Home About us Contact
|
Home About us Contact | ||
|
|
||
Result Leads (result + lead)
Selected AbstractsAn exploratory study of adolescent's perceptions of the WebJOURNAL OF COMPUTER ASSISTED LEARNING, Issue 4 2003J. Dinet Abstract The purpose of this exploratory study was to investigate the influence of two individual characteristics (Web experience and academic focus) of adolescents on the Web perception, using off-line questionnaires (a Lickert response scale) constituted on the basis of a series of interviews. Questions concerned: perceptions about the nature of information found in the Web; ,strategies' of access to the interesting Internet sites and the reliability of different information resources (libraries, television, Web, etc.). Results lead to the assumption that adolescents with high Web experience became more critical, less confident and less enthusiastic than adolescents with low Web experience and that, in some dimensions, perceptions of literature students are different to those of science students. Even if some interesting results were obtained, further research is needed to explore users' perceptions related to individuals' characteristics and to determine the generalisability of the influences identified in this exploratory study. [source] Highly Fluorescent Mesostructured Films that consist of Oligo(phenylenevinylene),Silica Hybrid Frameworks,ADVANCED FUNCTIONAL MATERIALS, Issue 22 2008Norihiro Mizoshita Abstract Highly fluorescent and visible-light-responsive mesostructured organosilica films are successfully obtained by acidic sol,gel polycondensation of oligo(phenylenevinylene) (OPV)-bridged organosilane and tetraethoxysilane precursors in the presence of a template surfactant. The OPV-bridged organosilane precursors with different lateral alkoxy substituents, hexyloxy and 2-ethylhexyloxy, and no substituent, are synthesized by Rh-catalyzed silylation of corresponding aromatic iodides. From the organosilane precursors, three kinds of mesostructured OPV,silica hybrid films are prepared by spin-casting using evaporation-induced self-assembly. UV-vis absorption and fluorescence behavior of the OPV,silica hybrid films show that the optical properties and intermolecular interactions of the OPV moieties embedded within the organosilica frameworks strongly depend on the lateral alkoxy substituents in the precursors. The hexyloxy and 2-ethylhexyloxy substituents prevent aggregation of the OPV units in the organosilica frameworks; this result leads to high fluorescence quantum yields of 0.48,0.61 and 0.63,0.66, respectively, while non-substitution leads to lower fluorescence quantum yields of 0.25,0.34. Fluorescence decay profiles of the organosilica hybrid films also confirm a suppression of the aggregation of OPV moieties by the lateral substituents. These mesostructured organosilica films with significant optical properties in the visible-light region are promising as a new class of phosphor materials. [source] Mechanism and structure,reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2-substituted ethyl methylcarbonates in the gas phaseJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2003Gabriel Chuchani Abstract The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323,435°C and pressure range 28.5,242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft,Topsom method, log(kZ/kH)=,(0.57±0.19),,+(1.34±0.49),R, (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the ,-hydrogen of the C,,H bond by the oxygen carbonyl. Because ,, is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, ,=,0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the ,-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd. [source] Test of Marginal Compatibility and Smoothing Methods for Exchangeable Binary Data with Unequal Cluster SizesBIOMETRICS, Issue 1 2007Zhen Pang Summary Exchangeable binary data are often collected in developmental toxicity and other studies, and a whole host of parametric distributions for fitting this kind of data have been proposed in the literature. While these distributions can be matched to have the same marginal probability and intra-cluster correlation, they can be quite different in terms of shape and higher-order quantities of interest such as the litter-level risk of having at least one malformed fetus. A sensible alternative is to fit a saturated model (Bowman and George, 1995, Journal of the American Statistical Association90, 871,879) using the expectation-maximization (EM) algorithm proposed by Stefanescu and Turnbull (2003, Biometrics59, 18,24). The assumption of compatibility of marginal distributions is often made to link up the distributions for different cluster sizes so that estimation can be based on the combined data. Stefanescu and Turnbull proposed a modified trend test to test this assumption. Their test, however, fails to take into account the variability of an estimated null expectation and as a result leads to inaccurate p -values. This drawback is rectified in this article. When the data are sparse, the probability function estimated using a saturated model can be very jagged and some kind of smoothing is needed. We extend the penalized likelihood method (Simonoff, 1983, Annals of Statistics11, 208,218) to the present case of unequal cluster sizes and implement the method using an EM-type algorithm. In the presence of covariate, we propose a penalized kernel method that performs smoothing in both the covariate and response space. The proposed methods are illustrated using several data sets and the sampling and robustness properties of the resulting estimators are evaluated by simulations. [source] Combustion of chlorinated hydrocarbons in catalyst-coated sintered metal fleece reactors,JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2-3 2003K Everaert Abstract Incinerators emit chlorinated hydrocarbons, such as polychlorinated benzenes (PCBz) and phenols (PCPh), polychlorinated biphenyls (PCB) and polychlorinated dibenzodioxins and furans (PCDD/F), as very dilute streams. High temperatures (>1000,°C) are required in traditional oxidizers. From an energy-saving perspective and to avoid de novo synthesis of PCDD/F, exhaust gas clean-up must be performed at low temperatures (250,350,°C). Catalytic combustion can be applied in this temperature range and different reactor layouts are used (eg monoliths, honeycomb). The present investigation uses a novel catalyst-coated sintered metal fleece. Thin metal fibers are sintered (non-woven) to fleece of various thickness, structure and porosity. V,Ti,W catalysts are examined. The paper will briefly review the catalyst coating method suitable to provide a structured fleece reactor with adequate characteristics. Experiments were carried out in the temperature range of 260,340,°C with various hydrocarbons injected in a carrier air stream. The experimental investigations demonstrated: (i) that the conversion of the hydrocarbons (volatile organic compounds, VOC) is independent of the oxygen concentration, corresponding to a zero-order dependence of the reaction rate; (ii) that the conversion of the hydrocarbons is a first-order reaction in the VOC; (iii) that the oxidation of the VOC proceeds to a greater extent with increasing temperature, with chlorine substitution enhancing the reactivity, and (iv) that the reaction rate constant follows an Arrhenius-dependence with activation energies between 37.3 and 58.4,kJ,mol,1. An assessment of the results leads to a model expression with kinetic reaction control. This model can be used in a scale-up strategy. © 2003 Society of Chemical Industry [source] Localization and interactions of melatonin in dry cholesterol/lecithin mixed reversed micelles used as cell membrane modelsJOURNAL OF PINEAL RESEARCH, Issue 4 2005David Bongiorno Abstract:, The state of melatonin confined in dry cholesterol/lecithin mixed reversed micelles dispersed in CCl4 was investigated using 1H-NMR and FT-IR spectroscopies as a function of the melatonin to lecithin molar ratio (RMLT) and of the cholesterol to lecithin molar ratio (RCHL). An analysis of experimental results leads to the hypothesis that, independent of RMLT and as a consequence of anisotropic melatonin/lecithin, melatonin/cholesterol and cholesterol/lecithin interactions, melatonin is totally solubilized in reversed micelles. Melatonin is mainly located in and oriented in the nanodomain constituted by the hydrophilic groups of cholesterol and lecithin. A competition of melatonin and cholesterol for the hydrophilic binding sites of the reversed micelles was observed by changing the RCHL. Some possible biological implications of the specific interactions governing the solubilization process, the preferential location and the peculiar properties of melatonin confined in cholesterol/lecithin mixed reversed micelles are discussed. [source] Generalized slit flow of an ellis fluidPOLYMER ENGINEERING & SCIENCE, Issue 11 2001Ryszard T. Steller Isothermal, steady-state and fully developed flows of Ellis fluids in planar and annular slits are discussed. The flow equations derived for Ellis fluids describe also the flows of Newtonian and power law fluids as specific cases. The most important flows resulting from thp general theory, i.e., the pressure flows in flat and annular slits for stationary channel walls and at transverse and longitudinal movements of a wall, are analyzed in detail. Numerical verification of the results leads to the conclusion that the planar and annular flows of Ellis and power law fluids are qualitatively similar The quantitative differences resulting from the slit curvature and the type of constitutive equation are relatively large only for flows of strongly non-New-tonian liquids. [source] | |||||||||||||