关键词： Clay   Epoxy resin   Exfoliated   Intercalated   Interphase   Molecular dynamics simulations  

摘要： Polymer/clay nanocomposites (PCNs) are multi-functional materials that have superior mechanical and thermal properties than polymer-based materials while maintaining their characteristic properties of lightweight and optical clarity. These materials are obtained by adding small amounts of clay nanofillers to a polymer matrix. In this work, we proposed a molecular model to investigate the morphology and thermomechanical properties of thermosetting clay nanocomposites. The epoxy matrix was composed of several representative cross-linked epoxy units, and different structures of PCNs, which correspond to intercalated or exfoliated status, were considered by varying the amount of polymer phase in the interlayer space. Molecular dynamics simulations of different nanocomposite structures were used to provide atomistic insights into the arrangement of the constitutive components of PCNs, the molecular interactions occurring in the interphase zone, and the influence of silicate layer on the thermodynamic and elastic properties of these multi-functional materials. (C) 2017 Elsevier B.V. All rights reserved.

关键词： MECHANICAL behavior of materials   TISSUE scaffolds   POROUS materials   COMPOSITE materials   DEGRADATION of plastics   DIGITAL image processing  

摘要： Tissue engineering combines artificial scaffolds and living cells in order to reconstruct damaged tissues and organs. The biodegradable scaffolds should maintain their mechanical properties during first stages of the regeneration. The aim of this study was to investigate the extent the degradation affects the mechanical stability of novel biodegradable composite scaffolds in relation to their composition. The scaffolds were made using fused deposition modeling. They were composed of ternary composites containing poly(epsilon-caprolactone) (PCL), 5 wt% of tricalcium phosphate (TCP) and 5, 15, and 25 wt% of poly(lactide-co-glycolide) (PLGA). Scaffolds made of pristine PCL and binary composite PCL-TCP were tested as reference samples. The degradation experiment was carried out in simulated body fluid at 37 degrees C for 12 weeks. Mechanical tests were carried out in a mechanical tester. Strain was measured using digital image correlation and crossbar displacement. Chemical composition had a significant effect on initial mechanical properties and their changes during degradation. The initial apparent Young's modulus of ternary composite scaffolds was two times higher than that of PCL-TCP. Higher PLGA concentration yielded faster decrease of the mechanical properties. At the end of the experiment, there were no significant differences of the modulus among all tested materials although degradation of the ternary composite scaffolds was significantly advanced. (C) 2015 Society of Plastics Engineers

关键词： Kevlar (R)   Aramid fibers   Molecular dynamics   Reactive potentials   Rate-dependence   Temperature-dependence   Fracture  

摘要： Molecular dynamic simulations are used to quantify how the mechanical behavior of PPTA crystallites, the fundamental building blocks of aramid fibers such as Kevlar (R), depend on strain-rate, temperature, and crystallite size. The (axial) crystallite elastic modulus is found to be independent of strain-rate and decreases with increasing temperature. The crystallite failure strain increases with increasing strain rate and decreases with increasing temperature and crystallite size. These observations are consistent with crystallite failure being driven by stress-assisted thermal fluctuations of bonds within PPTA crystallites and the concepts of the kinetic theory of fracture. Appealing to reliability theory, a model is proposed that predicts the onset of both primary and secondary bond failure within a crystallite as of function of strain rate, temperature, and crystallite size. The model is parameterized using bond failure data from constant strain-rate molecular dynamic strain-to-failure simulations and is used to compute the activation volume, activation energy, and frequency for both primary and secondary bond ruptures. (C) 2017 Published by Elsevier Ltd.

关键词： somitogenesis   segmentation   presomitic mesoderm   oscillations   excitable system   Yap   Notch   excitability   clock  

摘要： The periodic segmentation of the vertebrate body axis into somites, and later vertebrae, relies on a genetic oscillator (the segmentation clock) driving the rhythmic activity of signaling pathways in the presomitic mesoderm (PSM). To understand whether oscillations are an intrinsic property of individual cells or represent a population-level phenomenon, we established culture conditions for stable oscillations at the cellular level. This system was used to demonstrate that oscillations are a collective property of PSM cells that can be actively triggered in vitro by a dynamical quorum sensing signal involving Yap and Notch signaling. Manipulation of Yap-dependent mechanical cues is sufficient to predictably switch isolated PSM cells from a quiescent to an oscillatory state in vitro, a behavior reminiscent of excitability in other systems. Together, our work argues that the segmentation clock behaves as an excitable system, introducing a broader paradigm to study such dynamics in vertebrate morphogenesis.

关键词： 农业机械   劳动力投入产出   弹性技术   替代弹性  

摘要： 随着中国城镇化进程的不断推进和农村劳动力大量外出转移,中国农业生产正在朝着依靠更多机械投入的方向发展。认清农业机械对劳动力替代强度和粮食产出的影响以及这种影响在不同粮食品种、不同时空上的差异至关重要。本文基于全国农村固定观察点的农户面板数据,利用超越对数生产函数对机械和劳动力投入产出弹性以及两者间的技术替代弹性进行了测算。今后农业机技术创新与推进重点在结构上应该更多地向玉米和非平原地区倾料。

关键词： Mechanical effects of light on material media   Optomechanics   Photonic crystals   Solitons   Nonperturbative methods   Semiclassical methods  

摘要： A Kuznetsov-Ma soliton that exhibits an unusual pulsating dynamics has attracted particular attention in hydrodynamics and plasma physics in the context of understanding nonlinear coherent phenomena. Here, we demonstrate theoretically the formation of a novel form of Kuznetsov-Ma soliton in a microfabricated optomechanical array, where both photonic and phononic evolutionary dynamics exhibit periodic structure and coherent localized behavior enabled by radiation-pressure coupling of optical fields and mechanical oscillations, which is a manifestation of the unique property of optomechanical systems. Numerical calculations of the optomechanical dynamics show an excellent agreement with this theory. In addition to providing insight into optomechanical nonlinearity, optomechanical Kuznetsov-Ma soliton dynamics fundamentally broadens the regime of cavity optomechanics and may find applications in on-chip manipulation of light propagation.

关键词： 钢结构   动力机械   分类   建筑应用  

摘要： 钢结构建筑中动力机械合理应用,可以提高机械设备的运作效率。控制好动力机械设备的振动范围,可以减少对建筑物周围居民的影响,降低对邻近设备的干扰,对于提高钢结构建筑的建设具有重要的意义,而且动力机械设备的运用,能有效改善建筑结构,提高建筑结构的性能。本文首先对机械分类和基础类型等问题进行了阐述,在此基础上探讨了动力机械在钢结构建筑物中的应用。

关键词： CHEMISTRY periodicals   RUTHENIUM   CATALYSIS   DYNAMICS  

摘要： The mechanism of the dimerization of terminal aryl alkynes promoted by [{RuCl(mu-Cl)(eta(6)-p-cymene)}(2)] (1)/AcOH, under cooperative transition metal/Bronsted acid catalysis, has been investigated with regard to (i) the activation of the dinuclear ruthenium complex and (ii) the catalytic formation of the trans-1,4-diaryl-1,3-enyne products, by a detailed kinetic investigation of both processes. Complex 1 is subject to a slow solvolytic process in neat acetic acid or is transformed rapidly in the presence of sodium acetate to form the monomeric ruthenium(II) acetato complex [RuCl(eta(6)-pcymene)(OAc)]. The latter is the active catalytic species promoting the alkyne dimerization process, via initial pi-alkyne coordination and intramolecular C-H abstraction by the acetate ligand, as key steps of the catalytic cycle. The presence of additive acetate salts allows for the reaction to proceed at room temperature with short reaction times and high trans/cis stereoselectivity, thus rendering this catalytic system among the most active and selective procedures for the dimerization of terminal alkynes in a protic medium. The linear coupling of three molecules of phenylacetylene affords an organometallic ruthenium complex featuring a butenynyl ligand which has been characterized by X-ray crystallography.

关键词： MECHANICAL behavior of materials   IONS   MOLECULAR physics   IONOMERS   MOLECULAR dynamics   POLYMERS  

摘要： Using coarse-grained molecular dynamics simulations, we study ionomers in equilibrium and under uniaxial tensile deformation. The spacing of ions along the chain is varied, allowing us to consider how different ionic aggregate morphologies, from percolated to discrete aggregates, impact the mechanical properties. From the equilibrium simulations, we calculate the stress-stress auto correlation function, showing a distinct deviation from the Rouse relaxation due to ionic associations that depends on ion content. We then quantify the morphology during strain, particularly the degree to which both chains and ionic aggregates tend to align. We also track the location of the ionomer peak in the anisotropic structure factor during strain. The length scale of aggregate order increases in the axial direction and decreases in the transverse direction, in qualitative agreement with prior experimental results. Published by AIP Publishing.

关键词： molecular dynamics   physicochemical interaction   oxygen atom   phosphatidylcholine   plasma medicine   AMBER   quantum mechanics  

摘要： Plasma medicine is one of the most attractive applications using atmospheric pressure nonequilibrium plasma. With respect to direct contact of the discharge plasma with a biological membrane, reactive oxygen species play an important role in induction of medical effects. However, complicated interactions between the plasma radicals and membrane have not been understood well. In the present work, we simulated elemental processes at the first stage of physicochemical interactions between oxygen atom and phosphatidylcholine using the quantum mechanical molecular dynamics code in a general software AMBER. The change in the above processes was classified according to the incident energy of oxygen atom. At an energy of 1 eV, the abstraction of a hydrogen atom and recombination to phosphatidylcholine were simultaneously occurred in chemical attachment of incident oxygen atom. The exothermal energy of the reaction was about 80% of estimated one based on the bond energies of ethane. An oxygen atom over 10 eV separated phosphatidylcholine partially. The behaviour became increasingly similar to physical sputtering. The reaction probability of oxygen atom was remarkably high in comparison with that of hydrogen peroxide. These results suggest that we can uniformly estimate various physicochemical dynamics of reactive oxygen species against membrane lipids.