Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A24: Focus Session: Particle Dynamics & Organization: Polymer Mediated, Polymer Particles & Anisotropic Particles |
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Sponsoring Units: DPOLY Chair: Richard Register, Princeton University Room: Colorado Convention Center 201 |
Monday, March 5, 2007 8:00AM - 8:36AM |
A24.00001: Self-Assembly of Amphiphilic Colloids Invited Speaker: A rich physics appears when spherical particles in aqueous suspension possess patches of different surface chemical composition. We have explored the assembly of two types of micron-sized spherical particles: those with opposite electric charge on both hemispheres (``bipolar'') and those hydrophobic on one hemisphere and hydrophilic on the other (``amphiphilic''). Bipolar particles form clusters, not strings, because the particle diameter exceeds the electrostatic screening length. The cluster shapes are analyzed by combined epifluorescence microscopy and Monte Carlo computer simulations with excellent agreement, indicating that the particles assemble in aqueous suspension to form equilibrated aggregates. Translational and rotational diffusion are resolved at the single-particle level, with surprising conclusions. Work performed with Erik Luijten, Liang Hong, Angelo Cacciuto, Shan Jiang, Stephen Anthony, Minsu Kim, and Sung Chul Bae. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A24.00002: Dynamics of polymer microgel nanoparticles and polymer chains. Kiril Streletzky, John McKenna, Gerald Hillier Microgel nanoparticles were synthesized in aqueous solutions of neutral polymer hydroxypropylcellulose (HPC) through self-association of amphiphilic HPC molecules and subsequent cross linking at room temperature. We present a Dynamic Light Scattering study of transport properties of HPC polymer chains and HPC microgels made out of the same starting polymer solution. The spectra of both systems are highly non-exponential requiring a spectral time moment analysis. Our findings indicate the existence of at least two modes of relaxation in both systems. The comparison of the mean relaxation rates and diffusion coefficients of the different modes in two systems under good solvent conditions will be reported. Temperature induced volume phase transition of the polymer nanoparticles and its sensitivity to salt, polymer, and cross-linker concentration will be reported. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A24.00003: Icosahedral packing of polymer-tethered nanospheres and stabilization of the gyroid phase C.R. Iacovella, A.S. Keys, M.A. Horsch, S.C. Glotzer We present results of molecular simulations that predict the phases formed by the self-assembly of model nanospheres functionalized with a single polymer ``tether,'' including double gyroid, perforated lamella and crystalline bilayer phases. We show that microphase separation of the immiscible tethers and nanospheres causes confinement of the nanoparticles, which promotes local icosahedral packing that stabilizes the gyroid and perforated lamella phases. We present a new metric for determining the local arrangement of particles based on spherical harmonic ``fingerprints,'' which we use to quantify the extent of icosahedral ordering. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A24.00004: Bicontinuous Morphologies in Block Copolymer-Nanoparticle Composites Victor Pryamitsyn, Venkat Ganesan We present strong segregation approximation based analytical calculations and complementary computer simulation results on the ordering and structural characteristics of block copolymer-nanoparticle mixtures. We consider specifically the case of a symmetric block copolymer organized in a lamella phase, which is mixed with both selective and nonselective nanoparticles. We present results within the strong segregation approximation quantifying the density distribution of nanoparticles and the influence of the nanoparticles upon the lamella thickness and their elastic constants. The case of nonselective nanoparticles is treated in detail to account more accurately for both size effects as well as finite concentrations of nanoparticles. The latter results suggest the possibility of layer instabilities and morphological transitions to bicontinuous phases, resulting from the surfactant-like role of nonselective particles. Qualitative features of our model predictions are in agreement with our computer simulation results and recent experimental results. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A24.00005: Control of Nanoparticle Distribution with Directed Assembly of Block Copolymer Films Huiman Kang, Francois Detcheverry, Andrew Mangham, Mark Stoykovich, Robert Hamers, Juan de Pablo, Paul Nealey Model CdSe nanoparticles, functionalized with tetradecyl phosphonic acid, were synthesized so as to preferentially segregate into the polystyrene domains of polystyrene-\textit{block}-poly(methylmethacrylate) (PS-$b$-PMMA). Nanocomposites, composed with ternary blends (PS-$b$-PMMA/PS/PMMA) and CdSe, could be directed to assemble into defect-free and registered periodic and non-regular structures on chemically patterned substrates. CdSe nanoparticle arrays, replicating the block copolymer patterns, were obtained by removing the polymer using oxygen plasma. The location and distribution of nanoparticles in the PS domains controls depending on the blend composition, molecular weights of homopolymers and the commensurability between the chemical surface pattern and the bulk lamellar period of the composite and the experimental results are compared to prediction of single chain mean field (SCMF) theory extended to model nanocomposites. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A24.00006: Controlling Nanoparticle Location and Morphology in Polymer Blend and Copolymer Films. Russell Composto, Aysenur Corlu, Ranjan Deshmukh, Hyun-joong Chung, Kohji Ohno Polymer blends and block copolymers containing nanoparticles (NP) have potential as optoelectronic devices, chemical sensors and nanoreactors. Because structure governs device performance, self-regulating, stable structures ranging from the micro to nanoscale are highly desirable. This presentation shows how silica nanoparticles (NP) modified with polymer brushes partition in polymer blends. If NP partition into only one phase, phase separation slows down, whereas NP that jam the interface produce a bi-continuous metastable structure. As film thickness increases, jamming occurs at lower NP concentration. In symmetric block copolymer films, the addition of NP also slows down phase evolution and can even produce a metastable perpendicular morphology containing surface segregated NP. However, the mechanism differs from the polymer blend case and the resulting length scale is in the nanometer range. These studies demonstrate the interplay between NP distribution and phase morphology in both blends and copolymer systems that span the micro to nano length scales. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A24.00007: Molecular Theory Studies of Polymer/Nanoparticle Blends Near Surfaces Erin McGarrity, Amalie Frischknecht, Michael Mackay Recent experimental results have shown that nanoparticles added to supported thin polymer films can inhibit dewetting by migrating to the substrate. To better understand this phenomenon, we use a classical density functional theory developed by Tripathi and Chapman. The effects of nanoparticle radius and density are examined. Preliminary results for hard-particle hard-chain systems indicate that regular layered structures emerge when a critical density is reached and the particles displace the polymers near the substrate. The effects of particle and polymer attractions and substrate potentials are currently being studied. We also compare our results to molecular simulations. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A24.00008: Particle Dynamics in Polymer/Metal Nanocomposite Thin Films on Nanometer Length Scales Suresh Narayanan, Dong Ryeol Lee, Aleta Hagman, Xuefa Li, Sunil Sinha, Jin Wang X-ray photon correlation spectroscopy was used in conjunction with resonance-enhanced grazing-incidence small-angle x-ray scattering to probe the particle dynamics and kinetics in gold/polystyrene nanocomposite thin films. Such enhanced coherent scattering enables, for the first time, to measure the particle dynamics at wavevectors up to 1 \textit{nm}$^{-1}$ (or a few nm spatially), well in the regime where entanglement, confinement and particle interaction dominate the dynamics and kinetics. The dynamics at such length scales has been difficult, if not impossible to study, by any other probes. Measurements of the intermediate structure factor f(q,t) indicate a mechanism of particle motion very different from Brownian diffusion (governed by Stokes-Einstein equation). The measured dynamics is explained in terms of inter-particle and hydrodynamic interactions. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A24.00009: Investigation of Gold Nanoparticle Diffusion in Polymer Thin Films using X-ray Standing Waves Martin Tolkiehn, Ward Lope, Xuefa Li, Suresh Narayanan, Aleta Hagman, Heinrich Jaeger, Jin Wang Nanoparticle marker motion can be used to infer the ordering kinetics and nanoparticle dynamics in polymer/metal nanocomposite thin films. In current experiments, by using x-ray standing waves generated by total external reflection from the substrates, we elucidated the diffusion properties of thermally evaporated gold nanoparticles at homopolymer and diblock copolymer thin films. Different to recent results with gold particle monolayer embedded in a sandwich structure of polymer thin films, with a sub-nm spatial resolution we demonstrate that the monolayer at the surface does not diffuse into the polymer thin films even at a temperature well above the polymer glass transition temperature. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A24.00010: The Effect of Nanoparticle Shape on Polymer-Nanocomposite Rheology and Tensile Strength Scott T. Knauert, Jack F. Douglas, Francis W. Starr We investigate how nanoparticle shape influences the melt shear viscosity $\eta$ and the tensile strength $\tau$, with a focus on fullerene, carbon nanotube, and clay sheet nanocomposites. We simulate model nanoparticle dispersions of icosahedral, tube or rod-like, and sheet-like nanoparticles, all at a volume fraction $\approx 0.05$. Our results indicate an order of magnitude increase in the viscosity $\eta$ relative to the pure melt. This finding can not be explained by continuum hydrodynamics and we provide evidence that the $\eta$ increase has its origin in chain bridging between the nanoparticles. We find that this increase is the largest for the rod-like nanoparticles and least for the sheet-like nanoparticles. Curiously, the enhancements of $\eta$ and $\tau$ exhibit {\it opposite trends} with increasing chain length $N$ and with particle shape anisotropy. Evidently, the concept of bridging chains alone cannot account for the increase in $\tau$ and we suggest that the deformability or flexibility of the sheet nanoparticles contributes to nanocomposite strength and toughness by reducing the relative value of the Poisson ratio of the composite. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A24.00011: Stabilization of nanorods in polymer melts by end-adsorbed chains Amalie L. Frischknecht Adsorbed or grafted polymers are often used to provide steric stabilization of colloidal particles. When the particle size approaches the nanoscale, the curvature of the particles becomes relevant, and rules of thumb based on the behavior of polymers attached to flat surfaces may no longer apply. To investigate this effect for the case of cylindrical symmetry, I use a classical density functional theory applied to a coarse-grained model to study the polymer-mediated interactions between two nanorods. The rods are immersed in a polymer melt consisting of two kinds of chains: 1) a small fraction of chains of length N with ends that are attracted to the rods so that they form a polymer brush on the rods; and 2) a matrix of chains of length P which have no interactions with the rods. Calculations of the density profiles and potential of mean force reveal the effects of curvature compared to similar calculations for chains adsorbed to flat planar surfaces. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A24.00012: Dispersion and Percolation Transitions of Nanorods in Polymer Solutions Megha Surve, Victor Pryamitsyn, Venkat Ganesan We present effective pair-interaction potentials and resulting phase behavior, percolation transitions of nanorods dispersed in solutions of polymers. We use polymer self consistent field theory in conjunction with Derjaguin approximation to compute the polymer mediated orientation-dependent pair interaction potential between cylindrical nanorods. A modified Flory theory and a simple analytical model are used to delineate different equilibrium phases and the onset of percolation for nanorods in polymer solutions. Our results suggest that the topology of the phase diagram of mixture of polymer and rods is highly dependent on the anisotropy of the rods, relative sizes of the rods and polymers, concentration of the polymer and the strength of adsorption. For the case of non-adsorbing polymers, the polymer depletion-induced attractive interactions result in a large two phase region which widens with an increase in the polymer concentration. Addition of adsorbing polymers is observed to lead to a richer phase behavior where at high polymer concentrations, the polymer-induced repulsive interactions result in steric stabilization of the particles and lead to an isotropic-nematic transition which closely resembles the behavior for hard rod suspensions. As a model mimicking nanotube-polymer mixtures, we also discuss the influence of strong rod-rod van der Waals interactions on the stability characteristics. [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A24.00013: Self-assembly of anisotropic nanoparticles at oil/water interfaces Jinbo He, Qingling Zhang, Suresh Gupta, Todd Emrick, Thomas Russell, Zhongwei Niu, Qian Wang Self-assemblies of both bio- and synthetic nanorods with different aspect ratios have been studied at the oil/water interfaces. Tobacco mosaic virus (TMV) shows different geometries at the perfluorodecalin/water interface as the concentration changes in the bulk. With low concentration of TMV in the aqueous phase, TMV prefers lying randomly parallel to the interface to mediate as large interfacial tension per particle as possible. At high concentration, TMV prefers standing up at the interfaces, not only mediating the interfacial tension but also neutralizing the strong electrostatic interaction between each other. The similar phenomenon has also been observed with alkyl-chain-covered Cadmium Selenide nanorods at the toluene/water interface during solvent evaporation. These assemblies can be manipulated by controlling the interfacial tensions between different liquids; the surface properties, the aspect ratio and concentration of nanoparticles; and the ionic strength in solution. [Preview Abstract] |
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