Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D25: Theory and Simulations I |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Arthi Jayaraman, University of Illinois at Urbana-Champaign Room: Morial Convention Center 217 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D25.00001: Thermodynamically Consistent Nonrandom Mixing on a Bethe Lattice Scott Milner For over 40 years, engineering calculations of nonrandom mixing effects in lattice-based calculations of free energies of mixing in both small-molecule and polymeric solutions (e.g., the non-random two-liquid model, or NRTL) have been based on a strange approximation. By ``strange'', I mean that the approximation violates some commonsense sum rules in how the lattice is filled; namely, that ``something is next to everything'', and ``everything is next to something''. The resulting theories are thermodynamically inconsistent, and explicitly depend on combinations of interaction energies of which the exact mixing free energy is demonstrably independent. To remedy this, I have extended the exact solution of the Ising model on a Bethe lattice to an $n$-component mixture with arbitrary pairwise interactions. Explicit and practical expressions are obtained for the entropy and average energy per site, which incorporate nonrandom mixing in a thermodynamically consistent way. Although it still has mean-field exponents for the binary mixture critical point, the shape of the coexistence curve lies much closer to the exact results for the Ising model in $d=3$ dimensions than do previous engineering-level theories. In addition, the model may be generalized easily to deal with mixtures of species occupying different numbers of sites on the lattice. Thus the model can be used to compute phase behavior for mixtures of molecules of different sizes, including polymeric solutions. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D25.00002: Mixture Properties of Flexible Chains: Comparisons between Experiment, Simulation and Theory; Contrasts between Lattice and Continuum Ronald White, Jane Lipson We present new theoretical results for a series of binary chain-molecule mixtures using both the hard-sphere, and the square-well potentials. We compare these results to simulation data, and contrast them to those obtained using the analogous lattice version of the theory. We discuss all of our findings in the context of experimental data for hydrocarbon chain mixtures. In the course of these studies we consider mixtures of components with varied chain lengths and energetics, and examine the effects of changing composition, temperature, and density. In addition, by calculating the free energy over a wide range of the P,V,T and composition space, we are able to characterize coexistence, both liquid-vapor as well as liquid-liquid partial miscibility. [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D25.00003: A Multichain Self-Consistent Field Theory for Correlations in Polymers: Chain Swelling in Polymer Blends David Wu The self-consistent mean field theory of polymers has been highly successful as a tractable computational framework for capturing the thermodynamics and structure of polymer systems. One notable limitation has been the neglect of fluctuations and correlations, which can be important in a variety of physical circumstances. One such circumstance involves the non-Gaussian conformations (swelling) of branched polymers. We present a method for calculating these correlations with an extension of the SCF theory when applied to multiple chains. As an example of the methodology, we show how the crossover from swollen to screened conformations occurs in a blends of star and linear polymers. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D25.00004: Numerical Renormalization Group for Coarse Graining Field-Theoretic Fluid Models Michael Villet, Glenn Fredrickson Statistical field theory models have proven to be valuable tools for studying the equilibrium behavior of polymeric fluids, but direct simulation of these field theories without use of the mean field approximation is computationally demanding. Computational resources can be extended to simulate larger systems by discretizing the field variables with a coarsely spaced lattice, but indelicate coarse graining risks truncation of important short-wavelength physics. We investigate numerical renormalization group transformations in tandem with complex Langevin simulations as a systematic approach to coarse graining field-theoretic fluid models, using a simple repulsive Yukawa fluid as a test system. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D25.00005: Continuous translocations in connected chambers under pseudo-hydrodynamic force Erica Saltzman, Murugappan Muthukumar Experimental separation of polydisperse synthetic and biopolymers is frequently conducted via combination of electrophoretic or hydrodynamic flow with a series of obstacles or traps. In order to understand the interaction of entropic escape and biased diffusion processes, we conduct simulations on a generic model system. Brownian dynamics simulations are performed on linear chains confined in a series of chambers connected by narrow pores. A uniaxial force designed to mimic the effect of solvent flow acts on each bead of the chain, leading to translocation between chambers. Translocation events are separated by periods of trapping, which shorten with increasing chain length; for long chains individual translocations become indistinguishable. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D25.00006: A two-scale-two-mode dynamic self-consistent theory of entangled interfaces in polymer fluids under flow. Yitzhak Shnidman, Ismael Yacoubou-Djima Tracking conformation statistics on the Kuhn scale is essential for modeling interfacial phenomena in polymer fluids under flow. Successive entanglements partition entangled chains into strands that are in one of two modes: entangled or dangling. Strands follow different differential evolution equations for the second moment of their end-to-end distance, depending on their mode. Dangling strands are governed by the FENE-P equation. For entangled strands, a different evolution equation was proposed by G. Marrucci and G. Ianniruberto, \textit{Phil. Trans. R. Soc. Lond. A} \textbf{361}, 677 (2003). On the Kuhn scale, strand's conformation statistics is sampled by random walks in an effective potential, which are regulated by the evolving second moment of its end-to-end distance. Conformation statistics of a dangling strand is adequately modeled by Wiener (uncorrelated) random walks, but stretching of entangled strands under flow induce correlations between successive steps requiring a persistent random walk model (I. Yacoubou-Djima and Y. Shnidman, http://arxiv.org/abs/0708.2679v1). A two-scale-two-mode subchain propagation scheme, starting from free segments evolved by a probabilistic transport equation, allows a self-consistent calculation of evolving interfacial structure and rheology. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D25.00007: Twinkling Fractal Theory of the Glass Transition. Richard Wool A new approach to the glass transition temperature T$_{g}$ considers the interaction of particles with an anharmonic potential U(x), and Boltzmann population $\phi $(x) $\sim $ exp --U(x)/kT. As T$_{g}$ is approached from above, solid clusters of atoms form and percolate at T$_{g}$. However, the solid percolation cluster is in dynamic equilibrium with its surrounding liquid and ``\textit{twinkles}'' as solid and liquid atoms interchange. The \textit{twinkling} frequency F($\omega )$ is related to the vibrational density of states G($\omega )$ $\sim \quad \omega ^{df}$ and the energy difference $\Delta $U $\sim $ (T$^{2}$-T$_{g}^{2})$ via F($\omega ) \quad \sim $ G($\omega )$ exp -$\Delta $U/kT, where d$_{f}$ = 4/3 is the fracton dimension. F($\omega )$ controls the rate dependence of T$_{g}$, physical aging, yield stress, heat capacity C$_{p}$, T$_{g}$ of thin films, etc. When T $<$ T$_{g}$, the non-equilibrium volume development $\Delta $V, is determined by the fractal structure at T$_{g}$.$_{ }$The thermal expansion coefficients in the liquid and glass are related via $\alpha _{g}$ = p$_{c}\alpha _{L}$. For a Morse potential U(x) = D$_{o}$[1-exp $a$x]$^{2}$, we predict that T$_{g}$ = 2D$_{o}$/9k, and $\alpha _{L}$ = 3k/[4D$_{o}$aR$_{o}$]. For atoms with R$_{o}\approx $ 3 {\AA}, bond energy D$_{o} \quad \approx $ 2-10 kcal/mol and anharmonicity factor $a\approx $2/{\AA}, we obtain $\alpha _{L}$ T$_{g} \quad \approx $ 0.03, and modulus E $\sim $ 1/$\alpha _{L}$, which were found for a broad range of polymers. The yield stress $\sigma _{y}$ is determined by the onset of the twinkling fractal state as $\sigma _{y}$ = {\{}0.16 E [p$_{s}$-p$_{c}$] D$_{o}$/V$_{m}${\}}$^{1/2}$ where V$_{m}$ is the molar volume. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D25.00008: Nascent Polymerized Chain Crystallization on Surface Simulated by the Growing Chain Molecular Dynamics Xiaozhen Yang To understand nascent structure of polymerized chain on a catalyst surface, we have developed a code of growing chain molecular dynamics (GCMD), which describes aggregation behavior of growing chain with increase of repeat units during polymerization. This simulation shows that on the surface the growing chain has a nucleation process before certain chain length and an ordered structure growth process. Meantime, chain folding behavior was surprisingly observed. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D25.00009: Modeling Vapor Deposition Polymerization: Kinetic Monte Carlo Approach Sairam Tangirala, Yiping Zhao, David P. Landau A Kinetic Monte Carlo method is employed to model vapor deposition of growing, linear-polymer thin films which have applications ranging from microelectronic interconnects to biotechnology. Our 1+1 dimensional lattice model [1] implements various dynamical processes that occur during the film-growth, including random-angle deposition, monomer adsorption, free-monomer diffusion, and polymer-end flips. The temperature ($T$) is parametrized using the diffusion coefficient $(D=\exp(-\Delta E_a/k_BT))$, where $\Delta E_a$ is the activation energy for surface diffusion. The diffusion coefficient ($D$) and the deposition rate ($F$) play an important role in the growth process through the ratio $G$ ($=D/F$). We study the polymer chain length distribution, average polymer-chain length, film density, film height, surface-width, and radius of gyration as a function of $G$, system size ($L$), and time. Since polymers have much more complicated structures and interactions than those of organic materials, we find novel behaviors that are different from inorganic thin film growth. [1] W. Bowie and Y.-P. Zhao, \emph{Surf.\ Sci.\ Lett.} \textbf{563}, L245 (2004). [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D25.00010: Amphiphilic Systems under shear flow Hongxia Guo Phase behavior and the related physical and rheological properties of the amphiphilic systems including liquid crystals, diblock copolymers and surfactants are of wide-spread interest, e.g. in industrial processing of layered materials or biological applications of lipid membranes. For example, submitted to an applied shear flow, these lamellae show an interesting coupling of the layer orientation and the flow field. Despite an extensive literature dealing with the shear-induced transition, the underlying causes and mechanisms of the transition remain largely speculative. The experimental similarities between systems of different molecular constituents indicate, that the theoretical description of these reorientations can be constructed, from a common generic basis. Hence one can develop an efficient computer model which is able to reproduce the properties pertinent to real amphiphilic systems, and allows for a large-scale simulation. Here, I employed a simplified continuum amphiphilic computer model to investigate the shear--induced disorder-order, order-order and alignment flipping by large-scale parallelized (none) equilibrium molecular dynamics simulation [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D25.00011: Formation and structure of amorphous carbon char from polymer materials John Lawson, Deepak Srivastava Amorphous carbonaceous char produced from burning polymer solids has insulating properties that makes it valuable for aerospace thermal protection systems as well as for fire retardants. A pyrolytic molecular dynamics simulation method is devised to study the transformation of the local microstructure from virgin polymer to a dense, disordered char. Release of polymer hydrogen is found to be critical to allow the system to collapse into a highly coordinated structure. Mechanisms of the char formation process and the morphology of the resulting structure are elucidated. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D25.00012: Fluctuations in Confined Homopolymers Studied by Fast Off-Lattice Monte Carlo Simulations Yuhua Yin, Qiang Wang The conventional molecular simulations of many-chain systems are hindered by explicit excluded-volume interactions and expensive pair-potential calculations. The former greatly reduces the chain relaxation towards equilibrium configurations and the efficiency of sampling the configurational space, while the latter becomes computationally very expensive for concentrated polymeric systems. Fast off-lattice Monte Carlo (FOMC) simulations overcome these limitations, where individual polymer segments are modeled as volumeless points with the excluded-volume interactions modeled by either solvent quality, Helfand compressibility, or incompressibility constraint commonly used in polymer field theories. By dividing the simulation box into cells and assigning polymer segments to a cell, the short-range interactions can be readily evaluated without expensive pair-potential calculations. To demonstrate the great advantages of FOMC simulations, we have studied homopolymers confined between two parallel surfaces, and compared the results with self-consistent field calculations and field-theoretic simulations (FTS). Since FOMC simulation is particle-based, it avoids the unsolved ?sign problem? encountered in FTS. For the systems we have studied, FOMC simulations can sample the whole spectrum of fluctuations and are several orders of magnitude faster (more efficient) than FTS. [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D25.00013: Computational Modeling of Aging Effects of Epoxy Polymers. Thomas Clancy, Sarah-Jane Frankland, Thomas Gates Due to the increased usage of non-metallic materials in aircraft, there is interest in the effects of aging on the performance of these materials. In order to gain insight into the molecular mechanisms of failure or reduced performance of these materials, computational modeling has been performed. Crosslinked epoxy systems were studied at the atomistic level. Atomistic models of crosslinked epoxy polymers were built by performing molecular dynamics simulations of unreacted epoxy and crosslinker molecules, followed by the formation of chemical crosslinks. Further molecular dynamics simulations were employed to equilibrate the models. These atomistic crosslinked epoxy models were also built with a range of moisture content. In addition, the crosslinking density was varied. The mechanical properties of these atomistic models were calculated in order to assess the effect of hygrothermal aging on the epoxy models. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D25.00014: Conformation and collapse of a polymer chain in explicit solvent: A solvation potential approach Mark P. Taylor The conformation of a polymer chain in solution is intrinsically coupled to the chain's local solvent environment. In much of the theoretical work on polymers in solution the effects of solvent are treated implicitly and explicit chain-solvent coupling is ignored. Although a formally exact treatment of chain-solvent coupling can be constructed, the required many-body solvation potential is not practical to compute. We have recently shown that for short hard-sphere and square-well chain-in-solvent systems this many-body solvation potential can be made tractable via an ``exact'' decomposition into a \textit{set} of two-site potentials [1]. Here we use these exact short chain results, combined with the pure solvent potential of mean force, to construct approximate two-site solvation potentials for long chains under a range of solvent conditions [2]. Monte Carlo simulations for full chain-in-solvent systems verify the accuracy of our solvation potential mapping. We use this approach to study the role of solvent in both driving and inhibiting chain collapse in square-well systems and discuss the possibility of solvent driven chain collapse in the symmetric hard-sphere chain-in-solvent system. [1] M. P. Taylor and G. M. Petersen, J. Chem. Phys. \textbf{127}, 184901 (2007). [2] M. P. Taylor and S. Ichida, J. Polym. Sci., Part B: Polym. Phys. \textbf{45}, 3319 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D25.00015: Statistical Mechanical Theory of Phase Separation and Structure in Dense Polymer-Particle Mixtures Lisa Hall, Ken Schweizer Microscopic liquid state theory has been applied to investigate phase separation and structure of dense mixtures of hard spherical particles and flexible polymer chains in the presence of interfacial attractive interactions. The entire range of filler loading, from the dilute particle regime to the colloid science relevant case of ultra-high particle volume fraction with dilute polymer additives, has been studied for the first time. Many body effects can result in large quantitative, or even qualitative, changes of spinodal demixing boundaries compared to a low order virial treatment. In the temperature-particle volume fraction representation both upper and lower critical temperatures are present, separated by a miscibility window. Entropic effects dominate for weak interfacial attractions (high temperature) resulting in depletion phase separation with a critical point at roughly 10{\%} filler loading. At relatively high interfacial cohesion (low temperature) a network bridging transition occurs characterized by a highly asymmetric spinodal boundary which depends sensitively on attraction spatial range. Deep contact or bridging minima in the particle potential of mean force can occur, which raises the possibility that kinetic gelation or aggregation pre-empts equilibrium phase separation. The evolution of the real space correlations and scattering structure factors as phase separation is approached has been studied in detail. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700