关键词： Rubber   Sulphur vulcanization   Reversion   Kinetics   Mechanistic model  

摘要： A novel kinetic model of natural rubber sulphur vulcanization is proposed. The modeling approach takes into account current knowledge on the different polysulfidic structures present during vulcanization, and the associated individual reactions. A simplified scheme is proposed, giving a mechanistic view of the reversion phenomenon, which results in a decrease of the elastic modulus (related to the sulphur crosslink density) for long vulcanization times at high temperature. The resulting set of differential equations is solved by an appropriate numerical method to predict the evolution of the degree of vulcanization for isothermal cure conditions. The vulcanization kinetics of a model natural rubber compound was characterized experimentally by rheological measurements, in order to test the proposed kinetic model. A remarkable agreement between model predictions and experimental data is observed. The identified kinetic parameters corresponding to the individual reactions taken into account by the mechanistic model are consistent with those of an existing, less refined, pseudo-mechanistic model. The proposed model thus allows bridging the gap between the prediction of macroscopic variations of the elastic modulus and the evolution of molecular scale structure during vulcanization when the reversion phenomenon is present. (C) 2013 Elsevier Ltd. All rights reserved.

关键词： PHYTOPLANKTON   DYNAMICS   CLIMATIC changes   ECOSYSTEMS   COMMUNITIES   ZOOPLANKTON  

摘要： Driving factors of phytoplankton spring blooms have been discussed since long, but rarely analyzed quantitatively. Here, we use a mechanistic size-based ecosystem model to reconstruct observations made during the Kiel mesocosm experiments (2005-2006). The model accurately hindcasts highly variable bloom developments including community shifts in cell size. Under low light, phytoplankton dynamics was mostly controlled by selective mesozooplankton grazing. Selective grazing also explains initial dominance of large diatoms under high light conditions. All blooms were mainly terminated by aggregation and sedimentation. Allometries in nutrient uptake capabilities led to a delayed, post-bloom dominance of small species. In general, biomass and trait dynamics revealed many mutual dependencies, while growth factors decoupled from the respective selective forces. A size shift induced by one factor often changed the growth dependency on other factors. Within climate change scenarios, these indirect effects produced large sensitivities of ecosystem fluxes to the size distribution of winter phytoplankton. These sensitivities exceeded those found for changes in vertical mixing, whereas temperature changes only had minimal impacts.

关键词： Cross linking   Epoxy polymer   Mechanical properties   Molecular dynamics   Radial distribution function  

摘要： Recently, great attention has been focused on using epoxy polymers in different fields such as aerospace, automotive, biotechnology, and electronics, owing to their superior properties. In this study, the classical molecular dynamics (MD) was used to simulate the cross linking of diglycidyl ether of bisphenol-A (DGEBA) with diethylenetriamine (DETA) curing agent, and to study the behavior of resulted epoxy polymer with different conversion rates. The constant-strain (static) approach was then applied to calculate the mechanical properties (Bulk, shear and Young's moduli, elastic stiffness constants, and Poisson's ratio) of the uncured and cross-linked systems. Estimated material properties were found to be in good agreement with experimental observations. Moreover, the dependency of mechanical properties on the cross linking density was investigated and revealed improvements in the mechanical properties with increasing the cross linking density. The radial distribution function (RDF) was also used to study the evolution of local structures of the simulated systems as a function of cross linking density.

关键词： Carbon Nanocones   Molecular Dynamics   Mechanical Behaviors  

摘要： The paper presents the molecular dynamics simulation of the elastic, failure, buckling and vibrational behaviors of the carbon nanocones (CNCs) with different geometries. First, the elastic moduli of the CNCs are presented for different geometries and the effects of various loading rates and temperature variations on the rupture points of a CNC are investigated. Second, the buckling phenomena of the CNCs with different geometries are studied and the effects of mentioned conditions on the critical buckling stresses of a CNC are discussed. Third, the first natural frequencies of different CNCs are presented and the effects of temperature variations on the natural frequencies are studied.

关键词： DIFFERENCES   PHYTOPLANKTON   DYNAMICS   NICHE (Ecology)   BIOCOMPLEXITY   SEDIMENTATION & deposition   ALLOMETRY  

摘要： This paper aims to unravel the complexity of how cell size determines major ecological and physiological functions in unicellular algae. It assembles recent mechanistic descriptions for size dependencies of respiration and maximal resource uptake rates. New dependencies are derived for mortality due to sinking and grazing. The model also describes how internal nutrient quota varies within an autotrophic community due to the allometry of nutrient uptake. Net production, grazing, and sedimentation can be shown to be uni- or multi-modal functions of cell size, in accordance with literature data. Grazing mortality attains maxima at distinct prey sizes, which result from overlaying major zooplankton feeding spectra. Sedimentation vanishes for small cells due to Stokes' law, but also for huge diatoms as long as they are vital. Their greater degree of vacuolation possibly promotes the capacity of buoyancy regulation. After synthesizing the physiological, morphological, and ecological relationships, intricate and eventually multi-modal growth-size dependencies arise. This niche building may explain part of the great diversity in phytoplankton. Multi-modal allometries can also induce paradoxical community shifts or enlarge the variety of community responses to external changes.

关键词： Thiamine hydrochloride   Cerium(IV)   Manganese(II)   Oxidation   Catalysis   Kinetics  

摘要： The kinetics of the manganese(II)-catalyzed oxidation of thiamine hydrochloride by cerium(IV) in aqueous perchloric acid medium at a constant ionic strength of 1.10 mol dm(-3) was studied spectrophotometrically at 15, 25, 35, and 45 A degrees C by the stopped flow technique. The reaction between thiamine hydrochloride and cerium(IV) in the acid medium exhibits 1:3 stoichiometry. The main products were identified by spot test, IR, H-1 NMR, and GC-MS studies. The reaction is first order in cerium(IV) and manganese(II) and has less than unit order in thiamine hydrochloride. As the acid concentration increases the rate of reaction decreases. The added product cerium(III) retards the rate of reaction. The active catalyst and oxidant species were identified as [Mn(H2O)(4)](2+) and [Ce(OH)](3+), respectively. A probable mechanism involving free radicals and the formation of a complex between substrate and catalyst is proposed. The reaction constants, activation parameters, and thermodynamic quantities are calculated and discussed.

关键词： Hydrogenation  

摘要： Graphyne, a new type of carbon allotropes, has attracted considerable attention in recent years. Using molecular dynamics simulations, we investigate the mechanical properties of four different graphynes (alpha-, beta-, gamma-, and 6,6,12-graphynes) functionalized with hydrogen. The simulations results show that hydrogenation can greatly deteriorate the mechanical properties of the graphynes. For the different graphynes with 100% H-coverage, the reduction in fracture stress depends on the percentage of acetylenic linkages in the graphyne structures: The more the acetylenic linkages, the larger the reduction. For the same graphyne, the reduction in fracture stress depends on the hydrogenation location, distribution, and coverage. Hydrogenation on the acetylenic linkages causes a larger reduction in fracture stress than that on the hexagonal rings. A line hydrogenation perpendicular to the tensile direction leads to a larger reduction in fracture stress than that when the line hydrogenation is parallel to the tensile direction. For random hydrogenation, the fracture stress and Young's modulus decrease rapidly at low H-coverage (<10%), and then level off with increasing coverage. The reduction in the mechanical properties due to hydrogenation is found to be related to the formation of weakened out-of-plane C-C bonds, which leads to earlier breaking of those bonds and subsequent fracture of the graphynes. The present study not only offers an in-depth understanding in the mechanical properties of hydrogenated graphynes and their fracture mechanisms but it also presents an important database for the design and practical applications of hydrogenated graphynes. (C) 2013 AIP Publishing LLC.

关键词： Characterization   Growth from solutions   Organo-metallic compound   Nonlinear optical material  

摘要： L-proline manganese chloride (LPMC), an organo-metallic compound, was successfully grown from its aqueous solution by slow evaporation technique. The growth condition was optimized using nucleation kinetics. The grown crystal was subjected to XRD study to determine the crystal structure. The microstructure of the grown crystal and the presence of various elements in the crystal were studied using SEM and EDAX techniques. The UV-vis-NIR spectral analysis was carried out to measure the transparent range of the LPMC crystal. The modes of vibrations of different molecular groups present in the sample were identified using FTIR spectral analysis. The dielectric studies of the sample were carried out at different temperatures in the frequency range from 50 Hz to 5 MHz. The mechanical strength of the crystal was analyzed by Vickers hardness test. The second harmonic generation efficiency (SHG) was evaluated both theoretically and experimentally to understand the NLO activity of the grown crystal. (C) 2013 Published by Elsevier B.V.

关键词： Phonon spectra   RESEARCH   Young's modulus   Molybdenum disulfide   Molecular dynamics   Nanoribbons   Thermal conductivity  

摘要： We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2. (C) 2013 AIP Publishing LLC.

关键词： acid   base catalysis   autocatalysis   enzymes   molecular dynamics   zymogens  

摘要： Simulations: Molecular dynamics simulations showed that protonation of Asp164 triggers conformational changes and leads to reconstruction of the functional active site for the cleavage of the prodomain. Quantum mechanical/molecular mechanical (QM/MM) free-energy simulations revealed the catalytic mechanism and energetics for the acylation reaction of the autocatalytic process (see figure).