关键词： ageing   glass   glass transition   permittivity   polymers   stress relaxation   vitrification  

摘要： The dynamics of glass forming liquids as the glass transition temperature (T-g) is traversed has become of special interest because of the continuing question as to whether or not the dynamics diverge towards an ideal glass transition/Kauzmann temperature or if the apparent Vogel-Fulcher-Tammann (VFT) divergence is lost as one goes below the conventional T-g but remains in equilibrium. Here we examine the response of a poly(vinyl acetate) PVAc polymer glass-former using both dielectric and mechanical methods in the vicinity of T-g. Isothermal measurements were performed to study the aging behavior of the PVAc and to assure that the equilibrium state was achieved for temperatures as much as 16 K below the calorimetric T-g. Surprisingly, we found that the mechanical response took much longer to age into its equilibrium than did the dielectric response. Also, the temperature dependence of the time-temperature shift factors obtained from the two methods is different and the dielectric response shows a turnover to an apparent Arrhenius behavior rather than a continuation of the VFT extrapolated divergence at the lowest temperatures tested. (C) 2012 American Institute of Physics. [http://***/10.1063/1.3701736]

摘要： In vitro, attachment to the overlying membrane was found to affect the resting position of the hair cell bundles of the bullfrog sacculus. To assess the effects of such a deflection on mechanically decoupled hair bundles, comparable offsets were imposed on decoupled spontaneously oscillating bundles. Strong modulation was observed in their dynamic state under deflection, with qualitative changes in the oscillation profile, amplitude, and characteristic frequency of oscillation seen in response to stimulus. Large offsets were found to arrest spontaneous oscillation, with subsequent recovery upon reversal of the stimulus. The dynamic state of the hair bundle displayed hysteresis and a dependence on the direction of the imposed offset. The coupled system of hair bundles, with the overlying membrane left on top of the preparation, also exhibited a dependence on offset position, with an increase in the linear response function observed under deflections in the inhibitory direction.

关键词： Animals   Cytoskeleton/metabolism   Humans   Mechanotransduction, Cellular/physiology   Models, Theoretical   Stress Fibers/metabolism   Stress Fibers/metabolism   Stress, Mechanical   Viscosity  

摘要： Transmission of mechanical stimuli through the actin cytoskeleton has been proposed as a mechanism for rapid long-distance mechanotransduction in cells;however, a quantitative understanding of the dynamics of this transmission and the physical factors governing it remains lacking. Two key features of the actin cytoskeleton are its viscoelastic nature and the presence of prestress due to actomyosin motor activity. We develop a model of mechanical signal transmission through prestressed viscoelastic actin stress fibers that directly connect the cell surface to the nucleus. The analysis considers both temporally stationary and oscillatory mechanical signals and accounts for cytosolic drag on the stress fibers. To elucidate the physical parameters that govern mechanical signal transmission, we initially focus on the highly simplified case of a single stress fiber. The results demonstrate that the dynamics of mechanical signal transmission depend on whether the applied force leads to transverse or axial motion of the stress fiber. For transverse motion, mechanical signal transmission is dominated by prestress while fiber elasticity has a negligible effect. Conversely, signal transmission for axial motion is mediated uniquely by elasticity due to the absence of a prestress restoring force. Mechanical signal transmission is significantly delayed by stress fiber material viscosity, while cytosolic damping becomes important only for longer stress fibers. Only transverse motion yields the rapid and long-distance mechanical signal transmission dynamics observed experimentally. For simple networks of stress fibers, mechanical signals are transmitted rapidly to the nucleus when the fibers are oriented largely orthogonal to the applied force, whereas the presence of fibers parallel to the applied force slows down mechanical signal transmission significantly. The present results suggest that cytoskeletal prestress mediates rapid mechanical signal transmission and allows temporally

关键词： Reaction dynamics   Vibrational effects   Insertion and abstraction mechanisms   WP and QCT snapshots  

摘要： We present the quantum mechanical (QM) and quasiclassical trajectory (QCT) dynamics of the title reaction on two uncoupled surfaces, using a wavepacket (WP) method and considering some N-2(v,j) vibrational and rotational states. The reaction is investigated by calculating initial-state-resolved reaction probabilities, cross sections, and rate constants, which are explained in terms of energy profiles and collision mechanisms. These properties reflect the large endo-thermicity of the reaction and the features of the surfaces. Indeed, at low v values we obtain large thresholds and the lower surface is more reactive than the higher one, whereas at high v the thresholds decrease or disappear and the upper surface becomes more reactive. QM and QCT results are in satisfactory agreement, except some differences at low or high collision energy and temperature. QCT rate coefficients agree also with some published results. WP snapshots and movies of QCT time evolution show clearly abstraction and insertion mechanisms depending on the initial conditions. Insertion proceeds via a reaction complex and we observe a QM Feshbach resonance for a specific initial condition. On the overall, the dynamical observables are consistent with the collision mechanisms. (C) 2011 Elsevier B. V. All rights reserved.

关键词： DECARBOXYLATION   CHEMICAL kinetics   OXIDATION   FLUOROQUINOLONES   CHLORAMINES   AQUEOUS solutions   CHEMICAL reactions   FOURIER transform infrared spectroscopy  

摘要： The mechanistic investigation of oxidation of levofloxacin (LF) has been studied by chloramine-T(CAT) in aqueous perchloric acid medium at 25 degrees C. The reaction followed first-order kinetics with respect to [CAT], [LF], and [H+] in their lower concentrations range, tending to zero-order at their higher concentrations. First-order kinetics with respect to [CAT] was observed for the oxidation of levofloxacin. An increase in perchloric acid concentration increased the rate of reaction. The effect of added products, ionic strength, and dielectric constant of the medium was studied on the rate of reaction. The main products were identified by spot test, FT-IR, and NMR. A mechanism was proposed on the basis of experimental results. The activation parameters with respect to the slow step of the mechanism was evaluated, and the thermodynamic parameters were also determined and discussed.

关键词： Indigenous   Chromite mine water   Sorption capacity   Cr(VI) removal  

摘要： The current study focuses on the quantification and mechanistics of Cr(VI) biosorption with Acinetobacter junii, indigenous bacteria isolated from chromite mine sites of Sukinda valley, Orissa, India. The equilibrium sorption capacity 22.22, 13.88, 6.94 mg of Cr(VI) g(-1) of biomass was obtained for logarithmic phase, stationary phase and heat killed biomass respectively at optimum sorption conditions (pH: 2.0, contact time: 120 min, temperature: 27 degrees C, initial Cr(VI) concentration: 100 mg L-1, biosorbent dosage: 2 g L-1). The Langmuir isotherm model and pseudo first order (Lagergren kinetic model) were observed to hold good for the bio-sorbents. Three subsequent sorption/desorption cycles yielded 54.9, 38.4 and 34.5% regeneration of the biosorbent indicating an irreversible chromium(VI) binding property of the sorbent system. The mechanistic aspects of sorption by heat killed and log phase biomass were compared using FTIR and SEM techniques. The number of binding sites on biomass surface measured by acid/base titration (215 mu mol g(-1) and 99 mu mol g(-1) for logarithmic phase and heat killed biomass respectively) corroborated with the observations made by batch sorption studies. Detailed surface characterization studied through FT-IR, SEM, EDX and zeta potential measurement provided evidence for sorption through oxyanionic binding to the cell surface. The EPR spectroscopy revealed the reduction of Cr(VI) to Cr(V) on the biomass surface upon Cr(VI) sorption. (C) 2012 Elsevier B.V. All rights reserved.

关键词： 电动力农业机械   发展现状   趋势  

摘要： 电动力农业机械具有结构简洁紧凑、控制灵活、低碳高效、无尾气噪音污染和维护成本低等特点。根据能源发展方向、市场需求和技术条件,在目前国家给予的政策指导和扶植下,采用电动力是农业机械化的发展趋势之一。为此,分析了近几十年来我国农村用电规模、动力电功率情况和主要用电生产机械的情况,并分类论述了我国电动力农业机械的最新技术应用情况;最后,指出了电动力农业机械存在的主要问题和今后发展趋势。

关键词： N-Acetylneuraminic acid   Conformation   MD simulation   QM calculation   Geometry optimization  

摘要： N-Acetylneuraminic acid is a sugar molecule of biological significance due to its pivotal role in molecular recognition processes. The three dimensional structure and conformation of alpha-Neu5Ac in biological environments can be clearly observed by molecular dynamics (MD) simulation and quantum mechanical (QM) calculations. A 10 ns MD simulation on alpha-Neu5Ac yields two conformational models which are stabilized by water mediated hydrogen bond between O-8/O-9 hydroxyl oxygen and carbonyl of carboxylate group. The average life time of the conformers and the residual time of water which mediates the hydrogen bonding interactions are computed. Based on the amphiprotic nature of water, water mediation of each conformer is divided into two different modes, one donor-one acceptor mode and two donor modes. According to the analysis of simulation trajectories, the preferred mode of water mediation for conformers is the one donor-one acceptor mode. The energy and geometry of the MD derived conformational models of alpha-Neu5Ac are optimized using HF/6-31G* basis set of Gaussian03. QM calculations also resulted that alpha-Neu5Ac is preferentially stabilized by water mediated hydrogen bonding between O-8 hydroxyl and the carboxylate group where the mediation is one donor-one acceptor type. The optimized geometry of alpha-Neu5Ac which is in good agreement with the crystal structure of alpha-D-N-acetyl-1-O-methylneuraminic acid methyl ester is deposited in the public domain database 3DSDSCAR (http://***). This optimized structure can be used by biotechnologists, biophysicists and glycobiologists for modelling the sialylglycans and also to design drugs using sialic acid analog inhibitors. (C) 2012 Elsevier Ltd. All rights reserved.

关键词： 路面机械   高等级   江苏省   镇江市  

摘要： 日前从镇江市发改委获悉，江苏华通动力重工有限公司申报的高等级路面机械项目，经江苏省发改委工业发（2012）47号文批复核准建设。

关键词： 粉煤灰   机械活化   氧化铝   动力学  

摘要： 采用高能球磨活化粉煤灰,并对球级配、球料比、转速以及球磨时间进行正交试验,将经不同时间活化的粉煤灰与Na2CO3按一定比例煅烧溶出,得到Al2O3。通过测试分析,研究了高能球磨活化各工艺参数对Al2O3溶出率的影响,得到了最优化工艺条件,并对Al2O3溶出过程的动力学进行了验证分析。结果表明:当表面机械活化6h、溶出温度90℃、液固比L/S=6、硫酸浓度4mol/L时,效益最高;此反应属于收缩未反应芯模型,表观活化能为60.87kJ/mol,反应级数为0.1701。