Numerical Integration Techniques Based on a Geometric View and Application to Molecular Dynamics Simulations

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Simulation par dynamique moléculaire de la déformation élastique et plastique de polyéthylènes semi-cristallins

La déformation plastique d'un polyéthylène semi-cristallin est simulée par dynamique moléculaire. Les molécules sont décrites avec tous les atomes pour pouvoir décrire les phases cristalline et amorphe. Dans ce modèle, deux molécules de haute masse molaire participent à la formation de ces deux phases amorphe, avec des conditions triplement périodiques. C'est un des premiers modèles physiquement couplés qui inclus les propriétés d'organisation moléculaire connus. Une traction est appliquée perpendiculairement aux lamelles à vitesse de traverse imposée. Le mode de déformation des phases amorphe et cristalline, ainsi que l'évolution de la force, sont analysés en fonction de la déformation microscopique. Cette analyse permet de préciser les détails d'organisation de la phase amorphe compatibles avec les prédictions de comportement rhéologique. De plus, dans tous les cas apparaît un phénomène de cavitation dans la phase amorphe dont l'origine est alors expliquée

Molecular dynamics prediction of elastic and plastic deformation of semicrystalline polyethylene

International audienceThe elastic and large plastic deformations of semicrystalline polymers involve the multiscale organization of molecules inside spherulites and depend on the deformation path. Under a tensile test, as an effect of the lamellar organization, the first steps of elastic-plastic deformation are localized in a very thin layer in the equatorial zone, as shown by experiments. The molecular mechanism and the resulting stress-strain properties can be predicted by molecular dynamics simulations. An all-atom model is necessary to predict the behavior of polyethylene chains inside the amorphous and crystalline phases. Two large-molecular-weight polyethylene chains with a complex path are involved in crystalline and amorphous phases and in their interconnection with a 3D periodic condition. This paper explains the main physical characteristics of semicrystalline organization and the building process of this first molecular model which is fully coupled. This model, stretched along the thickness of the lamellae, is representative of the equatorial zone in a spherulite during the first steps of elastic and plastic deformation. The deformation mechanism of amorphous and crystalline phases is analyzed as a function of strain and strain-rate. A nanocavitation in the amorphous phase results from a topological constraint imposed by the crystalline phase. This mechanism is a natural consequence of the model and explains the cavitation observed at a macroscopic level

A Robust, Symmetric Operator-Composition Integrator for the Berendsen Temperature-Control Molecular Dynamics Equation

International audienceThe Berendsen equations of motion (EOM) are widely used for controlling the temperature of a target physical system in molecular dynamics simulations. Its numerical integration, however, has never raised much attention. Yet, a non-optimal integration scheme de…nitely lowers the e¢ ciency of the EOM. If the integration becomes more robust, then does the Berendsen method. To realize this, we propose an operator composition scheme having the following properties: symmetricity, i.e., time reversibility in the original di¤erential equation is kept; systematic, i.e., any higher order of the local accuracy can be attained; robustness, i.e., a new velocity scaling factor is bounded, which enables faster temperature control. Our extended EOM formalism, which provides an invariant function, also helps to observe the numerical error that cannot be detected solely by the temperature controllability. These properties of the proposed method were con…rmed by applying it into three molecular systems

Investigating Unusual Organic Functional Groups to Engineer the Surface Chemistry of Mesoporous Silica to Tune CO 2 –Surface Interactions

International audienceAs the search for functionalized materials forCO2 capture continues, the role of theoretical chemistry isbecoming more and more central. In this work, a strategy isproposed where ab initio calculations are compared andvalidated by adsorption microcalorimetry experiments for aseries of, so far unexplored, functionalized SBA-15 silicas withdifferent spacers (aryl, alkyl) and terminal functions (N3,NO2). This validation then permitted to propose the use of anitro-indole surface functionality. After synthesis of such amaterial the predictions were confirmed by experiment. This confirms that it is possible to fine-tune CO2-functional interactionsat energies much lower than those observed with amine species

Silica materials with wall-embedded nitroxides provide efficient polarization matrices for dynamic nuclear polarization NMR

CCC:000374035800006International audienceHybrid mesoporous silica materials with wall-embedded nitroxides are shown to efficiently polarize impregnated substrates in high-field dynamic nuclear polarization magic-angle spinning solid-state NMR experiments

Design of Wall-Functionalized Hybrid Silicas Containing Diazene Radical Precursors. EPR Investigation of Their Photolysis and Thermolysis

International audienceBis-silylated diazenes were designed to prepare wall-functionalized hybrid silicas via the sol–gel process. These precursors enabled the straightforward generation of arylsulfanyl radicals by either photolysis or thermolysis. Their location in the framework of the mesoporous materials was evidenced by spin trapping experiments. The incidence of confinement on the radical lifetime was investigated by electron paramagnetic resonance (EPR). An amazing persistence (t1/2 = 27 min) was recorded at 473 K