Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W31: Focus Session: Understanding Fluctuation and Correlation Effects in Polymers |
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Sponsoring Units: DPOLY Chair: Amalie Frischknecht, Sandia National Laboratories Room: 339 |
Thursday, March 21, 2013 2:30PM - 3:06PM |
W31.00001: Recent Developments in Field-Theoretic Polymer Simulations Invited Speaker: Glenn Fredrickson This presentation will address recent progress in methods and algorithms for conducting simulations of statistical field theory models of polymers and complex fluids beyond the mean-field approximation (as invoked, e.g., in self-consistent field theory). Topics to be discussed include regularization methods, improved stochastic integration algorithms for complex Langevin equations, techniques for locating phase boundaries, and systematic coarse-graining/renormalization techniques for multi-scale simulations. Early results on a promising ``coherent state'' formulation of polymer field theory will also be presented. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W31.00002: Understanding Fluctuation/Correlation Effects on the Order-Disorder Transition of Symmetric Diblock Copolymers with a Density-Functional Theory Jing Zong, Qiang Wang How fluctuations change the order-disorder transition (ODT) of symmetric diblock copolymers (DBC) is a classic yet unsolved problem in polymer physics.\footnote{\textit{L. Leibler}, \textbf{Macromolecules, 13}, 1602 (1980); \textit{G. H. Fredrickson and E. Helfand}, \textbf{J. Chem. Phys., 87}, 697 (1987). } Taking a model system of discrete Gaussian chains interacting with soft, finite-range repulsions as commonly used in dissipative-particle dynamics simulations we formulate a density-functional theory (DFT) based on the polymer integral equation theories,\footnote{\textit{D. Chandler and H. C. Andersen,}~\textbf{J. Chem. Phys.},~\textbf{57}, 1930 (1972);~\textit{K. S. Schweizer and J. G. Curro},~\textbf{Phys. Rev. Lett.},~\textbf{58},~246, (1987).} which includes the system fluctuations and correlations neglected by the mean-field theory (i.e., the widely applied self-consistent field theory) and can be reduced to the latter under the mean-spherical approximation. We then unambiguously reveal the fluctuation/correlation effects on the ODT of symmetric DBC by direct comparisons among the mean-field theory, DFT, and fast off-lattice Monte Carlo simulations,\footnote{\textit{Q. Wang and Y. Yin}, \textbf{J. Chem. Phys., 130}, 104903 (2009).} all using exactly the same model system (Hamiltonian) and thus without any parameter-fitting. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W31.00003: Computational Investigation of Block Copolymer Surfactants for Stabilizing Fluctuation-Induced Polymeric Microemulsions Kris Delaney, Glenn Fredrickson High molecular weight diblock copolymers introduced into a blend of immiscible homopolymers can act as a surfactant to suppress macroscopic two-fluid phase separation. With variation of block copolymer composition, the crossover between low-temperature ordering into microphase or macrophase separated states is marked by a mean-field isotropic Lifshitz multi-critical point. Strong fluctuations close to the Lifshitz point are observed[1,2] to suppress the low-temperature ordering; a microemulsion state emerges, with large, co-continuous domains of segregated fluid lacking any long-range order. We study this phenomenon with fully fluctuating field-theoretic simulations based on complex Langevin sampling, and we attempt to design new block polymer surfactants that can produce the microemulsion state with a wider composition tolerance. [1] Bates et al., PRL 79, 849 (1997) [2] Hillmyer et al., J Phys Chem B 103, 4814 (1999) [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W31.00004: Condensation of semiflexible polyelectrolytes in mixed solutions of mono- and multivalent salts Amelia A. Plunk, Erik Luijten The salt-dependent condensation of highly charged polyelectrolytes in aqueous solution is a topic of great biological and industrial importance that has been widely studied over the past decades. It is well established that interaction with multivalent counterions leads to the formation of bundle-like aggregates for rigid polyelectrolytes and to collapsed structures or disordered aggregates for flexible polyelectrolytes. Here, we investigate the behavior of semiflexible chain molecules, where the electrostatically induced aggregation is impeded by the intrinsic bending stiffness of the polymer. Moreover, we study the competition between monovalent and multivalent counterions in mixed solutions and establish the threshold salt concentration required for condensation. Our findings are relevant for a range of biomedical problems, including the fabrication of nanoparticles for gene delivery [1] and the packaging of DNA by histones. \\[4pt] [1] X. Jiang \emph{et al.}, Adv. Mater., DOI: 10.1002/adma.201202932. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W31.00005: Rattle, restrict, and release in entangled polymer solutions Subhalakshmi Kumar, Tsang Chi Hang Boyce, Sung Chul Bae, Steve Granick The nature of entanglement release and chain fluctuation is studied in entangled solutions of high molecular weight PEG in water. Reporter fluorescent polystyrene particles of radius comparable to the entanglement length are suspended in solution and tracked individually with with nm resolution using epifluorescence microscopy. Thousands of single particle trajectories are analyzed to quantify caging and hopping dynamics. The cage relaxation time changes by orders of magnitude depending on the polymer concentration, but is faster than and therefore more accessible within experimentally accessible time scales, than for colloidal glasses. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W31.00006: Direct imaging of fluctuations in a cross-linked biopolymer network Bo Wang, Lingxiang Jiang, Boyce Tsang, Steve Granick Cross-linked networks are ubiquitous in synthetic and biological polymer systems, such as rubbers and cytoskeletons. To model cross-linked networks, several theories have been developed on the basis of different assumptions as to fluctuations in the networks. Here we put these theories to direct test. This talk will describe direct single-molecule imaging of the dynamic fluctuations of junction points in a cross-linked semiflexible polymer (F-actin) network. The actin filaments are cross linked by biotin/avidin. The junction points are selectively labeled to allow nm spatial imaging resolution. The surprising results point to limitations of the prevailing network models. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W31.00007: Effect of Fluctuation on Order-Disorder Transition in Polydisperse Block Copolymer Melts Gunja Pandav, Venkat Ganesan We examine fluctuation effects on order-disorder transition (ODT) temperature in polydisperse block copolymer melts using single chain in mean field simulations. Diblock copolymer melts having monodisperse A blocks and polydisperse B blocks with symmetric composition on an average are examined. Increase in polydispersity at constant composition resulted in change in equilibrium morphology in accordance with the mean-field theory prediction. The dependence of ODT temperature on the strength of fluctuations as characterized by Ginzburg parameter is examined and scaling prediction for fluctuation induced shift in ODT is reported. Also, the qualitative shift in ODT as a function of increasing polydispersity in asymmetric copolymers is investigated. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W31.00008: Directed polymer liquids addressed via the two-dimensional one-component plasma: Developing the framework Anton Souslov, D. Zeb Rocklin, Paul M. Goldbart The distribution of \emph{small} density fluctuations in a directed polymer liquid is characterized by the equilibrium structure factor. By contrast, the distribution of \emph{large} density fluctuations embodies new information about the polymer state. Physically, large density fluctuations are closely related to particle inclusions, i.e., compact regions from which polymers are excluded. The highly correlated nature of directed polymer liquids complicates a single-chain approach to such issues and, instead, we invoke a quantum many-body technique to map the three-dimensional polymer system to a two-dimensional hard-core Bose fluid. Then, by using Chern-Simons field theory, we make the standard transformation of this Bose fluid into a system of non-interacting fermions that fill a single Landau level. The density distribution of these fermions is that of a classical two-dimensional one-component plasma (2DOCP), whose properties are well understood; we invoke them to obtain the entropy cost of particle inclusions in the polymer liquid. Along the way, we examine the validity of the various approximations that have been made. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W31.00009: Directed polymer liquids addressed via the two-dimensional one-component plasma: Implications for the density profile D. Zeb Rocklin, Anton Souslov, Paul Goldbart We consider the inclusion of one or more particles into a dense, three-dimensional liquid of long, directed polymers. The particles represent an excluded volume within the liquid which raises its free energy. As discussed in the accompanying talk, the statistical mechanics of such a polymer liquid can be described in terms of certain two-dimensional fluids of quantum particles and, hence, via an exactly solvable classical two-dimensional one-component plasma (2DOCP). The free energy cost of a particle inclusion is related to the probability of spontaneous formation of a large void within the quantum fluid or the plasma. We use these relationships to study the effect of particle inclusions in the polymer liquid, as well as large fluctuations of the liquid. We find that displaced polymers accumulate near the edge of the inclusion, in a manner similar to the accumulation of excess charge near the surface of a conductor. In addition, we are able to determine the equilibrium density profile for polymer liquids subject to more general constraints, e.g., ones that force some fixed number of polymers to pass through a ring. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W31.00010: Disentangle Model Differences and Fluctuation Effects in DPD Simulations of Diblock Copolymers David (Qiang) Wang, Paramvir Sandhu, Jing Jong, Delian Yang In the widely used dissipative particle dynamics (DPD) simulations [Hoogerbrugge and Koelman, \textbf{Europhys. Lett. 19}, 155 (1992); Groot and Warren, \textbf{J. Chem. Phys. 107}, 4423 (1997)], polymers are commonly modeled as discrete Gaussian chains interacting with soft, finite-range repulsions. In the original DPD simulations of microphase separation of diblock copolymer melts by Groot and Madden [\textbf{J. Chem. Phys. 108}, 8713 (1998)], the simulation results were compared and found to be consistent with the phase diagram for the ``standard model'' of continuous Gaussian chains with Dirac ?-function interactions obtained from self-consistent field (SCF) calculations. Since SCF theory is a mean-field theory neglecting system fluctuations/correlations while DPD simulations fully incorporate such effects, the model differences are mixed with the fluctuation/correlation effects in their comparison. Here we report the SCF phase diagram for exactly the same model system as used in DPD simulations. Comparing our phase diagram with that for the standard model highlights the effects of chain discretization and finite-range interactions, while comparing our phase diagram with DPD simulation results reveal without any parameter-fitting the effects of fluctuations/correlations neglected in the SCF theory. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W31.00011: Static Correlation Functions of Polymer Concentration Fluctuations in the Presence of an Interface Catherine Yeh, Philip Pincus We study static correlation functions of polymer solutions using the Cahn-de Gennes square gradient theory of interfacial energies. Fluctuations are considered for good, theta, and poor solvents at repulsive and adsorbing surfaces as well as at the interface of phase separated solutions. We predict the existence of bound state fluctuations associated with an interface under certain conditions. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W31.00012: Dynamical simulation of disordered micelles in a diblock copolymer melt with fluctuations Russell Spencer, Robert Wickham By including composition fluctuations in our dynamical simulation of the time-dependent Landau-Brazovskii model for a diblock copolymer melt, we find that disordered micelles form above the order-disorder transition to a BCC phase. At high-temperatures, the micelle number density is effectively zero, and the melt is disordered at the molecular level. As we lower the temperature, the micelle number density increases gradually and approaches the number density in the BCC phase. If we increase the strength of the fluctuations, the temperature range over which disordered micelles exist broadens, and the onset of BCC order is suppressed. We examine the dynamics of crystallization of disordered micelles into the BCC phase. By tracking trajectories, we also investigate the dynamical behaviour of individual micelles in an environment of disordered micelles. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W31.00013: X-ray imaging of wetting ridge on a soft solid Su Ji Park, Byung Mook Weon, Ji San Lee, Jung Ho Je, Robert W. Style, Guy K. German, Eric R. Dufresne, Steve Wang Softness of solids affects a microscopic deformation, called a `wetting ridge', at a three-phase contact line. We present a direct visualization of wetting ridges by high-resolution x-ray imaging, which shows a spatial transition between elastic and fluidic wetting behaviors on a soft solid. The fluidic behavior that corresponds to Neumann's triangle occurs at the vicinity of the triple point while the elastic deformation at \textbar x\textbar \textless le (the elasto-capillary length). Real-time x-ray imaging clearly shows temporal variation of wetting ridge. [Preview Abstract] |
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