Low Monomer Concentration Research Articles

Poly-(aminoethyl piperazine) membranes with ultra-thin selective layers prepared via a rate-retarded interfacial polymerization method

New monomers have been exploring to fabricate interfacial polymerized thin film composite membranes for decades. N-aminoethyl piperazine (AEP), an aliphatic amine with a heterocyclic structure, was used in this work as an amine monomer to synthesize nanofiltration membranes. Due to the high reactivity nature of AEP, a rate-retarded interfacial polymerization method was applied, which was realized by applying ultra-low AEP concentration (0.07 wt%), low reaction temperature (4 °C monomer solutions), and ice-water quench. The resulting AEP membrane had much better performance (3–4 times permeance and closed rejections) compared with the membranes prepared by the hot water post-treatment and the oven heat treatment. At lower monomer concentrations, an NF membrane with a selective layer thickness of only around 20 nm was prepared by the rate-retarded interfacial polymerization method. The membranes had a negatively charged surface with "willow leaf" like morphology. The MWCO was around 200 g mol−1, and the water permeance was up to 22.8 L m−2 h−1 bar−1, which is comparable with the commercial polyamide membrane NF270. Therefore, AEP is a very competitive alternative monomer for NF membrane synthesis.

Self-Polymerized Tough and High-Entanglement Zwitterionic Functional Hydrogels.

Zwitterionic hydrogels exhibit great potential in biomedical applications due to their antifouling properties and biocompatibility. However, the single-network structure of pure zwitterionic hydrogels leads to a low toughness and strength, limiting their application in biomedical fields. In this work, a high entanglement sulfobetaine methacrylate-dopamine hydrogel (SBMA-DA-PE) with low cross-linker content and high monomer concentration is prepared by using a dopamine oxidative radical polymerization method. Compared to a regular zwitterionic hydrogel, the SBMA-DA-PE hydrogel exhibits a 5-fold increase in tensile fracture stress and a 10-fold increase in compressive fracture stress. The SBMA-DA-PE hydrogel possesses excellent mechanical properties (the maximum compressive stress ≥4.85MPa, the maximum compressive strain ≥90%). Besides, the non-covalent interactions between catechol or ortho-quinones within the SBMA-DA-PE hydrogel, combined with strong intermolecular electrostatic interactions, endow the SBMA-DA-PE hydrogel with great self-healing capabilities and fatigue resistance. The SBMA-DA-PE hydrogel demonstrates low swellability and possesses good antifouling properties. Furthermore, the good printability and conductivity of the tough SBMA-DA-PE hydrogel endows it with new possibilities for developing biological 3D scaffolds and electronic devices. Overall, this work provides new insights into the preparation of zwitterionic hydrogels with high mechanical strength and multi-functionality for biomedical applications.

Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes.

Molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes of the type [Mo(C-p-C6H4Y)(OC(R)(CF3)2)2 (L)(NHC)][B(ArF)4] (Y = OMe, NO2; R = CH3, CF3; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH2), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl2), 1,3-diisopropylimidazol-2-ylidene (IiPr); B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With cis-cyclooctene (cCOE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(cCOE) became available at high monomer concentrations. Also, post-REMP allowed for converting low-molecular-weight cyclic poly(cCOE) into high-molecular-weight cyclic poly(cCOE). Tailored catalysts together with suitable additives offered access to the stereoselective REMP of functional norbornenes providing functional cis-isotactic (cis-it), cis-syndiotactic (cis-st) and trans-it poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.

Wetting-enhanced adhesion of photo-polymerized supramolecular adhesives for both smooth and rough surfaces.

Efficient interactions between an adhesive and a substrate surface at the molecular level are the basis for the formation of robust adhesion, which substantially relies on interfacial wetting. However, strong adhesives usually improve cohesion but compromise interfacial properties. Herein, we have reported a kind of robust supramolecular adhesive based on the outstanding mobility and interfacial wettability of adhesive precursors. In situ fast photopolymerization endows supramolecular adhesives with more outstanding adhesion for both smooth and rough surfaces in air and underwater in contrast to their counterparts from thermal polymerization. In addition to their low viscosity and high monomer concentration, supramolecular adhesive precursors without any organic solvents possess well-defined hydrogen bonding interactions. These superior properties consistently contribute to the wetting of the substrate and the formation of adhesive polymers with high molecular weights. This work highlights that enhancing interfacial wetting between an adhesive and a substrate is a promising route to achieving robust adhesion.

Enhancing oil repellency of electron beam‐grafted polyethylene terephthalate fabric with 2‐(perfluorohexyl) ethyl acrylate monomer via pre‐irradiation

AbstractElectron beam (EB)‐induced graft polymerization is advantageous for the surface modification of fabrics. We investigated the effect of monomer concentration and the addition of alkyl groups on the oil repellency of polyethylene terephthalate (PET) fabrics treated with monomers containing fluoroalkyl groups through EB‐induced graft polymerization via pre‐irradiation. We use 2‐(perfluorohexyl) ethyl acrylate (FEA) and stearyl acrylate (SA(C18)) with long alkyl chains as vinyl monomers to induce reaction with radicals generated from EB irradiation. The weight gain and surface morphology of the PET fabrics change with the FEA monomer concentration. The uniformity of the EB‐grafted PET fabric surface is determined at low monomer solution concentrations. Results of X‐ray photoelectron spectroscopy analysis show that adding 0.1 mol/L of FEA monomer to the EB‐grafted PET fabric yields the highest dodecane contact angle of 93.4° and a surface fluorine concentration of 39.8%. The addition of SA(C18) monomer to the FEA monomer decreases the dodecane contact angle by 77.5° and yields a surface fluorine concentration of 19.1%. EB graft polymerization via pre‐irradiation results in a uniformly treated surface, and stable oil repellency is achieved when using solely the FEA monomer at a lower monomer concentration than that used in a similar irradiation method reportedly previously.

Monodisperse oxidative-sensitive polymer nanoparticles from dispersion polymerization of N-acryloyl thiomorpholine

Aqueous emulsion or suspension polymerizations are well established industrial processes with widespread applications. Dispersion polymerizations, on the contrary, are rarely found for the preparation of stable particle dispersions, which is certainly related to the limited number of suitable monomers. Here, we report that N-acryloyl thiomorpholine (NAT) is an interesting candidate for aqueous dispersion polymerizations, as it is fully miscible with water but forms insoluble polymers at low degrees of polymerization. Polymerization in pure water results in an uncontrolled precipitation but with the addition of a surfactant well-defined dispersions with monomodal size distributions are obtained if low monomer concentrations (≤4 wt%) are used. Higher concentrations (10 wt%) broaden the dispersity of the resulting particles, but still result in stable dispersions, while 30 wt% cause a solidification of the solution as particles start to fuse. Crosslinked particles can be obtained by addition of a bisacrylamide comonomer. The thioether moiety of NAT further enables postpolymerization modifications including its oxidation to a more polar sulfoxide. Kinetic studies revealed that oxidation in H2O2 follows a bulk process causing a swelling of the particles after reaching a critical degree of oxidation (35%). In case of the non-crosslinked particles, disintegration occurs only at ≥ 60% conversion of the thioethers, while the particles remain intact for days at lower degrees of oxidation. The oxidation of the crosslinked particles results in water-swollen microgels. Overall, the straightforward process allows access to monodisperse reactive polymer nanoparticles of variable size (∼ 60 – 100 nm) that could be used as well-defined templates or carrier materials.

Distinct growth regimes of α-synuclein amyloid elongation

Addition of amyloid seeds to aggregation-prone monomers allows for amyloid fiber growth (elongation) omitting slow nucleation. We here combine Thioflavin T fluorescence (probing formation of amyloids) and solution-state NMR spectroscopy (probing disappearance of monomers) to assess elongation kinetics of the amyloidogenic protein, α-synuclein, for which aggregation is linked to Parkinson’s disease. We found that both spectroscopic detection methods give similar kinetic results, which can be fitted by applying double exponential decay functions. When the origin of the two-phase behavior was analyzed by mathematical modeling, parallel paths as well as stop-and-go behavior were excluded as possible explanations. Instead, supported by previous theory, the experimental elongation data reveal distinct kinetic regimes that depend on instantaneous monomer concentration. At low monomer concentrations (toward end of experiments), amyloid growth is limited by conformational changes resulting in β-strand alignments. At the higher monomer concentrations (initial time points of experiments), growth occurs rapidly by incorporating monomers that have not successfully completed the conformational search. The presence of a fast disordered elongation regime at high monomer concentrations agrees with coarse-grained simulations and theory but has not been detected experimentally before. Our results may be related to the wide range of amyloid folds observed.

Synthesis of PEG-PPG-PEG templated polydopamine nanoparticles under intensified conditions: Kinetics investigation, continuous process design and demonstration for photothermal application

Polydopamine is a nature inspired functional material with promising applications in a plethora of fields due to its structural, chemical, and optical properties. While there is significant interest in the preparation of polydopamine based nanomaterials that take advantage of its properties, less attention has been given to the optimisation of the synthetic process, which typically involves the oxidative self-polymerisation of dopamine under basic pH and requires 24–72 h for reaction completion. The present work investigated the kinetics of polydopamine formation in the presence of Pluronic P-123 (PEG-PPG-PEG) micelles acting as a soft template, in low monomer concentrations (that promote particle growth instead of nucleation) as a function of temperature and pressure. Simultaneous increase of pressure and temperature (up to 50 °C and 5 bar O2) was found to significantly reduce the reaction time to 20–40 min without compromising the particle quality. Based on the results of the kinetic investigation, the polydopamine synthesis was translated into a continuous process utilising a millifluidic co-axial membrane reactor with a focus on ease of use, process conditions’ reproducibility and safety of operation. The reactor produced nanoparticles similar to the batch synthesis and resisted fouling (which is generally expected in a compact flow reactor) due to the action of the P-123 surfactant. Due to the nontoxic process that utilises only biocompatible materials and oxygen as the oxidising agent, and the melanin-like structure of polydopamine, photothermal heating of the synthesised nanoparticles under concentrated IR irradiation at 808 nm light was studied, as this can potentially be used for photo-induced hyperthermia. The hyperthermia threshold of 10 °C temperature increase at relatively low laser power settings (fluence 1.77 W/cm2) was achieved, making it a promising candidate for this application.

Influence of Oblique Angle Deposition on Porous Polymer Film Formation.

In this study, we applied oblique angle deposition to a modified initiated chemical vapor deposition (iCVD) process to synthesize porous poly(methacrylic acid) (PMAA) films. During the modified iCVD process, frozen monomer molecules are first captured on a cooled substrate, then polymerization occurs via a free radical polymerization mechanism, and finally, the excess monomer is sublimated, resulting in a porous polymer film. We found that delivering the monomer through an extension at an oblique angle resulted in porous films with three morphological regions. Region 1 is located nearest to the monomer extension outlet and consists of porous polymer pillars; region 2 consists of densified pillars, which occur due to the recapturing and polymerization of the sublimated monomer; and region 3 is located furthest from the monomer extension outlet and consists of dendritic structures, which occur due to low monomer concentration. We investigated the role of substrate temperature and monomer deposition time on the growth process. We found that changing the extension angle influenced the location of the regions and the film coverage across the substrate. Our results provide useful guidelines for tuning the structures within porous polymer films by varying the angle of monomer delivery.

Measurement and Modeling of Semi-Batch Solution Radical Copolymerization of N-tert-Butyl Acrylamide with Methyl Acrylate in Ethanol/Water.

The synthetic polymer industry is transitioning from the use of organic solvents to aqueous media in order to reduce environmental impact. However, with radical polymerization kinetics affected by hydrogen-bonding solvents, there is limited information regarding the use of water as a solvent for sparingly soluble monomers. Thus, in this paper, the radical polymerization of methyl acrylate (MA) and N-tert-butylacrylamide (t-BuAAm) is studied in water and ethanol (EtOH), as the copolymer product is of commercial interest. A series of semi-batch reactions are conducted under a range of operating conditions (i.e., reaction temperature, solvent-to-monomer ratio, and comonomer composition) to demonstrate that the copolymer can be successfully synthesized without significant drifts in product molar masses or composition. The experiments provide additional data to probe the influence of the solvent on the polymerization rate and copolymer properties, as the low monomer concentration maintained under starved-feed operation leads to a solvent-to-monomer ratio different from that in a batch system. A model that captures the influence of backbiting and solvent effects on rate, previously developed and tested against batch polymerizations, also provides an excellent description of semi-batch operation, validating the set of mechanisms and kinetic coefficients developed to represent the system.

Self-healing and elastic polymer gel via terpyridine-metal coordination

A supramolecular polymer is prepared by tristerpyridine ligand and metal ion Zinc coordination. When the supramolecular polymer is dissolved in acetonitrile, a metallo-gel can be obtained at a very low monomer concentration (< 1.0 wt%). Remarkably, this metallo-gel exhibits rapid self-healing ability and excellent elasticity with high recovery. • Coordination gelation was observed at very low monomer concentrations, indicating gel- A /Zn 2+ was a highly potential supergelator. • The supramolecular gel exhibited a great macroscopic self-healing ability. • A good elastic behavior was observed from gel- A /Zn 2+ . Self-healing and elasticity are assiduously pursued qualities for substrate materials which are applicable in many fields such as wearable electronic devices, touch screens, and soft robotics. Up to now, the most reported self-healing and elastic gel materials were constructed by supramolecular polymers with covalent backbones. Herein, a supramolecular gel- A /Zn 2+ was built based on supramolecular polymerization through terpyridine-Zn 2+ coordination. Importantly, this metallo-gel exhibits the self-healing ability. In addition, a freshly prepared metallo-gel ball with a diameter of 1.0 cm could rebound back with 89.6% recovery from free-drop which displayed excellent elasticity. Superb features of gel- A /Zn 2+ including self-healing ability, elasticity and hydrophilia provide prospects for high-performance adaptive materials.

Polydioxanone Derivative Bearing Methoxy Groups toward Bio‐Functional Degradable Polymers Exhibiting Hydration‐Driven Biocompatibility

AbstractAn aliphatic poly(ester‐ether) with methoxy side chains from a glycerol derivative that exhibits increased hydration properties for potential applications in blood‐contacting medical devices is developed. The poly(ester‐ether) is prepared by ring‐opening polymerization (ROP) of an analogue of 1,4‐dioxan‐2‐one (DX) with a methoxy‐containing substituent in the presence of acidic organocatalysts. It is found that the ring–chain equilibrium resides in the ROP of the DX analogue at room temperature and at relatively low monomer concentration (≈2 m). The organocatalytic ROP has enabled more than 80% monomer conversion at temperatures below 0 °C in the bulk state. The methoxy‐bearing poly(ester‐ether) is insoluble in water, amorphous, and viscous, demonstrating a high level of interaction with water when evaluating the thermal properties of the hydrated polymer and water contact angles. The polymer also shows susceptibility to hydrolysis and end‐group dependence on thermal degradation. These results suggest a good potential for methoxy‐bearing poly(ester‐ether) as a biodegradable and biocompatible polymer derived from renewable feedstocks.

Ultrathin sulfonated mesoporous interlayer facilitates to prepare highly-permeable polyamide nanofiltration membranes

Highly-permeable nanofiltration (NF) membranes are strongly required and have become more crucial with increasing concerns in water treatment and purification. Herein a versatile approach is developed to prepare a highly-permeable polyamide (PA) NF membrane via the traditional interfacial polymerization (IP) process facilitated by an ultrathin sulfonated mesoporous interlayer. A series of the interlayers with a thickness of 110–250 nm and a cut-off around 12 nm is prepared and employed to prepare the PA NF membrane from piperazine (PIP) and 1, 3, 5-benzotriformyl chloride (TMC). Influence factors on the membrane preparation including the interlayer thickness and monomer concentrations are investigated in detail. Sulfonic groups on the interlayer greatly facilitate the PIP adsorption and thus increase the PIP storage in the interlayer, resulting in the formation of a defect-free, 10–50 nm-thick PA membrane at a low monomer concentration. The thickness and surface roughness of PA layer increase with the interlayer thickness simultaneously. Typically, the 40 nm-thick PA membrane has a pure water permeance of high to 36.3 L m−2 h−1 bar−1 with a Na2SO4 rejection of 98.5%. The newly developed approach has a wide application in the preparation of highly-permeable PA membranes for various fields including water treatment and chemical separation.

Aminated substrate based thin film composite nanofiltration membrane with high separation performance by chemically inhibiting the intrusion of polyamide

In this study, aminated PES substrates were prepared and applied for the investigation on the interfacial polymerization of piperazine (PIP) and 1,3,5-benzenetricarbonyl trichloride (TMC) thereon, with relatively low (PIP: 0.1 wt%, TMC: 0.01 wt%) and high (PIP: 0.5 wt%, TMC: 0.05 wt%) monomer concentration. In low concentration condition, thin, dense, and defect-free selective layers were synthesized on the aminated substrates. In high concentration condition, thin film composite (TFC) membranes with thin selective layers and without the intrusion of polyamide into the substrates were formed. The reactions between the aminated substrate and selective layer were confirmed by XPS analysis on the back surface of the selective layer. The aminated substrates based TFC membrane (TFC-48-H) showed high separation performance, including high water permeability as 20.6 ± 0.1 L/m2 h bar and high Na2SO4 to NaCl selectivity as 86.7, indicating the application potential in divalent to monovalent salts separation process. A conceptual model was proposed to highlight the role of the aminated substrates in the interfacial polymerization process as (1) PIP loaders, (2) PIP diffusion hinders, and (3) polyamide intrusion chemically inhibitors in high monomer concentration condition.

Near-Equilibrium Growth of Chemically Stable Covalent Organic Framework/Graphene Oxide Hybrid Materials for the Hydrogen Evolution Reaction.

Facile synthesis and post-processing of covalent organic frameworks (COFs) under mild synthetic conditions are highly sought after and important for widespread utilizations in catalysis and energy storage. Here we report the synthesis of the chemically stable aza-fused COFs BPT-COF and PT-COF by a liquid-phase method. The process involves the spontaneous polycondensation of vicinal diamines and vicinal diketones, and is driven by the near-equilibrium growth of COF domains at a very low monomer concentration. The method permits in situ assembly of COFs and COF-GO hybrid materials and leads to the formation of a uniform conducting film on arbitrary substrates on vacuum filtration. When used as electrocatalysts, the as-prepared membranes show a fast hydrogen evolution reaction (HER) with a low overpotential (45 mV at 10 mA cm-2 ) and a small Tafel slope (53 mV dec-1 ), which are the best among metal-free catalysts. Our results may open a new route towards the preparation of highly π-conjugated COFs for multifunctional applications.

Near‐Equilibrium Growth of Chemically Stable Covalent Organic Framework/Graphene Oxide Hybrid Materials for the Hydrogen Evolution Reaction

AbstractFacile synthesis and post‐processing of covalent organic frameworks (COFs) under mild synthetic conditions are highly sought after and important for widespread utilizations in catalysis and energy storage. Here we report the synthesis of the chemically stable aza‐fused COFs BPT‐COF and PT‐COF by a liquid‐phase method. The process involves the spontaneous polycondensation of vicinal diamines and vicinal diketones, and is driven by the near‐equilibrium growth of COF domains at a very low monomer concentration. The method permits in situ assembly of COFs and COF‐GO hybrid materials and leads to the formation of a uniform conducting film on arbitrary substrates on vacuum filtration. When used as electrocatalysts, the as‐prepared membranes show a fast hydrogen evolution reaction (HER) with a low overpotential (45 mV at 10 mA cm−2) and a small Tafel slope (53 mV dec−1), which are the best among metal‐free catalysts. Our results may open a new route towards the preparation of highly π‐conjugated COFs for multifunctional applications.

Mitigating Silica Fouling and Improving PPCP Removal by Modified NF90 Using In Situ Radical Graft Polymerization.

This study in-situ modified a commercial nanofiltration membrane, NF90, through the concentration-polymerization-enhanced radical graft polarization method by applying two agents of 3-sulfopropyl methacrylate potassium salt (SPM) and 2-hydroxyethyl methacrylate (HEMA) with different dosages. Surface characterization revealed that the modified membranes became rougher and more hydrophilic compared with the pristine membrane. The modified membranes exhibited considerably enhanced separation performance with 5.8–19.6% higher NaCl rejection and 17.2–19.9% higher pharmaceuticals and personal care products (PPCPs) rejection than the pristine membrane. When treating the feedwater with high silica concentration, the modified membranes exhibited relatively less flux decline with high percentage of reversible fouling, especially the ones modified using a lower monomer concentration (0.01 M SPM and 0.01 M HEMA). Moreover, membrane modification enhanced the PPCP rejection (1.3–5.4%) after silica fouling by mitigating foulant deposition on the membrane surface. The fouling mechanism was confirmed to be intermediate blocking of membrane pores. Therefore, the in-situ modification technique with a low monomer concentration proved to be effective for mitigating silica fouling and improving PPCP rejection, which can be easily performed and cost-effective in practical application.

Oil–Water–Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets

AbstractIonic covalent organic framework nanosheets (iCOFNs) with long‐range ordered and mono‐dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil–water biphase method, herein, we explore an oil–water–oil triphase method based on phase engineering strategy for the bottom‐up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few‐layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra‐high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect‐free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs.

Oil-Water-Oil Triphase Synthesis of Ionic Covalent Organic Framework Nanosheets.

Ionic covalent organic framework nanosheets (iCOFNs) with long-range ordered and mono-dispersed ionic groups hold great potential in many advanced applications. Considering the inherent drawbacks of oil-water biphase method, herein, we explore an oil-water-oil triphase method based on phase engineering strategy for the bottom-up synthesis of iCOFNs. The middle water phase serves as a confined reaction region, and the two oil phases are reservoirs for storing and supplying monomers to the water phase. A large aqueous space and low monomer concentration lead to the anisotropic gradual growth of iCOFNs into few-layer thickness, large lateral size, and high crystallinity. Notably, the resulting three cationic and anionic iCOFNs exhibit ultra-high aspect ratios of up to 20,000. We further demonstrate their application potential by processing into ultrathin defect-free COF membranes for efficient biogas separation. Our triphase method may offer an alternative platform technology for the synthesis and innovative applications of iCOFNs.

Ripening of two-dimensional colloidal CdSe nanocrystals into zero-dimensional nanodots

SummaryUnderstanding the ripening of two-dimensional (2D) colloidal nanocrystals (NCs) is important for the controllable synthesis of NCs with desired morphology and properties. In this study, we systematically investigate the ripening behavior of the 2D CdSe NCs in the presence of a short-chain acetate ligand and a long-chain oleate ligand. We find that a low acetate/oleate ratio, a low Cd/Se ratio, and a low monomer concentration help in the ripening of the 2D NCs to form 0D NCs. Moreover, a porous nanosheet intermediate is observed when there is a high Cd/Se ratio, whereas in the case of a low Cd/Se ratio, the ripening starts from the edge of the nanosheets, resulting in a saw-like nanosheet intermediate. These findings provide necessary insights into the growth and ripening of 2D CdSe NCs that allow for the controlled synthesis of 0D and 2D CdSe NCs.