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排序方式: 共有178条查询结果,搜索用时 15 毫秒
51.
Lucas J. T. Landherr Qing Zhang Claude Cohen Lynden A. Archer 《Journal of Polymer Science.Polymer Physics》2008,46(17):1773-1787
This study focuses on developing dry, surface‐tethered polymeric lubricant coatings capable of significantly decreasing friction and wear of nano‐ and micrometer scale machines. Vinyl‐terminated polydimethylsiloxane chains are spin‐coated with a crosslinking agent and platinum catalyst onto silicon wafers functionalized with a self‐assembling monolayer containing reactive vinyl groups. Lateral force microscopy (LFM) measurements employing a bead probe are used to quantify the coefficient of friction (COF) and adhesion characteristics of the PDMS‐SAM surface tethered networks. The combined polymer network and SAM layer manifest extremely low friction coefficients, μ = 4 × 10?3, which is nearly one order of magnitude lower than the friction coefficient of the bare silicon substrate. The lowest friction forces are measured using silicon substrates covered with nanometer thick, peroxide crosslinked PDMS networks; though poorly crosslinked, these networks display COFs as much as ten‐times lower than a solitary SAM coating layer. Micrometer thick end‐linked optimal networks also manifest attractive interfacial friction properties, with COFs approximately three times larger than the thinner, imperfect networks. These observations are discussed in terms of the structure of the polymer networks and the role of adhesion forces on interfacial friction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1773–1787, 2008 相似文献
52.
We prove a new stability property of the classical Hermite interpolation scheme in one variable. This stability property refines the usual convergence property of this interpolation scheme. Consequently, we obtain a natural proof of a Whitney extension theorem in one variable. 相似文献
53.
Yumeng Zhu William R. Archer Katlyn F. Morales Michael D. Schulz Yin Wang John B. Matson 《Angewandte Chemie (International ed. in English)》2023,62(22):e202302303
Inducing high levels of reactive oxygen species (ROS) inside tumor cells is a cancer therapy method termed chemodynamic therapy (CDT). Relying on delivery of Fenton reaction promoters such as Fe2+, CDT takes advantage of overproduced ROS in the tumor microenvironment. We developed a peptide-H2S donor conjugate, complexed with Fe2+, termed AAN - PTC – Fe2+ . The AAN tripeptide was specifically cleaved by legumain, an enzyme overexpressed in glioma cells, to release carbonyl sulfide (COS). Hydrolysis of COS by carbonic anhydrase formed H2S, an inhibitor of catalase, an enzyme that detoxifies H2O2. Fe2+ and H2S together increased intracellular ROS levels and decreased viability in C6 glioma cells compared with controls lacking either Fe2+, the AAN sequence, or the ability to generate H2S. AAN - PTC – Fe2+ performed better than temezolimide while exhibiting no cytotoxicity toward H9C2 cardiomyocytes. This study provides an H2S-amplified, enzyme-responsive platform for synergistic cancer treatment. 相似文献
54.
Analysis of underivatized cellodextrin oligosaccharides by capillary electrophoresis with direct photochemically induced UV‐detection
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Elodie Alinat Selma Jemmali Nathalie Delaunay Xavier Archer Pierre Gareil 《Electrophoresis》2015,36(14):1555-1563
This work focuses on the development of a CE method allowing, for the first time, the simultaneous separation of the underivatized first seven cellodextrin oligomers (glucose, cellobiose, cellotriose, cellotetraose, cellopentaose, cellohexaose, and celloheptaose), with a view to analyze the hydrolysates obtained after partial acid depolymerization of nitrocellulose, and eight carbohydrates (ribose, xylose, fructose, mannose, galactose, maltose, lactose, and sucrose), which might be potential interfering compounds in explosives samples. Separation was achieved with a highly alkaline BGE containing sodium chloride and direct mid‐UV‐absorbance detection was performed after photo‐oxidation in the detection window. EOF was reversed to speed up the analysis using a dynamic capillary coating by hexadimethrine bromide. A central composite design was carried out to determine the effects of BGE conductivity and sodium hydroxide concentration on resolutions between neighboring peaks, and analysis time. A desirability analysis on modeled responses was applied to maximize resolutions and to minimize analysis time. The simultaneous analysis in 20 min total runtime of the 15 carbohydrates plus internal reference (naphthalene sulfonate) was carried out at 25°C with a BGE composed of 77.4 mM NaOH and 183 mM NaCl to adjust the conductivity at the optimum value. Finally, the resolution robustness was checked. This new method should also be of interest to monitor food and nonfood crop products. 相似文献
55.
Landherr LJ Cohen C Agarwal P Archer LA 《Langmuir : the ACS journal of surfaces and colloids》2011,27(15):9387-9395
A bead-probe lateral force microscopy (LFM) technique is used to characterize the interfacial friction and adhesion properties of polymer brushes. Our measurements attempt to relate the physical structure and chemical characteristics of the brush to their properties as thin-film, tethered lubricants. Brushes are synthesized at several chain lengths and surface coverages from polymer chains of polydimethylsiloxane (PDMS), polystyrene (PS), and a poly(propylene glycol)-poly(ethylene glycol) block copolymer (PPG/PEG). At high surface coverage, PDMS brushes manifest friction coefficients (COFs) that are among the lowest recorded for a dry lubricant film (μ ≈ 0.0024) and close to 1 order of magnitude lower than the COF of a bare silicon surface. Brushes synthesized from higher molar mass chains exhibit higher friction forces than those created using lower molar mass polymers. Increased grafting density of chains in the brush significantly reduces the COF by creating a uniform surface of stretched chains with a decreased surface viscosity. Brushes with lower surface tension and interfacial shear stresses manifest the lowest COF. In particular, PDMS chains exhibit COFs lower than PS by a factor of 3.7 and lower than PPG/PEG by a factor of 4.7. A scaling analysis conducted on the surface coverage (σ) in relation to the fraction (ε) of the friction force developing from adhesion predicts a universal relation ε ~ σ(4/3), which is supported by our experimental data. 相似文献
56.
Elisabeth CW van Straaten Willem de Haan Hanneke de Waal Philip Scheltens Wiesje M van der Flier Frederik Barkhof Ted Koene Cornelis J Stam 《BMC neuroscience》2012,13(1):1-7
Background
This study examined the effects of dietary polyunsaturated fatty acids (PUFA) as different n-6: n-3 ratios on spatial learning and gene expression of peroxisome- proliferator-activated receptors (PPARs) in the hippocampus of rats. Thirty male Sprague?CDawley rats were randomly allotted into 3 groups of ten animals each and received experimental diets with different n-6: n-3 PUFA ratios of either 65:1, 22:1 or 4.5:1. After 10?weeks, the spatial memory of the animals was assessed using the Morris Water Maze test. The expression of PPAR?? and PPAR?? genes were determined using real-time PCR.Results
Decreasing dietary n-6: n-3 PUFA ratios improved the cognitive performance of animals in the Morris water maze test along with the upregulation of PPAR?? and PPAR?? gene expression. The animals with the lowest dietary n-6: n-3 PUFA ratio presented the highest spatial learning improvement and PPAR gene expression.Conclusion
It can be concluded that modulation of n-6: n-3 PUFA ratios in the diet may lead to increased hippocampal PPAR gene expression and consequently improved spatial learning and memory in rats. 相似文献57.
Bourlinos AB Giannelis EP Zhang Q Archer LA Floudas G Fytas G 《The European physical journal. E, Soft matter》2006,20(1):109-117
Ionically modified silica nanoparticles with large counter anions (sulfonate, isostearate) at two silica volume fractions
(13 and 27%) form a viscous fluid and a glass but not crystalline solids. Dielectric spectroscopy, Brillouin scattering and
shear rheometry were employed to investigate these new nanoparticle-based fluids. The glass transition temperature and hence
the local dynamics are governed by the large counter anions, whereas the flow properties can be controlled by the spatial
correlation between the nanoparticles, e.g. by tuning the volume fraction of hard cores and local interactions between segments in the soft corona. Liquid-like ordering
of the cores was revealed by X-ray scattering and found to influence significantly the macroscopic flow properties of these
salts. 相似文献
58.
Wajdi I. AlSadat Lynden A. Archer 《Angewandte Chemie (International ed. in English)》2019,58(9):2632-2637
Conversion of low‐value, but thermodynamically stable chemical byproducts such as alkanes or CO2 to more valuable feedstocks is of broad‐based interest. These so‐called up‐conversion processes are expensive because they require energy‐intensive and catalytic interventions to drive reactions against thermodynamic gradients. Here we show that the nucleophilic characteristics of superoxides, generated galvanostatically in an Aluminum/O2 electrochemical cell, can be used in tandem with the intrinsic catalytic properties of an imidazolium/AlCl3 electrolyte to facilely upgrade alkanes (n‐decane), alkenes (1‐decene), and CO2 feedstocks. The aluminum/O2 electrochemical cell used to generate the superoxide intermediate is also reported to deliver large amounts of electrical energy and therefore offers a system for high‐energy density storage and for chemical up‐conversion of low‐value compounds. Chronopotentiometry, mass spectrometry and nuclear magnetic resonance were used to investigate the electrochemical features of the system and to analyze the discharge products. We find that even at room temperature, alkanes and alkenes are facilely oligomerized and isomerized at high conversions (>97 %), mimicking the traditionally produced refined products. Incorporating CO2 in the alkane feed leads to formation of esters and formates at moderate yields (21 %). 相似文献
59.
Synchrotron X‐ray microbeam dosimetry with a 20 micrometre resolution scintillator fibre‐optic dosimeter
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James Archer Enbang Li Marco Petasecca Andrew Stevenson Jayde Livingstone Andrew Dipuglia Jeremy Davis Anatoly Rosenfeld Michael Lerch 《Journal of synchrotron radiation》2018,25(3):826-832
Cancer is one of the leading causes of death worldwide. External beam radiation therapy is one of the most important modalities for the treatment of cancers. Synchrotron microbeam radiation therapy (MRT) is a novel pre‐clinical therapy that uses highly spatially fractionated X‐ray beams to target tumours, allowing doses much higher than conventional radiotherapies to be delivered. A dosimeter with a high spatial resolution is required to provide the appropriate quality assurance for MRT. This work presents a plastic scintillator fibre optic dosimeter with a one‐dimensional spatial resolution of 20 µm, an improvement on the dosimeter with a resolution of 50 µm that was demonstrated in previous work. The ability of this probe to resolve microbeams of width 50 µm has been demonstrated. The major limitations of this method were identified, most notably the low‐light signal resulting from the small sensitive volume, which made valley dose measurements very challenging. A titanium‐based reflective paint was used as a coating on the probe to improve the light collection, but a possible effect of the high‐Z material on the probes water‐equivalence has been identified. The effect of the reflective paint was a 28.5 ± 4.6% increase in the total light collected; it did not affect the shape of the depth‐dose profile, nor did it explain an over‐response observed when used to probe at low depths, when compared with an ionization chamber. With improvements to the data acquisition, this probe design has the potential to provide a water‐equivalent, inexpensive dosimetry tool for MRT. 相似文献
60.
Bending properties of short (15-90 bp), double-stranded DNA fragments are quantified using fluorescence resonance energy transfer and small angle x-ray scattering. Results from both types of measurements indicate that short double-stranded DNA fragments exhibit surprisingly high flexibility. These observations are discussed in terms of base-pair-level length fluctuations originating from dynamic features of Watson-Crick base pairs. 相似文献