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Graphite oxide (GO) polymer nanocomposites were developed at 1, 5, and 10 wt % GO with polycarbonate (PC), acrylonitrile butadiene styrene, and high-impact polystyrene for the purpose of evaluating the flammability reduction and material properties of the resulting systems. The overall morphology and dispersion of GO within the polymer nanocomposites were studied by scanning electron microscopy and optical microscopy; GO was found to be well-dispersed throughout the matrix without the formation of large aggregates. Mechanical testing was performed using dynamic mechanical analysis to measure the storage modulus, which increased for all polymer systems with increased GO loading. Microscale oxygen consumption calorimetry revealed that the addition of GO reduced the total heat release and peak heat release rates in all systems, and GO-PC composites demonstrated very fast self-extinguishing times in vertical open flame tests, which are important to some regulatory fire safety applications.
ACS Appl Mater Interfaces 2009 Oct
PMID:Graphite oxide flame-retardant polymer nanocomposites. 2035 60

The preparation of multilumophore colorimetric luminescent oxygen sensors exhibiting red-green, red-blue, and red-green-blue color space responses across oxygen partial pressures in the range of 0-760 Torr is described. We show how the relative lumophore concentration, emission lifetime, and permeability of the polymer host matrix to oxygen may be used to control both the color space response and the sensitivity of these sensors, thus enabling the device to be optimized for operation over a specific oxygen concentration range or application. The Commission Internationale de L'Eclairage (CIE) xy color space response and oxygen sensitivity of these experimental multilumophore sensors has been modeled using a combination of CIE color coordinates and Stern-Volmer quenching kinetics, and generally good agreement between the experimental and modeled response is obtained. This approach will therefore provide a useful preexperimentation tool for the design of sensors with specific response characteristics.
ACS Appl Mater Interfaces 2009 May
PMID:Design and color response of colorimetric multilumophore oxygen sensors. 2035 87

The hydration of the surface of a highly bioactive silicate glass was modeled using ab initio (Car-Parrinello) molecular dynamics (CPMD) simulations, focusing on the structural and chemical modifications taking place at the glass-water interface immediately after contact and on the way in which they can affect the bioactivity of these materials. The adsorption of a water dimer and trimer on the dry surface was studied first, followed by the extended interface between the glass and liquid water. The CPMD trajectories provide atomistic insight into the initial stages relevant to the biological activity of these materials: following contact of the glass with an aqueous (physiological) medium, the initial enrichment of the surface region in Na+ cations establishes dominant Na+-water interactions at the surface, which allow water molecules to penetrate into the open glass network and start its partial dissolution. The model of a Na/H-exchanged interface shows that Ca2+-water interactions are mainly established after the dominant fraction of Na is leached into the solution. Another critical role of modifier cations was highlighted: they provide the Lewis acidity necessary to neutralize OH(-) produced by water dissociation and protonation of nonbridging oxygen (NBO) surface sites. The CPMD simulations also highlighted an alternative, proton-hopping mechanism by which the same process can take place in the liquid water film. The main features of the bioactive glass surface immediately after contact with an aqueous medium, as emerged from the simulations, are (a) silanol groups formed by either water dissociation at undercoordinated Si sites or direct protonation of NBOs, (b) OH(-) groups generally stabilized by modifier cations and coupled with the protonated NBOs, and (c) small rings, relatively stable and unopened even after exposure to liquid water. The possible role and effect of these sites in the bioactive process are discussed.
ACS Appl Mater Interfaces 2009 Jun
PMID:Modeling the water-bioglass interface by ab initio molecular dynamics simulations. 2035 29

Indium-tin-oxynitride (ITON) films have been fabricated by rf sputtering from an indium-tin-oxide target in nitrogen plasma. The influence of postdeposition annealing up to 800 degrees C is analyzed by electrical, optical, and surface characterization of the films in comparison to indium-tin-oxide (ITO) films fabricated in argon plasma. High-temperature annealing resulted in ITO(N) films with similar carrier concentrations. However, the resistivity and optical transmittance of the ITON films were higher than those of the ITO films. Photoelectron spectroscopy revealed that nitrogen is incorporated into the ITON structure in an unbound state as well as through the formation of metal-nitrogen and oxynitride bonds that decorate oxygen vacancies. When the core level electron spectra of ITO and ITON films are compared, a correlation between carrier concentration and the incorporated nitrogen is found. Changes in ITON electrical properties are mainly induced by the release of nitrogen at temperatures above 550 degrees C. In this context, ohmic contact behavior was achieved for ITON on p-type GaN after annealing at 600 degrees C, while no ohmic contact could be realized using ITO.
ACS Appl Mater Interfaces 2009 Jul
PMID:Effect of annealing on the properties of indium-tin-oxynitride films as ohmic contacts for gan-based optoelectronic devices. 2035 48

Mixed ionic-electronic conducting perovskite type oxides with a general formula ABO(3) (where A = Ba, Sr, Ca and B = Co, Fe, Mn) often have high mobility of the oxygen vacancies and exhibit strong ionic conductivity. They are key materials that find use in several energy related applications, including solid oxide fuel cell (SOFC), sensors, oxygen separation membranes, and catalysts. Barium/strontium cobaltite/ferrite (BSCF) Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) was recently identified as a promising candidate for cathode material in intermediate temperature SOFCs. In this work, we perform experimental and theoretical study of the local atomic structure of BSFC. Micro-Raman spectroscopy was performed to characterize the vibrational properties of BSCF. The Jahn-Teller distortion of octahedral coordination around Co(4+) cations was observed experimentally and explained theoretically. Different cations and oxygen vacancies ordering are examined using plane wave pseudopotential density functional theory. We find that cations are completely disordered, whereas oxygen vacancies exhibit a strong trend for aggregation in L-shaped trimer and square tetramer structure. On the basis of our results, we suggest a new explanation for BSCF phase stability. Instead of linear vacancy ordering, which must take place before the phase transition into brownmillerite structure, the oxygen vacancies in BSCF prefer to form the finite clusters and preserve the disordered cubic structure. This structural feature could be found only in the first-principles simulations and can not be explained by the effect of the ionic radii alone.
ACS Appl Mater Interfaces 2009 Jul
PMID:Structural characterization combined with the first principles simulations of barium/strontium cobaltite/ferrite as promising material for solid oxide fuel cells cathodes and high-temperature oxygen permeation membranes. 2035 54

A manometric permeation apparatus was used to study the "outgassing" or desorption of oxygen from polycarbonate (PC). A PC film was placed in the apparatus. Both sides were exposed to oxygen until the film was saturated. To simulate inert gas purging of a closed container or "microenvironment", oxygen was pumped from one side of the apparatus to reduce the concentration on that side to nearly zero. Oxygen concentrations on the freshly purged side rose quickly at first but then slowed. Eventually, a steady state was established and oxygen concentrations increased linearly with time. Mass-transport coefficients (permeation, diffusion, and solubility coefficients) were also estimated and then used to successfully predict the postpurge rise of the oxygen concentration.
ACS Appl Mater Interfaces 2009 Jul
PMID:Outgassing of oxygen from polycarbonate. 2035 58

Oxygen diffusion coefficients, D, have been measured in poly(methyl methacrylate) (PMMA) films containing small amounts of a cinnamic acid derived cross-linker. In the technique employed, the time evolution of oxygen sorption into the film was monitored using the phosphorescence of singlet oxygen as a spectroscopic probe. Although the effect of adding up to 1 mol % of the cross-linker is clearly manifested in the molecular weight and T(g) of these samples, values of D are only moderately influenced. Nevertheless, a systematic decrease in the value of D is discernible as the extent of cross-linking is increased. Although it is reasonable to expect that, in a glassy polymer, cross-linking should not significantly perturb the local motions and confined changes in free volume that influence the translational motion of a small penetrant such as oxygen, our data indicate that even small amounts of a cross-linker can, nevertheless, have a noticeable and potentially meaningful effect. Although UV irradiation of the polymer films disrupts the cinnamic acid derived cross-linker via a [2 + 2] cycloreversion reaction, the photoinduced changes observed in D appear, rather, to reflect degradation reactions in the PMMA-based polymer.
ACS Appl Mater Interfaces 2009 Mar
PMID:Effect of polymer cross-links on oxygen diffusion in glassy PMMA films. 2035 88

A simple and direct electrodeposition technique is employed to fabricate ZnO nanospikes and nanopillars on indium-tin oxide glass substrates at 70 degrees C without using any template, catalyst, or seed layer. Both ZnO nanospikes and nanopillars exhibit highly crystalline ZnO wurtzite structure with a preferred (0001) plane orientation in their high-resolution transmission electron microscopic images and X-ray diffraction patterns. The corresponding Raman spectra provide evidence for the presence of defects and oxygen vacancies in these nanostructures, which could produce the photoluminescence observed in the visible region. X-ray photoelectron spectroscopy further indicates the presence of a Zn(OH)2-rich surface region in these ZnO nanostructures and that a higher Zn(OH)2 surface moiety is found for nanospikes than nanopillars. In contrast to the nanopillars with flat tops, the nanospikes with tapered tips of 20-50 nm diameter provide a favorable geometry to facilitate excellent field-emission performance, with a low turn-on electric field of 3.2 V/microm for 1.0 microA/cm(2) and a threshold field of 6.6 V/microm for 1.0 mA/cm(2). The superior field-emission property makes the nanospikes among the best ZnO field emitters fabricated on a glass substrate at low temperature.
ACS Appl Mater Interfaces 2009 Apr
PMID:Fabrication of ZnO nanospikes and nanopillars on ITO glass by templateless seed-layer-free electrodeposition and their field-emission properties. 2035 3

This paper describes a novel surface engineering approach that combines oxygen plasma treatment and electrochemical activation to create micropatterned cocultures on indium tin oxide (ITO) substrates. In this approach, photoresist was patterned onto an ITO substrate modified with poly(ethylene) glycol (PEG) silane. The photoresist served as a stencil during exposure of the surface to oxygen plasma. Upon incubation with collagen (I) solution and removal of the photoresist, the ITO substrate contained collagen regions surrounded by nonfouling PEG silane. Chemical analysis carried out with time-of-flight secondary ion mass spectrometry (ToF-SIMS) at different stages in micropatterned construction verified removal of PEG-silane during oxygen plasma and presence of collagen and PEG molecules on the same surface. Imaging ellipsometry and atomic force microscopy (AFM) were employed to further investigate micropatterned ITO surfaces. Biological application of this micropatterning strategy was demonstrated through selective attachment of mammalian cells on the ITO substrate. Importantly, after seeding the first cell type, the ITO surfaces could be activated by applying negative voltage (-1.4 V vs Ag/AgCl). This resulted in removal of nonfouling PEG layer and allowed to attach another cell type onto the same surface and to create micropatterned cocultures. Micropatterned cocultures of primary hepatocytes and fibroblasts created by this strategy remained functional after 9 days as verified by analysis of hepatic albumin. The novel surface engineering strategy described here may be used to pattern multiple cell types on an optically transparent and conductive substrate and is envisioned to have applications in tissue engineering and biosensing.
ACS Appl Mater Interfaces 2009 Nov
PMID:Micropatterning of proteins and mammalian cells on indium tin oxide. 2035 32

This paper presents the fabrication of poly(ferrocenylmethylphenylsilane) (PFMPS) patterns by step-and-flash imprint lithography for use as high-contrast etch masks in dry etch processes. PFMPS was spin-coated onto a resist template made by UV nanoimprint lithography to create a reactive ion etch resist layer with a thickness variation corresponding to the imprinted pattern. Etching back the excess of PFMPS by argon sputtering revealed the imprinted organic resist material, which was subsequently removed by oxygen plasma. PFMPS lines down to 30 nm were obtained after removal of the organic resist by oxygen plasma. Because PFMPS contains iron and silicon atoms in its main chain, it possesses a high resistance to oxygen reactive ion etching and, e.g., CHF(3)/O(2) or SF(6)/O(2) reactive ion etch processes. PFMPS patterns formed after imprinting were subsequently transferred into the underlying silicon substrate, and etch rates of 300 nm/min into Si and around 1 nm/min into the PFMPS layer were achieved, resulting in an etch contrast of approximately 300.
ACS Appl Mater Interfaces 2009 Nov
PMID:Nanoscale patterning by UV nanoimprint lithography using an organometallic resist. 2035 38


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