annealing in neutral solvents was also effective to the perpendicular alignment of block copolymers with very high molecular weights,e.g., 362 000 Da. Remarkably, simply by immersing the annealed BCP films in hot ethanol followed by drying in air, the originally dense BCP films were nondestructively converted into porous membranes containing highly ordered, straight nanopores traversing the entire thickness of the membrane (up to 1.1 μm). Grazing incident small-angle X-ray spectroscopy confirmed the hexagonal ordering of the nanopores over large areas. We found that the overflow of P2VP chains from their reservoir P2VP cylinders and the deformation of the PS matrix in the swelling process contributed to the transformation of the solid P2VP cylinders to empty straight pores. The pore diameters can be tuned by either changing the swelling temperatures or depositing thin layers of metal oxides on the preformed membranes via atomic layer deposition with a subnanometer accuracy. To demonstrate the application of the obtained porous membranes, we used them as templates and produced centimeter-scale arrays of aligned nanotubes of metal oxides with finely tunable wall thicknesses.

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Plasma Activation of Porous Polytetrafluoroethylene Membranes for Superior Hydrophilicity and Separation Performances via Atomic Layer Deposition of TiO2

Qiang Xu, Yang Yang, Jie Yang, Xiaozu Wang, Zhaohui Wang, Yong Wang*, J. Membr. Sci. 2013, 443, 62-68

Conformal and smooth TiO2 thin layers were coated on the surface of porous PTFE membranes subjected to a plasma activation process by atomic layer deposition (ALD). In contrast, TiO2 was deposited on the PTFE surface as globular particulates without plasma activation due to the inert chemical nature of PTFE surface. X-ray photoelectron spectrometry confirmed the formation of functional groups on the plasma-activated PTFE surface which served as active sites for the homogeneous adsorption and reaction of ALD precursors. Scanning and transmission electron microscopy revealed the uniformity of the deposited layer and the precise control of its thickness by changing ALD cycle numbers. The water contact angle measurement showed that the hydrophobic surface gradually turned to be near superhydrophilic with the increment of ALD numbers. Also, the membrane mean pore size was progressively reduced by simply altering the ALD numbers. At proper deposition conditions, the modified membranes gained an increase of pure water flux of more than 150% and simultaneously a doubled retention compared to the original unmodified membrane. This research provides an efficient approach devoid of any harsh treatments to modifying PTFE membranes for achieving specific functions, which also can be applied to other organic materials especially with chemical inert surface.

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PVDF Membranes with Simultaneously Enhanced Permeability and Selectivity by Breaking the Tradeoff Effect via Atomic Layer Deposition of TiO2

Qianqian Wang, Xiaoting Wang, Zhaohui Wang, Jun Huang, Yong Wang*, J. Membr. Sci. 2013, 442, 57-64

Polyvinylidene fluoride (PVDF) is one of the most extensively used membrane materials, but PVDF membranes suffer deteriorating performances because of the intrinsic hydrophobic nature of PVDF. Significant efforts have been made to improve the hydrophilicity of PVDF membranes; however, there is frequently a tradeoff between the permeability and selectivity of the modified PVDF membranes. In this work, we broke the tradeoff effect by the atomic layer deposition (ALD) of TiO2 onto the PVDF membranes using TiCl4 and water as precursors. The vaporized precursors accessed the small membrane pores and produced a conformal and uniform thin layer of TiO2 that was tightly adhered to the pore walls. We progressively reduced the membrane pores by repeatedly cycling the deposition process until the membrane pores were completely blocked. The deposition of TiO2 enhanced the hydrophilicity and fouling resistance of the PVDF membranes, which was more evident at higher ALD cycle numbers. We simultaneously achieved greatly improved water flux and retention at 120 ALD cycles as a result of the competing effect of the increased hydrophilicity and reduced pore sizes. Moreover, the water flux could be sensitively tuned by changing the exposure time of the precursors. The thermal stability of the TiO2-deposited membranes was enhanced at a low ALD cycle number of 30.

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Responsive, Fluorescent Micellar Nanospheres of Amphiphilic Block Copolymers for the Characterization of Membrane Pores

Zhiming Yang, Zhaogen Wang, Xueping Yao, Xiaoqiang Chen, Yong Wang*, J. Membr. Sci. 2013, 441, 9-17

Fluorescent polymeric spheres with uniform sizes are of great interests in the measurement and calibration of the pore sizes and defects of porous media, including separating membranes. We reported the preparation of such nanospheres by dissolving amphiphilic block copolymers of polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) together with chromophores (perylene and pyrene) in acetic acid at elevated temperatures. Micellar nanosphere with the chromophores residing in the cores were formed through the heating-enabled micellization process. Following quaternization of the P2VP coronae gave the spheres a positively charged surface and a good water dispersibility. Scanning and transmission electron microscopy and dynamic light scattering analysis confirmed that the micellar nanospheres were uniform in size both in the dry and wet state. The micelle size and the thickness of coronae can be tuned by altering the molecular weight of the constituent blocks. Encapsulation of chromophores in the micelle cores provided a strong fluorescent emission to the micellar nanospheres, while quaternization of the P2VP coronae gave the nanospheres a greatly variable size according to pHs and salt concentrations in the surroundings. Taking advantages of their uniform size, water dispersibility, and fluorescent emission for accurate detection, we demonstrated that the micellar nanospheres were capable of characterizing pore sizes of separating membranes and detecting their large-pore defects.

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Selective-swelling-induced Porous Block Copolymers and Their Robust TiO2 Replicas via Atomic Layer Deposition for Antireflective Applications

Jie Yang, Ling Tong, Yang Yang, Xiaoqiang Chen, Jun Huang, Rizhi Chen, Yong Wang*, J. Mater. Chem. C 2013, 1, 5133-5141

We prepared antireflective nanoporous membranes by an extremely simple method which only involves the immersion of the optical substrates coated with amphiphilic block copolymers in hot ethanol for hours and a subsequent air drying process. Nanometer-sized pores were formed throughout the copolymer films through the swelling-induced pore generation mechanism. The pore structure and size, and consequently the antireflective properties of the porous copolymer membranes, could be modulated by changing the concentrations of thecopolymer solutions and also the swelling temperature and time. A transmittance higher than 99% of the copolymer-coated glass substrate could be obtained at optimized conditions. Furthermore, to obtain robust and easily cleanable antireflective layers, we deposited TiO2 on the porous copolymer membranes by atomic layer deposition followed by calcination in air to burn off the polymer components, producing highly porous TiO2 membranes composed of interconnected nanotubes with thin tube walls. The replicated TiO2 membranes also displayed a good antireflective function because of their high porosity. Thanks to the conformal deposition of TiO2 on the copolymer membranes, the TiO2 replicas maintained structural integrity and mechanical robustness after high temperature calcination, and were tightly adhered to the glass substrate. Moreover, the calcined TiO2 was transformed to anatase and exhibited a photocatalytic effect. We demonstrated that contaminated TiO2 layers could recover their original high transmittance, either by directly rinsing with water or organic solvents, or by UV exposure to degrade organic contaminants.

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Energy-saving, Responsive Membranes with Sharp Selectivity Assembled from Micellar Nanofibers of Amphiphilic Block Copolymers

Xueping Yao, Zhaogen Wang, Zhiming Yang, Yong Wang*, J. Mater. Chem. A 2013, 1, 7100-7110

Membrane technology contributes significantly in a large number of energy- and environment-related fields in a clean and efficient way. It remains a challenge to develop advanced membranes with simultaneous high flux and sharp selectivity. Such membranes require a thin selective layer, high porosity, and strong hydrophilicity. A promising strategy to fabricate such membranes is to build an integral selective layer of nanofibers on a macroporous support. We report an extremely simple method to produce uniform nanofibers with diameters of <30 nm by directly dissolving block copolymers of polystyrene-block-poly(4-pyridine) (PS-b-P4VP) with long, glassy PS blocks in a polar solvent. The fibers were cylindrical micelles with PS cores covered by P4VP coronae, formed through a heating-enabled micellization process. We deposited the fibers on the surface of macroporous supports by vacuum filtration to fabricate composite membranes. The gaps between the fibers served as mesh pores, with effective sizes down to several nanometers. Distinct from other types of nanofibers, the micellar fibers had a P4VP-covered surface, which not only enhanced the adhesion between fibers, but also endowed the membrane with a stimuli-responsive function. The fiber layers could be made very thin, with a thickness of ∼270 nm or even thinner, but were mechanically stable and exhibited a water flux as high as 940 L m−2 h−1 bar−1 at ∼100% rejection to bovine serum albumin. The fiber membrane displays an energy-saving characteristic as it provides high flux under low pressures compared with commercial ultrafiltration (UF) membranes. For example, it produces a flux over 10 times larger than that of commercial UF membranes. The membranes are promising for the removal of particulate contaminants from water as demonstrated by their excellent concentration capability for 5 nm gold colloidal nanoparticles.

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Highly Ordered TiO2 Nanostructures by Sequential Vapour Infiltration of Block Copolymer Micellar Films in an Atomic Layer Deposition Reactor

Jun Yin, Qiang Xu, Zhaogen Wang, Xueping Yao, Yong Wang*, J. Mater. Chem. C 2013, 1 (5), 1029-1036

Sequential vapour infiltration operated in an atomic layer deposition (ALD) reactor was used to controllably deliver precursors of TiO2 into the cores of micellar films of the amphiphilic block copolymer, poly(styrene-block-4-vinylpyridine) (PS-b-P4VP). Vaporized precursors diffused through the PS corona and were exclusively enriched into the P4VP cores. Arrays of hexagonally arranged TiO2 nanoparticles were produced by burning off the polymeric components after UV-crosslinking the TiO2-incorporated micellar films. The size of the TiO2 particles was tunable simply by repeating the cycle numbers, and the interparticle distances were dictated by the distances between neighboring micelles of the original micellar films and could be changed by using block copolymers with different molecular weights. Compared to the extensively used solution impregnation method, the sequential vapour infiltration strategy was distinct in terms of the simplicity accompanying the “dry” process and precise control in particle sizes. More importantly, growth of TiO2 particles inside micellar cores was not limited by the available pyridyl groups as TiO2 continued to grow on preformed TiO2 particles after the pyridyl groups were consumed. Consequently, the particle sizes could be tuned in a much broader range compared to the solution impregnation method in which the particle size was limited by the saturation of impregnated precursors bound to the pyridyl groups. Furthermore, we demonstrated the versatility of this sequential vapour infiltration strategy in producingnanostructures with different morphologies and chemical compositions.

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2012

Atomic Layer Deposition of Alumina on Porous Polytetrafluoroethylene Membranes for Enhanced Hydrophilicity and Separation Performances

Qiang Xu, Yang Yang, Xiaozu Wang, Zhaohui Wang, Wanqin Jin, Jun Huang, Yong Wang*, J. Membr. Sci. 2012, 415-416, 435-443

Porous polytetrafluoroethylene (PTFE) membranes are seeking extensive applications in gas and liquid purification whereas the strong hydrophobicity of PTFE limits their use in water system. We demonstrate that atomic layer deposition (ALD) is a simple and effective method to upgrade the separation performances of PTFE membranes by improving their surface hydrophilicity. Alumina was ALD-deposited on PTFE membranes following a subsurface nucleation and surface growth mechanism, leading to the formation of fine particulates with lower ALD cycles and continuous thick layers with higher ALD cycles on the surface of PTFE membranes, respectively. Thanking to the nanofibril-containing, porous surface morphology, the adhesion between the deposited alumina and PTFE substrate is strong, which can withstand harsh ultrasonication. The hydrophilicity of the deposited membranes was enhanced progressively with the rise of cycle numbers and the membrane with 500 cycles was completely water-wettable with a water angle less than 20°. Improved hydrophilicity of the alumina-deposited PTFE membranes not only affords remarkably enhanced fouling resistance, but also facilitates water permeation through the membrane, giving higher flux. Filtration experiments indicate that the deposited membranes prepared at optimal conditions possessed an increase of pure water flux of more than 50% and simultaneously an increase of retention of 12.4% compared to the pristine PTFE membrane.

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Swelling-induced Mesoporous Block Copolymer Membranes with Intrinsically Active Surfaces for Size-selective Separation

Zhaogen Wang, Xueping Yao, Yong Wang*, J. Mater. Chem. 2012, 22, 20542-20548

Block copolymers (BCPs) are receiving growing interest in the preparation of advanced membranes with regular pores due to their capability to form highly ordered, mesoscopic structures via microphase separation. We report on the fabrication of composite membranes with mesoporous amphiphilic BCPs as the size-selective layer and a macroporous membrane as the supporting layer by coating BCPs onto the supporting membrane. Mesopores were generated in the BCP layer by a selective swelling-induced pore-formation process. The composite membranes showed high pore regularity, strong mechanical robustness, and a separating property that can be tuned simply by changing the swelling time. Furthermore, the polyelectrolyte-natured blocks were exclusively relocated on the pore surface of the BCP layer during the swelling process, rendering an intrinsically active surface on the membrane. As a result, the hydrophilicity and fouling resistance of the membranes were significantly enhanced, and the membranes possessed a reversible pH-sensitive water flux. The membranes were used to separate nanoparticles of similar sizes and it was observed that the membrane subjected to 24 h of swelling was able to discriminate 10 nm gold particles from a mixture containing 2 nm gold particles with ∼100% yield, demonstrating its superior size selectivity.

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Highly Porous Metal Oxide Networks of Interconnected Nanotubes by Atomic Layer Deposition

Fengbin Li, Xueping Yao, Zhaogen Wang, Weihong Xing, Wanqin Jin, Jun Huang, Yong Wang*, Nano Lett. 2012, 12, 5033-5038

Mesoporous metal oxide networks composed of interconnected nanotubes with ultrathin tube walls down to 3 nm and high porosity up to 90% were fabricated by atomic layer deposition (ALD) of alumina or titania onto templates of swelling-induced porous block copolymers. The nanotube networks possessed dual sets of interconnected pores separated by the tube wall whose thickness could be finely tuned by altering ALD cycles. Because of the excellent pore interconnectivity and high porosity, the alumina nanotube networks showed superior humidity-sensing performances.

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Modification of Ceramic Membranes for Pore Structure Tailoring: The Atomic Layer Deposition Route

Fengbin Li, Yang Yang, Yiqun Fan, Weihong Xing, Yong Wang*, J. Membr. Sci. 2012, 397-398, 17-23

The technique of atomic layer deposition (ALD) is applied to the structural modification and pore tailoring of ceramic membranes. Ceramic microfiltration membranes with an average pore size of 50 nm are used as substrates, on which Al2O3 is deposited by ALD with the purpose to tailor their pore size. The thickness of the deposited Al2O3 layer increases with the number of ALD cycles, and it is confirmed by scanning electron microscopy that the pore size of the ceramic membrane decreases with increasing ALD cycles until the pores are completely sealed. It is found that an ultrathin selective layer with a gradient porous structure is formed, whose thickness can be tuned by varying the exposure time of percursors. With the increased ALD cycles, the Al2O3-deposited membranes have a decreasing water flux, and an increasing retention to bovine serum albumin. The membrane subjected to 600 ALD cycles has a reduced pure water flux from 1698 L (m2 h bar)−1 for an undeposited membrane to 118 L (m2 h bar)−1, while its BSA retention increases significantly from 2.9% to 97.1%.

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Water-dispersible, Uniform Nanospheres by Heating-enabled Micellization of Amphiphilic Block Copolymers in Polar Solvents

Zhiming Yang, Zhaogen Wang, Xueping Yao, Yong Wang*, Langmuir 2012, 28(5), 3011-3017

Uniform nanospheres with tunable size down to 30 nm were prepared simply by heating amphiphilic block copolymers in polar solvents. Unlike reverse micelles prepared in nonpolar, oily solvents, these nanospheres have a hydrophilic surface, giving them good dispersibility in water. Furthermore, they are present as individual, separated, rigid particles upon casting from the solution other than continuous thin films of merged micelles cast from micellar solution in nonpolar solvents. These nanospheres were generated by a heating-enabled micellization process in which the affinity between the solvent and the polymer chains as well as the segmental mobility of both hydrophilic and hydrophobic blocks was enhanced, triggering the micellization of the glassy copolymers in polar solvents. This heating-enabled micellization produces purely well-defined nanospheres without interference of other morphologies. The micelle sizes and corona thickness are tunable mainly by changing the lengths of the hydrophobic and hydrophilic blocks, respectively. The heating-enabled micellization route for the preparation of polymeric nanospheres is extremely simple, and is particularly advantageous in producing rigid, micellar nanospheres from block copolymers with long glassy, hydrophobic blocks which are otherwise difficult to prepare with high efficiency and purity. Furthermore, encapsulation of hydrophobic molecules (e.g., dyes) into micelle cores could be integrated into the heating-enabled micellization, leading to a simple and effective process for dye-labeled nanoparticles and drug carriers.

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2011

Precise Pore Size Tuning and Surface Modifications of Polymeric Membranes Using the Atomic Layer Deposition Technique

Fengbin Li, Ling Li, Xingzhi Liao, Yong Wang*, J. Membr. Sci. 2011, 385-386, 1-9

We demonstrated that atomic layer deposition (ALD) technique is an effective method in decorating pore walls of polymeric porous membranes (track etched polycarbonate membranes) by coating the membrane surface and pore walls with a uniform and conformal layer of aluminum oxide. Through ALD coating, the membrane obtained a thin, outer shell of oxide, showing improved hydrophilicity and resistance to acids and organic solvents. More importantly, the membrane pore size could be tuned continuously at a sub-angstrom preciseness simply by altering the numbers of ALD cycles, because the average growth rate of the deposited aluminum oxide was 0.8 Å/ALD cycle. Filtration experiments demonstrated that membranes subjected to increasing ALD cycles gave progressively rise in retention to the solute (bovine serum albumin) and drop in flux. Interestingly, the slightly deposited membrane (subjected to 10 ALD cycles) showed greatly enhanced retention at only a slight expense of the loss of flux because of the hydrophilic nature of the deposited aluminum oxide layer. The ALD approach can be extended to modify/functionalize other membranes since ALD is universal in depositing thin films on almost every type of polymeric and inorganic materials.

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Fabrication, Structural Characterization and Sensing Properties of Polydiacetylene Nanofibers Templated from Anodized Aluminum Oxide

Ling Tong, Bowen Cheng, Zengshe Liu, Yong Wang*, Sensor Actuat. B-Chem. 2011, 155 (2), 584-591

Polydiacetylene (PDA), as a unique conjugated polymer, has shown its potential in the application of chem/bio-sensors and optoelectronics. In this work, we first infiltrated PDA monomer (10, 12-pentacosadiynoic acid, PCDA) melt into anodized aluminum oxide template, and then illuminated the infiltrated template with UV light to initiate the polymerization of PCDA. After etching away the aluminum oxide templates, we obtained solid poly-PCDA nanofibers. We found that even tightly confined in template pores with diameter as small as ∼35 nm, PCDA crystals were able to be polymerized through a solid polymerization mechanism induced by UV light. Poly-PCDA nanofibers, both in the released form and embedded in the template, showed characteristic blue to red color change in sensing exposure of organic solvents and temperature increasing, and the red colored fibers possessed strong fluorescence. Moreover, poly-PCDA nanofibers were highly crystalline, and oriented favorably in some certain direction due to the confinement of nanopores, as demonstrated by X-ray diffraction texture analysis and orientation distribution analysis by scanning confocal optical microscopy.

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An Emerging Pore-Making Strategy: Confined Swelling-Induced Pore Generation in Block Copolymer Materials

Yong Wang*, Fengbin Li, Adv. Mater. 2011, 23, 2134-2148

Block copolymers (BCPs) composed of two or more thermodynamically incompatible homopolymers self-assemble into periodic microdomains. Exposing self-assembled BCPs with solvents selective to one block causes a swelling of the domains composed of this block. Strong swelling in the confinement imposed by the matrix of the other glassy block leads to well-defined porous structures via morphology reconstruction. This confined swelling-induced pore-making process has emerged recently as a new strategy to produce porous materials due to synergic advantages that include extreme simplicity, high pore regularity, involvement of no chemical reactions, no weight loss, reversibility of the pore forming process, etc. The mechanism, kinetics, morphology, and governing parameters of the confined swelling-induced pore-making process in BCP thin films are discussed, and the main applications of nanoporous thin films in the fields of template synthesis, surface patterning, and guidance for the areal arrangements of nanomaterials and biomolecules are summarized. Recent, promising results of extending this mechanism to produce BCP nanofibers or nanotubes and bulk materials with well-defined porosity, which makes this strategy also attractive to researchers outside the nanocommunity, are also presented.

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Swelling-induced Morphology Reconstruction in Block Copolymer Nanorods: Kinetics and Impact of Surface Tension During Solvent Evaporation

Yong Wang*, Ling Tong, Martin Steinhar*, ACS Nano 2011, 5(3), 1928-1938

Nanoscopic domain structures of BCP nanorods can be converted into well-defined mesopore systems by swelling the BCP minority component with a selective solvent at temperatures below the bulk glass transition temperature of the nonswelling matrix. The initial stage of this process involves rapid morphology reconstruction of the nonswelling majority domains to accommodate the increased volume of the swelling minority domains caused by rapid solvent uptake. Morphology reconstruction slows down once entropic restoring forces of the swelling chains impede further uptake of swelling agent. Upon evaporation of the swelling agent, mesopores form in place of the swollen domains as the swollen minority blocks undergo entropic relaxation while intermediate nonequilibrium morphologies in the BCP nanorods are fixated by the reconstructed majority component. The surface area of mesopores developing when swollen cylindrical minority domains collapse may be minimized by the growth of Rayleigh instabilities. Depending on swelling temperature, swelling agent, and BCP architecture, BCP nanorods with one or several cylindrical channels undulated or uniform in diameter running along their long axes, linear strings of spherical cavities, and continuous mesopore systems can be obtained.

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2010

Synthesis of Two-Dimensional Carbon and Carbon–Metal Nanocomposites Using a Natural Cellular Material as the Carbon Precursor

Fengbin Li, Ling Li, Xingzhi Liao, Ling Tong, Zhiquan Chen, Yong Wang*, Eur. J. Inorg. Chem. 2010, 4367-4373

Two-dimensional carbon nanostructures, including carbon nanoflakes that are formed from the disassembly of carbon foams and carbon–metal composites, were directly synthesized using the stem pith of the rice-paper plant (SPRP) as the carbon precursor and also the synthesis template. SPRP has a foamlike porous structure. SPRP was pyrolyzed to prepare carbon foam with ultralow apparent density, and this foam was ruptured to produce carbon nanoflakes with a lateral size up to hundreds of micrometers and a thickness of approximately 100 nm. Palladium and platinum were added to the SPRP foams by wet impregnation, and the impregnated foams were subsequently converted into composites of metal nanoparticles and carbon in the form of foams or nanoflakes. The composites were characterized using electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and Raman spectroscopy. The metal nanoparticles were found to be highly crystalline with diameters in the range of several tens of nanometers (palladium) and several nanometers (platinum), and were uniformly distributed throughout the carbon nanoflakes. The amount of metal in the composite was tunable by changing the impregnation time and the concentration of the impregnation solution.

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(Inside Cover) Nanoporous Metal Membranes with Bicontinuous Morphology from Recyclable Block Copolymer Templates

Yong Wang*, Changcheng He, Weihong Xing*, Fengbin Li, Ling Tong, Zhiquan Chen, Xingzhi Liao, Martin Steinhart*, Adv. Mater. 2010, 22(18), 2068-2072

Nondestructive preparation of bicontinuous nanoporous metal membranes by replication of bicontinuous nanoporous polymeric membranes consisting of recoverable asymmetric block copolymers (BCPs) is reported. The BCP membranes are generated by swelling the minority domains of the BCP with selective solvents accompanied by reconstruction of the glassy matrix formed by the majority component (see figure).

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Responsive Micellar Films of Amphiphilic Block Copolymer Micelles: Control on Micelle Opening and Closing

Zhiquan Chen, Changcheng He, Fengbin Li, Ling Tong, Xingzhi Liao, Yong Wang*, Langmuir 2010, 26(11), 8869-8874

We reported the deliberate control on the micelle opening and closing of amphiphilic polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micellar films by exposing them to selective solvents. We first treated the micellar films with polar solvents including ethanol and water (pH = 4, 8, and 12) that have different affinities to P2VP. We observed opening of the micelles in all the cases. Both the size of opened pores and the opening rate are dependent on the solvency of different solvents for P2VP. We then explored the closing behavior of the opened micelles using solvents having different affinities to PS. We found that the opened micelles were recovered to their initial closed micelle forms. The recovery was accompanied by a slow micelle disassociation process which gradually reduced the micelle size. The rates of the micelle closing and disassociation are also dependent on the solvency of different solvents for PS.

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2009-2003

Nanostructured Gold Films for SERS by Block Copolymer-Templated Galvanic Displacement Reactions

Yong Wang*, Michael Becke, Li Wang, Jinquan Liu, Roland Scholz, Juan Peng, Ulrich Gosele, Silke Christiansen, Dong Ha Kim*, Martin Steinhart*, Nano Lett. 2009, 9, 2384-2389

Up to now, little effort has been made to exploit large-area high-throughput patterning by block copolymer (BCP) lithography to generate nanostructured substrates with periods well below 100 nm for surface-enhanced Raman scattering (SERS). We show that simple BCP-templated galvanic displacement reactions yield dense arrays of mushroom-shaped gold nanopillars with a period of 50 nm. The nanoporous BCP films used as templates were obtained by swelling-induced reconstruction of reverse micelle monolayers deposited on silicon wafers. Coupling of adjacent mushroom caps almost impinging on each other combined with their strong local curvature results in a high spatial density of hot spots in the narrow gaps between them. Thus, substrates characterized by high SERS efficiencies are obtained.

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(Frontspiece) Nanoscopic Morphologies in Block Copolymer Nanorods as Templates for Atomic Layer Deposition of Semiconductors

Yong Wang*, Yong Qin, Andreas Berger, Eric Yau, Changcheng He, Lianbing Zhang, Ulrich Gosele, Mato Knez, Martin Steinhart*, Adv. Mater. 2009, 21, 2763-2766

Block-copolymer nanorods containing mesopore structures derived from confinement-induced nanoscopic morphologies were used as templates for atomic-layer deposition. Diffusion of the ALD precursors through the polymeric scaffold and deposition of ZnO on the walls of the internal mesopores yielded 1D ZnO nanostructures with hierarchical architectures containing helices and stacked doughnuts as structure motifs.

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Nanopatterned Carbon Films with Engineered Morphology by Direct Carbonization of UV-Stabilized Block Copolymer Films

Yong Wang*, Jinquan Liu, Silke Christiansen, Dong Ha Kim, Ulrich Gosele, Martin Steinhart*, Nano Lett. 2008, 8, 3993-3997

Nanopatterned thin carbon films were prepared by direct and expeditious carbonization of the block copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) without the necessity of slow heating to the process temperature and of addition of further carbon precursors. Carbonaceous films having an ordered “dots-on-film” surface topology were obtained from reverse micelle monolayers. The regular nanoporous morphology of PS-b-P2VP films obtained by subjecting reverse micelle monolayers to swelling-induced surface reconstruction could likewise be transferred to carbon films thus characterized by ordered nanopit arrays. Stabilization of PS-b-P2VP by UV irradiation and the concurrent carbonization of both blocks were key to the conservation of the film topography. The approach reported here may enable the realization of a broad range of nanoscaled architectures for carbonaceous materials using a block copolymer ideally suited as a template because of the pronounced repulsion between its blocks and its capability to form highly ordered microdomain structures.

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Mesoporous Block Copolymer Nanorods by Swelling-Induced Morphology Reconstruction

Yong Wang*, Ulrich Gosele, Martin Steinhart*, Nano Lett. 2008, 8, 3548-3553

Engineering the topography of thin block copolymer (BCP) films by surface reconstruction associated with selective swelling of one of the blocks has been investigated intensively. Here we show that swelling-induced structural transitions in nanorods consisting of amphiphilic BCPs involve pronounced reshaping of the nonswollen glassy domains in the course of the transition from the equilibrium morphology of the molten BCP in cylindrical confinement to that of the BCP dissolved in the swelling agent. The reconstruction process can be quenched to retain intermediate nonequilibrium morphologies. The collapse of the swollen chains upon drying yields polymeric nanorods exhibiting complex nanoscopic architectures characterized by a variety of mesopore structures and surface topographies, including channels along the nanorods, bunches of partially interconnected strands, and strings of spheres. The complex BCP nanorods thus obtained can be used as soft templates for the rational arrangement of metal nanoparticles.

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Mesoporous Polymer Nanofibers by Infiltration of Block Copolymers with Sacrificial Domains into Porous Alumina

Yong Wang*, Ulrich Gosele, Martin Steinhart*, Chem. Mater. 2008, 20, 379-381

Mesoporous polymer nanofibers were prepared by degrading sacrificial domains of block copolymers (BCPs) infiltrated into porous alumina. Surface-induced ordering influences the formation of the mesoscopic morphology in porous alumina with pore diameters 1 order of magnitude larger than the BCP period. In smaller pores, nanotubes were obtained.

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Micropatterned Polymer Surfaces Enduced by Nonsolvent

Yong Wang, Zhimin Liu*, Ying Huang, Buxing Han*, Guanying Yang, Langmuir 2006, 22, 1928-1931

In this work, we present a facile method for fabricating polymer thin films with micropatterned surfaces by evaporating polymer solution containing a small amount of nonsolvent of polymer in air. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) films with densely packed micropores on the surfaces were fabricated. This method was also used to prepare three-dimensional PMMA films with micropatterned surfaces. The effects of nonsolvent content; evaporation temperature; and interactions between the polymer, solvent, and nonsolvent on the specific patterns were investigated, and the formation mechanism of the pores is discussed. This simple route can potentially be used, for example, in the large-scale production of patterned surfaces, three-dimensional painting, and hydrophobicity-enhancing coatings.

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Facile Synthesis of Polyaniline Nanofibers Using Chloroaurate Acid as the Oxidant

Yong Wang, Zhimin Liu*, Buxing Han*, Zhenyu Sun, Ying Huang, Guanying Yang, Langmuir 2005, 21, 833-836

This work demonstrated a facile route to the synthesis of polyaniline (PANI) nanofibers by polymerization of aniline using chloroaurate acid (HAuCl4) as the oxidant. The reduction of AuCl4- is accompanied by oxidative polymerization of aniline, leading to uniform PANI nanofibers with a diameter of 35 ± 5 nm and aggregated gold nanoparticles which can precipitate from the liquid phase during the reaction. The resultant PANI nanofibers and gold particles were characterized by means of different techniques, such as UV−vis, FTIR spectroscopy, and scanning and transmission electron microscopy methods. It is found that the gold aggregates are capped with polyaniline, and the conductivity of the fibers is around 0.16 S/cm.

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Carbon Microspheres with Supported Silver Nanoparticles Prepared from Pollen Grains

Yong Wang, Zhimin Liu*, Buxing Han*, Ying Huang, Guanying Yang, Langmuir 2005, 21, 10846-10849

In this paper, we present a new method to fabricate carbon microspheres with supported silver nanoparticles on the surfaces. In this method, pollen grains were first treated with AgNO3 aqueous solution, then preoxidized in air at 300 °C and carbonized in nitrogen at 600 °C, resulting in the silver/carbon nanocomposites. The silver/carbon nanocomposites were characterized by means of SEM, TEM, TG, and XRD. The size and distribution of the silver nanoparticles on the carbon microsphere surface could be controlled by tuning the AgNO3 treatment conditions.

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Compressed-CO2-Assisted Patterning of Polymers

Yong Wang, Zhimin Liu*, Buxing Han*, Ying Huang, Jianling Zhang, Donghai Sun, Jimin Du, J. Phys. Chem. B 2005, 109, 12376-12379

We report a facile route to pattern polymer surfaces with the aid of compressed CO2, termed the compressed-CO2-assisted imprint method. In this method, compressed CO2 serves as a plasticizer for polymers (such as poly(methyl methylate) and polystyrene), which leads to a tremendous reduction in the glass transition temperature and viscosity of the polymers. Nylon fabrics and anode aluminum oxide porous membranes are used as molds, respectively, to pattern the softened polymers at relatively low temperatures, resulting in patterns at the scale of micrometers and nanometers on the surface of polymer films. The patterned structures can be tuned by changing CO2 pressure and temperature in the imprinting process. This method is simple and environmentally benign. It also can be operated at low temperatures, for instance, ambient temperature.

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Phase-Separation-Induced Micropatterned Polymer Surfaces and Their Applications

Yong Wang, Zhimin Liu*, Buxing Han*, Zhenyu Sun, Jianling Zhang, Donghai Sun, Adv. Funct. Mater. 2005, 15, 655-663

We report a route to fabricate micropatterned polymer films with micro- or nanometer-scale surface concavities by spreading polymer solutions on a non-solvent surface. The route is simple, versatile, highly efficient, low-cost, and easily accessible. The concavity density of the patterned films is tuned from 106 to 109  features cm-2, and the concavity size is controlled in the range from several micrometers to less than 100 nm, by changing the film-forming parameters including the polymer concentration, the temperature of the non-solvent and the interactions between polymer, solvent, and non-solvent. We further demonstrate that these concavity-patterned films have significantly enhanced hydrophobicity, owing to the existence of the surface concavities, and their hydrophobicity could be controlled by the concavity density. These films have been used as templates to successfully fabricate convex-patterned polymer films, inorganic TiO2 microparticles, and NaCl nanocrystals. Their other potential applications are also discussed.

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Replication of Biological Organizations through a Supercritical Fluid Route

Yong Wang, Zhimin Liu*, Buxing Han*, Zhenyu Sun, Jimin Du, Jianling Zhang, Tao Jiang, Weize Wu, Zhenjiang Miao, Chem. Commun. 2005, 23, 2948-2950

A novel and simple method to replicate biological organizations (cotton and pollen grains) with high precision was proposed, in which the precursor dissolved in supercritical CO2 reacted with the surface active groups and adsorbed surface water on biological templates, followed by in situ SCFextraction of the byproducts and unreacted precursor, resulting in inorganic replicas faithfully copying both the macro- and microstructures of the biotemplmatchates.

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Fabrication of Flowerlike Polymer Superstructures Using Polymer/Zeolite Composites Prepared with Supercritical CO2

Yong Wang, Zhimin Liu*, Buxing Han, Junchun Li, Haixiang Gao, Jiaqiu Wang, Jianling Zhang, J. Phys. Chem. B 2005, 109, 2605-2609

We report a route to the fabrication of unique flowerlike polymer superstructures with uniform petals at the nanoscale. In this method, polymer/zeolite composite is first prepared by loading corresponding monomer and initiator into the channels of the host zeolite with the aid of supercritical (SC) CO2, followed by thermal polymerization of monomers in the channels of the zeolite. The resultant polymer/zeolite composite is then treated with HF aqueous solution to allow the self-aggregation of the polymer and the inorganic components to form the polymeric layers and inorganic layers. Unique microscale flowerlike polymer superstructures are obtained after further treatment with HF aqueous solution. Different techniques, such as scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetry (TG), have been used to characterize the microflowers.

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A Simple Route to Micropatterned Polymer Surfaces

Yong Wang, Zhimin Liu*, Buxing Han*, Haixiang Gao, Jianling Zhang, Xun Kuang, Chem. Commun. 2004, 800-801

Polymer surfaces with uniform microscale concave arrays were fabricated by spreading polymersolution on the nonsolvent surface, and the concave-patterned surface could further be used as a template to structure convex-patterned polymer surfaces.

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pH Sensitive Polypropylene Porous Membrane Prepared by Grafting Acrylic Acid in Supercritical Carbon Dioxide

Yong Wang, Zhimin Liu*, Buxing Han*, Zexuan Dong, Jiaqiu Wang, Donghai Sun, Ying Huang, Guanwen Chen, Polymer 2004, 45, 855-860

pH sensitive membrane was prepared by grafting acrylic acid (AA) on the porous polypropylene (PP) membrane using supercritical (SC) CO2 as a solvent. The monomer (AA) and the initiator (benzyl peroxide, BPO) were impregnated into the PP substrate with the aid of SC CO2, and were grafted onto the microporous PP substrate. The grafted membranes were characterized by Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), and the water permeability of the virgin and grafted membranes were determined at different pH values. It was demonstrated that the grafting degree (Dg) could be easily controlled by varying operating conditions, such as the monomer concentration, pressure, and temperature during the soaking process. The water permeation of the virgin membrane is nearly independent of pH. However, the water permeation of grafted membranes decreases dramatically with pH as the pH varies from 3 to 6 because the conformation of the PAA changes significantly with the pH of the contacting solution.

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Synthesis of Polypropylene/ZnS Composite Using the Template Prepared by Supercritical CO2

Yong Wang, Zhimin Liu*, Buxing Han, Jiangling Zhang, Tao Jiang, Weize Wu, Guanying Yang, Ying Hang, Chem. Phys. Lett. 2003, 381, 271-277

A new template membrane, acrylic acid grafted microporous polypropylene (PP) membrane, was prepared using the supercritical (SC) CO2 as a solvent. Then zinc ions were anchored in the membrane by forming a monodentate complex with the carboxyl groups in the template membrane. Subsequent treatment with Na2S aqueous solution led to the formation of ZnS nanoparticles within the PP matrix, resulting in ZnS/PP nanocomposite. UV–Vis and Fourier Transform Infrared (FTIR) spectra, and energy-dispersive X-ray analysis (EDS) confirmed the existence of ZnS in the grafted membrane. Field emission scanning electronic microscopy (FE-SEM) and transmission electron microscopy (TEM) images showed that the ZnS nanoparticles distributed uniformly in the composite.

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