Solvent-free synthesis of Janus colloidal particles

Taking advantage of the quick and efficient access of vapor to surfaces, a simple, solvent-free method is demonstrated to synthesize Janus colloidal particles in large quantity and with high efficiency. First, at the liquid-liquid interface of emulsified molten wax and water, untreated silica particles adsorb and are frozen in place when the wax solidifies. The exposed surfaces of the immobilized particles are modified chemically by exposure to silane vapor and, in principle, subsequent dissolution of the wax opens up the inner particle surface for further chemical modification. Applying this scheme, this paper describes the production of amphiphilic Janus particles (hydrophobic on one side, hydrophilic on the other) and dipolar Janus particles (positively charged on one side, negatively charged on the other). Janus geometry is confirmed by fluorescence microscopy and flow cytometry. Amphiphilic Janus particles are found to adsorb strongly to the water-oil interface, whereas dipolar particles assemble into chains in the aqueous phase.

A new self-healing epoxy with tungsten (VI) chloride catalyst

Using self-healing materials in commercial applications requires healing chemistry that is cost-effective, widely available and tolerant of moderate temperature excursions. We investigate the use of tungsten (VI) chloride as a catalyst precursor for the ring-opening metathesis polymerization of exo-dicyclopentadiene (exo-DCPD) in self-healing applications as a means to achieve these goals. The environmental stability of WCl6 using three different delivery methods was evaluated and the associated healing performance was assessed following fracture toughness recovery protocols. Both as-received and recrystallized forms of the WCl6 resulted in nearly complete fracture recovery in self-activated tests, where healing agent is manually injected into the crack plane, at 12wt% WCl6 loading. In situ healing using 15wt% microcapsules of the exo-DCPD produced healing efficiencies of approximately 20%.

Life extension of self-healing polymers with rapidly growing fatigue cracks

Self-healing polymers, based on microencapsulated dicyclopentadiene and Grubbs' catalyst embedded in the polymer matrix, are capable of responding to propagating fatigue cracks by autonomic processes that lead to higher endurance limits and life extension, or even the complete arrest of the crack growth. The amount of fatigue-life extension depends on the relative magnitude of the mechanical kinetics of crack propagation and the chemical kinetics of healing. As the healing kinetics are accelerated, greater fatigue life extension is achieved. The use of wax-protected, recrystallized Grubbs' catalyst leads to a fourfold increase in the rate of polymerization of bulk dicyclopentadiene and extends the fatigue life of a polymer specimen over 30 times longer than a comparable non-healing specimen. The fatigue life of polymers under extremely fast fatigue crack growth can be extended through the incorporation of periodic rest periods, effectively training the self-healing polymeric material to achieve higher endurance limits.

Self-healing kinetics and the stereoisomers of dicyclopentadiene

While original epoxy resin-based self-healing systems used the commercially available endo-isomer of dicyclopentadiene (DCPD), the exo-stereoisomer is known to have much faster olefin metathesis reaction rates with first-generation Grubbs' catalyst. Here, we measure the energy to failure of healed specimens as a function of healing time and compare the kinetics of damage repair for endo- and exo-DCPD, and mixtures of the two isomers. Using catalyst loading levels previously reported to be effective for endo-DCPD, exo-DCPD was found to heal approximately 20 times faster than the endo-isomer, but with a lower healing efficiency. The fracture toughness of the repaired specimens decreased when the exo content of the blends was greater than 40% and, for the pure exo-DCPD, when the catalyst loadings were below 1%. Possible causes of the reduced healing efficiencies of the exo-DCPD healing agent are discussed.

Performance characterization of nanofiltration membranes based on rigid star amphiphiles

The objective of this study was to develop new nanofiltration (NF) membranes capable of providing significantly greater water permeability and higher rejection of water contaminants compared to state-of-the-art NF membranes. The active layer of the new NF membranes is prepared with rigid star amphiphiles (RSAs) synthesized as part of this study. Performance characterization for a first generation of RSA membranes in a bench-scale apparatus reveals that most of the new membranes provide water permeability of 1.3-3.1 times that of two commercial NF membranes with polyamide active layers while providing comparable rejection of the organic contaminant surrogate Rhodamine WT. However, the rejection of arsenious acid (H3AsO3) by most new NF membranes was found to be lower than that by the two commercial NF membranes tested. Future research efforts of this study will focus on exploring if H3AsO3 rejection could be significantly increased, without negatively affecting water permeability and organic contaminant rejection, by addition of various chemical groups to RSA hydrophobic cores and hydrophilic branches, and by RSA cross-linking.

Self-healing materials with microvascular networks

Self-healing polymers composed of microencapsulated healing agents exhibit remarkable mechanical performance and regenerative ability, but are limited to autonomic repair of a single damage event in a given location. Self-healing is triggered by crack-induced rupture of the embedded capsules; thus, once a localized region is depleted of healing agent, further repair is precluded. Re-mendable polymers can achieve multiple healing cycles, but require external intervention in the form of heat treatment and applied pressure. Here, we report a self-healing system capable of autonomously repairing repeated damage events. Our bio-inspired coating-substrate design delivers healing agent to cracks in a polymer coating via a three-dimensional microvascular network embedded in the substrate. Crack damage in the epoxy coating is healed repeatedly. This approach opens new avenues for continuous delivery of healing agents for self-repair as well as other active species for additional functionality.

"Click" modification of silica surfaces and glass microfluidic channels

This paper demonstrates a chemical surface modification method for covalent attachment of various polymers by using silane-based "click" chemistry on silica surfaces and within glass microchannels suitable for CE systems. Modified surfaces are characterized by contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared-attenuated total reflection spectroscopy. Electroosmotic flow (EOF) measurements in modified and unmodified channels are provided. Spectroscopic and transport data show that various polymers can be covalently attached to glass surfaces with a measurable change in EOF.

Foldamers as Reactive Sieves: Reactivity as a Probe of Conformational Flexibility

A series of m-phenyleneethynylene (mPE) oligomers modified with a dimethylaminopyridine (DMAP) unit were treated with methyl sulfonates of varying sizes and shapes, and the relative reactivities were measured by UV spectrophotometry. Using a small-molecule DMAP analogue as a reference, each of the methyl sulfonates was shown to react at nearly identical rate. In great contrast, oligomers that are long enough to fold, and hence capable of binding the methyl sulfonate, experience rate enhancements of 18-1600-fold relative to that of the small-molecule analogue, depending on the type of alkyl chain attached to the guest. Three different oligomer lengths were studied, with the longest oligomers exhibiting the fastest rate and greatest substrate specificity. Even large, bulky guests show slightly enhanced methylation rates compared to that with the reference DMAP, which suggests a dynamic nature to the oligomer's binding cavity. Several mechanistic models to describe this behavior are discussed.

Biasing reaction pathways with mechanical force

During the course of chemical reactions, reactant molecules need to surmount an energy barrier to allow their transformation into products. The energy needed for this process is usually provided by heat, light, pressure or electrical potential, which act either by changing the distribution of the reactants on their ground-state potential energy surface or by moving them onto an excited-state potential energy surface and thereby facilitate movement over the energy barrier. A fundamentally different way of initiating or accelerating a reaction is the use of force to deform reacting molecules along a specific direction of the reaction coordinate. Mechanical force has indeed been shown to activate covalent bonds in polymers, but the usual result is chain scission. Here we show that mechanically sensitive chemical groups make it possible to harness the mechanical forces generated when exposing polymer solutions to ultrasound, and that this allows us to accelerate rearrangement reactions and bias reaction pathways to yield products not obtainable from purely thermal or light-induced reactions. We find that when placed within long polymer strands, the trans and cis isomers of a 1,2-disubstituted benzocyclobutene undergo an ultrasound-induced electrocyclic ring opening in a formally conrotatory and formally disrotatory process, respectively, that yield identical products. This contrasts with reaction initiation by light or heat alone, in which case the isomers follow mutually exclusive pathways to different products. Mechanical forces associated with ultrasound can thus clearly alter the shape of potential energy surfaces so that otherwise forbidden or slow processes proceed under mild conditions, with the directionally specific nature of mechanical forces providing a reaction control that is fundamentally different from that achieved by adjusting chemical or physical parameters. Because rearrangement in our system occurs before chain scission, the effect we describe might allow the development of materials that are activated by mechanical stress fields.

Optical transduction of chemical forces

We describe a plasmonic crystal device possessing utility for optically transducing chemical forces. The device couples complex plasmonic fields to chemical changes via a chemoresponsive, surface-bound hydrogel. We find that this architecture significantly enhances the spectroscopic responses seen at visible wavelengths while enabling capacities for sensitive signal transduction, even in cases that involve essentially no change in refractive index, thus allowing analytical detection via colorimetric assays in both imaging and spectroscopic modes.

A Mo(VI) Alkylidyne Complex with Polyhedral Oligomeric Silsesquioxane Ligands: Homogeneous Analogue of a Silica-Supported Alkyne Metathesis Catalyst

A highly active alkyne metathesis catalyst is realized by replacing the amide ligands of a molybdenum(VI) trisamide alkylidyne complex with silanol groups from incompletely condensed POSS (polyhedral oligomeric silsesquioxane) ligands. This catalyst serves as an effective homogeneous mimic of an amorphous silica-supported catalyst. Reactivities of various catalytic mixtures are reported along with an X-ray structure of the aniline-coordinated amidodisiloxymolybdenum(VI) alkylidyne complex.

Synthesis and aggregation behavior of thermally responsive star polymers

To mimic the three-dimensional (3-D) globular architecture resulting from the precise positioning of hydrophobic/hydrophilic domains (blocks) of naturally occurring proteins, water-soluble linear and star homopolymers of N,N'-dimethylacrylamide (DMA) were synthesized with prescribed molecular weights via reversible addition-fragmentation chain transfer (RAFT) polymerization and subsequently used as macro chain transfer agents for block copolymerization with N-isopropylacrylamide (NIPAM). For the star block copolymers, the interior block consisted of NIPAM while the exterior block was DMA. Since polyNIPAM thermally switches from hydrophilic to hydrophobic, the 3-D solution conformations of the polymers were studied as a function of temperature using differential scanning calorimetry (DSC), static light scattering (SLS), and dynamic light scattering (DLS). The polymers were observed to form monodisperse aggregates in an aqueous pH 4 buffer solution when heated above the lower critical solution temperature (LCST) of polyNIPAM. The temperature at which the polymers aggregated and the size of the aggregates were dependent on the NIPAM block length and the core architecture. A simple model based on an optimal area per headgroup was used to analyze our experimental findings and was useful for predicting the final size and molecular weight of the aggregates formed.