Thin-Film Composite Membranes for Hydrogen Evolution with a Saline Catholyte Water Feed.

Hydrogen gas evolution using an impure or saline water feed is a promising strategy to reduce overall energy consumption and investment costs for on-site, large-scale production using renewable energy sources. The chlorine evolution reaction is one of the biggest concerns in hydrogen evolution with impure water feeds. The "alkaline design criterion" in impure water electrolysis was examined here because water oxidation catalysts can exhibit a larger kinetic overpotential without interfering chlorine chemistry under alkaline conditions. Here, we demonstrated that relatively inexpensive thin-film composite (TFC) membranes, currently used for high-pressure reverse osmosis (RO) desalination applications, can have much higher rejection of Cl- (total crossover of 2.9 ± 0.9 mmol) than an anion-exchange membrane (AEM) (51.8 ± 2.3 mmol) with electrolytes of 0.5 M KOH for the anolyte and 0.5 M NaCl for the catholyte with a constant current (100 mA/cm2 for 20 h). The membrane resistances, which were similar for the TFC membrane and the AEM based on electrochemical impedance spectroscopy (EIS) and Ohm's law methods, could be further reduced by increasing the electrolyte concentration or removal of the structural polyester supporting layer (TFC-no PET). TFC membranes could enable pressurized gas production, as this membrane was demonstrated to be mechanically stable with no change in permeate flux at 35 bar. These results show that TFC membranes provide a novel pathway for producing green hydrogen with a saline water feed at elevated pressures compared to systems using AEMs or porous diaphragms.

Improving the efficiency of CO2 electrolysis by using a bipolar membrane with a weak-acid cation exchange layer.

The efficient conversion of electricity to chemicals is needed to mitigate the intermittency of renewable energy sources. Driving these electrochemical conversions at useful rates requires not only fast electrode kinetics, but also rapid mass and ion transport. However, little is known about the effect of local environments on ionic flows in solid polymer electrolytes. Here, we show that it is possible to measure and manipulate the local pH in membrane electrolysers with a resolution of tens of nanometres. In bipolar-membrane-based gas-fed CO2 electrolysers, the acidic environment of the cation exchange layer results in low CO2 reduction efficiency. By using ratiometric indicators and layer-by-layer polyelectrolyte assembly, the local pH was measured and controlled within an ~50-nm-thick weak-acid layer. The weak-acid layer suppressed the competing hydrogen evolution reaction without affecting CO2 reduction. This method of probing and controlling the local membrane environment may be useful in devices such as electrolysers, fuel cells and flow batteries, as well as in operando studies of ion distributions within polymer electrolytes.

Fourier transform infrared spectroscopy investigation of water microenvironments in polyelectrolyte multilayers at varying temperatures.

Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of oppositely charged polyelectrolytes. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully quantified with respect to both temperature and ionic strength. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25-1.5 M NaCl and 35-70 °C. The OD stretch peak is deconvoluted into three peaks: (1) high frequency water, which represents a tightly bound microenvironment, (2) low frequency water, which represents a loosely bound microenvironment, and (3) bulk water. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. Increasing temperature slightly reduces the amount of absorbed water, while addition of salt increases the amount of absorbed water. Finally, a van't Hoff analysis is applied to estimate the enthalpy (11-22 kJ mol-1) and entropy (48-79 kJ mol-1 K-1) of water exchanging from low to high frequency states.

Balancing Water Dissociation and Current Densities To Enable Sustainable Hydrogen Production with Bipolar Membranes in Microbial Electrolysis Cells.

Hydrogen production using two-chamber microbial electrolysis cells (MECs) is usually adversely impacted by a rapid rise in catholyte pH because of proton consumption for the hydrogen evolution reaction. While using a bipolar membrane (BPM) will maintain a more constant electrolyte pH, the large voltage loss across this membrane reduces performance. To overcome these limitations, we used an acidic catholyte to compensate for the potential loss incurred by using a BPM. A hydrogen production rate of 1.2 ± 0.7 L-H2/L/d (jmax = 10 ± 0.4 A/m2) was obtained using a Pt cathode and BPM with a pH difference (ΔpH = 6.1) between the two chambers. This production rate was 2.8 times greater than that of a conventional MEC with an anion exchange membrane (AEM, 0.43 ± 0.1 L-H2/L/d, jmax = 6.5 ± 0.3 A/m2). The catholyte pH gradually increased to 11 ± 0.3 over 9 days using the BPM and Pt/C, which decreased current production (jmax = 2.5 ± 0.3 A/m2). However, this performance was much better than that obtained using an AEM as the catholyte pH increased to 10 ± 0.4 after just one day. The use of an activated carbon cathode with the BPM enabled stable performance over a longer period of 12 days, although it reduced the hydrogen production rate (0.45 ± 0.1 L-H2/L/d).

Solvent-non-solvent rapid-injection for preparing nanostructured materials from micelles to hydrogels.

Due to their distinctive molecular architecture, ABA triblock copolymers will undergo specific self-assembly processes into various nanostructures upon introduction into a B-block selective solvent. Although much of the focus in ABA triblock copolymer self-assembly has been on equilibrium nanostructures, little attention has been paid to the guiding principles of nanostructure formation during non-equilibrium processing conditions. Here we report a universal and quantitative method for fabricating and controlling ABA triblock copolymer hierarchical structures using solvent-non-solvent rapid-injection processing. Plasmonic nanocomposite hydrogels containing gold nanoparticles and hierarchically-ordered hydrogels exhibiting structural color can be assembled within one minute using this rapid-injection technique. Surprisingly, the rapid-injection hydrogels display superior mechanical properties compared with those of conventional ABA hydrogels. This work will allow for translation into technologically relevant areas such as drug delivery, tissue engineering, regenerative medicine, and soft robotics, in which structure and mechanical property precision are essential.

Anion Exchange Membranes with Dynamic Redox-Responsive Properties.

Redox-responsive anion exchange membranes were developed using photoinitiated free-radical polymerization and reversible oxidation and reduction of viologen. The membranes were formulated using poly(ethylene glycol diacrylate) and diurethane dimethacrylate oligomers, dipentaerythritol penta-/hexa-acrylate cross-linker, photoinitiators, and 4-vinylbenzyl chloride as precursors for functionalization. In the membrane, 4,4'-bipyridine reacted with the 4-vinylbenzyl chloride residues, and subsequently, unreacted amines were methylated with iodomethane to obtain viologen as both the ion carrier and redox-responsive group. Upon oxidation, viologen supports two cations, where the reduced form only contains one cation. Thus, the redox responsiveness changed the membrane ionicity by a factor of 2. The area-specific resistance of the membranes in the oxidized, +2, state was lower than in the reduced, +1, state. The resistance increased between 40.6 ± 0.1 and 111.6 ± 0.1%, depending on membrane thickness, with the most significant increment being a resistance change from 4.88 × 10-4 Ω m2 in the oxidized state to 1.03 × 10-3 Ω m2 in the reduced state. Membrane permselectivity in the reduced, +1, state was between 15.9 ± 0.1 and 26.5 ± 0.01% lower than in the oxidized, +2, state, with no change in water uptake, spanning an average of 0.87 ± 0.02 in the oxidized state to an average of 0.7 ± 0.01 in the reduced state. Upon reduction, membrane ion-exchange capacity decreases, increasing ionic resistance and decreasing membrane permselectivity due to a reduction in fixed charge concentration without a measurable change in water uptake. This trend is not generally observed for ion-exchange membranes and explains that the changes in transport properties result from changes in ionicity, not water uptake or domain size. The reversibility and stability of the stimuli responsiveness were confirmed by the absence of transport property changes after redox cycling.

Biomimetic Separation of Transport and Matrix Functions in Lamellar Block Copolymer Channel-Based Membranes.

Cell membranes control mass, energy, and information flow to and from the cell. In the cell membrane a lipid bilayer serves as the barrier layer, with highly efficient molecular machines, membrane proteins, serving as the transport elements. In this way, highly specialized transport properties are achieved by these composite materials by segregating the matrix function from the transport function using different components. For example, cell membranes containing aquaporin proteins can transport ∼4 billion water molecules per second per aquaporin while rejecting all other molecules including salts, a feat unmatched by any synthetic system, while the impermeable lipid bilayer provides the barrier and matrix properties. True separation of functions between the matrix and the transport elements has been difficult to achieve in conventional solute separation synthetic membranes. In this study, we created membranes with distinct matrix and transport elements through designed coassembly of solvent-stable artificial (peptide-appended pillar[5]arene, PAP5) or natural (gramicidin A) model channels with block copolymers into lamellar multilayered membranes. Self-assembly of a lamellar structure from cross-linkable triblock copolymers was used as a scalable replacement for lipid bilayers, offering better stability and mechanical properties. By coassembly of channel molecules with block copolymers, we were able to synthesize nanofiltration membranes with sharp selectivity profiles as well as uncharged ion exchange membranes exhibiting ion selectivity. The developed method can be used for incorporation of different artificial and biological ion and water channels into synthetic polymer membranes. The strategy reported here could promote the construction of a range of channel-based membranes and sensors with desired properties, such as ion separations, stimuli responsiveness, and high sensitivity.

Resistance and Permselectivity of 3D-Printed Micropatterned Anion-Exchange Membranes.

It has been demonstrated that a micropatterned surface can decrease the resistance of anion-exchange membranes (AEMs) and can induce desirable flow properties in devices, such as mixing. Previously, a model that related the resistance of flat and patterned membranes with the same equivalent thickness was proposed, which used the patterned area and thickness ratio of the features to describe the membrane resistance. Here, we explored the validity of the parallel resistance model for a variety of membrane surface designs and area ratios. We demonstrated that the model can predict the resistance of a wide range of patterned AEMs. We showed that the resistance is independent of the spatial ordering of the design by examining random patterns, which is relevant for applications that require, for example, increased mixing in multilayered devices. Some experimental values of resistance obtained for patterned membranes presented deviations from the model. Scanning electron microscopy (SEM) images of the patterned membranes revealed resolution variations and pattern replication errors due to the stereolithographic process. A geometric correction of the target ratios improved the fit of the modeled data to the experimental values, showing that light bleeding during curing was a source of error. Two additional experimental factors were not accounted for in the model: a distinct interface between the bottom and top layer and overcuring of the bottom layer during successive steps. These sources of error were investigated by examining the resistance of single- and double-layered membranes, as well as single-layer membranes with different curing times. The differences obtained in the resistances for control samples demonstrated that both the interface and the overcuring influenced the resistance of the membrane. The results obtained in this study enlighten the discussion relating membrane-surface morphology and transport properties, as well as the optimization of 3D-printed membranes using a stereolithography process.

Improved ATR-FTIR detection of hydrocarbons in water with semi-crystalline polyolefin coatings on ATR elements.

In situ measurement of hydrocarbons in water is critical for assuring the safety and quality of drinking water and in environmental remediation activities such as the cleanup of oil spills. Thus, effective detection methods of hydrocarbons in aqueous environments are important and several methods have been used for this type of sensing, including spectroscopic techniques, fiber optic sensors, and chromatography. However, under aqueous conditions, small amounts of hydrocarbons are difficult to detect due to their low concentration in water and robust sensing of these types of compounds in an aqueous environment remains a challenging analytical task. Hydrophobic polymer coatings have been widely used to concentrate hydrocarbons for attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) detection at the surface of an ATR crystal by preventing water molecules from penetrating into the polymer coating while absorbing hydrocarbons. However, in typical coating designs only thin films (<5 µm) can be applied onto the ATR sensor due to the decrease in detection limit and sensitivity to hydrocarbons with increasing film thickness. This paper demonstrates that a semi-crystalline linear low-density polyethylene (LLDPE) polymer coating with thicker thickness (40 µm) can be applied effectively for in situ ATR-FTIR detection of hydrocarbons in aqueous solution. The ATR signal is enhanced by the polymer coating which swells in response to the hydrocarbons and prevents water accumulation at the IR detection interface. Coating the ATR element with a LLDPE film (crystallinity = 12%) reduced the detection time for various hydrocarbons, including toluene, benzene and chloroform. The detection limits and kinetics of the ATR-FTIR detection were not significantly altered when the thickness of the LLDPE coating was increased to improve its mechanical properties which represents a significant improvement from coatings published in the literature. The LLDPE coating described in this research has the potential to be applied as a sensor coating for rapid detection of hydrocarbon-based substances or non-polar biomolecules in aqueous environments.

Creating cross-linked lamellar block copolymer supporting layers for biomimetic membranes.

The long-standing goal in membrane development is creating materials with superior transport properties, including both high flux and high selectivity. These properties are common in biological membranes, and thus mimicking nature is a promising strategy towards improved membrane design. In previous studies, we have shown that artificial water channels can have excellent water transport abilities that are comparable to biological water channel proteins, aquaporins. In this study, we propose a strategy for incorporation of artificial channels that mimic biological channels into stable polymeric membranes. Specifically, we synthesized an amphiphilic triblock copolymer, poly(isoprene)-block-poly(ethylene oxide)-block-poly(isoprene), which is a high molecular weight synthetic analog of naturally occurring lipids in terms of its self-assembled structure. This polymer was used to build stacked membranes composed of self-assembled lamellae. The resulting membranes resemble layers of natural lipid bilayers in living systems, but with superior mechanical properties suitable for real-world applications. The procedures used to synthesize the triblock copolymer resulted in membranes with increased stability due to the crosslinkability of the hydrophobic domains. Furthermore, the introduction of bridging hydrophilic domains leads to the preservation of the stacked membrane structure when the membrane is in contact with water, something that is challenging for diblock lamellae that tend to swell, and delaminate in aqueous solutions. This new method of membrane fabrication offers a practical model for making channel-based biomimetic membranes, which may lead to technological applications in reverse osmosis, nanofiltration, and ultrafiltration membranes.

Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity.

As the field of anion exchange membranes (AEMs) employs an increasing variety of cations, a critical understanding of cation properties must be obtained, especially as they relate to membrane ion conductivity. Here, to elucidate such properties, metal cation-based AEMs, featuring bis(norbornene) nickel, ruthenium, or cobalt complexes, were synthesized and characterized. In addition, isothermal titration calorimetry (ITC) was used to probe counterion exchange thermodynamics in order to understand previously reported differences in conductivity. The ion conductivity data reported here further demonstrated that nickel-complex cations had higher conductivity as compared to their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation-counterion association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger thermodynamic driving force for chloride counterion release was observed, shown through a smaller Δ Htot for counterion exchange, which indicated weaker cation-counterion association. The use of ITC to study cation-counterion association was further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation, which demonstrated small variances in their enthalpic response, but an overall Δ Htot similar to that of the nickel-based cation. The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in determining the strength of the initial cation-counterion pair. This report identifies for the first time how ITC can be used to experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in AEMs.

Substrate-Dependent Physical Aging of Confined Nafion Thin Films.

The humidity-induced physical aging, or structural relaxation, of spin-cast Nafion thin films on gold, carbon, and native oxide silicon (n-SiO2) substrates was examined using spectroscopic ellipsometry (SE). Physical aging rates, ß, were calculated from the change in measured sample thickness, h, upon exposure to controlled humidity. Three Nafion films, h = 188, 57, and 27 nm, deposited on gold substrates demonstrated an increased ß with decreasing thickness due to confinement. The Nafion film on n-SiO2, h = 165 nm, also showed a humidity-induced aging, while a Nafion film deposited on carbon, h = 190 nm, exhibited no measurable humidity-induced aging. The reported rate of aging was related to the strength of the polymer/substrate interactions during film formation. Strong interactions between Nafion and the gold and n-SiO2 substrates anchored the thin film to the substrate during film formation, resulting in a nonequilibrium as-cast film and subsequent relaxation upon exposure to water vapor until complete plasticization. Weak interactions between the carbon substrate and Nafion resulted in fully relaxed as-cast films which displayed no relaxation upon hydration.

Quantifying Carboxylic Acid Concentration in Model Polyamide Desalination Membranes via Fourier Transform Infrared Spectroscopy.

Carboxylic acid groups impart hydrophilicity and ionizable moieties to polyamide membranes for desalination, hence influencing water and ion transport through the material. Model polyamide films were synthesized via molecular layer-by-layer deposition on planar substrates to study the formation process of these materials and overcome the chemical and topological inhomogeneity inherent to conventional interfacially polymerized polyamide membranes. The carboxylic acid content in these model films was characterized using Fourier transform infrared (FTIR) spectroscopy by quantifying the C=O band at 1718 cm-1. The concentration of carboxylic acid groups decreased as the thickness of the membrane increased, suggestive of an increase in crosslink density as the polyamide network develops. For the thinnest molecular layer-by-layer (mLbL) samples, the carboxylic acid concentration for films on gold was 0.35 mmol g-1, whereas analogous films on silicon had an acid content of 0.56 mmol g-1, indicating a clear influence of the substrate on the initial network formation. As the thickness of the membrane increased, the influence of the substrate and initial layer growth became less significant as the carboxylic acid concentration on both substrates reached a value of 0.12 mmol g-1. We demonstrate that FTIR spectroscopy is a practical and accessible way to quantify the carboxylic acid content in these types of extremely thin polyamide membranes to help quantify network formation in these materials.

Multivariate analysis of attachment of biofouling organisms in response to material surface characteristics.

Multivariate analyses were used to investigate the influence of selected surface properties (Owens-Wendt surface energy and its dispersive and polar components, static water contact angle, conceptual sign of the surface charge, zeta potentials) on the attachment patterns of five biofouling organisms (Amphibalanus amphitrite, Amphibalanus improvisus, Bugula neritina, Ulva linza, and Navicula incerta) to better understand what surface properties drive attachment across multiple fouling organisms. A library of ten xerogel coatings and a glass standard provided a range of values for the selected surface properties to compare to biofouling attachment patterns. Results from the surface characterization and biological assays were analyzed separately and in combination using multivariate statistical methods. Principal coordinate analysis of the surface property characterization and the biological assays resulted in different groupings of the xerogel coatings. In particular, the biofouling organisms were able to distinguish four coatings that were not distinguishable by the surface properties of this study. The authors used canonical analysis of principal coordinates (CAP) to identify surface properties governing attachment across all five biofouling species. The CAP pointed to surface energy and surface charge as important drivers of patterns in biological attachment, but also suggested that differentiation of the surfaces was influenced to a comparable or greater extent by the dispersive component of surface energy.

Spectroscopic Characterization of Sulfonate Charge Density in Ion-Containing Polymers.

The charge density and hydrogen bonding with water of five different polymer membranes functionalized with various sulfonate side-chain chemistries were investigated using Fourier transform infrared (FTIR) techniques and density functional theory (DFT) calculations. The peak position of the OD stretch of dilute HOD absorbed into the sulfonated poly(sulfone) membranes was studied using FTIR to compare the charge density of the sulfonate headgroup across the different samples, which can ultimately be related to the acidity of the proton-form sulfonate moieties. The OD peak was deconvoluted to determine the percentage of headgroup-associated, intermediate, and bulk water. DFT modeling was used to calculate the charge density of each headgroup and visualize how the chemistry of the headgroup influenced the conformation of the side-chain tether. FTIR-determined OD peak positions and charge density calculations demonstrated that a perflurosulfonate containing a thioether linkage produced the most acidic sulfonate headgroup. However, the amount of headgroup-associated water calculated for this side chain was low due to the unique cis conformation of the thioether side chain. The biperfluorosulfonate side chain had very low calculated headgroup-associated water due to its bulkiness and water molecules bridging the two sulfonates. These detailed insights on local hydration of sulfonate side chains will point towards new headgroup designs for advanced membranes.

First-Principles Calculation of Pt Surface Energies in an Electrochemical Environment: Thermodynamic Driving Forces for Surface Faceting and Nanoparticle Reconstruction.

Platinum is a widely used catalyst in aqueous and electrochemical environments. The size and shape of Pt nanoparticles and the faceting (and roughness) of extended Pt surfaces change during use in these environments due to dissolution, growth, and reconstruction. Further, many Pt nanoparticle synthesis techniques are carried out in an aqueous environment. The surface structures formed are impacted by the relative surface energies of the low index facets in these environments. Density functional theory is used to calculate the surface energy of the low index facets of platinum as a function of electrochemical potential and coverage of adsorbed hydrogen, hydroxide, oxygen, and the formation of surface oxide in an aqueous environment. Whereas Pt(111) is the lowest energy bare surface in vacuum, the strong adsorption of hydrogen to Pt(100) at low potentials and of hydroxide to Pt(110) and oxygen to Pt(100) at high potentials drives these surfaces to be more stable in an electrochemical environment. We experimentally conditioned a polycrystalline platinum electrode by cycling the potential and find a growth in the total area as well as in the fraction of 110 and 100 sites, which are lower in energy at potentials where dissolved Pt is deposited or surface oxide is reduced. Further, we find that the lower surface energy of Pt(100) at low potentials may play a role in the growth of tetrahexahedral nanoparticles seen on square wave cycling of spherical Pt nanoparticles. Wulff constructions are presented as a function of Pt electrode potential.

Water Sorption in Electron-Beam Evaporated SiO2 on QCM Crystals and Its Influence on Polymer Thin Film Hydration Measurements.

Spectroscopic ellipsometry (SE) and quartz crystal microbalance (QCM) measurements are two critical characterization techniques routinely employed for hydration studies of polymer thin films. Water uptake by thin polymer films is an important area of study to investigate antifouling surfaces, to probe the swelling of thin water-containing ionomer films, and to conduct fundamental studies of polymer brush hydration and swelling. SiO2-coated QCM crystals, employed as substrates in many of these hydration studies, show porosity in the thin electron-beam (e-beam) evaporated SiO2 layer. The water sorption into this porous SiO2 layer requires correction of the optical and mass characterization of the hydrated polymer due to changes in the SiO2 layer as it sorbs water. This correction is especially important when experiments on SiO2-coated QCM crystals are compared to measurements on Si wafers with dense native SiO2 layers. Water adsorption filling void space during hydration in ∼200-260 nm thick SiO2 layers deposited on a QCM crystal resulted in increased refractive index of the layer during water uptake experiments. The increased refractive index led to artificially higher polymer swelling in the optical modeling of the hydration experiments. The SiO2-coated QCM crystals showed between 6 and 8% void as measured by QCM and SE, accounting for 60%-85% of the measured polymer swelling in the low humidity regime (<20% RH) and 25%-40% of the polymer swelling in the high humidity regime (>70% RH) from optical modeling for 105 and 47 nm thick sulfonated polymer films. Correcting the refractive index of the SiO2 layer for its water content resulted in polymer swelling that successfully resembled swelling measured on a silicon wafer with nonporous native oxide.

Click Cross-Linking-Improved Waterborne Polymers for Environment-Friendly Coatings and Adhesives.

Waterborne polymers, including waterborne polyurethanes (WPU), polyester dispersions (PED), and polyacrylate emulsions (PAE), are employed as environmentally friendly water-based coatings and adhesives. An efficient, fast, stable, and safe cross-linking strategy is always desirable to impart waterborne polymers with improved mechanical properties and water/solvent/thermal and abrasion resistance. For the first time, click chemistry was introduced into waterborne polymer systems as a cross-linking strategy. Click cross-linking rendered waterborne polymer films with significantly improved tensile strength, hardness, adhesion strength, and water/solvent resistance compared to traditional waterborne polymer films. For example, click cross-linked WPU (WPU-click) has dramatically improved the mechanical strength (tensile strength increased from 0.43 to 6.47 MPa, and Young's modulus increased from 3 to 40 MPa), hardness (increased from 59 to 73.1 MPa), and water resistance (water absorption percentage dropped from 200% to less than 20%); click cross-linked PED (PED-click) film also possessed more than 3 times higher tensile strength (∼28 MPa) than that of normal PED (∼8 MPa). The adhesion strength of click cross-linked PAE (PAE-click) to polypropylene (PP) was also improved (from 3 to 5.5 MPa). In addition, extra click groups can be preserved after click cross-linking for further functionalization of the waterborne polymeric coatings/adhesives. In this work, we have demonstrated that click modification could serve as a convenient and powerful approach to significantly improve the performance of a variety of traditional coatings and adhesives.

Highly conductive side chain block copolymer anion exchange membranes.

Block copolymers based on poly(styrene) having pendent trimethyl styrenylbutyl ammonium (with four carbon ring-ionic group alkyl linkers) or benzyltrimethyl ammonium groups with a methylene bridge between the ring and ionic group were synthesized by reversible addition-fragmentation radical (RAFT) polymerization as anion exchange membranes (AEMs). The C4 side chain polymer showed a 17% increase in Cl(-) conductivity of 33.7 mS cm(-1) compared to the benzyltrimethyl ammonium sample (28.9 mS cm(-1)) under the same conditions (IEC = 3.20 meq. g(-1), hydration number, λ = ∼7.0, cast from DMF/1-propanol (v/v = 3 : 1), relative humidity = 95%). As confirmed by small angle X-ray scattering (SAXS), the side chain block copolymers with tethered ammonium cations showed well-defined lamellar morphologies and a significant reduction in interdomain spacing compared to benzyltrimethyl ammonium containing block copolymers. The chemical stabilities of the block copolymers were evaluated under severe, accelerated conditions, and degradation was observed by (1)H NMR. The block copolymer with C4 side chain trimethyl styrenylbutyl ammonium motifs displayed slightly improved stability compared to that of a benzyltrimethyl ammonium-based AEM at 80 °C in 1 M NaOD aqueous solution for 30 days.

3D Printing of Micropatterned Anion Exchange Membranes.

Micropatterned anion exchange membranes (AEMs) have been 3D printed via a photoinitiated free radical polymerization and quaternization process. The photocurable formulation, consisting of diurethane dimethacrylate (DUDA), poly(ethylene glycol) diacrylate (PEGDA), dipentaerythritol penta-/hexa- acrylate, and 4-vinylbenzyl chloride (VBC), was directly cured into patterned films using a custom 3D photolithographic printing process similar to stereolithography. Measurements of water uptake, permselectivity, and ionic resistance were conducted on the quaternized poly(DUDA-co-PEGDA-co-VBC) sample series to determine their suitability as ion exchange membranes. The water uptake of the polymers increased as the ion exchange capacity (IEC) increased due to greater quaternized VBC content. Samples with IEC values between 0.98 to 1.63 mequiv/g were synthesized by varying the VBC content from 15 to 25 wt %. The water uptake was sensitive to the PEGDA content in the network resulting in water uptake values ranging from 85 to 410 wt % by varying the PEGDA fractions from 0 to 60 wt %. The permselectivity of the AEM samples decreased from 0.91 (168 wt %, 1.63 mequiv/g) to 0.85 (410 wt %, 1.63 mequiv/g) with increasing water uptake and to 0.88 (162 wt %, 0.98 mequiv/g) with decreasing IEC. Permselectivity results were relatively consistent with the general understanding of the correlation between permselectivity, water uptake, and ion content of the membrane. Lastly, it was revealed that the ionic resistance of patterned membranes was lower than that of flat membranes with the same material volume or equivalent thickness. A parallel resistance model was used to explain the influence of patterning on the overall measured ionic resistance. This model may provide a way to maximize ion exchange membrane performance by optimizing surface patterns without chemical modification to the membrane.