Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A40: New Experimental, Theoretical, and Computational Methods in Polymer and Soft Matter Physics |
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Sponsoring Units: DPOLY Chair: Amalie Frischknecht, Sandia National Laboratories Room: A122/123 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A40.00001: A First Principle Approach to Rescale the Dynamics of Simulated Coarse-Grained Macromolecular Liquids Ivan Lyubimov, Marina Guenza A first-principle approach has been developed to rescale dynamical data from mesoscopic molecular dynamics simulations of polymer liquids. We derive rescaling factors from Generalized Langevin Equations (GLE) for the coarse-grained at the monomer level representation and coarse-grained at the mesoscopic level representation of the liquid, exploiting the Mori-Zwanzig projection operator formalism. The rescaling factors explicitly depend on coarse-grained model parameters and thermodynamic parameters. Two corrections need to be accounted to compensate the acceleration effect on dynamics caused by higher level of coarse-graining: change in entropy and change in friction. After applying our rescaling to data from mesoscopic simulations of unentangled and weakly entangled polyolefin melts we observe a good agreement with data of translational diffusion measured experimentally and from UA simulations. The method is used to predict self-diffusion coefficients for systems not yet investigated experimentally. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A40.00002: Phase behavior of disk-coil molecules YongJoo Kim, Alfredo Alexander-Katz Using Monte Carlo simulations, we investigate the self-assembly of disk-coil molecules in the NPT ensemble. By changing the interaction parameters between the disk and the coil portion of the molecules, a full phase diagram of these four phases is constructed. Furthermore, we study the ordering of disks within the crystal phase and we find that the confinement imposed by the mesophase segregation induces stronger order compared to the pure disk case, which was also explicitly simulated. Our results show that by reducing the dimensionality of a system it is possible to induce higher order of the molecules and help orient the disks in the crystal phase. Furthermore, we simulated molecules with additional interaction and obtained interesting additional phases. Our results are relevant for organic photoactive (typically planar) molecules that are functionalized with tails to improve their processability and long-range order in the solid phase. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A40.00003: A unified model Hamiltonian for polythiophene, polypyrrole, polyfuran, free base porphyrin, and polyaniline: Accuracy, transferability, and computational efficiency Andre Botelho, Xi Lin Two fully transferable physical parameters are incorporated into the historical Su-Schrieffer-Heeger Hamiltonian to model conducting polymers beyond polyacetylene, one parameter $\gamma$ scales the electron-phonon coupling strength in aromatic rings and the other parameter $\varepsilon$ specifies the heterogeneous core charges. This generic Hamiltonian predicts the fundamental band gaps of polythiophene, polypyrrole, polyfuran, free base porphyrin, polyaniline, and their oligomers of all lengths with an accuracy exceeding the time-dependent density functional theory. Additionally, its computational costs are four orders of magnitude or more lower than first-principles approaches. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A40.00004: Fluctuating lattice-Boltzmann model for complex fluids Santtu Ollila, Colin Denniston, Mikko Karttunen, Tapio Ala-Nissila We develop, and test numerically, a lattice-Boltzmann (LB) model for non-ideal fluids that incorporates thermal fluctuations through a random component in the local stress tensor. The fluid model is a momentum-conserving thermostat, for which we demonstrate how the temperature can be made equal at all length scales present in the system by having noise both in the stress tensor of the fluid and by shaking the whole system in accord with the local temperature. The validity of the model is extended to a broad range of values of the sound velocity. Furthermore, our model features a consistent coupling scheme between the fluid and solid molecular dynamics objects, which allows us to use the LB fluid as a heat bath for solutes evolving in time {\it without} external Langevin noise added to the solute. This property expands the applicability of LB models to dense, strongly correlated systems with thermal fluctuations and potentially non-ideal equations of state. We benchmark our model by performing tests on the fluid itself and on the static and dynamic properties of a coarse-grained polymer chain under strong hydrodynamic interactions. We find that our model produces results for single-chain diffusion that are in quantitative agreement with theory [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A40.00005: Polymer network stretching during electrospinning Israel Greenfeld, Arkadii Arinstein, Kamel Fezzaa, Miriam Rafailovich, Eyal Zussman Fast X-ray phase contrast imaging is used to observe the flow of a semi-dilute polyethylene oxide solution during electrospinning. Micron-size glass particles mixed in the polymer solution allow viewing of the jet flow field, and reveal a high-gradient flow that has both longitudinal and radial components that grow rapidly along the jet. The resulting hydrodynamic forces cause substantial longitudinal stretching and transversal contraction of the polymer network within the jet, as confirmed by random walk simulation and theoretical modeling. The polymer network therefore concentrates towards the jet center, and its conformation may transform from a free state to a fully-stretched state within a short distance from the jet start. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A40.00006: Microscopic theory of topological entanglement constraints in fluids of rigid macromolecules Daniel Sussman, Ken Schweizer A theoretical description of the slow dynamics of an ideal gas of infinitely thin, non-rotating rods or three-dimensional crosses is presented. As objects with no excluded volume their equilibrium structure is trivial, and thus slow dynamics are determined solely by bond uncrossability and macromolecular connectivity. Our work builds on the dynamic mean-field theory of Szamel, which successfully predicted tube localization and reptation for non-rotating uncrossable rods. We derive an effective diffusion constant by exactly enforcing uncrossability at the two-molecule level in conjunction with a self-consistent renormalization to account for many-particle effects. For crosses and isotropically translating rods a topological localization transition is predicted at a critical density above which macromolecules are localized by a confinement potential with very strong anharmonicities. The spatial nature of the latter, including the density-dependent localization length, is analyzed and contrasted with recent experiments on entangled F-actin filaments. The stability of the localization transition to both an external force (yielding) and macromolecular collective density fluctuations is examined, and comparison with simulations on entangled crosses is performed. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A40.00007: Tying Polymer Knots to Find the Entanglement Length Jian Qin, Scott Milner We propose two relations between the entanglement length and the probability distribution of topological states accessed by topologically equilibrated ring polymer melts. The first states that the rings are most likely entangled when the ring length exceeds the entanglement length. The second states that the topological entropy measuring the number of accessible topological states is about $k_B$ per entanglement strand. To test these ideas, we simulated melts of ring polymers with hybrid MC/MD moves, and sampled their topological states by using various ring rebridging moves. Topological states are identified by mapping the molecular configurations to knots, and knots are distinguished by computing their invariant polynomials. We accumulated the state statistics, their ring length dependence, and extracted the entanglement length using these two approaches. The results are consistent with each other, and agree with those from the heuristic methods. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A40.00008: Towards simulation of charges in the presence of varying dielectric response Vikram Jadhao, Francisco Solis, Guillermo Guerrero-Garcia, Monica Olvera de la Cruz A variational formulation of electrostatics suitable for carrying out simulations of charges interacting in the presence of different dielectric media is developed. The variational principle employs the polarization charge density as the only variational field. A true energy functional is constructed to yield the correct electrostatic energy at its minimum and the correct force during the approach to the minimum. In the hope of applying the method to more realistic and complicated geometries, some numerical investigations of the variational procedure are also presented. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A40.00009: Colloidal Particle Vibration Spectroscopy Tim Still, George Fytas, Maurizio Mattarelli, Maurizio Montagna Brillouin light scattering (BLS) on dry colloidal particles resolves a large number of resonance vibrations (eigenmodes),\footnote{Cheng et al., \textit{J. Chem. Phys.} \textbf{123}, 121104, 2005.} allowing determining the elastic properties at meso- and nanoscale and measure (polymer) physical properties not accessible by other methods.\footnote{Still et al., \textit{Nano Lett.} \textbf{8}, 3194, 2008; \textit{J. Coll. Int. Sci.} \textbf{340}, 42, 2009; \textit{Macromolecules} \textbf{43}, 3422, 2010.} So far, only the frequencies of the different eigenmodes, labeled by the ``quantum numbers'' ($n$,$l$) and calculated following Lamb's 19th century approach, were taken to identify the nature of the measured signals, however leading to some ambiguities. Herein, we present the first full theoretical representation of BLS eigenmode spectra, allowing an unprecedentedly precise access to the individual colloid's thermomechanical properties.\footnote{Still et al., \textit{J. Phys. Chem. Lett.} \textbf{1}, 2440, 2010.} A longstanding discussion is resolved, showing that both even and odd $l$ spheroidal modes are active. The theoretically predicted scattering angle dependence of the BLS intensity is verified. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A40.00010: Physical properties of two-dimensional directed polymer systems obtained via bosonization and related techniques David Zeb Rocklin, Paul M. Goldbart Classical directed polymers in 2 dimensions are well known to be equivalent to quantum particles in 1+1 dimensions, with polymer configurations corresponding to particle worldlines. This equivalence motivates the use of techniques designed for one-dimensional quantum systems for exploring many-polymer systems, as first exploited by de Gennes [1]. We discuss how thermodynamic quantities and certain correlation functions of a particular model polymer system can be calculated exactly from the Bethe ansatz solution for the Lieb-Liniger model of bosons with repulsive local interactions. We also discuss how the universal properties of more general polymer systems can be captured via Haldane's harmonic-fluid approach. Via this approach, we address various properties of strongly interacting many-polymer systems, focusing on aspects that display qualitative differences from those displayed by single polymers. \\[4pt] [1] P.-G. de Gennes, J. Chem Phys. 48, 2257-2259 (1968) [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A40.00011: Adapted Su-Schrieffer-Heeger Hamiltonian for PPV, PPP, and polyacenes Yongwoo Shin, Xi Lin This work presents a unified model Hamiltonian for poly-$\it{p}$-phenylenevinylene (PPV), poly-$\it{p}$-phenylene (PPP), and polyacenes based on the classical Su- Schrieffer-Heeger Hamiltonian for polyacetylene, with one single extra electron- phonon coupling parameter. Predicted band gaps of all these polymers and their oligomers of all lengths closely match to the available experimental results, with an accuracy exceeding the time-dependent density functional theory. Self-localized polaron states and their mobility are computed without any constraints. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A40.00012: Rigid body constraints in HOOMD-Blue, a general purpose molecular dynamics code on graphics processing units Trung D. Nguyen, Carolyn L. Phillips, Joshua A. Anderson, Sharon C. Glotzer Rigid body constraints are commonly used in a wide range of molecular modeling applications from the atomistic scale, modeling the bonds in molecules such as water, carbon dioxide, and benzene, to the colloidal scale, modeling macroscopic rods, plates and patchy nanoparticles. While the parallel implementations of rigid constraints for molecular dynamics simulations for distributed memory clusters have poor performance scaling, on shared memory systems, such as multi-core CPUs and many-core graphics processing units (GPUs), rigid body constraints can be parallelized so that significantly better performance is possible. We have designed a massively parallel rigid body constraint algorithm and implemented it in HOOMD-Blue, a GPU-accelerated, open-source, general purpose molecular dynamics simulation package. For typical simulations, the GPU implementation running on a single NVIDIA$^{\textregistered}$ GTX 480 card is twice as fast as LAMMPS running on 32 CPU cores. In the HOOMD-blue code package, rigid constraints can be used seamlessly with non-rigid parts of the system and with different integration methods, including NVE, NVT, NPT, and Brownian Dynamics. We have also incorporated the FIRE energy minimization algorithm, reformulated to be applicable to mixed systems of rigid bodies and non-rigid particles. [Preview Abstract] |
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