Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A18: Focus Session: Multiscale Modeling in Polymer and Soft Matter Physics |
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Sponsoring Units: DPOLY DCOMP Chair: Mesfin Tsige, University of Illinois at Carbondale Room: B117 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A18.00001: Structural Properties of Polystyrene Adsorbed onto a Solid Surface Yergou Tatek, Mesfin Tsige The understanding of the structural properties of polymer layers adsorbed onto a solid surface is of paramount importance in a wide variety of technical applications. For instance, an area where surface coating is important is the solar cell technology. It has been shown that the performance of solar cells can be greatly enhanced by polymer coating. In the present work we are studying the local conformation of chains in a thin film of polystyrene (PS) adsorbed on a solid substrate by using atomistically detailed simulations. The simulations are carried out by using the readily available and massively parallel molecular dynamics code known as LAMMPS. In particular, a special emphasis is given to the density and orientation of side chains (which consist of phenyl and methylene groups) at solid/polymer and air/polymer interfaces. Three types of substrate were used in our study: $\alpha $-quartz, graphite and amorphous silica. Moreover, we investigated the adsorption of PS chains of different tacticity. Our preliminary results show the presence of a peak of concentration of phenyl rings near the substrate/PS interface. This is tantamount to the existence of a local ordering in that region. Rings at both interfaces tend to point outward the film whereas rings away from the interfaces have no preferred orientation. Our results are in a large part in good agreement with previous experimental and simulation results. [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A18.00002: Analytical rescaling of dynamics from mesoscale simulations of coarse-grained polymer melts to their atomic description Ivan Lyubimov, Marina Guenza Mesoscale simulations of coarse-grained polymeric systems enable the description of dynamics on much longer time scales than united atom simulations. However, because the energy landscape of the coarse-grained system is artificially smooth, the dynamics from mesoscale simulations is accelerated and needs to be rescaled. Starting from our analytical coarse-graining formalism, we derive a novel rescaling approach, which allows for the direct measurement of ``real'' dynamics from mesoscale simulations of a coarse-grained polymer liquid. The rescaling procedure is obtained in two steps, first through the inclusion of the intramolecular vibrational degrees of freedom, which were averaged out during coarse-graining, and second through the rescaling of the friction coefficient from the approximated solution of the memory kernel. Comparison of theoretical predictions of the diffusion coefficients and rotational decorrelation of the end-to-end vectors, for polyethylene liquids of increasing molecular weights, shows quantitative agreement against united atom simulations. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A18.00003: Classical density functional theory of fluids as a multi-scale modeling tool for charged fluids: Electrical double layers, biological ion channels, dielectric interfaces Dirk Gillespie Classical density functional theory (DFT) of fluids (not quantum mechanical DFT of electron orbitals) has the potential to be a powerful modeling technique in many areas of science including ionic liquids, colloids, polymers, and proteins. DFT of fluids is a thermodynamic (statistical) theory in the grand canonical ensemble; given the particle interactions, equations are solved directly for the ensemble-averaged quantities. Because DFT is a thermodynamic theory where the ensemble-averaged quantities are computed directly, it computes results quickly (minutes for 1D problems, $\sim $1 hour for 3D) and in arbitrarily low concentrations and it produces steady-state results. The DFT method is generally applicable to fluids in confining geometries or at interfaces. In this presentation the focus is on new results of DFT applications to electrolytes at charged interfaces and ion current through biological ion channels. A new technical advance of a DFT for dielectric interfaces will also be presented. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 9:12AM |
A18.00004: Structure, Self-assembly, Solvation, and Phase Equilibria in Hydrogen-bonding Fluids Invited Speaker: This talk will focus on applications of efficient particle-based simulation methods and accurate force fields to obtain molecular-level insights on structure and solvation in complex chemical systems. These simulations help to reconcile often conflicting views based on macroscopic measurements. In particular, the following applications will be discussed: (i) aggregation of alcohols in dilute solutions, (ii) influence of water saturation on structure and solvation in 1-octanol, and (iii) retention in reversed-phase liquid chromatography. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A18.00005: Geometric cluster algorithm for anisotropic particles Daniel W. Sinkovits, Erik Luijten Complex fluids typically contain components of different sizes. In simulations of such systems, the relaxation time increases strongly with the size ratio, effectively limiting the type of fluids that can be studied. The geometric cluster algorithm is a Monte Carlo method that eliminates this limitation for mixtures of particles with isotropic interaction potentials, but cannot be applied to anisotropic particles. We present an elegant modification of the geometric cluster algorithm which has the ability to efficiently relax the rotational degrees of freedom of anisotropic particles in a rejection-free manner. This greatly expands the applicability of this method, as is illustrated by means of simulations of self-assembly in a multicomponent mixture of nanoparticles and non-adsorbing polymer. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A18.00006: Nucleationand surface induced crystallization in supercooled liquid water Giovanna Russo, Tianshu Li, Davide Donadio, Giulia Galli Understanding crystallization of water into ice is a very challenging problem, both experimentally and theoretically; in particular, the spatial and temporal resolutions required to characterize the crystallization process at the atomic scale are not yet accessible to experiment. Here we employ a combination of molecular dynamics simulations and advanced sampling techniques to study nucleation in supercooled liquid water. Recently, such an approach has been successfully applied to study nucleation in supercooled liquid silicon [1,2]. The results of our simulations, carried out using a coarse grain potential [3], are used to analyze nucleation rates at various temperatures and to investigate the role played by the presence of surfaces in the freezing processes. \\[4pt] [1] T. Li, D. Donadio and G. Galli, Nat. Mat. 9, 726730 (2009)\\[0pt] [2] T. Li, D. Donadio and G. Galli, J. Chem. Phys., in press\\[0pt] [3] V. Molinero and E. B. Moore J. Phys. Chem. B 113, 40084016 (2009) [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A18.00007: Numerical Coarse-Graining of Polymer Field Theories Michael Villet, Glenn Fredrickson Field theoretic models of polymers are widely used to investigate polymer self-assembly, but numerical simulations of these models that include full fluctuation physics are computationally demanding and infrequently conducted. To reduce this computational cost, we propose the use of systematically coarse-grained field theories that can be simulated on a coarsely spaced lattice without truncation of important short-wavelength physics. We present a variational method for numerically executing this coarse-graining, in which fine-grained simulation data is used to parameterize trial coarse-grained models, and results from the application of this method to some model systems. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:24AM |
A18.00008: Structure and Transport in Soft Materials Studied by Multiscale Simulation Invited Speaker: The mapping across length scales is now an established method in particle-based simulations of polymers and other soft-matter systems. Concepts known as coarse-graining and the inverse procedures known as reverse mapping, backmapping or fine-graining allow the bridging of scales from the electronic to the mesoscopic level. They use as basic building blocks or degrees of freedom anything from the electron (quantum chemistry), the atom (atomistic models), larger groups of atoms or superatoms (coarse-grained models), up to large sections of polymer chains (soft particle models such as dissipative particle dynamics). To devise such models which are simple enough for faster computation and yet complex enough to be material specific, a number of coarse-graining techniques have been devised. This talk covers the so-called Iterative Boltzmann Inversion, which has the reproduction of polymer structure as its primary target, its successes in the prediction of structural and thermodynamic properties, and the remaining challenges. One of them is the prediction of dynamic and transport properties. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A18.00009: Concurrent coupling between a particle simulation and a continuum description: case study of a polymer blend Marcus Mueller We introduce a numerical scheme that concurrently couples a particle simulation of a multi-component polymer melt to a continuum description. We use a soft, coarse-grained model in the particle simulation and a time-dependent Ginzburg-Landau approach for the continuum description. The coupling between the particle coordinates and the order-parameter field, $m$, (e.g., composition) allows us to estimate the parameters the free-energy functional, ${\cal F}_{\rm GL}[m]$, and the non-local Onsager coefficient of the Ginzburg-Landau approach. It makes the particle model follow the time-evolution of the order-parameter field and can be exploited to speed-up the particle simulation. The algorithm is based on the separation between the strong bonded interactions, which dictate the dynamics of the particle simulation, and the weak non-bonded interactions, which control the kinetics of the order-parameter field. A detailed analysis is presented for the spinodal decomposition of a binary polymer blend based on the Random-Phase Approximation and Monte-Carlo simulations. Generalizations to other systems will be discussed. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A18.00010: Effective Pair Potentials and Mesoscale Simulations of Binary Polymer Blends James McCarty, Marina Guenza Macromolecular mixtures are complex fluids characterized by an extended range of relevant length scales, such as the diverging length scale of concentration fluctuations, which develops when two chemically distinct polymers in a blend approach the spinodal. Such macroscopic phenomena readily exceed box sizes commonly used to model polymer ensembles, yet depend specifically on local interactions between monomers. To overcome this problem, we present an implementation of our analytical coarse-graining procedure, which maps a binary mixture of polymers into a system of interacting colloidal particles. By utilizing the hypernetted-chain closure, we derive a ``soft'' effective potential acting between coarse-grained units, which is explicitly parameter dependent. From computer simulations of various polymer systems under different thermodynamic conditions we calculate pair correlation functions and show that our mesoscale simulations capture the relevant trends expected for demixing as the thermodynamic conditions are changed. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A18.00011: Multiscale simulations of chain dynamics in polymeric liquids undergoing shear Bamin Khomami, Jun Kim, Brian Edwards Multiscale simulations were performed of a polyethylene liquid at the atomistic, mesoscopic, and continuum levels of description. Molecular dynamics of shear flow at the atomistic level provided a fundamental description of the rheological properties and the dynamical information concerning individual chains. At low shear rates, the dynamics are dominated by the chain-stretching mode, whereas correlations of the chain end-to-end vectors indicated distinct tumbling frequencies at high shear rates. A mesoscopic model was developed based on the concept of a bead-spring chain in a mean field representing the surrounding chains using an anisotropic diffusion matrix. Brownian dynamics simulations revealed quantitative agreement between these two models for the rheological properties, the tumbling frequencies of the individual chains, and the extensions of the chains. The mesoscopic model was reduced to a continuum model by averaging the bead-spring chain model. This model predicted the simulation data quantitatively at low shear rates, but greatly over-predicted the data at high shear rates where the tumbling dynamics of the individual chains dominated the system response. [Preview Abstract] |
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