Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H25: Focus Session: Particle Dynamics and Organization |
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Sponsoring Units: DPOLY Chair: Lynn Walker, Carnegie Mellon University Room: Baltimore Convention Center 322 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H25.00001: Noncontinuum Effects in Dynamics of Nanoparticles in Polymer Matrices Ganesan Venkatraghavan, Victor Pryamitsyn The dynamics and rheology of particulate suspensions have long been well-understood in a continuum fluid-mechanical framework dating back to Stokes and Einstein. These theories predict that the mobility of the particles decreases with an increase in the viscosity of the fluid and that the addition of particles increases the overall viscosity of the suspension. However, in many recent applications such as polymer-nanoparticle composites, the particle size is comparable to or smaller than the structural scale of the solvent and the dynamics exhibits many ``non-continuum'' effects, such as the mobility becoming independent of the molecular weight (and viscosity) of the polymer, a decrease in the suspension viscosity due to the addition of nanoparticles etc. Here we propose a new theoretical formulation for the dynamics of particulate suspensions, which within a single framework encompasses for the first time both the ``macroparticle'' limit of particles larger than the structural scale of solvent and the ``nanoparticle'' limit where the particles are smaller than such scales. We present theoretical predictions for the case of polymer-nanoparticle suspensions which quantitatively explain the preceding observations and also delineates the size scales at which the particles cross-over from behaving dynamically as a ``particle suspension in polymers'' to a ``solvent for the polymers.'' [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H25.00002: Mechanisms of Steady State Rheological Behavior of Polymer Nanoparticle Composites Victor Pryamitsyn, Venkat Ganesan We use a mesoscale computer simulation framework to delineate the the mechanisms behind the linear and nonlinear dynamical properties of polymer-nanoparticle composites. Our studies specifically focus on the regimes where the particle sizes and the interparticle distances become comparable to the polymer sizes. Our results indicate that the zero shear viscosities of the composite can be described in a manner similar to colloidal suspensions in a simple fluid when both the particle-induced changes in the polymer relaxation spectra and the polymer slip effects are accounted. The nonlinear shear rheology and the first normal stresses of the composite exhibits a subtle interplay between the shear thinning arising from the polymer and particle stresses. Overall, at dilute and semidilute nanoparticle concentrations, the composite shear rheology is dominated by the shear thinning of the polymer chains which in turn is modified by the presence of the particles. For higher particle loads, especially in regimes where particle jamming manifests, the polymeric contribution to the rheology becomes much less important and the shear thinning is dominated by the particles stresses. These competing efffects lead to intriguing polymer molecular weight, particle loading dependencies for the steady shear rheological behavior. Our results and mechanistic explanations are in excellent agreement with associated experimental observations. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H25.00003: Surface Modification of Plate-Like Nanoparticles and Their Assembly into Nematic Organogels Bani Cipriano, Srinivasa Raghavan Plate-like clay nanoparticles (e.g., laponite) form gels in water at sufficiently high concentration. A remarkable feature of these gels is the appearance of birefringent textures characteristic of nematic liquid crystals. Here we report the counterpart of this phenomenon in organic solvents, i.e., the formation of nematic textures by adding organically modified clay nanoparticles into non-polar liquids such as toluene and chloroform. We present the phase diagram (isotropic/nematic phases) for these gels. The viscoleastic properties of the resulting gels are characterized by use of rheological methods. The quality of the dispersions and the birefringent textures are evaluated using optical microscopy and x-ray scattering techniques. The finding that organoclays self assemble into a nematic phase in non-polar mediums may well provide a route for in-situ formation of nematic polymer nanocomposites. We also describe preliminary efforts towards achieving nematic ordering of particles in polymers. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H25.00004: Gelation and structural characteristics of nanoparticles in solutions of adsorbing polymers Megha Surve, Victor Pryamitsyn, Venkat Ganesan In the present talk, we examine the gelation, clustering behavior and structural characteristics of nanoparticles in presence of adsorbing polymers. We implement a polymer self consistent field theory to obtain the structural conformations of the polymer chains between two spherical particles. By solving the self-consistent field theory equations numerically in bispherical coordinates we account for size disparities between the particles and polymers. We present the effect of different molecular parameters such as adsorption strength, particle size, polymer concentration on the size, number and probability distribution of polymer bridges and anchors. The structure of polymer-nanoparticle mixtures is examined using Monte Carlo simulations. By incorporating the microscopic bridging statistics into the simulation framework, we predict the structure and clustering statistics of polymer-nanoparticle gels formed in mixtures of nanoparticles and adsorbing polymers. We further extend our formalism to predict the elastic and mechanical properties of the polymer-nanoparticle mixtures. Our analysis provides a molecular insight into the nanoscale phenomena such as prevalent gelation and significant lowering of percolation thresholds observed in nanoparticle regime and quantifies the role played by various nanoscale parameters such as particle curvature and strength of polymer-particle interactions in determining the macroscopic properties of polymer nanocomposites. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H25.00005: Shear induced alignment in thin bilayer films: a simulation study Joerg Rottler, David J. Srolovitz, Paul Chaikin Motivated by recent experimental progress, we study ordering mechanisms in confined bilayer systems under shear via Brownian dynamics simulations. In sphere-forming thin block copolymer films, spherical nanodomains arrange in grains of local hexagonal symmetry, but random orientation. If the film has precisely the bilayer thickness, the application of simple shear leads to global, macroscopic alignment in a triangular lattice (D. Angelescu et al., Adv. Mat. 17, 1878 (2005)). By representing each nanodomain a as purely repulsive sphere on a coarse-grained level (therefore making the model also applicable to colloidal assemblies), our simulations reveal that the long range order can develop through the shear induced motion of grain boundaries rather than global melting and recrystallization. The time dependence of this process may be described by Avrami growth laws. In order to gain further insight into the microscopic mechanism of shear induced ordering, we study the motion of single domain walls in simplified geometries and determine the dependence of the grain boundary migration velocity on geometry, shearing conditions and the resulting local stress gradients. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H25.00006: Field-theoretic nanocomposite simulations: preliminary results of hybrid particle/SCFT simulations Scott Sides, Glenn Fredrickson Using block copolymers as mesoscale templates for organic/inorganic nanoparticles has the potential to create patterned particle aggregates that could be used as magnetic storage media and semiconductor materials. More generally, polymer nanocomposites such as these are being investigated for the potential to develop materials with specifically tailored optical, electrochemical, thermal and mechanical properties. The overall behavior of the nanocomposite system depends on the morphology of the polymer chains as well as the arrangement of the particles in the polymer matrix. Self-consistent field theory (SCFT) for dense polymer melts has been highly successful in describing complex morphologies in block copolymers. Field-theoretic simulations such as these are able to access large length and time scales that are difficult or impossible for particle-based simulations such as molecular dynamics, while still incorporating more realistic polymer models than many macroscopic, continuum simulations. In this talk I will outline the SCFT method, discuss some efficient methods of numerically solving the SCFT equations and present results for spherical nanoparticles embedded in an AB diblock copolymer melt. These results have been obtained using a hybrid particle/SCFT approach that treats the polymer in a field theory framework while explicitly retaining the individual nanoparticle coordinates as degrees of freedom. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H25.00007: Distributed Polymeric Stickers and their Role in Colloidal Interactions and Selective Adhesion Dynamics. Maria Santore, Natalia Kozlova, Bing Mei Polyelectrolytes are frequently added to colloidal dispersions as flocculants. When they are high in molecular weight and low in backbone charge, as they adsorb to particles they extend (at least temporarily) into solution from the surface of a first particle to trap a second. This is the basis for bridging flocculation, which is often reversible to the extent that backbone relaxation ultimately causes the chain to release the second particle and more completely coat and stabilize the first. By contrast, addition of small amounts of densely-charged low molecular weight chains produces irreversible flocculation via a patchwise mechanism: A localized region on a first particle is charge-overcompensated by an adsorbing chain, and adheres to a bare spot on an approaching particle. We show here, with a model system, how the dynamics of patchwise flocculation depend on fluctuations in the distribution of adsorbing chains, and gives rise to a curvature-based selectivity for particle destabilization. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H25.00008: Barrier Hopping, Viscous Flow and Kinetic Gelation in Nanoparticle-Polymer Suspensions Kenneth Schweizer, Yeng-Long Chen, Vladimir Kobelev Ideal mode coupling theory is combined with the polymer reference interaction site model theory of structural correlations to predict depletion-induced physical gelation and elasticity of suspensions of nonadsorbing polymers and hard particles. The approach has been extended to treat the alpha relaxation process in the ultra-slow activated barrier hopping regime. The dynamic barrier is a rich function of polymer concentration, polymer-particle size asymmetry ratio and particle volume fraction. All these dependences can be collapsed on to a single master curve that is an effective power law function of a composite variable. Over a range of material parameters commonly studied experimentally the alpha relaxation time is an exponential function of colloid volume fraction and reduced polymer concentration. The theory has been applied to compute kinetic gel boundaries and the shear viscosity, and also to address the phenomenon of delayed sedimentation. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H25.00009: Soft X-ray Resonant Scattering of Structured Polymer Nanoparticles Harald Ade, T. Araki, G. Mitchell, J. Stubbs, D. Sundberg, J. Kortright, A.L.D. Kilcoyne We present the development and application of resonant soft x-ray scattering (RXS) to chemically heterogeneous, i.e. structured, soft condensed matter materials. Polymer composite latex particles $\sim $200 nm in diameter were utilized to determine the potential utility of this technique. Two styrene-acrylic polymer latex systems were prepared on silicon nitride membranes. Scattering measurements were performed in transmission. Angular scans at photon energies corresponding to strong scattering contrast between specific chemical moieties made it possible to infer the relative effective radii corresponding to the two polymer phases in the structured latex particles. The results show that resonant soft x-ray scattering should be a powerful complementary tool to neutron- and hard x-ray scattering and NEXAFS spectromicroscopy for the characterization of structured soft condensed matter nanomaterials. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H25.00010: Studies of the formation of microporous polymer films in `breath figure' condensation processes Mohan Srinivasarao, Jung Park, Matthew Barrow, Richard Jones, Chris Wright, P. Rhodri Williams We report studies of the formation of ordered microporous polymer films by the evaporation of polymer solutions following exposure to a humid atmosphere. High speed microphotographic (HSMP) studies of the formation process reveal that near the surface of the polymer solution, vapor condensation produces near monodisperse water droplets which form a close-packed monolayer (or `breath figure'). Following the evaporation of the solvent, characterization of the solid by Atomic Force Microscopy confocal microscopy and white light interferometry reveals that the surface of the polymer film features extensive regions of hexagonally close-packed microscopic pores, whose spatial arrangement replicates that of the initial droplet monolayer. Defects recorded by HSMP in the packing of the colloidal monolayer of liquid droplets formed above the surface of the polymer solution are found to correspond to those transferred into the eventual solid film, providing the first direct evidence of the structure templating role of the droplet monolayer. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H25.00011: Filling Small Pores With Polymer Melts Priyanka Dobriyal, Mingfu Zhang , Jiun Tai Chen, Thomas Russell Wetting of nanoporous inorganic templates by polymer melts is a well known and convenient way to generate polymeric nanostructures. In this work, we carried out a systematic study of wetting of nanoporous aluminum oxide membranes by polystyrene melts and obtained very different polymeric nanostructures (rods or tubes) under different conditions. When the annealing of polystyrene was done at temperature slightly above its glass transition temperature (T$_{g})$, nanorods were generated. When the annealing temperature was well above T$_{g}$, the resultant nanostructure depended on polymer molecular weight: for low molecular weight polystyrenes, nanotubes were obtained; for polystyrenes with very high molecular weights, however, nanorods formed. Thus, the viscosity of polymer melt controlled the nanostructure formation. When the viscosity of polymer melt was high, the capillary force was the dominant driving force which gave rise to the formation of nanorods; but when the melt viscosity was low, nanotubes formed and the wetting of the nanopores was achieved through the formation of precursor film. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H25.00012: Shear Rate Dependent Structure of Polymer Stabilized TiO$_{2 }$ Dispersions - 1. TiO$_{2}$ Structure Alan Nakatani, Antony VanDyk, Lionel Porcar, John Barker We measured the shear rate dependent structure of TiO$_{2}$ dispersions (37{\%} volume fraction) stabilized with commercial polymer dispersants in water. The TiO$_{2}$ is a commercial grade TiO$_{2}$ with a particle diameter of approximately 400 nm (when the dispersions were diluted and measured by Nanotrac 150). The solvent compositions were adjusted to the contrast match point of each dispersant to isolate the behavior of the TiO$_{2}$. Two different molecular weight poly acid dispersants (Referred to as ``low MW acrylate polymer salt'' and ``high MW acrylate polymer salt'') and two different molecular weight alkali soluble acrylate copolymers (Referred to as ``low MW hydrophobic acrylate copolymer salt'' and ``high MW hydrophobic acrylate copolymer salt'') were used in this study. The experiments were conducted at the NIST Center for Neutron Research (NCNR) on the perfect crystal diffractometer (PCD) for ultra-high resolution small-angle neutron scattering (USANS) measurements. The results for the TiO$_{2}$ scattering show that shear induced agglomeration occurs for certain dispersants and shear induced breakup of TiO$_{2}$ aggregates is observed for other dispersants. The results suggest new ways to consider dispersion of particles. [Preview Abstract] |
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