Continuous Ligand-Free Catalysis Using a Hybrid Polymer Network Support.

Although the pharmaceutical and fine chemical industries primarily utilize batch homogeneous reactions to carry out chemical transformations, emerging platforms seek to improve existing shortcomings by designing effective heterogeneous catalysis systems in continuous flow reactors. In this work, we present a versatile network-supported palladium (Pd) catalyst using a hybrid polymer of poly(methylvinylether-alt-maleic anhydride) and branched polyethyleneimine for intensified continuous flow synthesis of complex organic compounds via heterogeneous Suzuki-Miyaura cross-coupling and nitroarene hydrogenation reactions. The hydrophilicity of the hybrid polymer network facilitates the reagent mass transfer throughout the bulk of the catalyst particles. Through rapid automated exploration of the continuous and discrete parameters, as well as substrate scope screening, we identified optimal hybrid network-supported Pd catalyst composition and process parameters for Suzuki-Miyaura cross-coupling reactions of aryl bromides with steady-state yields up to 92% with a nominal residence time of 20 min. The developed heterogeneous catalytic system exhibits high activity and mechanical stability with no detectable Pd leaching at reaction temperatures up to 95 °C. Additionally, the versatility of the hybrid network-supported Pd catalyst is demonstrated by successfully performing continuous nitroarene hydrogenation with short residence times (<5 min) at room temperature. Room temperature hydrogenation yields of >99% were achieved in under 2 min nominal residence times with no leaching and catalyst deactivation for more than 20 h continuous time on stream. This catalytic system shows its industrial utility with significantly improved reaction yields of challenging substrates and its utility of environmentally-friendly solvent mixtures, high reusability, scalable and cost-effective synthesis, and multi-reaction successes.

Enhanced Triboelectric Charge Stability by Air-Stable Radicals.

This paper demonstrates that air-stable radicals enhance the stability of triboelectric charge on surfaces. While charge on surfaces is often undesirable (e.g., static discharge), improved charge retention can benefit specific applications such as air filtration. Here, it is shown that self-assembled monolayers (SAMs) containing air-stable radicals, 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), hold the charge longer than those without TEMPO. Charging and retention are monitored by Kelvin Probe Force Microscopy (KPFM) as a function of time. Without the radicals on the surface, charge retention increases with the water contact angle (hydrophobicity), consistent with the understanding that surface water molecules can accelerate charge dissipation. Yet, the most prolonged charge retention is observed in surfaces treated with TEMPO, which are more hydrophilic than untreated control surfaces. The charge retention decreases with reducing radical density by etching the TEMPO-silane with tetrabutylammonium fluoride (TBAF) or scavenging the radicals with ascorbic acid. These results suggest a pathway toward increasing the lifetime of triboelectric charges, which may enhance air filtration, improve tribocharging for patterning charges on surfaces, or boost triboelectric energy harvesting.

A Scalable Microstructure Photonic Coating Fabricated by Roll-to-Roll "Defects" for Daytime Subambient Passive Radiative Cooling.

The deep space's coldness (∼4 K) provides a ubiquitous and inexhaustible thermodynamic resource to suppress the cooling energy consumption. However, it is nontrivial to achieve subambient radiative cooling during daytime under strong direct sunlight, which requires rational and delicate photonic design for simultaneous high solar reflectivity (>94%) and thermal emissivity. A great challenge arises when trying to meet such strict photonic microstructure requirements while maintaining manufacturing scalability. Herein, we demonstrate a rapid, low-cost, template-free roll-to-roll method to fabricate spike microstructured photonic nanocomposite coatings with Al2O3 and TiO2 nanoparticles embedded that possess 96.0% of solar reflectivity and 97.0% of thermal emissivity. When facing direct sunlight in the spring of Chicago (average 699 W/m2 solar intensity), the coatings show a radiative cooling power of 39.1 W/m2. Combined with the coatings' superhydrophobic and contamination resistance merits, the potential 14.4% cooling energy-saving capability is numerically demonstrated across the United States.

UV- and thermally-active small bi-functional gelator for creating gradient polymer network coatings.

We present a versatile one-pot synthesis method for creating surface-anchored orthogonal gradient networks using a small bi-functional gelator, 4-azidosulfonylphenethyltrimethoxysilane (4-ASPTMS). The sulfonyl azide (SAz) group of 4-ASPTMS is UV (≤254 nm) and thermally active (≥100 °C) and, thus, enables us to vary the cross-link density in orthogonal directions by controlling the activation of SAz groups via UV and temperature means. We deposit a thin layer (∼200 nm) of a mixture comprising ∼90% precursor polymer and ∼10% 4-ASPTMS in a silicon wafer. Upon UV irradiation or annealing the layers, SAz releases nitrogen by forming singlet and triplet nitrenes that concurrently react with any C-H bond in the vicinity leading to sulfonamide cross-links. Condensation among trimethoxy groups in the bulk connects 4-ASPTMS units and completes the cross-linking. Simultaneously, 4-ASPTMS near the substrate reacts with surface-bound -OH motifs that anchor the cross-linked polymer chains to the substrate. We demonstrate the generation of orthogonal gradient network coatings exhibiting cross-link density (or stiffness) gradients in orthogonal directions using such a simple process.

PDMS networks meet barnacles: a complex and often toxic relationship.

The biological impact of chemical formulations used in various coating applications is essential in guiding the development of new materials that directly contact living organisms. To illustrate this point, an investigation addressing the impact of chemical compositions of polydimethylsiloxane networks on a common platform for foul-release biofouling management coatings was conducted. The acute toxicity of network components to barnacle larvae, the impacts of aqueous extracts of crosslinker, silicones and organometallic catalyst on trypsin enzymatic activity, and the impact of assembled networks on barnacle adhesion was evaluated. The outcomes of the study indicate that all components used in the formulation of the silicone network alter trypsin enzymatic activity and have a range of acute toxicity to barnacle larvae. Also, the adhesion strength of barnacles attached to PDMS networks correlates to the network formulation protocol. This information can be used to assess action mechanisms and risk-benefit analysis of PDMS networks.

Nonwoven Membranes with Infrared Light-Controlled Permeability.

This study presents the development of the first composite nonwoven fiber mats (NWFs) with infrared light-controlled permeability. The membranes were prepared by coating polypropylene NWFs with a photothermal layer of poly(N-isopropylacrylamide) (PNIPAm)-based microgels impregnated with graphene oxide nanoparticles (GONPs). This design enables "photothermal smart-gating" using light dosage as remote control of the membrane's permeability to electrolytes. Upon exposure to infrared light, the GONPs trigger a rapid local increase in temperature, which contracts the PNIPAm-based microgels lodged in the pore space of the NWFs. The contraction of the microgels can be reverted by cooling from the surrounding aqueous environment. The efficient conversion of infrared light into localized heat by GONPs coupled with the phase transition of the microgels above the lower critical solution temperature (LCST) of PNIPAm provide effective control over the effective porosity, and thus the permeability, of the membrane. The material design parameters, namely the monomer composition of the microgels and the GONP-to-microgel ratio, enable tuning the permeability shift in response to IR light; control NWFs coated with GONP-free microgels displayed thermal responsiveness only, whereas native NWFs showed no smart-gating behavior at all. This technology shows potential toward processing temperature-sensitive bioactive ingredients or remote-controlled bioreactors.

Degradable Anti-Biofouling Polyester Coatings with Controllable Lifetimes.

To achieve degradable, anti-biofouling coatings with longer lifetimes and better mechanical properties, we synthesized a series of degradable co-polyesters composed of cyclic ketene acetals, di-(ethylene glycol) methyl ether methacrylate, and a photoactive curing agent, 4-benzoylphenyl methacrylate, using a radical ring-opening polymerization. The precursor co-polyesters were spin-coated on a benzophenone-functionalized silicon wafer to form ca. 60 nm films and drop-casted on glass to form ∼32 µm films. The copolymers were cross-linked via UV irradiation at 365 nm. The degradation of films was studied by immersing the specimens in aqueous buffers of different pH values. The results show that both the pH of buffer solutions and gel fractions of networks affect the degradation rate. The coatings show good bovine serum albumin resistance capability. By adjusting the fractions of monomers, the degradation rate and degree of hydration (e.g., swelling ratio) are controllable.

Counterpropagating Gradients of Antibacterial and Antifouling Polymer Brushes.

We report on the formation of counterpropagating density gradients in poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) brushes featuring spatially varying quaternized and betainized units. Starting with PDMAEMA brushes with constant grafting density and degree of polymerization, we first generate a density gradient of quaternized units by directional vapor reaction involving methyl iodide. The unreacted DMAEMA units are then betainized through gaseous-phase betainization with 1,3-propanesultone. The gas reaction of PDMAEMA with 1,3-propanesultone eliminates the formation of byproducts present during the liquid-phase modification. We use the counterpropagating density gradients of quaternized and betainized PDMAEMA brushes in antibacterial and antifouling studies. Completely quaternized and betainized brushes exhibit antibacterial and antifouling behaviors. Samples containing 12% of quaternized and 85% of betainized units act simultaneously as antibacterial and antifouling surfaces.

Dynamic Surfaces-Degradable Polyester Networks that Resist Protein Adsorption.

We synthesized a series of novel degradable alternating copolyesters composed of diglycolic anhydride (DGA) and two epoxides, epoxymethoxytriethylene glycol (ETEG) and a photoactive crosslinking agent epoxy benzophenone (EBP). After UV crosslinking, soaking the films in a good solvent (tetrahydrofuran) removed uncrosslinked material, and the resulting film gel fractions were calculated. These network films were then degraded in buffer solutions of varying pH values. The degradation of networks with lower gel fraction (fewer crosslinks) was faster and followed first-order kinetics. In contrast, the denser network degraded slower and followed zeroth-order kinetics. The lower gel fraction networks possess a higher swelling ratio and resist bovine serum albumin (BSA) adsorption better by entropic shielding and faster degradation. In comparison, higher gel fraction networks with higher EBP mole fractions adsorb more BSA due to hydrophobic interactions and slower degradation.

DFT Analysis of Organotin Catalytic Mechanisms in Dehydration Esterification Reactions for Terephthalic Acid and 2,2,4,4-Tetramethyl-1,3-cyclobutanediol.

Polyesters synthesized from 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) and terephthalic acid (TPA) are improved alternatives to toxic polycarbonates based on bisphenol A. In this work, we use ωB97X-D/LANL2DZdp calculations, in the presence of a benzaldehyde polarizable continuum model solvent, to show that esterification of TMCD and TPA will reduce and subsequently dehydrate a dimethyl tin oxide catalyst, becoming ligands on the now four-coordinate complex. This reaction then proceeds most plausibly by an intramolecular acyl-transfer mechanism from the tin complex, aided by a coordinated proton donor such as hydronium. These findings are a key first step in understanding polyester synthesis and avoiding undesirable side reactions during production.

Antipathogenic properties and applications of low-dimensional materials.

A major health concern of the 21st century is the rise of multi-drug resistant pathogenic microbial species. Recent technological advancements have led to considerable opportunities for low-dimensional materials (LDMs) as potential next-generation antimicrobials. LDMs have demonstrated antimicrobial behaviour towards a variety of pathogenic bacterial and fungal cells, due to their unique physicochemical properties. This review provides a critical assessment of current LDMs that have exhibited antimicrobial behaviour and their mechanism of action. Future design considerations and constraints in deploying LDMs for antimicrobial applications are discussed. It is envisioned that this review will guide future design parameters for LDM-based antimicrobial applications.

UV- and Thermally-Active Bifunctional Gelators Create Surface-Anchored Polymer Networks.

A versatile one-step synthesis of surface-attached polymer networks using small bifunctional gelators (SBG), namely 4-azidosulfonylphenethyltrimethoxysilane (4-ASPTMS) and 6-azidosulfonylhexyltriethoxysilane (6-ASHTES) is reported. A thin layer (≈200 nm) of a mixture comprising ≈90% precursor polymer and 10% of 4-ASPTMS or 10% 6-ASHTES on a silicon wafer is deposited. Upon UV irradiation (≈l-254 nm) or annealing (>100 °C) layers, sulfonyl azides (SAz) release nitrogen by forming singlet and triplet nitrenes that concurrently react with any C─H bond in the vicinity resulting in sulfonamide crosslinks. Condensation among tri-alkoxy groups (i.e., methoxy or ethoxy) in bulk connects the SBG units, which completes the crosslinking. Concurrently, when such functionalities react with hydroxyl groups at the surface, which enable the covalent attachment of the crosslinked polymer chains. A systematic investigation on reaction mechanism and gel formation using spectroscopic ellipsometry (SE) and Fourier-transform infrared spectroscopy in the attenuated total reflection mode (FTIR-ATR) is performed. Analogous thermally initiated gelation for both 4-ASPTMS and 6-ASHTES is found. The 6-ASHTES is UV inactive at ≈l-254 nm, while the 4-ASPTMS is active and forms gels. The difference is attributed to the aromatic nature of 4-ASPTMS that absorb UV light at ≈l-254 nm due to π-π* transition.

Effect of Poly(vinyl butyral) Comonomer Sequence on Adhesion to Amorphous Silica: A Coarse-Grained Molecular Dynamics Study.

Modulating a comonomer sequence, in addition to the overall chemical composition, is the key to unlocking the true potential of many existing commercial copolymers. We employ coarse-grained molecular dynamics (MD) simulations to study the behavior of random-blocky poly(vinyl butyral-co-vinyl alcohol) (PVB) melts in contact with an amorphous silica surface, representing the interface found in laminated safety glass. Our two-pronged coarse-graining approach utilizes both macroscopic thermophysical data and all-atom MD simulation data. Polymer-polymer nonbonded interactions are described by the fused-sphere SAFT-γ Mie equation of state, while bonded interactions are derived using Boltzmann inversion to match the bond and angle distributions from all-atom PVB chains. Spatially dependent polymer-surface interactions are mapped from a hydroxylated all-atom amorphous silica slab model and all-atom monomers to an external potential acting on the coarse-grained sites. We discovered an unexpected complex relationship between the blockiness parameter and the adhesion energy. The adhesion strength between PVB copolymers with intermediate VA content and silica was found to be maximal for random-blocky copolymers with a moderately high degree of blockiness rather than for diblock copolymers. We attribute this to two main factors: (1) changes in morphology, which dramatically alter the number of VA beads interacting with the surface and (2) a non-negligible contribution of vinyl butyral (VB) monomers to adhesion energy because of their preference to adsorb to zones with low hydroxyl density on the silica surface.

Attributes, Fabrication, and Applications of Gallium-Based Liquid Metal Particles.

This work discusses the attributes, fabrication methods, and applications of gallium-based liquid metal particles. Gallium-based liquid metals combine metallic and fluidic properties at room temperature. Unlike mercury, which is toxic and has a finite vapor pressure, gallium possesses low toxicity and effectively zero vapor pressure at room temperature, which makes it amenable to many applications. A variety of fabrication methods produce liquid metal particles with variable sizes, ranging from nm to mm (which is the upper limit set by the capillary length). The liquid nature of gallium enables fabrication methods-such as microfluidics and sonication-that are not possible with solid materials. Gallium-based liquid metal particles possess several notable attributes, including a metal-metal oxide (liquid-solid) core-shell structure as well as the ability to self-heal, merge, and change shape. They also have unusual phase behavior that depends on the size of the particles. The particles have no known commercial applications, but they show promise for drug delivery, soft electronics, microfluidics, catalysis, batteries, energy harvesting, and composites. Existing challenges and future opportunities are discussed herein.

Controlled heating and alignment platform enhances versatility in colloidal probe fabrication.

A colloidal probe, comprising a colloidal particle attached to an atomic force microscope cantilever, is employed to measure interaction forces between the particle and a surface. It is possible to change or even destroy a particle while attaching it to a cantilever, thus limiting the types of systems to which the colloidal probe technique may be applied. Here, we present the Controlled Heating and Alignment Platform (CHAP) for fabricating colloidal probes without altering the original characteristics of the attached particle. The CHAP applies heat directly to the atomic force microscope chip to rapidly and precisely control the cantilever temperature. It minimizes particle heating and enables control over the viscosity of the thermoplastic adhesive to prevent it from contaminating the particle surface. 3D-printed components made the CHAP compatible with standard optical microscopes and streamlined the fabrication process, while increasing the platform's versatility. To demonstrate the utility of CHAP, we conducted a case study using a thermoplastic wax adhesive to fabricate colloidal probes bearing polystyrene and silica particles between 0.7 and 40 µm in diameter. We characterized the properties and interactions of the adhesive and particles, as well as the properties of the completed probes, to demonstrate the retention of particle features throughout fabrication. Pull-off tests with CHAP's probes measured adhesive force values in the expected ranges and demonstrated that particles were firmly attached to the cantilevers.

Extending the fused-sphere SAFT-γ Mie force field parameterization approach to poly(vinyl butyral) copolymers.

SAFT-γ Mie, a molecular group-contribution equation of state with foundations in the statistical associating fluid theory framework, is a promising means for developing accurate and transferable coarse-grained force fields for complex polymer systems. We recently presented a new approach for incorporating bonded potentials derived from all-atom molecular dynamics simulations into fused-sphere SAFT-γ Mie homopolymer chains by means of a shape factor parameter, which allows for bond distances less than the tangent-sphere value required in conventional SAFT-γ Mie force fields. In this study, we explore the application of the fused-sphere SAFT-γ Mie approach to copolymers. In particular, we demonstrate its capabilities at modeling poly(vinyl alcohol-co-vinyl butyral) (PVB), an important commercial copolymer widely used as an interlayer in laminated safety glass applications. We found that shape factors determined from poly(vinyl alcohol) and poly(vinyl butyral) homopolymers do not in general correctly reproduce random copolymer densities when standard SAFT-γ Mie mixing rules are applied. However, shape factors optimized to reproduce the density of a random copolymer of intermediate composition resulted in a model that accurately represents density across a wide range of chemical compositions. Our PVB model reproduced copolymer glass transition temperature in agreement with experimental data, but heat capacity was underpredicted. Finally, we demonstrate that atomistic details may be inserted into equilibrated fused-sphere SAFT-γ Mie copolymer melts through a geometric reverse-mapping algorithm.

Effect of surface interactions on the settlement of particles on a sinusoidally corrugated substrate.

Naturally-occurring surface topographies abound in nature and endow diverse properties, i.e., superhydrophobicity, adhesion, anti-fouling, self-cleaning, anti-glare, anti-bacterial, and many others. Researchers have attempted to replicate such topographies to create human-made surfaces with desired functionalities. For example, combining the surface topography with judicial chemical composition could provide an effective, non-toxic solution to combat non-specific biofouling. A systematic look at the effect of geometry, modulus, and chemistry on adhesion is warranted. In this work, we use a model system that comprises silica (SiO x ) beads interacting with a substrate made of a commercial polydimethylsiloxane kit (PDMS, Sylgard 184) featuring a sinusoidal topography. To examine the impact of interactions on particle settlement, we functionalize the surfaces of both the PDMS substrate and the SiO x beads with polyacrylic acid (PAA) and polyethyleneimine (PEI), respectively. We also use the PDMS commercial kit coated with liquid glass (LG) to study the effect of the substrate modulus on particle settlement. Substrates with a higher aspect ratio (i.e., amplitude/periodicity) encourage adsorption of particles along the sides of the channel compared with substrates with lower aspect ratio. We employ colloidal probe microscopy to demonstrate the effect of interaction between the substrate and the particle. The interplay among the surface modulus, geometry, and interactions between the surface and the particle governs particle settlement on sinusoidally-corrugated substrates.

Charge Density Gradients of Polymer Thin Film by Gaseous Phase Quaternization.

We report on the rapid formation of charge density gradients in polymer films by exposing poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) films resting on flat silica substrates to methyl iodide (i.e., MI, also known as iodomethane) vapors. We adjust the charge gradient by varying the MI concentration in solution and the process time. The thickness of the parent PDMAEMA film does not affect the diffusion of MI through and the reaction kinetics in the films. Instead, the diffusion of MI through the gaseous phase constitutes the limiting step in the overall process.

Dependence of deposition method on the molecular structure and stability of organosilanes revealed from degrafting by tetrabutylammonium fluoride.

We probe the structure of self-assembled monolayers (SAMs) comprising organosilanes deposited on flat silica-based surfaces prepared by liquid and vapor deposition by removing the organosilane molecules gradually from the underlying substrate via tetrabutylammonium fluoride (TBAF). Removal of organosilanes from the surface involves the cleavage of all pertinent Si-O bonds that anchor the organosilane molecules to the SAM, i.e., direct organosilane-surface linkages and in-plane crosslinks between neighboring organosilanes. We gain insight into the organosilane structure and stability by monitoring the organosilane density as a function of exposure time to TBAF. Degrafting of trifunctional chloro- and methoxy-alkylsilanes deposited from solution yields similar degrafting kinetics. We observe fast degrafting for organosilane SAMs deposited from the vapor phase, indicating that SAMs prepared in this manner form more loosely packed arrays, with less in-plane connectivity, compared to their solution-deposited counterparts. Bulkier, fluorinated silanes form more stable SAMs due to their ability to readily align and form a network with few aggregates and a relatively high fraction of surface bonds. The addition of a polymer brush to an anchored organosilane molecule demonstrates that increased bond tension accelerates the degrafting process despite the increased diffusion resistance.

Mechanochemical Degrafting of a Surface-Tethered Poly(acrylic acid) Brush Promoted Etching of Its Underlying Silicon Substrate.

The stability of surface-tethered polyelectrolyte brushes has been investigated during the past few years. We have previously reported on the degrafting of poly(acrylic acid) (PAA) polymer brushes from flat silicon substrates. Here, we present a detailed study on the effects of NaCl concentration and the grafting density and molecular weight on the stability of PAA brushes during incubation in 0.1 M ethanolamine buffer (pH 9.0) solutions. Without NaCl in the buffer solution, the PAA brushes remain intact. Adding NaCl facilitates etching of the substrate due to accelerating dissolution of the top silica layer and promoting degrafting of the PAA chains. The PAA grafting density and molecular weight play an important role in the substrate etching by affecting the penetration barrier and local concentration of the etchants. We also tested the stability of self-assembled monolayers (SAMs) made of hydrophobic alkyltrichlorosilanes anchored on silicon substrates. The results demonstrated that the SAMs were too thin to protect the substrates from etching, in contrast to thick poly(methyl methacrylate) brushes. Our findings suggest that both polymer brushes (especially polyelectrolyte brushes) and SAMs anchored to silicon substrates may undergo erosion/etching on the substrates in basic environments, which compromises their stability and therefore jeopardizes their applications in coating, biosensing, and so forth.