Reversibly Cross-Linked Isoporous Membranes Fabricated by the Recyclable Block Copolymer with Pendent Dithiolane Groups.

The reversible formation and cleavage of disulfide bonds under physical/chemical stimuli make it a valuable motif in constructing dynamically cross-linked materials. In the present work, the block copolymer bearing pendent dithiolanes was synthesized and fabricated into isoporous membranes by the combination of self-assembly and nonsolvent-induced phase separation strategy. The cross-linking within the membrane was realized by the thiol-initiated ring-opening cascades of cyclic disulfides. Successful formation of disulfide bond networks within the isoporous membranes was proved by the Raman spectra, UV-vis diffuse reflectance spectroscopy, differential scanning calorimetry, and rheological analysis. The cross-linking in membranes was further demonstrated by the notably improved toughness and obviously enhanced swelling resistance to acid/alkaline solution as well as organic solvents. Importantly, the cross-linked isoporous membranes were fully dissolvable in solution containing dithiothreitol, which enabled the complete cleavage of disulfide bonds and successful recovery of the block copolymer that was able to be repeatedly fabricated into isoporous membranes with pore sizes identical to membranes prepared from the freshly synthesized copolymer. Our results indicate that dynamically cross-linked isoporous membranes with improved durability and good recyclability can be custom-made by simply incorporating active dithiolane moieties into self-assembling block copolymers.

Ionic Dendrimer Based Polyamide Membranes for Ion Separation.

Separating low/high-valent ions with sub-nanometer sizes is a crucial yet challenging task in various areas (e.g., within environmental, healthcare, chemical, and energy engineering). Satisfying high separation precision requires membranes with exceptionally high selectivity. One way to realize this is constructing well-designed ion-selective nanochannels in pressure-driven membranes where the separation mechanism relies on combined steric, dielectric exclusion, and Donnan effects. To this aim, charged nanochannels in polyamide (PA) membranes are created by incorporating ionic polyamidoamine (PAMAM) dendrimers via interfacial polymerization. Both sub-10 nm sizes of the ionic PAMAM dendrimer molecules and their gradient distributions in the PA nanofilms contribute to the successful formation of defect-free PA nanofilms, containing both internal (intramolecular voids) and external (interfacial voids between the ionic PAMAM dendrimers and the PA matrix) nanochannels for fast transport of water molecules. The external nanochannels with tunable ionizable groups endow the PA membranes with both high low/high-valent co-ion selectivity and chemical cleaning tolerance, while the ion sieving/transport mechanism was analyzed by employing the Donnan steric pore model with dielectric exclusion.

Enhancing the antifouling property of polymeric membrane via surface charge regulation.

In this study, positively charged monomers were grafted onto negatively charged membranes via UV radiation to improve the antifouling/antibiofouling properties of the polymeric membrane and the stability of the modification layer. The surface properties, morphologies, antifouling and antibiofouling properties, and stability of the modified membranes were systematically characterized. Results indicated that the introduction of [2-(methacryloyloxy) ethyl] trimethylammonium chloride (MTAC) monomers onto polyethersulfone (PES)/sulfonated polyethersulfone (SPES) membranes effectively increased the surface hydrophilicity. Meanwhile, the surfaces were neutralized with ~0 mV zeta potential in pH 3-10. Moreover, the formation of a polyampholytic copolymer and the antibacterial ability of MTAC considerably improved the antibiofouling properties of the modified membranes. The MTAC-grafted PES/SPES membranes showed excellent antifouling/antibiofouling properties during the treatment of various types of wastewater, including bovine serum albumin solution, oil/water emulsion, and bacterial suspension. Therefore, this study provides a simple and effective method of constructing stable and antifouling membranes for sustainable water treatment.

Antifouling and antibacterial behavior of membranes containing quaternary ammonium and zwitterionic polymers.

To overcome the organic-/bio- fouling of the membrane, a dual-functional ultrafiltration membrane containing quaternary ammonium and zwitterionic polymers via quaternization and surface radical polymerization was designed, and its antifouling and antibacterial behavior was studied. In this work, poly(vinylidene fluoride)/poly(methyl methacrylate-co-dimethylamino-2-ethyl methacrylate) (PVDF/P(MMA-co-DMAEMA)) blend membrane was quaternized by p-chloromethyl styrene (p-CMS), and the double bonds were introduced onto the membrane surface, which further participated in the polymerization of zwitterionic monomers on the membrane surface. The results indicated that the resultant membrane exhibited obviously improved hydrophilicity and weak positive charge (isoelectric point, 7.49). The membrane presented higher flux recovery ratio and lower protein adhesion compared with the pure PVDF membrane. Meanwhile, the membrane showed high-efficiency broad-spectrum antibacterial performance, that is, the bacteria killing efficiency of S. aureus and E. coli reached 98.2% and 97.0%, respectively. Moreover, the membrane effectively inhibited bacterial adhesion, which is important for the long-term antibacterial properties of membrane. This antifouling and antibacterial PVDF membrane may have potential in the long-term filtration process, especially when dealing with microbiologically contaminated water.

Improved permeability and antifouling properties of polyvinyl chloride ultrafiltration membrane via blending sulfonated polysulfone.

To improve the permeability and antifouling properties of polyvinyl chloride (PVC) ultrafiltration (UF) membrane, amphiphilic sulfonated polysulfone (SPSF) was introduced into PVC matrix. Three types of PVC/SPSF blend membranes containing different SPSF with the sulfonation degree (SD) of 20%, 30%, and 50% were fabricated by non-solvent induced phase separation (NIPS) process. The excellent compatibility between PVC and SPSF was confirmed by differential scanning calorimetry (DSC). Surface chemical compositions, morphology, roughness, charge, hydrophilicity, permeability and antifouling properties of the pristine PVC membrane and the PVC/SPSF blend membranes were systematically compared and characterized. Due to the improved hydrophilicity and endowed negative charge, the blend membrane showed high water permeability (i.e. 880 L m-2h-1 bar-1), high bovine serum albumin (BSA) rejection (i.e. 95.7%), and high flux recovery ratio (i.e. 96%), which outperformed ever reported and commercialized PVC membranes. Furthermore, the permeability and rejection properties of PVC/SPSF UF membranes were maintained after soaking in acidic and alkaline solutions for 30 days, indicating their outstanding acid and alkali tolerance. Therefore, SPSF was expected as a potential versatile modifier for fabricating high performance UF membranes.

Electrostatic Adsorption Behavior of Zwitterionic Copolymers on Negatively Charged Surfaces.

To investigate the effect of the surface properties and the coating layer properties on surface modification via electrostatic adsorption, the electrostatic adsorption behavior of zwitterionic copolymers on negatively charged surfaces was studied. A series of positively charged zwitterionic copolymers and a series of negatively charged surfaces, including porous substrates and dense films, were fabricated. The electrostatic adsorption behavior of the zwitterionic copolymers on the negatively charged porous substrates was confirmed using the contact angles and fluorescently labeled protein adsorption experiments. The adsorption behavior of the zwitterionic copolymers on the negatively charged dense films was confirmed using quartz crystal microbalance determination and a fluorescently labeled protein adsorption experiment. The results indicated that a lower charge density on the zwitterionic copolymer brings about a higher adsorption mass on the charged surface, whereas an extremely low charge density on the coating layer results in a lower adsorption mass on the charged surface, due to weak interaction. A high density of the film surface charge is beneficial for surface adsorption, whereas an extremely high density of the film surface charge leads to low surface adsorption due to steric hindrance of the negatively charged sites. This work provides an insight into the best strategy for surface modification via electrostatic adsorption.

Negatively-charged nanofiltration membrane and its hexavalent chromium removal performance.

To enhance hexavalent chromium (Cr(VI)) removal performance under acidic conditions, the nanofiltration (NF) membrane with enhanced negative charge was fabricated via introducing 2, 5-diaminobenzenesulfonic acid (DABSA) into polyamide layer. The control membrane (NF-P) was directly prepared from piperazine and 1, 3, 5-benzenetricarbonyltrichloride. Surface chemical compositions, morphology, surface charge, pore size, permeability and pH-dependent separation performance of the fabricated membranes were characterized. The membranes showed the similar water permeance (∼11.5 L m-2 h-1 bar-1) and Na2SO4 rejections (∼98%) under neutral environments. The DABSA introduced NF membrane (NF-PD) was negatively charged in the pH range of 2.5-11, while the isoelectric point for NF-P was ∼pH 4.0. Cr(VI) removal ability was then evaluated under various concentrations and pH environments. The results indicated that NF-PD showed the better Cr(VI) rejection performance in all tested conditions than NF-P, especially under acidic environments (e.g., pH 4 and pH 5). Moreover, there was a fluctuation of the rejection with the increase of Cr(VI) concentration under acidic environments, which was mainly caused by the formation of Cr2O72- species. The harmful Cr(VI) was efficiently removed by the NF membrane with enhanced negative charge under acidic environments, which indicated the wider application range of the NF membrane.

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.

A positively charged tight UF membrane and its properties for removing trace metal cations via electrostatic repulsion mechanism.

The development of highly efficient membranes technology using low-pressure driven filtration process, is one of the principal challenges in the wastewater treatment field, especially those aimed at the removal of trace heavy metals. In this work, a novel positively charged tight ultrafiltration (PCTUF) membrane was developed to remove heavy metal cations (Mn2+, Co2+, Ni2+, Zn2+ and Cd2+) from contaminated waters via electrostatic repulsion mechanism. The PCTUF membrane was fabricated from a new polymer with poly (vinyl chloride co dimethylaminoethyl methacrylate), P (VC-co-DMA) via a nonsolvent induce phase separation (NIPS) process and following facile surface quaternization. The quaternization conditions, the pore structures and chemical properties of the membranes were investigated in detail. The optimally quaternized membrane possessed a positively charged surface and 3.27 nm charged channel with the water permeability of 84 L m-2 h-1 bar-1. The rejections of heavy metal cations surpassed 95% for feed solutions containing 10 ppm heavy metal. Moreover, the influences of feed concentrations and the operating condition with pressure and pH on the membrane performances were also investigated. The results revealed that the prepared PCTUF membrane with its high perm-selectivity performance provides a worthy reference for highly efficient removal of heavy metal cations.

Fouling-Resistant and Self-Cleaning Aliphatic Polyketone Membrane for Sustainable Oil-Water Emulsion Separation.

The cost-effective treatment of emulsified oily wastewater discharged by many industries and human societies is a great challenge. Herein, based on an aliphatic polyketone (PK) polymer with a good membrane formation ability and an intrinsic intermediate hydrophilicity, a new class of reduced PK (rPK) membranes combining an all hydrophilic and electrically neutral surface chemistry comprising ketone and hydroxyl groups, and a fibril-like morphology featuring re-entrant structure, was facilely prepared by phase separation and following fast surface reduction. The synergetic cooperation of surface chemistry and surface geometry endowed the prepared membranes with excellent superhydrophilicity, underwater superoleophobicity, and underoil superhydrophilicity, in addition to antiprotein-adhesion property. Thus, fouling-resistant and self-cleaning filtrations of challenging oil-in-water emulsions containing adhesive oil, surfactant, high salinity, and proteins were effortlessly realized with high flux (up to ∼50 000 L m-2 h-1 bar-1), slow and reversible flux decline, and low oil permeate (<20 ppm). In contrast, a commercial superhydrophilic microporous membrane made of mixed cellulose ester suffered severe fouling gradually or immediately when carrying out the emulsion filtrations due to its less than ideal surface properties. It is believed that this class of membranes with desirable superwettability, high flux, and preparation simplicity can be a potential new benchmark for high performance and large-scale oil-water separation in complex environments.

Dual Superlyophobic Aliphatic Polyketone Membranes for Highly Efficient Emulsified Oil-Water Separation: Performance and Mechanism.

Efficient treatment of difficult emulsified oil-water wastes is a global challenge. Membranes exhibiting unusual dual superlyophobicity (combined underwater superoleophobicity and underoil superhydrophobicity) are intriguing to realize high-efficiency separation of both oil-in-water and water-in-oil emulsions. For the first time, a robust polymeric membrane demonstrating dual superlyophobicity to common apolar oils was facilely fabricated via a simple one-step phase separation process using an aliphatic polyketone (PK) polymer, thanks to a conjunction of intermediate hydrophilicity and re-entrant fibril-like texture upon the prepared PK membrane. Further chemical modification to improve surface hydrophilicity slightly can enable dual superlyophobicity to both apolar and polar oils. It is found that a nonwetting composite state of oil against water or water against oil was obtainable on the membrane surfaces only when the probe liquids possess an equilibrium contact angle (θow or θwo) larger than the critical re-entrant angle of the textured surfaces (73°), which can explain the existences of dual superlyophobicity and also the nonwetting to fully wetting transitions. A simple design chart was developed to map out the operational windows of material hydrophilicity and re-entrant geometry, that is, a possible zone, to help in the rational design of similar interfacial systems from various materials. Switchable filtrations of oil-in-water and water-in-oil nanoemulsions were achieved readily with both high flux and high rejection. The simplicity and scalability of the membrane preparation process and the well-elucidated underlying mechanisms illuminate the great application potential of the PK-based superwetting membranes.

One-step fabrication of polyamide 6 hollow fibre membrane using non-toxic diluents for organic solvent nanofiltration.

We developed new polyamide 6 hollow fibre membranes using a green process to fabricate cutting-edge "organic solvent nanofiltration" membranes by one-step spinning process for organic solvent separation. This economic and sustainable membrane showed good rejection and durability performance in various organic solvents.