Kinetics of film formation by interfacial polycondensation

Polyamide nanofiltration membrane with highly uniform sub-nanometre pores for sub-1 Å precision separation

Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Achieving such a precise separation using membranes requires Angstrom scale pores with a high level of pore size uniformity. Herein, we demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization (SARIP). The dynamic, self-assembled network of surfactants facilitates faster and more homogeneous diffusion of amine monomers across the water/hexane interface during interfacial polymerization, thereby forming a polyamide active layer with more uniform sub-nanometre pores compared to those formed via conventional interfacial polymerization. The polyamide membrane formed by SARIP exhibits highly size-dependent sieving of solutes, yielding a step-wise transition from low rejection to near-perfect rejection over a solute size range smaller than half Angstrom. SARIP represents an approach for the scalable fabrication of ultra-selective membranes with uniform nanopores for precise separation of ions and small solutes.

Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Here, the authors demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization.

Intrinsic Nanoscale Structure of Thin Film Composite Polyamide Membranes: Connectivity, Defects, and Structure-Property Correlation

Transport of water, solutes, and contaminants through a thin film composite (TFC) membrane is governed by the intrinsic structure of its polyamide separation layer. In this work, we systematically characterized the nanoscale polyamide structure of four commercial TFC membranes to reveal the underlying structure-property relationship. For all the membranes, their polyamide layers have an intrinsic thickness in the range of 10-20 nm, which is an order of magnitude smaller than the more frequently reported apparent thickness of the roughness protuberances due to the ubiquitous presence of nanovoids within the rejection layers. Tracer filtration tests confirmed that these nanovoids are well connected to the pores in the substrates via the honeycomb-like opening of the backside of the polyamide layers such that the actual separation takes place at the frontside of the polyamide layer. Compared to SW30HR and BW30, loose membranes XLE and NF90 have thinner intrinsic thickness and greater effective filtration area (e.g., by the creation of secondary roughness features) for their polyamide layers, which correlates well to their significantly higher water permeability and lower salt rejection. With the aid of scanning electron microscopy, transmission electron microscopy, and tracer tests, the current study reveals the presence of nanosized defects in a polyamide film, which is possibly promoted by excessive interfacial degassing. The presence of such defects not only impairs the salt rejection but also has major implications for the removal of pathogens and micropollutants.

Monitoring the Interfacial Polymerization of Piperazine and Trimesoyl Chloride with Hydrophilic Interlayer or Macromolecular Additive by In Situ FT-IR Spectroscopy

The interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC) has been extensively utilized to synthesize nanofiltration (NF) membranes. However, it is still a huge challenge to monitor the IP reaction, because of the fast reaction rate and the formed ultra-thin film. Herein, two effective strategies were applied to reduce the IP reaction rate: (1) the introduction of hydrophilic interlayers between the porous substrate and the formed polyamide layer, and (2) the addition of macromolecular additives in the aqueous solution of PIP. As a result, in situ Fourier transform infrared (FT-IR) spectroscopy was firstly used to monitor the IP reaction of PIP/TMC with hydrophilic interlayers or macromolecular additives in the aqueous solution of PIP. Moreover, the formed polyamide layer growth on the substrate was studied in a real-time manner. The in situ FT-IR experimental results confirmed that the IP reaction rates were effectively suppressed and that the formed polyamide thickness was reduced from 138 ± 24 nm to 46 ± 2 nm according to TEM observation. Furthermore, an optimized NF membrane with excellent performance was consequently obtained, which included boosted water permeation of about 141-238 (L/m2·h·MPa) and superior salt rejection of Na2SO4 > 98.4%.

Thin-film composite membrane breaking the trade-off between conductivity and selectivity for a flow battery

A membrane with both high ion conductivity and selectivity is critical to high power density and low-cost flow batteries, which are of great importance for the wide application of renewable energies. The trade-off between ion selectivity and conductivity is a bottleneck of ion conductive membranes. In this paper, a thin-film composite membrane with ultrathin polyamide selective layer is found to break the trade-off between ion selectivity and conductivity, and dramatically improve the power density of a flow battery. As a result, a vanadium flow battery with a thin-film composite membrane achieves energy efficiency higher than 80% at a current density of 260 mA cm⁻², which is the highest ever reported to the best of our knowledge. Combining experiments and theoretical calculation, we propose that the high performance is attributed to the proton transfer via Grotthuss mechanism and Vehicle mechanism in sub-1 nm pores of the ultrathin polyamide selective layer.

Low-cost flow batteries with high power density are promising for energy storage, but membranes with simultaneously high ion conductivity and selectivity should be developed. Here the authors report a thin-film composite membrane that breaks the trade-off between ion conductivity and selectivity.

Preparation of a novel zwitterionic striped surface thin-film composite nanofiltration membrane with excellent salt separation performance and antifouling property

Thin-film composite (TFC) nanofiltration (NF) membranes were fabricated via the co-deposition of taurine, tannic acid (TA), and polyethyleneimine (PEI), followed by subsequent interfacial polymerization with trimesoyl chloride (TMC) on the surface of the polysulfone ultrafiltration substrates. The surface properties, including the roughness, hydrophilicity, surface potential, and NF performances were facilely tuned by varying the taurine content for the prepared TFC membranes. In addition, the as-prepared TFC NF membranes had an excellent antifouling property and flux recovery ratio (FRR) in humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) filtration tests. These results also revealed that the taurine content controlled the formation of the striped surface. Thus, this work provided a viable strategy for fabricating TFC NF membranes with high selectivity and outstanding antifouling ability.

Thin-film composite (TFC) nanofiltration (NF) membranes with zwitterionic striped surface were fabricated via the co-deposition and interfacial polymerization.

Interfacial Polymerization of Cellulose Nanocrystal Polyamide Janus Nanocomposites with Controlled Architectures

The widespread use of renewable nanomaterials has been limited due to poor integration with conventional polymer matrices. Often, chemical and physical surface modifications are implemented to improve compatibility, however, this comes with environmental and economic cost. This work demonstrates that renewable nanomaterials, specifically cellulose nanocrystals (CNCs), can be utilized in their unmodified state and presents a simple and versatile, one-step method to produce polyamide/CNC nanocomposites with unique Janus-like properties. Nanocomposites in the form of films, fibers, and capsules are prepared by dispersing as-prepared CNCs in the aqueous phase prior to the interfacial polymerization of aromatic diamines and acyl chlorides. The diamines in the aqueous phase not only serve as a monomer for polymerization, but additionally, adsorb to and promote the incorporation of CNCs into the nanocomposite. Regardless of the architecture, CNCs are only present along the surface facing the aqueous phase, resulting in materials with unique, Janus-like wetting behavior and potential applications in filtration, separations, drug delivery, and advanced fibers.

Controlled Growth of Polyamide Films atop Homogenous and Heterogeneous Hydrogels using Gel-Liquid Interfacial Polymerization

Controlled growth of crosslinked polyamide (PA) thin films is demonstrated at the interface of a monomer-soaked hydrogel and an organic solution of the complementary monomer. Termed gel-liquid interfacial polymerization (GLIP), the resulting PA films are measured to be chemically and mechanically analogous to the active layer in thin film composite membranes. PA thin films are prepared using the GLIP process on both a morphologically homogeneous hydrogel prepared from poly(2-hydroxyethylmethacrylate) (PHEMA) and a phase-separated, heterogeneous hydrogel prepared from poly(acrylamide) (PAAm). Two monomer systems are examined: trimesoyl chloride (TMC) reacting with m-phenylene diamine (MPD) and TMC reacting with piperazine (PIP). Unlike the self-limiting growth behavior in TFC membrane fabrication, diffusion-limited, continuous growth of the PA films is observed, where both the thickness and roughness of the PA layers increase with reaction time. A key morphological difference is found between the two monomer systems using the GLIP process: TMC/MPD produces a ridge-and-valley surface morphology whereas TMC/PIP produces nodule/granular structures. The GLIP process represents a unique opportunity to not only explore the pore characteristics (size, spacing, and continuity) on the resulting structure and morphology of interfacially polymerized thin films, but also a method to modify the surface of (or encapsulate) hydrogels.

Controllable Interfacial Polymerization for Nanofiltration Membrane Performance Improvement by the Polyphenol Interlayer

It is a huge challenge to have a controllable interfacial polymerization in the fabrication process of nanofiltration (NF) membranes. In this work, a polyphenol interlayer consisting of polyethyleneimine (PEI)/tannic acid (TA) was simply assembled on the polysulfone (PSf) substrate to fine-tune the interfacial polymerization process, without additional changes to the typical NF membrane fabrication procedures. In addition, three decisive factors in the interfacial polymerization process were examined, including the diffusion kinetics of fluorescence-labeled piperazine (FITC-PIP), the spreading behavior of the hexane solution containing acyl chloride, and the polyamide layer formation on the porous substrate by in situ Fourier transform infrared (FT-IR) spectroscopy. The experimental results demonstrate that the diffusion kinetics of FITC-PIP is greatly reduced, and the spreading behavior of the hexane solution is also impeded to some extent. Furthermore, in situ FT-IR spectroscopy demonstrates that by the mitigation of this PEI/TA interlayer, the interfacial polymerization process is greatly controlled. Moreover, the as-prepared NF membrane exhibits an increased water permeation flux of 65 L m^-2 h^-1 (at the operation pressure of 0.6 MPa), high Na₂SO₄ rejection of >99%, and excellent long-term structural stability.

Nanoparticle-Assembled Thin Film with Amphipathic Nanopores for Organic Solvent Nanofiltration

Polymeric thin film composite (TFC) membranes have been proven promising for a wide range of separation applications. However, their development is significantly hindered by low permeance (below 8.0 L m^-2 h^-1 bar^-1). Here, we report the fabrication of new films with nanoparticle-assembled structure via interfacial polymerization using quantum dots (QDs) as building blocks. The tailored QDs with hydrophobic and hydrophilic regions permit cross-linking into nanoparticle-assembled defect-free thin films. Significantly, amphipathic QDs show good affinity to polar and nonpolar molecules, facilitating their fast dissolution into film. Meanwhile, the nanopores (∼1.4 nm) render fleet diffusion of molecules, which highly promotes the transfer of molecules within the film. This synergetic effect endows the resultant TFC membrane with high permeance, over 2 orders of magnitude higher than the conventional polyamide films. The permeances for acetonitrile and n-hexane reach 46.9 and 50.8 L m^-2 h^-1 bar^-1, respectively. We demonstrate that films fabricated by hydrophilic and hydrophobic QDs exhibit different molecular transfer mechanisms, and the corresponding model equations are established. The film fabricated by amphipathic QDs shows a combination transfer mechanism of the two models. Furthermore, those QD-based TFC membranes display favorable structural and operational stability, holding promise for industrial separation applications.

Influence of l-arginine on performances of polyamide thin-film composite reverse osmosis membranes

To prepare polyamide thin-film composite reverse osmosis (PA-TFC-RO) membranes with high performance, l-arginine (Arg) was used as an additive in m-phenylenediamine (MPD) aqueous solution. Arg with active amine groups can react with 1,3,5-benzenetricarboxylic chloride (TMC) to be incorporated into the polyamide selective layer during interfacial polymerization. X-ray photoelectron spectroscopy verified the successful introduction of Arg into the polyamide selective layer. Scanning electron microscopy, atomic force microscopy, contact angle and zeta potential measurements manifested that the polyamide selective layer was thinner, smoother, more hydrophilic and less negatively charged after the incorporation of Arg. The thinner and more hydrophilic polyamide selective layers favor the boosting of the permeability of the RO membrane by decreasing the hydraulic resistance to water permeation. Consequently, when the content of Arg was 0.5 wt%, the water flux and salt rejection of the resulting membranes increased from the original 46.46 L m⁻² h⁻¹ and 96.34% to 54.13 L m⁻² h⁻¹ and 98.36%. Besides, the modified membranes showed excellent fouling-resistance and easy-cleaning properties when tested by using bovine serum albumin (BSA) and dodecyltrimethyl ammonium bromide (DTAB) as model foulants.

l-Arginine (Arg) as an aqueous additive was incorporated into the polyamide selective layer during interfacial polymerization, thereby the separation performance and anti-fouling properties of the resulting RO membranes were enhanced.

Measuring Interfacial Polymerization Kinetics Using Microfluidic Interferometry

A range of academic and industrial fields exploit interfacial polymerization in producing fibers, capsules, and films. Although widely used, measurements of reaction kinetics remain challenging and rarely reported, due to film thinness and reaction rapidity. Here, polyamide film formation is studied using microfluidic interferometry, measuring monomer concentration profiles near the interface during the reaction. Our results reveal that the reaction is initially controlled by a reaction-diffusion boundary layer within the organic phase, which allows the first measurements of the rate constant for this system.

Optical Activity of Homochiral Polyamides in Solution and Solid State: Structural Function for Chiral Induction

In this work, we have explored a simple and facile approach to prepare optically active helical polyamides. The hydroxyl groups of l-TA and d-TA were protected by O-alkoyl ester, and the resulting enantiomers, l-2,3-di-O-acetyl-tartaric acid (l-ATA) and d-2,3-di-O-acetyl-tartaric acid (d-ATA) crystals, were obtained. A pair of aliphatic homochiral polyamides of PA-l and PA-d are prepared using l-ATA, d-ATA, and achiral 1,11-undecanediamine as building blocks via interfacial polycondensation. PA-l and PA-d display negative and positive mirror circular dichroism (CD) spectra images in both solution and solid state. Moreover, the polyamides in solid state display different CD signals and stronger optical activity compared to those in ethanol and even the related chiral monomers in solid state, which was due to the helical conformation of the polyamides in solid state. Scanning electron microscopy results indicated that the aggregations of PA-l express left-handed helical sense, whereas those of PA-d express right-handed helix. In addition, the induced CD signals from the chiral conformation of the backbone become weaker when increasing the temperature from 0 to 60 °C in dilute solution. Either of the polyamides displays relatively stable CD images in solid state when elevating the temperature from 0 to 90 °C.

Mesh-Embedded Polysulfone/Sulfonated Polysulfone Supported Thin Film Composite Membranes for Forward Osmosis

In this work, mesh-embedded polysulfone (PSU)/sulfonated polysulfone (sPSU) supported thin film composite (TFC) membranes were developed for forward osmosis (FO). The robust mesh integrated in PSU/sPSU sublayer imparts impressive mechanical durability. The blending of hydrophilic sPSU in PSU sublayer affects the hydrophilicity, porosity, pore structure, and pore size of mesh-embedded PSU/sPSU substrates, and the total thickness, cross-linking degree, and roughness of the corresponding TFC-FO membrane active layers. An appropriate incorporation of sPSU not only significantly decreases the structural parameter, S of the mesh-embedded substrate to 220 μm, which is the lowest reported value for fabric backed FO membrane, but also optimizes the permselectivity of the formed active layer. Regarding the osmosis performance, TFC membranes with sPSU modified substrates gain a higher water flux (J_w) while keeping the specific reverse salt flux (J_s/J_w) low. The optimal TFC-FO membrane has a J_w of 31.76 LMH with J_s/J_w of 0.19 g/L in FO mode when using deionized water feed and 1 M NaCl draw solution. This paper is practical for developing TFC-FO membrane on hydrophilic support membrane materials.

Preparation and application of flavor and fragrance capsules

The preparation methods and applications of flavor and fragrance capsules based on polymeric, inorganic and polymeric–inorganic wall materials are summarized.

Preparation and characterization of a novel nanofiltration membrane with chlorine-tolerant property and good separation performance

High water flux, good separation property and excellent chlorine resistance are crucial factors affecting the development of nanofiltration (NF) membranes. To obtain these properties, NF membranes were fabricated via interfacial polymerization using m-xylylenediamine (m-XDA) and polyethyleneimine (PEI) as aqueous monomers. By controlling the concentration ratio of m-XDA and PEI in the aqueous solution, it was found that the addition of PEI to the aqueous solution can increase the rejection of the NF membrane to magnesium chloride (MgCl₂) and magnesium sulfate (MgSO₄) from 18.3%, 54.5% to 84.4%, 94.1%, respectively. Meanwhile, the rejection to sodium sulphate (Na₂SO₄) and sodium chloride (NaCl) remain essentially unchanged. On the other hand, the addition of m-XDA to the aqueous solution can improve the chlorine resistance of the NF membrane, but it decreased the water flux of NF membrane. Sodium hypochlorite (NaClO) solution was used to evaluate chlorine resistance of NF membranes. After 10 000 ppm h NaClO immersion, the rejections to Na₂SO₄ of NF membranes prepared from the pure m-XDA and the blend of m-XDA and PEI were basically unchanged and the water flux increased. In conclusion, the obtained membranes not only exhibited good separation performance but also had good chlorine resistance.

High water flux, good separation property and excellent chlorine resistance are crucial factors affecting the development of nanofiltration (NF) membranes.