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The influence of middle B-block polydispersity on the molecular configurations and lamellar structure characteristics of lamella-forming ABA triblock copolymers

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EPL 2013年第6期103卷

作者： Li, Y. Qian, H. J. Lu, Z. Y. Jilin Univ Inst Theoret Chem State Key Lab Theoret & Computat Chem Changchun 130023 Peoples R China

A recent experimental study (Widin J. M., Schmitt A. K., Schmitt A. L., Im K. and Mahanthappa M. K., J. Am. Chem. Soc., 134 (2012) 3834) showed that polydispersity of the middle B-block has large effects on the phase behavior of ABA triblock copolymers. It is well known that the intriguing properties of ordered structures are associated with the molecular configurations and their distribution at microscale. By using a comprehensive dissipative particle dynamics (DPD) simulation method, we study the effect of middle B-block polydispersity of lamella-forming ABA triblock copolymers on the molecular configurations and the characteristics of lamellar structures. The results show that molecules with short B-blocks in a polydisperse system mainly adopt a looped configuration and preferably locate at the A/B interfaces. On the contrary, the long blocks can adopt either looped or bridged configurations with an equal probability. The bridging fraction of triblock copolymers will decrease with the increase of middle B-block polydispersity, which is due to the fact that there will be more short triblock copolymers adopting looped configurations. Such triblock copolymers with short B-blocks accumulating at the interface region can act as compatibilizers and reduce the interfacial free energy. On the other hand, the domain spacing is mainly determined by the long blocks filling in the domain center. With an increase in middle B-block polydispersity, the domain spacing will expand due to the molecular stretching of the long blocks in the domain center as well as to the reduction in interfacial free energy. Copyright (C) EPLA, 2013

关键词： Copolymers, phase transitions, structure General molecular conformation and symmetry, stereochemistry computer simulation of molecular and particle dynamics

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Topology of force networks in compressed granular media

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EPL 2012年第5期97卷

作者： Kondic, L. Goullet, A. O'Hern, C. S. Kramar, M. Mischaikow, K. Behringer, R. P. New Jersey Inst Technol Dept Math Sci Newark NJ 07102 USA New Jersey Inst Technol Ctr Appl Math & Stat Newark NJ 07102 USA Yale Univ Dept Mech Engn & Mat Sci New Haven CT 06520 USA Yale Univ Dept Phys New Haven CT 06520 USA Rutgers State Univ Dept Math Piscataway NJ 08854 USA Duke Univ Dept Phys Durham NC 27708 USA Duke Univ Ctr Nonlinear & Complex Syst Durham NC 27708 USA

Using numerical simulations, we investigate the evolution of the structure of force networks in slowly compressed model granular materials in two spatial dimensions. We quantify the global properties of the force networks using the zeroth Betti number B0, which is a topological invariant. We find that B0 can distinguish among force networks in systems with frictionless vs. frictional disks and varying size distributions. In particular, we show that 1) the force networks in systems composed of frictionless, monodisperse disks differ significantly from those in systems with frictional, polydisperse disks and we isolate the effect (friction, polydispersity) leading to the differences;2) the structural properties of force networks change as the system passes through the jamming transition;and 3) the force network continues to evolve as the system is compressed above jamming, e.g., the size of connected clusters with forces larger than a given threshold decreases significantly with increasing packing fraction. Copyright (C) EPLA, 2012

关键词： Granular systems computer simulation of molecular and particle dynamics

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Instabilities and turbulence-like dynamics in an oppositely driven binary particle mixture

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EPL 2012年第6期99卷

作者： Ikeda, Masahiro Wada, Hirofumi Hayakawa, Hisao Kyoto Univ Yukawa Inst Theoret Phys Kyoto 6068502 Japan Ritsumeikan Univ Dept Phys Kusatsu Shiga 5258577 Japan

Using extensive particle-based simulations, we investigate out-of-equilibrium pattern dynamics in an oppositely driven binary particle system in two dimensions. A surprisingly rich dynamical behavior including lane formation, jamming, oscillation and turbulence-like dynamics is found. The ratio of two friction coefficients is a key parameter governing the stability of lane formation. When the friction coefficient transverse to the external force direction is sufficiently small compared to the longitudinal one, the lane structure becomes unstable to shear-induced disturbances, and the system eventually exhibits a dynamical transition into a novel turbulence-like phase characterized by random convective flows. We numerically construct an out-of-equilibrium phase diagram. Statistical analysis of complex spatio-temporal dynamics of the fully nonlinear turbulence-like phase suggests its apparent reminiscence to the swarming dynamics in certain active matter systems. Copyright (C) EPLA, 2012

关键词： computer simulation of molecular and particle dynamics Granular models of complex systems, traffic flow molecular dynamics and particle methods

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Tumbling of polymers in semidilute solution under shear flow

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EPL 2011年第5期93卷

作者： Huang, C. -C. Sutmann, G. Gompper, G. Winkler, R. G. Forschungszentrum Julich Inst Complex Syst D-52425 Julich Germany Forschungszentrum Julich Julich Supercomp Ctr D-52425 Julich Germany Forschungszentrum Julich Inst Adv Simulat D-52425 Julich Germany

The tumbling dynamics of individual polymers in semidilute solution is studied by large-scale non-equilibrium mesoscale hydrodynamic simulations. We find that the tumbling time is equal to the non-equilibrium relaxation time of the polymer end-to-end distance along the flow direction and strongly depends on concentration. In addition, the normalized tumbling frequency as well as the widths of the alignment distribution functions for a given concentration-dependent Weissenberg number exhibit a weak concentration dependence in the cross-over regime from a dilute to a semidilute solution. For semidilute solutions a universal behavior is obtained. This is a consequence of screening of hydrodynamic interactions at polymer concentrations exceeding the overlap concentration. Copyright (c) EPLA, 2011

关键词： Polymers and polymer solutions computer simulation of molecular and particle dynamics Multi-scale methods

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Multiscale simulation of history-dependent flow in entangled polymer melts

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EPL 2011年第1期96卷

作者： Murashima, T. Taniguchi, T. Kyoto Univ Dept Chem Engn Nishikyo Ku Kyoto 6158510 Japan JST Core Res Evolut Sci & Technol Kawaguchi Saitama 3320012 Japan

Predicting the flow of an entangled polymer melt is still difficult because of its multiscale characteristics. We have developed a novel multiscale simulation technique to investigate the history-dependent flow behavior of entangled polymer melts. The technique involves using a smoothed particle hydrodynamics simulation that is coupled at each fluid element to microscopic simulators that can accurately account for the dynamics of entangled polymers. The multiscale simulation is used to investigate the flow of an entangled polymer melt around a cylindrical obstacle subject to periodic boundary conditions. It is found that the macroscopic flow behavior is dependent on the history of the microscopic states of the polymers and that this memory causes nonlinear behavior even in the regions where the local Weissenberg number defined using the local strain-rate is less than unity. The spatial distribution of the entanglements < Z > suggests that, in a region around the obstacle, a slight depletion of the entanglements is observed and that this region broadens along the downstream direction. The totality of the presented results suggests that we have succeeded in describing the entangled polymer melt flow without using any constitutive equation. Copyright (C) EPLA, 2011

关键词： computer simulation of molecular and particle dynamics Polymer melts Nonlinear viscoelasticity

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Flow generation by rotating colloids in planar microchannels

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EPL 2010年第6期92卷

作者： Goetze, I. O. Gompper, G. Forschungszentrum Julich Inst Festkorperforsch D-52425 Julich Germany

Non-equilibrium structure formation and conversion of spinning to translational motion of magnetic colloids driven by an external rotating magnetic field in microchannels is studied by particle-based mesoscale hydrodynamics simulations. For straight channels, laning is found. In ring channels, the channel curvature breaks symmetry and leads to a net fluid transport around the annulus with the same rotational direction as the colloidal spinning direction. The dependence of the translational velocity on channel width, ring radius, colloid concentration, and thermal motion is predicted. Copyright (C) EPLA, 2010

关键词： Micro- and nano- scale flow phenomena computer simulation of molecular and particle dynamics Colloids

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Hybrid description of complex molecules

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EPL 2009年第4期88卷

作者： Espanol, P. Univ Nacl Educ Distancia Dept Fis Fundamental E-28040 Madrid Spain Freiburg Inst Adv Studies D-79104 Freiburg Germany

We present a hybrid description of complex molecules in which one of the molecules is described in full atomistic detail, while the rest of molecules is represented at a coarse-grained level with their centers-of-mass variables. Based on Zwanzig's projection method, we derive the equations of motion for the coupled detailed/coarse-grained system, which turn out to be of the form of Dissipative particle dynamics. Copyright (C) EPLA, 2009

关键词： Statistical mechanics of classical fluids computer simulation of molecular and particle dynamics Brownian motion

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Structural and dynamical heterogeneities in two-dimensional melting

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EPL 2009年第6期86卷

作者： Shiba, H. Onuki, A. Araki, T. Kyoto Univ Dept Phys Kyoto 6068502 Japan

Using molecular-dynamics simulation, we study structural and dynamical heterogeneities at melting in two-dimensional one-component systems with 36000 particles. Between crystal and liquid we find intermediate hexatic states, where the density fluctuations are enhanced at small wave number k as well as those of the sixfold orientational order parameter. Their structure factors both grow up to the smallest wave number equal to the inverse system length. The intermediate scattering function of the density S( k, t) is found to relax exponentially with decay rate Gamma(k) alpha k(z) with z similar to 2.6 at small k in the hexatic phase. Copyright (C) EPLA, 2009

关键词： Solid-liquid transitions Point defects and defect clusters computer simulation of molecular and particle dynamics

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simulation studies of soft matter: generic statistical properties and chemical details

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EUROPEAN PHYSICAL JOURNAL B 2008年第3-4期64卷 525-529页

作者： Kremer, K. Max Planck Inst Polymer Res D-55128 Mainz Germany

The relation between atomistic structure, architecture, molecular weight and material properties is a basic concern of modern soft material science. This by now goes far beyond standard properties of bulk materials. A typical additional focus is on surface or interface aspects or on the relation between structure and function in nanoscopic molecular assemblies. This all implies a thorough understanding on many length and correspondingly time scales ranging from (sub)-atomic to macroscopic. At this point computer simulations are playing an increasingly important, if not the central role. Traditionally simulations have been separated in two main groups, namely simplified models to deal with generic or universal aspects of polymers, i.e. critical exponents, and those employing classical force field simulations with (almost) all atomistic detail, i.e. for the diffusion of small additives in a small "sample". Still characteristic problems, which require huge systems and/or long times in combination with a chemistry specific model, cannot be tackled by these methods alone. More recently with the development of scale bridging or multi scale simulation techniques, these different approaches have been combined into an emerging rather powerful tool. It is the purpose of this contribution to give a few examples of how such an approach can be used to understand specific material properties.

关键词： Structural modeling: serial-addition models, computer simulation computer simulation of molecular and particle dynamics

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Thermalization in open classical systems with finite heat baths

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EUROPEAN PHYSICAL JOURNAL B 2008年第3期61卷 271-275页

作者： Smith, S. T. Onofrio, R. Dartmouth Coll Dept Phys & Astron Hanover NH 03755 USA Univ Padua Dipartimento Fis Galileo Galilei I-35131 Padua Italy Ctr Stat Mech & Complex CNR INFM Unita Roma 1 I-00185 Rome Italy

We discuss thermalization of a test particle schematized as a harmonic oscillator and coupled to a Boltzmann heat bath of finite size and with a finite bandwidth for the frequencies of its particles. We find that complete thermalization only occurs when the test particle frequency is within a certain range of the bath particle frequencies, and for a certain range of mass ratios between the test particle and the bath particles. These results have implications for the study of classical and quantum behaviour of high-frequency nanomechanical resonators.

关键词： Brownian motion Nonequilibrium and irreversible thermodynamics Mechanical properties of nanoscale systems computer simulation of molecular and particle dynamics

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