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11.
Yang J Cyr PW Wang Y Soong R Macdonald PM Chen L Manners I Winnik MA 《Photochemistry and photobiology》2006,82(1):262-267
We describe experiments that determine the quenching kinetics by poly(ferrocenylsilane) (PFS) for platinum octaethylporphine (PtOEP) phosphorescence in toluene solution. The phosphorescence quenching process was interpreted in terms of diffusion-controlled kinetics. Pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR) and dynamic light scattering (DLS) were used to characterize the diffusion behavior of PFS and PtOEP in toluene solution. We found that the ferrocene group present in the repeat unit of polymer backbone is a good quencher for PtOEP phosphorescence. Quenching by the polymer involves the entire PFS polymer chain instead of individual ferrocene groups. The intrinsic quenching ability of PFS was found to be higher than that of a model compound, Bu-FS, that contains a single ferrocene group. 相似文献
12.
In the present study, a novel theoretical model is developed for the analysis of rotating thermal-fluid flow characteristics in the presence of electrokinetic effects in the microscale gap region between two parallel disks under specified electrostatic, rotational, and thermal boundary conditions. The major flow configuration considered is a rotor-stator disk system. Axisymmetric Navier-Stokes equations with consideration of electric body force stemming from streaming potential are employed in the momentum balance. Variations of the fluid viscosity and permittivity with the local fluid temperature are considered. Between two disks, the axial distribution of the electric potential is determined by the Poisson equation with the concentration distributions of positive and negative ions obtained from Nernst-Planck equations for convection-diffusion of the ions in the flow field. Effects of disk rotation and electrostatic and thermal conditions on the electrokinetic flow and thermal characteristics are investigated. The electrohydrodynamic mechanisms are addressed with an interpretation of the coupling nature of the electric and flow fields. Finally, solutions with electric potential determined by employing nonlinear or linearized Poisson-Boltzmann equation and/or invoking assumptions of constant properties are compared with the predictions of the present model for justification of various levels of approximation in solution of the electrothermal flow behaviors in rotating microfluidic systems. 相似文献
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14.
Tony Y. Liu David S. Soong Michael C. Williams 《Journal of Polymer Science.Polymer Physics》1984,22(9):1561-1587
The kinetic network (KN) model discussed previously in the context of monodisperse and bimodal polymer systems is extended to polymers of arbitrary molecular weight distribution. A generalization is proposed for the flow-dependent entanglement loss term in the structure equation, replacing the shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} by a new variable \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \Gamma $\end{document} which reduces to\documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} for simple shear and is more appropriate for elongational and other flows. New data are obtained on shear stress transients of many kinds, using a recently developed parallel-plate rheometer. These data and others on steady and transient flows of well-characterized polydisperse polymers in shear and in elongation are used to demonstrate that the KN model predictions are valid. Comparisons with predictions for monodisperse polymers having the same as M w polydisperse systems show that transient behavior—especially stress overshoot—is particularly sensitive to details of the molecular weight distribution. Further possible improvements in the theory are suggested, and the relationship of the KN model to other recent network models is discussed. The KN model has greater data fitting capabilities, with fewer parameters, than any other model available at present. 相似文献
15.
Fafarman AT Koh WK Diroll BT Kim DK Ko DK Oh SJ Ye X Doan-Nguyen V Crump MR Reifsnyder DC Murray CB Kagan CR 《Journal of the American Chemical Society》2011,133(39):15753-15761
Ammonium thiocyanate (NH(4)SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH(4)SCN is also effective in replacing ligands on NCs after their assembly into the solid state. The spectroscopic properties of this ligand provide unprecedented insight into the chemical and electronic nature of the surface of the NCs. Spectra indicate that the thiocyanate binds to metal sites on the NC surface and is sensitive to atom type and NC surface charge. The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 10(3) Ω(-1) cm(-1) for Au NC thin films deposited from solution. NH(4)SCN treatment of PbTe NC films increases the conductivity by 10(13), allowing the first Hall measurements of nonsintered NC solids, with Hall effect mobilities of 2.8 ± 0.7 cm(2)/(V·s). Thiocyanate-capped CdSe NC thin films form photodetectors exhibiting sensitive photoconductivity of 10(-5) Ω(-1) cm(-1) under 30 mW/cm(2) of 488 nm illumination with I(photo)/I(dark) > 10(3) and form n-channel thin-film transistors with electron mobilities of 1.5 ± 0.7 cm(2)/(V·s), a current modulation of >10(6), and a subthreshold swing of 0.73 V/decade. 相似文献
16.
The diffusion of various molecular weight poly(ethyleneglycol)s (PEG) confined between the lamellae of magnetically aligned bicelles has been measured using stimulated echo (STE) pulsed field gradient (PFG) 1H nuclear magnetic resonance (NMR) spectroscopy. Bicelles were formulated to contain dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), and dihexanoylphosphatidylcholine (DHPC) in the proportion DMPG/DMPC = 0.05 and q = (DMPC + DMPG)/DHPC = 4.5. PEG diffusion within the interlamellar spaces between such bicelles was found to be unrestricted over diffusion distances of tens of microns. Two confinement regimes could be differentiated according to the dependence of the reduced PEG diffusivity D/D0, where D0 is the unconfined PEG diffusion coefficient, on the relative confinement Rh/H, where Rh is the unperturbed hydration radius of the particular PEG and H approximately 60 A is the separation between apposing lamellae of the magnetically aligned bicelles. In the regime Rh/H < 0.4, the reduced PEG diffusivity was altered only in proportion to the viscosity increase associated with the bicelle dispersion relative to bulk solution. In the regime Rh/H > 0.4, the reduced PEG diffusivity scaled as (Rh/H)-2/3, in agreement with scaling theories for confined polymers. 相似文献
17.
ABSTRACTIn the present study, molecular dynamics (MD) simulation was used to investigate the relationship between wetting behaviour and slip length on patterned substrates. We adopted two solid surfaces of Si(100) and graphite due to similarities in their intrinsic contact angle. Contact angle and apparent slip length were obtained using discrete simulations with the same thermodynamic states. In the present study, a number of questions regarding surface roughness and the problem of contact angle (θ) and slip length (Ls) are discussed. These questions include the relationship between θ and surface roughness, the characteristics used to describe the difference between static and dynamic fluid fields and the reason for a lack of multilayer sticking observed in the current cases. Our results indicate that the quasi-universal θ ? Ls equation proposed by Hung et al. (2008) is applicable to cases involving a Cassie-like nanoscale roughened surface. In contrast, in cases with a Wenzel-like nanostructure, the no-slip boundary conditions are independent of variations in the contact angle. The adoption of a Wenzel–Cassie hybrid model helped to verify that the fluid density inside the cavity is a critical indicator of wettability of the wall–fluid interface. Our results also demonstrate that ρf, cav is a critical property in the measurement of hydrodynamic effects and thus its importance as an indicator of the validity of the equation θ ? Ls. The average time that water molecules are trapped and the number of averaged hydrogen bonds within cavities in a dynamic fluid field were also investigated. 相似文献
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19.
A method to analyze polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/DFs, dioxins) and coplanar polychlorinated biphenyls in environmental samples is developed and used to determine the levels of PCDD/DFs in 13 soil and 4 fish samples collected around a waste pentachlorophenol manufacturing plant. The 2,3,7,8-tetrachlorodioxin toxic equivalents (TEQ) values range from 0.239 ng-TEQ/g to 1357 ng-TEQ/g in soil samples and 0.041 ng-TEQ/g to 0.247 ng-TEQ/g in fish samples. A broader survey of PCDD/DF contamination in the vicinity around the manufacturing plant is strongly recommended. 相似文献
20.
Huey-Jan Chuang Lee-Lin Soong George E. Leroi Alexander I. Popov 《Journal of solution chemistry》1989,18(8):759-770
Sodium-23 NMR chemical shifts and linewidths have been measured for 0.1M NaClO4 in binary mixtures of N-methylformamide (NMF) with a series of other solvents, as a function of the solvent mole fraction. The relative solvent composition at the isosolvation point, the mid-value of the Na-23 chemical shift between those measured in the respective pure solvents, reveals preferential solvation of the sodium cation in many cases. The isosolvation composition correlates well with the relative solvating abilities of the two solvents-as characterized by their donicities-provided that the cation-solvent interactions are of the hard-hard type and that they are not complicated by interionic interactions. The variation in the electric field gradient around the sodium nucleus, as the composition of the solvent changes, results in broadening of the resonance line. Maximum broadening occurs close to the solvent mole fraction corresponding to the isosolvation point. 相似文献