Tailor-made ß-ketoenamine-linked covalent organic polymer nanofilms for precise molecular sieving.

The membranes that accurately separate solutes with close molecular weights in harsh solvents are of crucial importance for the development of highly-precise organic solvent nanofiltration (OSN). The physicochemical structures of the membrane need to be rationally designed to achieve this goal, such as customized crosslinked networks, thickness, and pore size. Herein, we synthesize a type of covalent organic polymer (COP) nanofilms with tailor-made thickness and pore structure using a cyclic deposition strategy for precise molecular sieving. By elaborately designing monomer structures and controlling deposition cycle numbers, the COP nanofilms linked by robust ß-ketoenamine blocks were endowed with sub-nanometer micropores and a linearly tunable thickness of 10-40 nm. The composite membranes integrating COP nanofilms exhibited adjustable solvent permeance. The membranes further demonstrated steep and finely-regulated rejection curves within the molecular weight range of 200 to 400 Da, where the difference value was as low as 40 Da. The efficient purification and concentration of the antibacterial drug and its intermediate was well achieved. Therefore, the exploited COP nanofilms markedly facilitate the application of microporous organic polymers for precise molecular separation in OSN.

Ultrathin Polyamide Nanofilms with Controlled Microporosity for Enhanced Solvent Permeation.

Organic solvent nanofiltration (OSN) technology shows reduced energy consumption by almost 90% with great potential in achieving low-carbon separation applications. Polyamide nanofilms with controlled intrinsic and extrinsic structures (e.g., thickness and porosity) are important for achieving such a goal but are technically challenging. Herein, ultrathin polyamide nanofilms with controlled microporosity and morphology were synthesized via a molecular layer deposition method for OSN. The key is that the polyamide synthesis is controlled in a homogenous organic phase, rather than an interface, not only involving no monomer kinetic diffusion but also broadening the applicability of amine monomers. The particular nonplanar and rigid amine monomers were superbly used to increase microporosity and the nanofilm was linearly controlled at the nanometer scale to decrease thickness. The composite membrane with the polyamide nanofilms as separation layers displayed highly superior performance to current counterparts. The ethanol and methanol permeances were up to 5.5 and 14.6 L m-2 h-1 bar-1, respectively, but the molecular weight cutoff was tailored as low as 300 Da. Such separation performance remained almost unchanged during a long-term operation. This work demonstrates a promising alternative that could synergistically control the physicochemical structures of ultrathin selective layers to fabricate high-performance OSN membranes for efficient separations.

Robust and Scalable In Vitro Surface Mineralization of Inert Polymers with a Rationally Designed Molecular Bridge.

The artificial integration of inorganic materials onto polymers to create the analogues of natural biocomposites is an attractive field in materials science. However, due to significant diversity in the interfacial properties of two kinds of materials, advanced synthesis methods are quite complicated and the resultant materials are always vulnerable to external environments, which limits their application scenarios and makes them unsuitable for scalable production. Herein, we report a simple and universal approach to achieve robust and scalable surface mineralization of polymers using a rationally designed triple functional molecular bridge of fluorosilane, 3-[(perfluorohexyl sulfonyl) amino] propyltriethoxy silane (PFSS). In a two-step solution deposition, the fluoroalkyl and siloxane of the PFSS take charge of its adhesion and immobilization onto polymers by hydrophobic interaction and wrapping-like chemical cross-linking, and then the assembly and growth of inorganic nanoclusters for integration are achieved by strong chemical coordination of PFSS sulfonamide. The versatile mineralization of inorganic oxides (e.g., TiO2, SiO2, and Fe2O3) onto chemically inert polymer surfaces was realized very well. The resultant mineralized materials exhibit robust and multiple functionalities for hostile applications, such as hydrophilic membranes for removing oils in strong acidic and alkaline wastewaters, fabrics with advanced anti-bacteria for healthy wearing, and plates with strong mechanical performance for better use. Experimental results and theoretical calculations confirmed the homogenous distribution of the PFSS onto polymers via cross-linking for robust coordination with inorganic oxides. These results demonstrate a skillful enlightenment in the design of high-performance mineralized polymer materials used as membranes, fabrics, and medical devices.

Microporous Matrimid/PIM-1 Thin Film Composite Membranes with Narrow Pore Size Distribution used for Molecular Separation in Organic Solvents.

Polymers of intrinsic microporosity (PIMs) are a class of microporous organic materials that contain interconnected pores of less than 2 nm in diameter. Such materials are of great potential used in membranes for molecular separation, such as drug fractionation in pharmaceutical industry. However, the PIMs membranes are often susceptible to low separation selectivity toward different molecules due to their wide pore size distribution. Herein, a linear polyimide, Matrimid, is incorporated with PIM-1 (a typical member of PIMs) by solution blending, and the blends are dip-coated onto a polyimide P84 support membrane to prepare thin-film composite (TFC) membranes to control pore size distribution while keep high microporosity. The component miscibility, pore characteristics, and molecular separation performances of the Matrimid/PIM-1 TFC membranes are investigated in detail. The Matrimid and PIM-1 are partially miscible due to their similar Hansen solubility parameters. The Matrimid endows the selective layers (coatings) with narrower pore size distribution due to more compact chain packing. The prepared Matrimid/PIM-1 TFC membranes show high selectivity for separation of riboflavin (80% of retention) and isatin (only 5% of retention). The developed membranes exhibit great potential for separating molecules with different molecular weights.

Ultrathin metal-organic framework nanosheets as building blocks of lamellar nanofilms for ultrafast molecular sieving.

Metal-organic framework (MOF) nanosheets have significant potential applications including separation, catalysis, and sensors. However, the on-demand design with tunable thickness and morphology remains a great challenge, leading to difficulties in modulating their hierarchical assembly for the preparation of macroscopic films. Herein, we report the successful synthesis of smooth and ultrathin MOF (Cu-TCPP (TCPP = 4,4,4,4-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid))) nanosheets used in lamellar nanofilms for the rejection of organic molecules from water. Dopamine hydrochloride (DA-HCl) is used as an adjuvant in the synthesis. Facilitated by a HCl acid environment and DA competitive coordination, the normal and lateral growths of Cu-TCPP nanosheets are modulated to achieve the desired thickness and morphology. DA-HCl can be also easily removed from the nanosheets without affecting their physicochemical properties. The as-synthesized nanosheets are utilized as nanofilm building blocks in which they are stacked into ordered bricks. The obtained membrane displays an ultrahigh water permeance of 2540 L m-2 h-1 bar-1, which is two orders of magnitude higher than the currently reported polymer membranes, while it does not sacrifice the solute rejection as completely determined by the intrinsic pore size of the nanosheets (i.e., 98.8% for molecules larger than 1.3 nm). This work provides a novel and facile strategy to tailor the morphology of the MOF nanosheets for maximizing their functionalities and structure superiority in many engineering applications.

Enhancing membrane surface antifouling by implanting amphiphilic polymer brushes using a swelling induced entrapment technique.

In this work, a swelling induced entrapment technique was developed to enhance the hydrophilicity and antifouling performances of polypropylene (PP) microfiltration membranes. By this method, three amphiphilic polymers with different chemical structures (i.e., a homopolymer (polypropylene glycol), a di-block copolymer (oligoethylene glycol monooctadecylether), and a tri-block copolymer of ethylene glycol (EO) and propylene glycol) were successfully implanted onto membrane surfaces to be polymer brushes with high density, without having a significant effect on the membrane pore structure. The polymer brushes significantly enhanced the hydrophilicity and protein fouling resistance of the membrane. In particular, when using the di-block copolymer with a short hydrophilic EO chain, the modified membrane showed a low water contact angle, down to 20°, and low adsorption of bovine serum albumin of 1.1 µg cm-2. Furthermore, the implanted polymer brushes exhibited excellent durability. The hydrophobic segments of amphiphilic polymers played a leading role in the implantation and stability of the brushes on the PP membrane surface. This work provides a feasible strategy to achieve surface hydrophilicity and antifouling performances in a hydrophobic membrane for use in high-efficiency water treatment.

Macroporous membranes doped with micro-mesoporous ß-cyclodextrin polymers for ultrafast removal of organic micropollutants from water.

Herein, we report novel macroporous membranes doped with micro-mesoporous ß-cyclodextrin polymers (ß-CDP), named ß-CDP membranes, for water decontamination by the flow-through process. These membranes combine excellent adsorption behavior of ß-CDP and the advantages of membranes filtration including low energy consumption and easy scale-up. Filtration adsorption results demonstrated that the optimal ß-CDP membrane removed > 99.9% of bisphenol A with ultrahigh water flux (3000 L m-2 h-1) or high concentration (50 mg L-1). The dynamic adsorption capacity of the membrane was close to the static maximum adsorption capacity of membrane, suggesting the effective accessibility of adsorption sites. The outstanding adsorption performance was attributed to the synergistic effect of the fast adsorption of ß-CDP, abundant ß-CDP nanoparticles and large contact area offered by spongy pores. Furthermore, not only single other organic micropollutants but also mixture was completely removed by the ß-CDP membranes. In addition, the membranes were easily regenerated by simple ethanol filtration.

Antifouling Double-Skinned Forward Osmosis Membranes by Constructing Zwitterionic Brush-Decorated MWCNT Ultrathin Films.

Pressure retarded osmosis (PRO) process is hindered by severe fouling occurring within the porous support of the forward osmosis (FO) membranes. We designed a novel double-skinned FO membrane containing a polyamide salt-rejecting layer and a zwitterionic brush-decorated, multiwalled carbon nanotube (MWCNT/PSBMA) foulant-resisting layer on the back side. Our results demonstrated that the coating of the MWCNT/PSBMA layer on the porous polyketone (PK) support imparted enhanced hydrophilicity and smaller membrane pore size, thereby providing excellent resistance toward both protein adhesion and bacterial adsorption. We also further evaluated this resultant double-skinned membrane (i.e., TFC-MWCNT/PSBMA) in dynamic PRO fouling experiments using protein and alginate as model organic foulants. Compared to the pristine TFC-PK and hydrophobic TFC-MWCNT membranes, the TFC-MWCNT/PSBMA membrane exhibited not only the lowest water flux decline but also the highest water flux recovery after simple physical flushing. These results shed light on fabrication of antifouling PRO membranes for water purification purposes.

Preparation of positively charged composite nanofiltration membranes by quaternization crosslinking for precise molecular and ionic separations.

Developing nanofiltration (NF) membranes with highly efficient and precise separation ability is of great significance for the molecular and ionic separations. In this work, positively charged composite NF membranes were engineered via soaking of polysulfone (PSF) ultrafiltration membranes in poly(N-vinyl imidazole) (PVI) solutions followed by a quaternization crosslinking step. The PVI was firmly attached to the PSF membrane by this method and acted as an active separation layer of the composite NF membrane. The obtained composite NF membrane featured a high rejection (83%) to vitamin B12 (molecular radius: 0.74 nm) but a low rejection (24.6%) to vitamin B2 (molecular radius: 0.47 nm), exhibiting a great potential in precisely molecular separation. Furthermore, the ionic separation ability of the composite NF membrane was confirmed with a rejection order of Na2SO4 < MgSO4 < NaCl < MgCl2 and the MgCl2 rejection reached up to 90.1%. Compared to conventional polyamide NF membranes, the developed PVI/PSF composite NF membranes were characterized with high separation precision to organic molecules, higher rejection over cationic ions than over anionic ones, better chlorine resistance and stability in long-term operation. In addition, the membrane fabrication process is convenient and easily scaled up in industry. This work offers a novel alternative of NF membranes for high precision in molecular and ionic separations.