Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F32: Polymer Nanocomposites II |
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Sponsoring Units: DPOLY Chair: Nigel Clarke, University of Sheffield Room: 340 |
Tuesday, March 19, 2013 8:00AM - 8:36AM |
F32.00001: POLYMER PHYSICS PRIZE BREAK
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Tuesday, March 19, 2013 8:36AM - 8:48AM |
F32.00002: Polystyrene Nanocomposites: Shear and Bulk Rheology and PVT Behavior Ran Tao, Sindee Simon One potential strategy to mitigate thermal residual stresses in polymer materials is to reduce the thermal pressure coefficient, a product of the bulk modulus and thermal expansion coefficient. Recent model predictions show that the liquid bulk modulus could decrease by incorporation of well-dispersed spherical nanoparticles into polymer matrix. In this work, the pressure-volume-temperature (PVT) behavior and pressure relaxation response of a 10 wt{\%} silica nanoparticle-filled polystyrene nanocomposite sample are measured using a custom-built pressurizable dilatometer. The glass transition temperature (T$_{g})$ is calculated as a function of pressure from the PVT data, and the PVT data are fitted to the Tait equation. Isothermal pressure relaxation experiments are performed in the vicinity of the pressure-dependent T$_{g}$, from which the time-dependent bulk modulus is calculated. The temperature dependence of the horizontal shift factors is examined and compared to those obtained from the shear response. In addition, the retardation spectra for the bulk and shear responses are compared and the implications are discussed. The results are consistent with literature prediction indicating that bulk modulus will increase in aggregated nanocomposites system. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F32.00003: Simulating Fiber Aggregation in Shear Flow with Dissipative Particle Dynamics Justin Stimatze, David Egolf, Jeffrey Urbach We have developed a mesoscale simulation of fiber aggregation using LAMMPS and its implementation of dissipative particle dynamics. Fiber-fiber interactions are approximated by combinations of standard pairwise forces, allowing exploration of multiple interaction-influenced fiber behaviors such as aggregation and bundling. We determine viscosity, stresses, fluid velocity field, and fiber forces while simulating the evolution of a model fiber system in shear flow. Preliminary simulations supported by AFOSR HPC resources have demonstrated several aggregate types dependent on system parameters. Explorations of fiber interaction mechanisms and parameters may enable greater insight into processes such as nanocomposite material manufacturing and silk fibrillation. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F32.00004: Effects of functional groups and ionization on the structure of alkanethiol coated gold nanoparticles Dan S. Bolintineanu, J. Matthew D. Lane, Gary S. Grest We report fully atomistic molecular dynamics simulations of alkanethiol coated gold nanoparticles solvated in water and decane. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in different neutralization states. We study the effects of charge in the end groups for two different chain lengths (10 and 18 carbons) and different counterions (mono- and divalent). For the longer alkanes we find significant local phase segregation of chains on the nanoparticle surface, which results in highly asymmetric coating structures. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. Based on the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. The asymmetry in the alkanethiol coatings is also likely to have a significant effect on aggregation behavior. More importantly, our simulations suggest the ability to modulate end group charge states (e.g. by changing the pH of the solution) in order to control coating structure, and therefore control solubility and aggregation behavior. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F32.00005: Shear and Extensional Flow-Induced Particle Orientation in Polypropylene/Clay Nanocomposites Wesley Burghardt, Erica McCready Synchrotron-based in situ x-ray scattering is used to monitor the orientation of dispersed particles in molten polypropylene/clay nanocomposite melts during flow. Nanocomposite samples were prepared via twin screw extrusion processing, and the degree of clay exfoliation assessed in terms of the magnitude of the low frequency enhancement in viscoelasticity. In shear flow, an annular cone and plate flow cell is used which allows measurement of the degree and direction of particle orientation in the flow-gradient (1-2) plane. Samples were also studied in extensional flow, using an SER extensional flow fixture installed in a custom-built convection oven that provides x-ray access. In both shear and extensional flow, only a moderate degree of particle orientation is observed. Extensional flow studies are complicated by (i) the tendency of samples to fail at moderate Hency strain, and (ii) a heterogeneous initial distribution of particle orientation in the SER specimens, prepared by compression molding of extruded pellets of the nanocomposite. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F32.00006: Effect of Ionic Groups on the Assembly of Polymer-Grafted Magnetic Nanoparticles Yang Jiao, Pinar Akcora Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains at low grafting density assemble into long strings of nanoparticles. Brush-brush entanglement and the effective dipolar interactions of these elongated clusters drive this aggregation process. In this work, we investigate the influence of ionic attractions on the morphologies of these polymer functionalized nanoparticles at different grafting densities. The effect of sulfonic group locations incorporated into poly(styrene) chains on the aggregation state of nanoparticles will be discussed with small-angle x-ray scattering measurements in solution and melts. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F32.00007: Synthesis and Assembly of Janus Gold Nanorods in Polymer Matrices Robert C. Ferrier, Hyun-Su Lee, Michael J.A. Hore, Matthew Caporizzo, David M. Eckmann, Russell J. Composto Gold nanorods (AuNRs) possess unique optical properties that depend on the local orientation and separation of the individual rods. Previous research focused on assembling AuNRs either end-to-end or side-by-side in solution. Our group has explored the dispersion of polymer grafted AuNRs in polymer matrices. The present work investigates the end-to-end assembly of polymer grafted Janus AuNRs (JNRs) in polymer thin films. JNRs are synthesized by exploiting the anisotropic surface chemistry of CTAB-coated AuNRs. Poly(ethylene oxide) (PEO) brushes are grafted to the side of the AuNR, while leaving the ends unmodified. Using alkane dithiols of different lengths, the JNRs are covalently linked in various solutions and the optical properties are characterized by UV/visible spectroscopy. Linked JNRs are spin-cast in poly(methyl metracrylate) (PMMA) or PEO thin films and characterized via electron microscopy and UV/visible spectroscopy. Using this procedure, linked JNRs can be dispersed in a polymer matrix and linked end-to-end to control the optical properties of coating. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F32.00008: Dispersion of Polymer-Grafted Nanorods in Polymer Films Amalie L. Frischknecht, Michael J. A. Hore, Russell J. Composto Gold nanorods (NRs) exhibit unique optical properties, i.e. their surface plasmon resonances, which can be tuned by the separation between the NRs. One strategy for controlling the assembly of NRs in a polymer film is to coat them with a polymer brush. The resulting dispersion or aggregation of the rods depends on the details of their interactions, which we examine using both theory and experiment. Classical density functional theory (DFT) and self-consistent field theory calculations of the structure of the brush around an isolated NR in a polymer melt predict a gradual transition from a ``wet'' to a ``dry'' brush as the NR radius, the grafting density, and/or the ratio of matrix to brush chain lengths is increased. DFT calculations of the interaction free energy between two NRs find an attractive well at intermediate NR separations. The strength of the attraction increases as the brushes become more dry. Including the van der Waals attractions between the NRs gives an estimate of their total interaction free energy, which can be used to predict when the NRs are dispersed or aggregated. A dispersion map shows good agreement between DFT calculations and experimental observations. Our calculations can be used as a guide to the design rules for tuning NR assembly in polymer films. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F32.00009: Dispersion of small nanoparticles in random copolymer melts Debapriya Banerjee, Kenneth S. Schweizer, Bobby Sumpter, Mark D. Dadmun Microscopic PRISM integral equation theory is applied to study the structure and miscibility of extremely small nanoparticles (e.g., C60 buckyballs) dissolved at low concentrations in a chemically heterogeneous random AB copolymer melt. The effects of polymer stiffness, melt isothermal compressibility, and the strength and spatial range of polymer-particle, polymer-polymer, and filler-filler attractions on the miscibility of the nanoparticles are studied. Complex, subtle and highly nonuniversal behavior is found. Appropriate tuning of the chemical interactions can result in the emergence of an intermediate range of random copolymer compositions where miscibility is maximized and larger than in either homopolymer limit. The physical origin involves a competition between depletion, steric stabilization, and bridging polymer-mediated interactions. When the direct interaction between the small fillers is tuned to model fullerenes, the potentials of mean force exhibit a competition between contact aggregation and bridging, and miscibility is enhanced with decreasing contact aggregation until the onset of bridging. Qualitative comparisons to recent experiments have been performed using attractive interaction strengths motivated by quantum chemical calculations. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F32.00010: Dispersion of Soft Nanoparticles in a Chemically Identical Polymer Matrix Dilru Ratnaweera, D. Baskaran, D. Holley, M. Ruppel, J. Mays, V. Urban, Mark Dadmun The mechanical, thermal and rheological properties of polymers can be improved by embedding organic nanoparticles (NPs). However, controlling the dispersion of NPs is often challenging due to thermodynamic and kinetic incompatibilities between particles and matrices. The current work focuses on the dispersion of chemically identical NPs in a polystyrene matrix. These NPs were made through a micro-emulsion technique using styrene and divinylbenzene (DVB) monomers. Polystyrene nanoparticles with controlled interfacial roughness and targeted styrene densities were achieved by controlling DVB volume fraction during synthesis. The dispersion of these NPs in deuterated polystyrene matrices was followed by Small Angle Neutron Scattering as a function of NP concentration and matrix molecular weight. At low NP concentrations, individual NPs are well distributed in the matrix, while aggregates were formed at higher concentrations in high molecular weight polymer matrices. Shape of the aggregates as well as the minimum concentration of NPs required to form aggregates were affected by the surface roughness and softness of the NPs. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F32.00011: Using Polydispersity in Polymer Grafted Nanoparticles for Tuning Morphology in Polymer Nanocomposites Tyler Martin, Arthi Jayaraman Polymer nanocomposites, consisting of nanoscale additives in a polymer matrix, are used in many applications where high thermal and wear resistance is important e.g. automotive tires. To achieve uniform mechanical and thermal properties of the nanocomposite, the nanoparticles need to be well dispersed in the polymer matrix. One way to control the nanoparticle spatial organization in the polymer matrix is by grafting the nanoparticle surface with polymers that are chemically similar to the matrix polymer and tuning the effective interactions between the particles by simply tuning the grafting density, graft length, matrix length, particle size, filler concentration, and matrix density. In this study, we demonstrate that polydisperse polymer grafts can stabilize dispersions of polymer grafted nanoparticles in a polymer matrix in cases where monodisperse grafts would cause aggregation of particles. The change in the effective inter-particle interactions with increasing polydisersity is because of increased wetting of the grafted polymers by the matrix polymers. The implication that polydispersity can stabilize particle dispersions in matrix shows that it can be used as a design tool to program inter-particle interactions in a polymer matrix. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F32.00012: Synchrotron radiation studies of the evolution dynamics of self-assembled nanoparticle Langmuir films Yeling Dai, Oleg Shpyrko, Binhua Lin, Mati Meron, Kyungil Kim, Brian Leahy Nanoparticle Langmuir films self-assembled on a liquid sub-phase represent a class of systems that is of great interest for studies of phase transitions in quasi-2D systems, chemical self-assembly, surfactant behavior and biologically relevant monolayers and membranes. We utilize Grazing Incidence X-ray Off-Specular (GIXOS) scattering to study elastic properties, structure and surface fluctuating modes of these systems. We present here a comparison between the GIXOS and the X-ray Reflectivity (XR) measurements, where XR is conventionally used to provide structural information of samples along the surface-normal direction. We further present a detailed analysis of GIXOS data from the self-assembled nanoparticle films and describe how we use it to obtain quantitative, Angstrom-resolution details of the electron density profile normal to the surface, complementary to that obtained with XR. Additionally, GIXOS provides us with improved temporal resolution that allows us to directly study the evolution dynamics of self-assembled nanoparticle films in response to lateral compression. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F32.00013: HipGISAXS: A Massively Parallel Code for GISAXS Simulation Slim Chourou, Abhinav Sarje, Xiaoye Li, Elaine Chan, Alexander Hexemer Grazing Incidence Small-Angle Scattering (GISAXS) is a valuable experimental technique in probing nanostructures of relevance to polymer science. New high-performance computing algorithms, codes, and software tools have been implemented to analyze GISAXS images generated at synchrotron light sources. We have developed flexible massively parallel GISAXS simulation software ``HipGISAXS'' based on the Distorted Wave Born Approximation (DWBA). The software computes the diffraction pattern for any given superposition of custom shapes or morphologies in a user-defined region of the reciprocal space for all possible grazing incidence angles and sample rotations. This flexibility allows a straightforward study of a wide variety of possible polymer topologies and assemblies whether embedded in a thin film or a multilayered structure. Hence, this code enables guided investigations of the morphological and dynamical properties of relevance in various applications. The current parallel code is capable of computing GISAXS images for highly complex structures and with high resolutions and attaining speedups of 200x on a single-node GPU compared to the sequential code. Moreover, the multi-GPU (CPU) code achieved additional 900x (4000x) speedup on 930 GPU (6000 CPU) nodes. [Preview Abstract] |
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