Polyamide Network Research Articles

Advanced Lithium Extraction Membranes Derived from Tagged‐Modification of Polyamide Networks

AbstractEfficient Mg2+/Li+ separation is crucial to combating the lithium shortage worldwide, yet current nanofiltration membranes suffer from low efficacy and/or poor scalability, because desirable properties of membranes are entangled and there is a trade‐off. This work reports a “tagged‐modification” approach to tackle the challenge. A mixture of 3‐bromo‐trimethylpropan‐1‐aminium bromide (E1) and 3‐aminopropyltrimethylazanium (E2) was designed to modify polyethylenimine – trimesoyl chloride (PEI‐TMC) membranes. E1 and E2 reacted with the PEI and TMC, respectively, and thus, the membrane properties (hydrophilicity, pore sizes, charge) were untangled and intensified simultaneously. The permeance (34.3 L m−2 h−1 bar−1) and Mg2+/Li+ selectivity (23.2) of the modified membranes are about 4 times and 2 times higher than the pristine membrane, and they remain stable in a 30‐days test. The permeance is the highest among all analogous nanofiltration membranes. The tagged‐modification method enables the preparation of large‐area membranes and modules that produce high‐purity lithium carbonate (Li2CO3) from simulated brine.

Synthesis and Characterization of Amorphous Selenium, Cadmium and Silver Selenide Thin Films on Polyamide-6

Increasing photon absorption by capturing light is an important way to increase the efficiency of photovoltaic devices. In this regard, the small optical band gap (Eg) and high absorption coefficient of Se-containing thin nanofilms make them ideal for next generation photovoltaic devices based on selenides. Amorphous selenium was introduced into polyamide-6 (PA 6) via a chemical synthesis in a bath and the influence of the products of its reaction with Cd2+ and Ag+ ions on the film phase composition, topographic and optical properties were evaluated. AFM data have revealed that the surface roughness of the a-Se/PA 6 composite noticeably increases compared to that of unreacted PA 6. However, at later stages of film deposition, the roughness decreases, and the thin film becomes smoother and uniform. The incorporation of solid inorganic nanoparticles into flexible polyamide network causes chain stretching, which has been confirmed by ATR-FTIR spectroscopy data. The data of X-ray diffraction analysis, depending on the stage of synthesis, showed the crystalline composition of the film with peaks of Se8, CdSe, Ag2Se and Ag, which may explain the observed optical properties. The optical properties of the composites indicate a shift in the band gap from 4.46 eV for PA 6 to 2.23–1.64 eV upon the stepwise deposition of amorphous Se, CdSe and Ag2Se. Eg is conveniently located in the visible region of solar energy, making the obtained nanofilms ideal for solar energy harvesting.

In-situ growth of poly(m-phenylenediamine) in polyethylene (PE) matrix for nanofiltration membranes enabling outstanding selectivity of both mono-/di-valent ions and dyes/salt mixtures with enhanced water flux

Membrane-based nanofiltration is a crucial technique for treating dyestuff-salts wastewater, owing to its excellent selectivity towards mono/multivalent ions and organic matter. In this study, a facile and highly practical method was employed to enhance the hydrophilicity of the PE matrix through in-situ growth of poly(m-phenylenediamine) on the PE fibrils via the chemical oxidative polymerization of MPD. The inclusion of the poly(m-phenylenediamine) on PE enables the formation of homogenous and defect-free PA layer showing a unique honeycomb-like Turing morphology with characteristic crater-belt structure. Furthermore, the pendant amine groups from the poly(m-phenylenediamine) chain promotes the formation of a PA layer with a smaller effective mean pore size and a sharper pore size distribution, resulting from the formation of an extra polyamide network (reaction of PMPD with TMC) within the PA layer. The as-prepared PE-based NF membrane exhibits a high pure water permeance of 17.7 LMH bar−1, exceptional Na2SO4 rejection of 99.3%, and a long-term stability for cross-flow filtration for over 10 days. The PE-based NF membrane exhibits the water flux 2.2 times higher than that of the commercially available DK and DL membranes while maintaining a high salt rejection. Additionally, the PE-based NF membrane demonstrates exceptional selectivity, with a value of 115 for the separation of NaCl/Na2SO4 and 339.8 for Cl-/SO42- in a binary salt mixture. Furthermore, the PE-based NF membrane shows ultra-high selectivity values of 990 for CR/NaCl and 1039 for MB/NaCl in the treatment of dye/salt mixtures, respectively, which are significantly higher than those of the DK and DL membranes. The findings of this study highlight the potential of the in-situ growth of poly(m-phenylenediamine) on PE fibrils as a simple and cost-effective method for the modulation of hydrophilicity to enhance the NF membrane performance.

Benzene Tetraamide: A Covalent Supramolecular Dual Motif in Dynamic Covalent Polymer Networks.

In dynamic polyamide networks, 1,2,4,5-benzene tetraamide (B4A) units act simultaneously as a dynamic covalent cross-linker and as supramolecular stacking motif. This results in materials with a rubbery plateau modulus that is about 20 times higher than that of a corresponding reference network in which the supramolecular interaction is suppressed. In branched polyamides with the same B4A dynamic motif, hydrogen bonding and stacking lead to strong and reversible supramolecular networks, whereas a branched polyamide with the nonstacking reference linker is a viscous liquid under the same conditions. Wide-angle X-ray scattering and variable-temperature infrared experiments confirm that covalent cross-linking and stacking cooperatively contribute to the dynamics of the network. Stress relaxation in the reference network is dominated by a single mode related to the dynamic covalent chemistry, whereas relaxation in the B4A network has additional modes assigned to the stacking dynamics.

In-situ rapid construction of aliphatic amine combined polyamide seawater reverse osmosis membrane for efficient boron removal

Polyamide (PA) reverse osmosis membranes are commonly employed in seawater desalination owing to their effective salt rejection and water permeability; however, the elimination of small and neutral boron molecules from seawater remains a significant hurdle in energy-efficient and cost-effective desalination processes. In this work, a seawater reverse osmosis (SWRO) membrane with powerful boron removal competence was designed by adopting an in-situ rapid integration protocol, which utilized aliphatic amines as hydrophobic barriers by bonding the residual chloride groups upon the membrane surface and as molecular plugs by embedding in the PA networks. Consequently, it resulted in a notable improvement in the rejection of neutral boron molecules due to enhanced steric hindrance caused by immobilized amine plugs and synergistically tunned hydrophobic interactions. The permeability coefficient of boron decreased from 4.8 to 0.9 L m−2 h−1, and the boron rejection increased from 80.7 to 90.5% under the modification conditions with the optimal type and concentration of amines, while displaying a NaCl rejection of 99.8% and an acceptable water permeability of 0.55 L m−2 h−1 bar−1. Meanwhile, the alteration of membrane chemical compositions and structure properties was kept to a minimum. This study offers intuitive insights into the critical roles played by the aliphatic amines in the selective layer of the membrane for the removal of neutral boron molecules and salts, thereby enabling the fabrication of highly selective SWRO membranes, which may have significant implications for more efficient membrane-based seawater desalination and boron removal.

Employing Visible Light To Construct Photoinitiated Polymerized Nanofiltration Membranes for High Permeation and Environmental Micropollutant Removal.

Improving the nanofiltration (NF) performance of membrane-based treatment is conducive to promoting environmental water recycling and addressing water resource depletion. Combinations of light, electricity, and heat with traditional techniques of preparing membranes should optimize membrane performance. Interfacial polymerization and photopolymerization were integrated to construct a photopolymerized thin-film composite NF membrane with a ridged surface morphology. Under visible light initiation, 2-acrylamido-2-methyl-1-propanesulfonic acid was crosslinked with the polyamide network. The control effects of light on the membrane surface and physicochemical properties were revealed via infrared thermal images and response surface methodology. To present the diffusion motion of piperazine molecules, molecular dynamics simulations were implemented. Through density functional theory simulations, the crosslinking mechanism of the photoinduced NF network was identified and verified. The surface physicochemical characteristics and perm-selectivity performance were systematically illustrated. The photopolymerized membrane outperformed the pristine in permeability and selective separation competence; without degradation of solute repulsion, the water permeation was enhanced to 33.5 L m-2 h-1 bar-1, 6.6 times that of the initial membrane. In addition, the removal of organic contaminants and antifouling capacities were improved. This work represents a novel lead for applying sustainable resources in constructing high-performance membranes for environmental challenges.

Polyamide–based membranes with nanoscale homogeneity and asymmetric structure for ultrafast ion separation

Polymeric membranes are the most practical separation technology for extracting target ionic species necessary for water purification and clean energy, but are limited by the multi-scale heterogeneity, and transient and dynamic free volume pores. Modulated interfacial polymerization techniques are emergent concepts for preparing polyamide composite membranes with reduced thickness and homogeneous pore structures to satisfy the requirements for efficient ion separation applications. Herein, N–Cyclic cation-assisted kinetically regulated interfacial polymerization strategy to generate polyamide nanofilm composite membranes with tuned pore network and asymmetric structures for water purification is reported. Experimental characterization techniques and molecular dynamic simulations revealed that the N-Cyclic cations control the kinetic diffusion behaviour of the amine monomers to the aqueous/organic interface conducing to the formation of nanofilm membranes with fine-tuned polyamide network structures. Meanwhile, the N–Cyclic cation–regulated IP process not only affords to decrease the thickness of the formed polyamide nanofilm selective layers but also effectively narrows the effective pore size of the membranes. These membranes with thinner, homogeneous and asymmetric polyamide network structures exhibit enhanced water permeance of 55.7–60.42 L m−2 h−1 bar−1, and showed Na2SO4 rejection of 99.59–99.66% and MgCl2 rejection of 98.94–98.96% in nanofiltration separations, thereby outperforming the permeance–selectivity performance of the state-of-the-art polyamide-based membranes. The membranes also exhibit improved forward osmosis performances. We envision that this research will provide insightful foundations for the molecular tuning and construction of the next-generation structurally modulated polyamide nanofilms for various applications in molecular separation and water purification.

Visible light-induced crosslinking enables highly permeable and chlorine-resistant polyamide membranes for water purification

Nanofiltration (NF) based purification technology plays a growing role in water softening, wastewater reuse, and drinking water production, whose core component is the thin-film composite (TFC) polyamide (PA) surface separating layer with high water permeation and significant antifouling and chemical resistance. Strengthening water transport with simultaneously improving chemical tolerance and fouling resistance accords with the demands for sustainable water treatment. In this work, a photo-assisted crosslinked NF membrane with a regulated interlaced stripe surface morphology was fabricated through grafting 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) into the PA network during visible light irradiation. Surface structures and chemical features were characterized, and the NF performance, antifouling and chlorine resistance were systematically evaluated. The optimal membrane with AMPS molecules carrying sulfonyl groups crosslinked in the PA network was rendered electronegative charge, uniformly narrowed pore size, improved hydrophilicity, and higher crosslinking degree. The water permeation was augmented to 30.5 L m−2 h−1·bar−1 for the optimal membrane, four times more advanced than the pristine membrane with a water permeability of 5.9 L m−2 h−1·bar−1, and the divalent salt retention was improved to 99.7%. In addition, the optimal membrane also exhibited outstanding chlorine tolerance, fouling removal, and antifouling capacity. The flux recovery ratio (FRR) of the best membrane outperformed that of the pristine membrane and the flux declined more slowly in the whole fouling cycle testing. This work provides a novel avenue for fabricating high-efficiency TFC membranes with great water permeation and fouling resistance.

High-flux polyamide nanofiltration membranes via phosphate saline-buffered polymerization

Controlling the structure and charge of nanofiltration membranes is vital for removing emerging contaminants in both point-of-use and industrial wastewater treatment. However, the challenge is finding a viable method to regulate interfacial polymerization for high-permeance polyamide membranes with tailored selectivity and reduced scaling potential. Here we propose a simple and straightforward approach to creating an ultra-thin, highly charged polyamide membrane through protonation-regulated interfacial polymerization with phosphate saline buffer (PB-IP). The resulting thin films exhibit a long-term permeation performance with permeance >110 L m−2 h−1 bar−1 without pore collapsing. Within these membranes, the polyamide network is composed of loosely crosslinked chains with high charge density, providing sub-1 nm pore sizes that are effective for separating small molecules, such as perfluorooctanoic acid (PFOA) and different antibiotics. The PB-IP methodology allows for tailored membrane functionality, particularly for reducing the rate of surface mineral deposition. We examined the inorganic scaling propensity of these membranes by systematically quantifying the rejection of different salt species and studying the change in hydraulic flux using the solution with high gypsum saturation index. Furthermore, the fouling characteristics of the membranes were stimulated with common foulants to demonstrate the antifouling characteristic. This work provides a feasible strategy to adjust the structure-property-performance relationship profile for polyamide membranes, and to enable their use in water purification under a more dynamic range of water conditions.

Chlorine resistance property improvement of polyamide reverse osmosis membranes through cross-linking degree increment

Highly permeable polyamide reverse osmosis (RO) membranes are desirable for reducing the energy burden and ensuring future water resources in arid and semiarid regions. One notable drawback of thin film composite (TFC) polyamide RO/NF membranes is the polyamide's sensitivity to degradation by free chlorine, the most used biocide in water purification trains. This investigation demonstrated a significant increase in the crosslinking-degree parameter by the m-phenylenediamine (MPD) chemical structure extending in the thin film nanocomposite (TFN) membrane without adding extra MPD monomers to enhance the chlorine resistance and performance. Membrane modification was carried out according to monomer ratio changes and Nanoparticle embedding into the PA layer approaches. A new class of TFN-RO membranes incorporating novel aromatic amine functionalized (AAF)-MWCNTs embedded into the polyamide (PA) layer was introduced. A purposeful strategy was carried out to use cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) as an intermediate functional group in the AAF-MWCNTs. Thus, amidic nitrogen, connected to benzene rings and carbonyl groups, assembles a structure similar to the standard PA, consisting of MPD and trimesoyl chloride. The resulting AAF-MWCNTs were mixed in the aqueous phase during the interfacial polymerization to increase the susceptible positions to chlorine attack and improve the crosslinking degree in the PA network. The characterization and performance results of the membrane demonstrated an increase in ion selectivity and water flux, impressive stability of salt rejection after chlorine exposure, and improved antifouling performance. This purposeful modification resulted in overthrowing two tradeoffs; i) high crosslink density-water flux and ii) salt rejection-permeability. The modified membrane demonstrated ameliorative chlorine resistance relative to the pristine one, with twice the increase in crosslinking degree, more than four times the enhancement of the oxidation resistance, negligible reduction in the salt rejection (0.83 %), and only 5 L/m2.h flux loss following a rigorous static chlorine exposure of 500 ppm.h under acidic conditions. The excellent performance of new chlorine resistant TNF RO membranes fabricated via AAF-MWCNTs together with the facile membrane manufacturing process offered the possibility of postulating them in the desalination field, which could eventually help the current freshwater supply challenge.

Ultra‐Permeable Dual‐Mechanism‐Driven Graphene Oxide Framework Membranes for Precision Ion Separations

AbstractTwo‐dimensional graphene oxide (GO) membranes are gaining popularity as a promising means to address global water scarcity. However, current GO membranes fail to sufficiently exclude angstrom‐sized ions from solution. Herein, a de novo “posterior” interfacial polymerization (p‐IP) strategy is reported to construct a tailor‐made polyamide (PA) network in situ in an ultrathin GO membrane to strengthen size exclusion while imparting a positively charged membrane surface to repel metal ions. The electrostatic repulsion toward metal ions, coupled with the reinforced size exclusion, synergistically drives the high‐efficiency metal ion separation through the synthesized positively charged GO framework (PC‐GOF) membrane. This dual‐mechanism‐driven PC‐GOF membrane exhibits superior metal ion rejection, anti‐fouling ability, good operational stability, and ultra‐high permeance (five times that of pristine GO membranes), enabling a sound step towards a sustainable water‐energy‐food nexus.

Ultra-Permeable Dual-Mechanism-Driven Graphene Oxide Framework Membranes for Precision Ion Separations.

Two-dimensional graphene oxide (GO) membranes are gaining popularity as a promising means to address global water scarcity. However, current GO membranes fail to sufficiently exclude angstrom-sized ions from solution. Herein, a de novo "posterior" interfacial polymerization (p-IP) strategy is reported to construct a tailor-made polyamide (PA) network in situ in an ultrathin GO membrane to strengthen size exclusion while imparting a positively charged membrane surface to repel metal ions. The electrostatic repulsion toward metal ions, coupled with the reinforced size exclusion, synergistically drives the high-efficiency metal ion separation through the synthesized positively charged GO framework (PC-GOF) membrane. This dual-mechanism-driven PC-GOF membrane exhibits superior metal ion rejection, anti-fouling ability, good operational stability, and ultra-high permeance (five times that of pristine GO membranes), enabling a sound step towards a sustainable water-energy-food nexus.

Correlating the diffusion of water to performance in model reverse osmosis polyamides with controlled crosslink densities

We systematically reduce the crosslink density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride crosslinking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (PA) separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating 15 mol % of the difunctional chain extender, we reduce the crosslink density of the network by a factor of two, which leads to an 80% increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30% increase in the salt passage through the membrane. Surprisingly, this leads to a 30% decrease in the overall permeance of water through the membrane. This conundrum is resolved by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. In the highest and lowest crosslink density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest crosslinked density network, consistent with the reduced water permeance. This is interpreted as water molecules interacting more strongly with the increased free amine content. Over longer length scales the water diffusion is confined, exhibiting mobility that is independent of length scale. The length scales of confinement from the quasi-elastic neutron scattering experiments at which this transition from confined to translational diffusion occurs is on the order of (5–6) Å, consistent with complementary X-ray scattering, small angle neutron scattering, and positron annihilation lifetime spectroscopy measurements. The confinement appears to come from heterogeneities in the average inter-atomic distances, suggesting that diffusion occurs by water bouncing between chains and occasionally sticking to the polar functional groups. The results obtained here are compared with similar studies of water diffusion through both rigid porous silicates and ion exchange membranes, revealing robust design cues for engineering high-performance RO membranes.

Coaxial electrospun nanofibrous substrates with tunable wettability for constructing cucurbit[n]uril-embedded organic solvent nanofiltration membranes

Solvent resistance and high perm-selectivity have always been the pursuit in organic solvent nanofiltration field. Insoluble polyimide nanofibrous membranes feature superior solvent resistance. However, its high hydrophobicity usually limits the uniform spread of the aqueous amine monomers during the interfacial polymerization process, resulting in the non-uniform reaction with the acyl chloride and the defective selective layer. Herein, coaxial electrospinning was employed to construct the core–shell nanofibrous substrates, in which aminated multi-walled carbon nanotubes were added to the shell layer. This strategy endowed the nanofibrous substrates with tunable wettability without sacrificing stability, which guaranteed the one-step construction of defect-free thin-film nanofibrous composite membranes via interfacial polymerization (IP). Further, cucurbit[6]uril was embedded into the polyamide network by coupling host–guest chemistry and IP, providing the wider solvent transport channels and achieving the four times enhanced methanol permeance (22.34 L·m−2·h−1·bar−1) without sacrificing the selectivity. The membrane shows application prospects for solvent recovery in chemical, pharmaceutical, and other fields. This work provides new thoughts for the design and modification of novel nanofiltration membranes.

Imparting Outstanding Dispersibility to Nanoscaled 2D COFs for Constructing Organic Solvent Forward Osmosis Membranes.

In the context of thin-film nanocomposite membranes with interlayer (TFNi), nanoparticles are deposited uniformly onto the support prior to the formation of the polyamide (PA) layer. The successful implementation of this approach relies on the ability of nanoparticles to meet strict requirements regarding their sizes, dispersibility, and compatibility. Nevertheless, the synthesis of covalent organic frameworks (COFs) that are well-dispersed, uniformly morphological, and exhibit improved affinity to the PA network, while preventing agglomeration, remains a significant challenge. In this work, a simple and efficient method is presented for the synthesis of well-dispersed, uniformly morphological, and amine-functionalized 2D imine-linked COFs regardless of the ligand composition, group type, or framework pore size, by utilizing a polyethyleneimine (PEI) shielded covalent self-assembly strategy. Subsequently, the as-prepared COFs are incorporated into TFNi for the recycling of pharmaceutical synthetic organic solvents. After optimization, the membrane exhibits a high rejection rate and a favorable solvent flux, making it a reliable method for efficient organic recovery and the concentration of active pharmaceutical ingredient (API) from the mother liquor through an organic solvent forward osmosis (OSFO) process. Notably, this study represents the first investigation of the impact of COF nanoparticles in TFNi on OSFO performance.

High-Performance Polyamide Reverse Osmosis Membrane Containing Flexible Aliphatic Ring for Water Purification

A reverse osmosis (RO) membrane with a high water permeance and salt rejection is needed to reduce the energy requirement for desalination and water treatment. However, improving water permeance while maintaining a high rejection of the polyamide RO membrane remains a great challenge. Herein, we report a rigid-flexible coupling strategy to prepare a high-performance RO membrane through introducing monoamine with a flexible aliphatic ring (i.e., piperidine (PPR)) into the interfacial polymerization (IP) system of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). The resulted polyamide film consists of a robust aromatic skeleton and soft aliphatic-ring side chain, where the aliphatic ring optimizes the microstructure of polyamide network at a molecular level. The obtained membranes thereby showed an enhanced water permeance of up to 2.96 L·m-2 h-1 bar-1, nearly a 3-fold enhancement compared to the control group, meanwhile exhibiting an ultrahigh rejection toward NaCl (99.4%), thus successfully overcoming the permeability-selectivity trade-off limit. Furthermore, the mechanism of the enhanced performance was investigated by molecular simulation. Our work provides a simple way to fabricate advanced RO membranes with outstanding performance.

Positively-charged PEI/TMC nanofiltration membrane prepared by adding a diamino-silane coupling agent for Li+/Mg2+ separation

Positively charged nanofiltration membrane has gained significant interest in magnesium and lithium separation due to its separation mechanism, especially in salt lake-based lithium industry. To obtain a high-density positive charged surface and high Li+/Mg2+ selectivity, a novel thin-film nanocomposite (TFN) nanofiltration (NF) membrane for recovery of Li+ from Mg/Li mixtures with Mg2+/Li+ ratios of 20 was successfully prepared by using polymeric amine polyethyleneimine (PEI) and monomeric amine silane coupling agent (3-diamino-methyl-cyclohexyl triethoxysilane, DTES). Then the effect of DTES on the surface properties and performance of the prepared DTES/PEI/TMC TFN membranes were investigated. Chemical composition analysis revealed that DTES not only hydrolyzed to Si–OH and then self-polymerized to Si–O–Si, but also reacted with the self-carrying acyl chloride of 1,3,5-benzene trichloromethane (TMC). Moreover, SiO2 nanoparticles were in-situ generated from DTES inside the PA layer with high compatibility. Due to the compatible properties with the positively charged PEI polymer, the reacted DTES was distributed uniformly in the formed TFN selective layer with no noticeable aggregation. The doping of SiO2 into the polyamide network gave rise to the enlargement of the membrane pore size, enhanced the surface hydrophilicity, and reinforced the surface positive charges. The water flux of the DTES/PEI/TMC membrane improved from 36.9 L m−2 h−1 to 49.58 L m−2 h−1 compared to the PEI/TMC membrane. Additionally, the Mg2+ and Li + rejection of the optimized membranes was 91.46% and −10.5%, respectively, with permeability and selectivity (SLi, Mg = 12.95) superior to that of the commercial NF membranes. Considering the commercial availability of DTES, this study not only offers a high-performance nanofiltration membrane for recovering lithium from brines but also provides a feasible strategy for industrial separation of other monovalent and divalent ions.

Incorporating ionic carbon dots in polyamide nanofiltration membranes for high perm-selectivity and antifouling performance

Recent advances in nanotechnology have brought great opportunities to the developing next-generation nanofiltration membranes for addressing the concern of global water scarcity and energy crisis. Carbon dots have been deemed as a promising nanomaterial for enhancing the nanofiltration performance of polymeric membranes. Herein, carbon dots with cationic amine groups (PEI-CDs) and those with anionic sulfonate groups (PS-CDs) are incorporated into the polyamide selective layer of thin-film nanocomposite (TFN) membranes to create charged nanovoids (free volume) between the carbon dots and the surrounding polyamide network for high-efficiency nanofiltration. The positively charged amine group-based and negatively charged sulfonate group-based nanovoids in the resultant TFN-PEI-CDs and TFN-PS-CDs membranes, respectively, provide alternative pathways for efficient water permeation while effectively rejecting divalent ions via the Donnan and dielectric exclusion effects, thereby overcoming the recurring trade-off between permeability and selectivity encountered by dense polymeric membranes. In particular, the creation of negatively charged nanovoids enables the TFN-PS-CDs membrane to achieve one of the best performances among the recently reported nanofiltration membranes, by showing a nearly tripled pure water permeability of 30.9 L m−2 h−1 bar−1 as well as a higher Na2SO4 rejection rate of 99.4% than the membrane with a pristine polyamide selective layer. Moreover, the TFN membranes demonstrate greater resistance to foulants with charges of the same sign as the nanovoids. This work provides insights into the design of nanovoids with desired properties in other polymeric membranes for high-efficiency filtration processes.

Heterogeneous polyamide composite membranes based on aromatic poly(amidoamine) dendrimer for molecular sieving

Membrane separation technologies featuring environmentally friendly and highly efficient processes have been widely adopted for sustainable development. Non-linear macromolecules, represented by aromatic poly(amidoamine) dendrimers (ArPD), have good potential for increasing the porosity of the active layer in thin-film composite (TFC) membranes. However, they are often difficult to use as the sole monomer in interfacial polymerization to prepare membranes with high separation accuracy. Herein, the DMF/H2O system was determined to resolve the problem of ArPD dissolution, and the behavioral differences of ArPD for different generations in film-forming were then elucidated. For G4 (the fourth generation of ArPD) -TMC (trimesoyl chloride) membranes, the good sieving performance is attributed to the relatively dense polyamide networks from interfacial polymerization, and the continuous pore channels from the tightly packed macromolecules. Therefore, the final polyamide layer shows significant heterogeneity. Moreover, a “patching” strategy was employed to further improve the separation performance. The ArPD-based membranes exhibit about 3 times higher permeance than MPD (m-phenylenediamine)-based membranes under the comparable NaCl rejection. For the repaired membrane, the water permeance reaches 3.2 L m−2 h−1 bar−1 with the NaCl rejection of about 94.9%, which is advantageous compared with reported macromolecule-based membranes. The repaired membrane also shows excellent performance in the removal of small organics in organic solvent system. For example, the methanol permeance achieves 1.5 L m−2 h−1 bar−1 with the rejection to potassium cinnamate (186.25 g mol−1) about 90.3%. This work provides a typical paradigm for the application of macromolecules in the preparation of TFC membranes.

Biomimetic hydroxypropyl-β-cyclodextrin (Hβ-CD) / polyamide (PA) membranes for CO2 separation

Carbonic anhydrase (CA)-inspired membranes for CO2 separation were constructed by introducing hydroxypropyl-β-cyclodextrins (Hβ-CDs) into the polyamide networks, which were formed via interfacial polymerization reactions between trimesoyl chloride and tertiary amino-containing monomers. Hydrophobic inner cavities of Hβ-CDs with the hydrophilic exterior can be used to simulate the hydrophobic pockets of CA enzymes. Hydrophilic and hydrophobic regions were constructed via introducing Hβ-CDs into membranes, while the transport of nonpolar CO2 molecules and polar components (H2O and HCO3−) in CO2 hydration reactions could be enhanced. Meanwhile, tertiary amino groups in biomimetic membranes can act as the base catalysts for CO2 hydration reactions, which were deemed as carriers for facilitated transport membranes. Biomimetic membranes containing Hβ-CDs and tertiary amino groups exhibited outstanding CO2 permeance of 2792 GPU and CO2/N2 selectivity of 171 at 0.15 MPa, showing the potentials for low-pressure CO2 capture. Particularly, CO2 transport mechanisms were also analyzed for biomimetic facilitated-transport membranes, which can supply new visions for developing novel membrane materials for CO2 separation.