Molecular simulation of polyamide synthesis by interfacial polymerization

Polyarylates containing phthalazinone moieties with excellent thermal resistance

Polyarylates containing phthalazinone moieties are synthesized by interfacial polymerization of 2,4-(4-hydroxyphenyl)-2,3-phthalazin-1-one with isophthaloyl dichloride (IPC) and terephthaloyl dichloride (TPC). The effects of organic solvents and phase transfer catalysts (PTC) on the intrinsic viscosity ( η int) are systematically investigated for polymers with η int up to 1.52 dL g−1. The polyarylate has a high η int with 1,2-dichloroethane and cetyltrimethylammonium bromide used as the organic phase solvent and PTC. It is found that polyarylates prepared from BPPZ with IPC and TPC have excellent thermal resistance, with glass transition temperatures of 292 and 337°C, respectively. The polyarylates exhibit excellent thermal stability with 5% mass-loss temperature above 469°C in both N2 and air, and residual mass ratios at 800°C in N2 and air above 54.1% and 4.0%, respectively.

Multimodal confined water dynamics in reverse osmosis polyamide membranes

While polyamide (PA) membranes are widespread in water purification and desalination by reverse osmosis, a molecular-level understanding of the dynamics of both confined water and polymer matrix remains elusive. Despite the dense hierarchical structure of PA membranes formed by interfacial polymerization, previous studies suggest that water diffusion remains largely unchanged with respect to bulk water. Here, we employ neutron spectroscopy to investigate PA membranes under precise hydration conditions, and a series of isotopic contrasts, to elucidate water transport and polymer relaxation, spanning ps-ns timescales, and Å-nm lengthscales. We experimentally resolve, for the first time, the multimodal diffusive nature of water in PA membranes: in addition to (slowed down) translational jump-diffusion, we observe a long-range and a localized mode, whose geometry and timescales we quantify. The PA matrix is also found to exhibit rotational relaxations commensurate with the nanoscale confinement observed in water diffusion. This comprehensive ‘diffusion map’ can anchor molecular and nanoscale simulations, and enable the predictive design of PA membranes with tuneable performance.

Polymeric membranes are extensively used in water desalination, but the effect of membrane nanostructure on water transport is still elusive. The authors, using quasi-elastic neutron scattering and contrast variation techniques, provide detailed insight into the dynamics of the polymer network and confined water across a wide range of length and timescales.

Impacts of Surface Hydrophilicity of Carboxylated Polyethersulfone Supports on the Characteristics and Permselectivity of PA-TFC Nanofiltration Membranes

Our current study experimentally evaluates the impacts of surface hydrophilicity of supports on the properties of polyamide (PA) thin-film composite (TFC) nanofiltration (NF) membranes. A series of "carboxylated polyethersulfone" (CPES) copolymers with an increasing "molar ratio" (MR) of carboxyl units were used to prepare supports with diverse surface hydrophilicities by the classical nonsolvent-induced phase separation (NIPS) method. Then, the PA-TFC NF membranes were finely fabricated atop these supports by conventional interfacial polymerization (IP) reactions. The linkages between the surface hydrophilicity of the supports and the characteristics of the interfacially polymerized PA layers as well as the permselectivity of NF membranes were investigated systematically. The morphological details of the NF membranes indicate that the growth of PA layers can be adjusted through increasing the surface hydrophilicity of the supports. Moreover, the separation results reveal that the NF membrane fabricated on the relatively hydrophobic support exhibits lower permeability (7.04 L·m^-2·h^-1·bar^-1) and higher selectivity (89.94%) than those of the ones prepared on the hydrophilic supports (14.64~18.99 L·m^-2·h^-1·bar^-1 and 66.98~73.48%). A three-stage conceptual scenario is proposed to illustrate the formation mechanism of the PA layer in NF membranes, which is due to the variation of surface hydrophilicity of the supports. The overall findings specify how the surface hydrophilicity of the supports influences the formation of PA layers, which ultimately defines the separation performances of the corresponding NF membranes.

Visualizing and monitoring interfacial polymerization by aggregation-induced emission

The aggregation-induced emission effect is used to visualize and monitor interfacial polymerization at the alkane–ionic liquid interface by virtue of the quantitative fluorescence of arylamine luminogens.

Thickness-Dependent Permeance of Molecular Layer-By-Layer Polyamide Membranes

We present the thickness-dependent permeance of highly cross-linked polyamide (PA) membranes formed by a molecular layer-by-layer (mLbL) deposition process. The deposition allows for the synthesis of extremely smooth, uniform PA films of tunable thickness, which is counter to the less controlled interfacial polymerization process used commercially. The ability to control and measure the membrane thickness allows us to elucidate the relationships among network structure, transport properties, and separation performance. In this work, a series of large-area mLbL PA membranes is prepared with thickness ranging from less than 5 nm to greater than 100 nm, which can be transferred defect-free via a film floating technique onto a macroporous support layer and challenged with salt solutions. A critical thickness of 15 nm is identified for efficient desalination, and water permeance is described using a multi-layer solution diffusion model that allows for the extraction of material properties relevant to transport. Finally, the model demonstrates the existence of two distinct layers in the mLbL films, one layer comprised of a (5 to 10) nm graded or less cross-linked layer at the surface and a more densely cross-linked layer in the interior of the film. This graded layer appears inherent to the mLbL deposition process and is observed at all film thicknesses.

Highly Permeable Reverse Osmosis Membranes via Molecular Layer-by-Layer Deposition of Trimesoyl Chloride and 3,5-Diaminobenzoic Acid

We present a series of polyamide membranes synthesized via molecular layer-by-layer (mLbL) deposition of trimesoyl chloride (TMC) and 3,5-diaminobenzoic acid (BA). These membranes exhibit superior NaCl rejection compared to previously reported TMC-BA membranes prepared via interfacial polymerization, with the improved performance of the mLbL films attributable to higher cross-link density facilitated by the stepwise deposition process in good solvents. We compare the TMC-BA series with membranes synthesized from TMC and m-phenylenediamine (MPD), a conventional reverse osmosis membrane chemistry. At the minimum thickness capable of 90 % NaCl rejection, mLbL TMC-BA membranes exhibit 50 % greater water permeance than mLbL TMC-MPD.

Molecular Understanding of Ion Effect on Polyzwitterion Conformation in an Aqueous Environment

Polyzwitterions (PZs) are promising materials for the antifouling in reverse osmosis and nanofiltration membrane technology for water treatment. Fundamental understanding of the structure and molecular interactions involving zwitterions is crucial to the optimal design of antifouling in membrane separation. Here we employ the umbrella sampling and molecular dynamics simulations to investigate molecular interactions between sulfobetaine/carboxybetaine zwitterions and different metal ions (Na⁺, K⁺, and Ca²⁺) in an aqueous solution. The simulation results show that these ions can form stable or metastable contact ionic/solvent-shared-ionic pairs with zwitterions. Simulations at different grafting densities of PZ brush arrays reveal complex competitive association mechanisms, which are attributed to nonbonded electrostatic and van der Waals interactions among zwitterions, water molecules, and different metal ions in an aqueous environment. While the high-grafting density of the PZ brush array leads to a strong branch association between different zwitterions in water, this association is decreased at intermediate- and low-grafting densities due to strong zwitterion-water interactions. More importantly, adding ions into water at intermediate- and low-grafting densities further breaks down the zwitterion branch association, resulting in a randomly oriented and dispersed branch configuration with significant swelling of the polymers. The degree of swelling depends on the type of ions, which further changes the surface electrostatic potential of PZ coatings.

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.

Molecular Simulations of the Hydration Behavior of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment

We carried out umbrella sampling and molecular dynamics (MD) simulations to investigate molecular interactions between sulfobetaine zwitterions or between sulfobetaine brushes in different media. Simulation results show that it is more energetically favorable for the two sulfobetaine zwitterions or brushes to be fully hydrated in aqueous solutions than in vacuum where strong ion pairs are formed. Structural properties of the hydrated sulfobetaine brush array and its antifouling behavior against a foulant gel are subsequently studied through steered MD simulations. We find that sulfobetaine brush arrays with different grafting densities have different structures and antifouling mechanisms. At a comparably higher grafting density, the sulfobetaine brush array exhibits a more organized structure which can hold a tightly bound hydration water layer at the interface. Compression of this hydration layer results in a strong repulsive force. However, at a comparably lower grafting density, the brush array exhibits a randomly oriented structure in which the antifouling of the brush array is through the deformation of the sulfobetaine branches.

Sustainable Process for the Preparation of High-Performance Thin-Film Composite Membranes using Ionic Liquids as the Reaction Medium

A new form of interfacial polymerization to synthesize thin-film composite membranes realizes a more sustainable membrane preparation and improved nanofiltration performance. By introducing an ionic liquid (IL) as the organic reaction phase, the extremely different physicochemical properties to those of commonly used organic solvents influenced the top-layer formation in several beneficial ways. In addition to the elimination of hazardous solvents in the preparation, the m-phenylenediamine (MPD) concentration could be reduced 20-fold, and the use of surfactants and catalysts became redundant. Together with the more complete recycling of the organic phase in the water/IL system, these factors resulted in a 50 % decrease in the mass intensity of the top-layer formation. Moreover, a much thinner top layer with a high ethanol permeance of 0.61 L m(-2)  h(-1)  bar(-1) [99 % Rose Bengal (RB, 1017 Da) retention; 1 bar=0.1 MPa] was formed without the use of any additives. This EtOH permeance is 555 and 161 % higher than that for the conventional interfacial polymerization (without and with additives, respectively). In reverse osmosis, high NaCl retentions of 97 % could be obtained. Finally, the remarkable decrease in the membrane surface roughness indicates the potential for reduced fouling with this new type of membrane.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

Visualization and characterization of interfacial polymerization layer formation

We present a microfluidic platform to visualize the formation of free-standing films by interfacial polymerization. A microfluidic device is fabricated, with an array of micropillars to stabilize an aqueous-organic interface that allows a direct observation of the films formation process via optical microscopy. Three different amines are selected to react with trimesoyl chloride: piperazine, JEFFAMINE(®)D-230, and an ammonium functionalized polyhedral oligomeric silsesquioxane. Tracking the formation of the free-standing films in time reveals strong effects of the characteristics of the amine precursor on the morphological evolution of the films. Piperazine exhibits a rapid reaction with trimesoyl chloride, forming a film up to 20 μm thick within half a minute. JEFFAMINE(®)D-230 displays much slower film formation kinetics. The location of the polymerization reaction was initially in the aqueous phase and then shifted into the organic phase. Our in situ real-time observations provide information on the kinetics and the changing location of the polymerization. This provides insights with important implications for fine-tuning of interfacial polymerizations for various applications.

Fabrication and characterization of a novel nanofiltration membrane by the interfacial polymerization of 1,4-diaminocyclohexane (DCH) and trimesoyl chloride (TMC)

A novel thin-film composite polyamide membrane for nanofiltration is prepared, and the addition of sodium N-cyclohexylsulfamate is found to have a significant influence on its performance.

Molecular dynamics simulations of polyamide membrane, calcium alginate gel, and their interactions in aqueous solution

We perform molecular dynamics (MD) simulations to investigate the cross-linked polyamide (PA) membrane, the aggregation of alginate molecules in the presence of Ca(2+) ions, and their molecular binding mechanism in aqueous solution. We use a steered molecular dynamics (SMD) approach to simulate the unbinding process between a PA membrane and an alginate gel complex. Simulation results show that Ca(2+) ions are strongly associated with the carboxylate groups in alginate molecules, forming a web structure. The adhesion force between alginate gel and PA surface during unbinding originates from several important molecular interactions. These include the short-range hydrogen bonding and van der Waals attraction forces, and the ionic bridge binding that extends much longer pulling distances due to the significant chain deformations of alginate gel and PA membrane.

Preparation, modification and characterization of polymeric hollow fiber membranes for pressure-retarded osmosis

The present study evaluated the performance of polymeric hollow fiber membranes in the pressure-retarded osmosis (PRO) process for power generation.

Hydrated polyamide membrane and its interaction with alginate: a molecular dynamics study

The properties of the hydrated amorphous polyamide (PA) membrane and its binding with alginate are investigated through molecular dynamics simulations. The density of the hydrated membrane, surface morphology, and water diffusion near and inside the membrane are compared to other studies. Particular focus is given to the steered molecular dynamics (SMD) simulation of the binding between the PA membrane and an alginate model. The PA surface composition is determined on the basis of experimental measurements of the oxygen/nitrogen (O/N) ratio. The surface model is built using a configurational-bias Monte Carlo technique. The consistent valence force field (CVFF) is used to describe the atomic interactions in the membrane-foulant system. Simulation results show that the carboxylate groups in both the PA surface and alginate exhibit strong binding with metal ions. This binding mechanism plays a major role in the PA-alginate fouling through the formation of an ionic binding bridge. Specifically, Ca(2+) ions have stronger binding with the carboxylate group than Na(+) ions, while the binding breakdown time is shorter for Ca(2+) than Na(+) because of the comparably higher hydration free energy of Ca(2+) ions with water molecules.