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ABSTRACT: Large area uniform nanofibers have been fabricated from a hexameric arylene-ethynylene macrocycle (1) through in situ self-assembly on a glass substrate during solvent evaporation. The fibril morphology is controlled by the solvophilic core of 1, in conjunction with the interfacial interactions between the side chains of 1 and the substrate.
Chemical Communications 07/2012; 48(71):8904-6. · 6.17 Impact Factor
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ABSTRACT: A novel mechanophore with acid-releasing capability is designed to produce a simple catalyst for chemical change in materials under mechanical stress. The mechanophore, based on a gem-dichlorocyclopropanated indene, is synthesized and used as a cross-linker in poly(methyl acrylate). Force-dependent rearrangement is demonstrated for cross-linked mechanophore samples loaded in compression, while the control shows no significant response. The availability of the released acid is confirmed by exposing a piece of insoluble compressed polymer to a pH indicator solution. The development of this new mechanophore is the first step toward force-induced remodeling of stressed polymeric materials utilizing acid-catalyzed cross-linking reactions.
Journal of the American Chemical Society 07/2012; 134(30):12446-9. · 9.91 Impact Factor
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ABSTRACT: Electrical conductivity of mechanically damaged silver ink circuits is automatically restored using core-shell microcapsules. Upon mechanical damage to the circuit and microcapsules, silver particles reorganize by dissolution of the polymer binder layer upon release of solvent, hexyl acetate, from microcapsule cores. Conductivity is restored within minutes of damage, and no short-circuiting is revealed during the healing of closely spaced lines.
Advanced Materials 04/2012; 24(19):2578-81, 2509. · 13.88 Impact Factor
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ABSTRACT: A dynamic combinatorial approach for the synthesis of arylene ethynylene macrocycles (AEMs) from linear polymers is described. By using readily available carbazolyl-ethynylene copolymers as starting materials we obtained a number of novel macrocycles that would be difficult to prepare by traditional methods.
Chemical Communications 03/2012; 48(37):4426-8. · 6.17 Impact Factor
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ABSTRACT: Development of simple, cost-effective, and sensitive fluorescence-based sensors for explosives implies broad applications in homeland security, military operations, and environmental and industrial safety control. However, the reported fluorescence sensory materials (e.g., polymers) usually respond to a class of analytes (e.g., nitroaromatics), rather than a single specific target. Hence, the selective detection of trace amounts of trinitrotoluene (TNT) still remains a big challenge for fluorescence-based sensors. Here we report the selective detection of TNT vapor using the nanoporous fibers fabricated by self-assembly of carbazole-based macrocyclic molecules. The nanoporosity allows for time-dependent diffusion of TNT molecules inside the material, resulting in further fluorescence quenching of the material after removal from the TNT vapor source. Under the same testing conditions, other common nitroaromatic explosives and oxidizing reagents did not demonstrate this postexposure fluorescence quenching; rather, a recovery of fluorescence was observed. The postexposure fluorescence quenching as well as the sensitivity is further enhanced by lowering the highest occupied molecular orbital (HOMO) level of the nanofiber building blocks. This in turn reduces the affinity for oxygen, thus allocating more interaction sites for TNT. Our results present a simple and novel way to achieve detection selectivity for TNT by creating nanoporosity and tuning molecular electronic structure, which when combined may be applied to other fluorescence sensor materials for selective detection of vapor analytes.
Journal of the American Chemical Society 03/2012; 134(10):4978-82. · 9.91 Impact Factor
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ABSTRACT: When heated, poly(lactic acid) (PLA) fibers depolymerize in a controlled manner, making them potentially useful as sacrificial fibers for microchannel fabrication. Catalysts that increase PLA depolymerization rates are explored and methods to incorporate them into commercially available PLA fibers by a solvent mixture impregnating technique are tested. In the present study, the most active catalysts are identified that are capable of lowering the depolymerization temperature of modified PLA fibers by ca. 100 °C as compared to unmodified ones. Lower depolymerization temperatures allow PLA fibers to be removed from a fully cured epoxy thermoset resin without causing significant thermal damage to the epoxy. For 500 μm diameter PLA fibers, the optimized treatment involves soaking the fibers for 24 h in a solvent mixture containing 60% trifluoroethanol (TFE) and 40% H(2)O dispersed with 10 wt % tin(II) oxalate and subsequent air-drying of the fibers. PLA fibers treated with this procedure are completely removed when heated to 180 °C in vacuo for 20 h. The time evolution of catalytic depolymerization of PLA fiber is investigated by gel permeation chromatography (GPC). Channels fabricated by vaporization of sacrificial components (VaSC) are subsequently characterized by scanning electron microscopy (SEM) and X-ray microtomography (Micro CT) to show the presence of residual catalysts.
ACS Applied Materials & Interfaces 02/2012; 4(2):503-9. · 4.53 Impact Factor
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ABSTRACT: Self-healing of an electrical circuit is demonstrated with nearly full recovery of conductance less than one millisecond after damage. Crack damage breaks a conductive pathway in a multilayer device, interrupting electron transport and simultaneously rupturing adjacent microcapsules containing gallium-indium liquid metal (top). The released liquid metal flows to the area of damage, restoring the conductive pathway (bottom).
Advanced Materials 12/2011; · 13.88 Impact Factor
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ABSTRACT: A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum-assisted microfluidic filling protocol that employs dilutions of a carbon-black-containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self-supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin-based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular-level grayscale patterns are characterized by condensation microscopy and imaging mode time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof-of-concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure-sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.
Small 12/2011; 7(23):3350-62. · 8.35 Impact Factor
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ABSTRACT: We report a new core-shell microcapsule system for the visual detection of mechanical damage. The core material, 1,3,5,7-cyclooctatetraene, is a conjugated cyclic olefin and a precursor to intensely colored polyacetylene. A combination of poly(urea-formaldehyde) and polyurethane is required to effectively encapsulate the volatile core material. Increasing the outer shell wall thickness and including a core-side prepolymer improves the thermal stability and free-flowing nature of these capsules, which tend to leach and rupture with thinner shell walls. Capsules ruptured in the presence of the Grubbs-Love ruthenium catalyst show immediate color change from nearly colorless to red-orange and dark purple over time, and color change in thin films resulted from scratch damage.
ACS Applied Materials & Interfaces 11/2011; 3(12):4547-51. · 4.53 Impact Factor
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ABSTRACT: Ultrasound activation of mechanophores embedded in polymer backbones has been extensively studied of late as a method for realizing chemical reactions using force. To date, however, there have been few attempts at systematically investigating the effects of mechanophore structure upon rates of activation by an acoustic field. Herein, we develop a method for comparing the relative reactivities of various cyclobutane mechanophores. Through the synthesis and ultrasonic irradiation of a molecular weight series of poly(methyl acrylate) polymers in which each macromolecule has a single chain-centered mechanophore, we find measurable and statistically significant shifts in molecular weight thresholds for mechanochemical activation that depend on the structure of the mechanophore. We also show that calculations based on the constrained geometries simulate external force method reliably predict the trends in mechanophore reactivity. These straightforward calculations and the experimental methods described herein may be useful in guiding the design and the development of new mechanophores for targeted applications.
Journal of the American Chemical Society 11/2011; 133(46):18992-8. · 9.91 Impact Factor
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ABSTRACT: We used fluorescence and electronic absorption spectroscopy to study the molecular weight dependence of macromolecule-induced folding in a chain-centered meta-phenylene ethynylene (mPE) oligomer. Analogous to the ability of intrinsically unstructured proteins (IUPs) to induce folding of globular proteins in cellular environments, we show that macromolecules attached to both ends of an mPE dodecamer induce the foldamer to collapse into a presumed helical conformation. The collapse is especially prominent once the macromolecule segments become larger than ca. 50 kDa. For sufficiently large macromolecules, the conformational structuring occurs even in solvents that normally denature the foldamer. Based on these findings, chain-centered foldamers might find use as models to investigate the fundamental macromolecular physics of IUPs.
Journal of the American Chemical Society 11/2011; 133(49):19650-2. · 9.91 Impact Factor
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Advanced Materials 08/2011; 23(32):3653. · 13.88 Impact Factor
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Advanced Materials 08/2011; 23(32):3654-8. · 13.88 Impact Factor
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ABSTRACT: We present a crystallographic study that systematically investigates the effects of the n-alkyl side-chain length on the crystal packing in shape-persistent macrocycles. The solid-state packing of carbazole-ethynylene-containing macrocycles is sensitive to the alkyl-chain length. In macrocycles containing n-alkyl side chains up to nine carbons in length, face-on aromatic π interactions predominate, while the addition of one carbon leads to a completely different packing arrangement. Macrocycles with C(10) or C(11) chains exhibit a novel packing motif wherein the alkyl chains intercalate between macrocycles, leading in one case to continuous solvent-filled channels. When crystals of the C(10) macrocycle are bathed in solvent, the included molecules exchange with the external solvent, and the alkyl chain disorder changes in response to changes in the guest volume in order to retain crystallinity. Powder X-ray diffraction data indicate that alkyl-macrocycle interactions in the longer chains "emulate" the distances typical of face-to-face π interactions, leading to deceptive indicators of π stacking.
Journal of the American Chemical Society 08/2011; 133(35):14063-70. · 9.91 Impact Factor
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ABSTRACT: Dimethylnorbornene ester (DNE) is successfully used as a noncovalent adhesion promoter. DNE was confirmed to copolymerize with dicyclopentadiene (DCPD) to yield a copolymer with better adhesion to an EPON 828 epoxy matrix relative to poly(DCPD) alone. The mechanical properties of the copolymer were comparable to that of poly(DCPD) alone. An optimized blend of the monomers was encapsulated using a urea-formaldheyde microencapsulation procedure and the resulting capsules were used for in situ self-healing experiments. Improved healing efficiency was observed for samples containing the DCPD/DNE capsules under conditions in which the monomers were efficiently polymerized.
ACS Applied Materials & Interfaces 07/2011; 3(8):3072-7. · 4.53 Impact Factor
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ABSTRACT: Stimuli-responsive capsules are of interest in drug delivery, fragrance release, food preservation, and self-healing materials. Many methods are used to trigger the release of encapsulated contents. Here we highlight mechanisms for the controlled release of encapsulated cargo that utilize chemical reactions occurring in solid polymeric shell walls. Triggering mechanisms responsible for covalent bond cleavage that result in the release of capsule contents include chemical, biological, light, thermal, magnetic, and electrical stimuli. We present methods for encapsulation and release, triggering methods, and mechanisms and conclude with our opinions on interesting obstacles for chemically induced activation with relevance for controlled release.
07/2011;
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Nature 04/2011; 472(7343):299-300. · 36.28 Impact Factor
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ABSTRACT: Scrambling experiments suggest that the self-assembly of 2D ladders via imine metathesis is kinetically trapped at four or more rungs. Consequently, ladders containing five or more rungs cannot be synthesized in high yield under the conditions used, as misaligned out-of-register byproducts cannot self-correct.
Chemical Communications 03/2011; 47(17):5028-30. · 6.17 Impact Factor
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ABSTRACT: The substantial kinetic barrier to molybdenum nitride-alkyne metathesis is facilitated by precomplexation of the borane Lewis acid B(C(6)F(5))(3), providing convenient access to metathesis-active molybdenum alkylidynes. Spectroscopic and X-ray structural analysis suggest Mo≡N bond weakening upon borane complexation.
Chemical Communications 11/2010; 46(42):7939-41. · 6.17 Impact Factor
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ABSTRACT: An efficient synthesis of hyperbranched poly(m-phenylene)s is achieved by Suzuki polycondensation of a m-terphenyl-derived branched AB2 monomer. The resulting hyperbranched polymers show high molecular weight and relatively low polydispersity and are readily soluble in common organic solvents. A catalyst system composed of Pd(OAc)2 and S-Phos was found to be highly active in the present Suzuki polycondensation protocol. The molecular weight and polydispersity of the polymer are controllable by varying the catalyst loading and the starting monomer concentration. Experimental results are consistent with a pseudo-chain-growth pathway that involves intramolecular catalyst transfer during polymerization. Furthermore, the triflate end groups of the hyperbranched poly(m-phenylene)s can be efficiently converted to other functionalities via in situ Suzuki−Miyaura cross-couplings.
10/2010;
