Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M32: Focus Session: Polymer Nanocomposites: Dynamics |
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Sponsoring Units: DPOLY Chair: Laura Clarke, North Carolina State University Room: 340 |
Wednesday, March 20, 2013 8:00AM - 8:36AM |
M32.00001: Polymer Dynamics in Nanocomposites and under Confinement Invited Speaker: Dieter Richter In this talk I will present neutron spin echo investigations on polymers interacting attractively with nanoparticles or confining surfaces. Polyethylene-oxide (PEO) was filled with neat SiO$_{2}$ nanoparticles up to 15 vol{\%}. Investigating a short chain matrix we realised that a fraction of chains is adsorbed at the nanoparticle surface suppressing completely its translational diffusion. Nevertheless these adsorbed chains undergo an unchanged segmental dynamics seemingly forming a micelle like corona of chains connected with their OH-end groups. Changing to methylene terminated chains the picture changes drastically now showing a tightly adsorbed layer that however is not glassy as often assumed but undergoes pico second local dynamics. These results are corrobated and extended in studying the dynamics of Polydimethylsiloxane (PDMS) confined in nanoporous Alumina. There a partly anchored chain fraction is found that undergoes restricted Rouse motions with segmental mobilities as in the bulk phase. The size of this layer exceeds significantly the length scale of the directly adsorbed polymer, presenting a first direct microscopic evidence for the hypothetical interphase. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M32.00002: Universal Scaling of Polymer Diffusion in Nanocomposites Jihoon Choi, Michael J.A. Hore, Jeffrey S. Meth, Nigel Clarke, Karen I. Winey, Russell J. Composto The tracer diffusion of deuterated polystyrene (dPS) is measured in a polystyrene (PS) nanocomposite containing hard and soft silica nanoparticles (NPs). The soft NPs are grafted with a PS brush (87 kg/mol). The matrix for both NPs is PS (160 kg/mol). The diffusion coefficients for dPS (23 - 1,866 kg/mol) decrease as the hard and soft NP volume fractions increase. To accurately determine the interparticle distances (ID) relevant to each dPS (M) diffusing through the PS(160k):soft NP matrix, self consistent field theory and small angle neutron scattering studies were performed; both theory and experiment show that short dPS chains can deeply penetrate the brush, whereas longer dPS chains only penetrate the periphery and mainly remain in the matrix. The reduced diffusion coefficient (D/D$_{\mathrm{0}}$), plotted against the confinement parameter, namely ID relative to tracer size (2R$_{\mathrm{g}}$), collapses onto a master curve independent of NP type. These experiments demonstrate that polymer diffusion in nanocomposites is captured by the confinement parameter over an extremely wide range of ID/2R$_{\mathrm{g}}$ and, hopefully, motivate new models to capture the dynamics in confined (ID/2R$_{\mathrm{g}}$ \textless\ 10) regimes. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M32.00003: Hopping Diffusion of Nanoparticles Subjected to Topological Constraints Li-Heng Cai, Sergey Panyukov, Michael Rubinstein We describe a novel hopping mechanism for diffusion of large non-sticky nanoparticles subjected to topological constraints in polymer solids (networks and gels) and entangled polymer liquids (melts and solutions). Probe particles with size larger than the mesh size of unentangled polymer networks (tube diameter of entangled polymer liquids) are trapped by the network (entanglement) cages at time scales longer than the relaxation time of the network (entanglement) strand. At long time scales, however, these particles can move further by hopping between neighboring confinement cages. This hopping is controlled by fluctuations of surrounding confinement cages, which could be large enough to allow particles to slip through. The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size slightly larger than the network mesh size (tube diameter). Very large particles in polymer solids will be permanently trapped by local network cages, whereas they can still move in polymer liquids by waiting for entanglement cages to rearrange on the relaxation time scale of the liquids. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M32.00004: The Role of Excluded Volume on the Reduction of Polymer Diffusion into Nanocomposites Jeff Meth, Sangah Gam, Russell Composto, Karen Winey An analytic model for the reduction of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chain diffuses. The distribution of cylinder diameters in the ensemble is predicted from statistical mechanical theories based on the packing of spheres. The reduction in polymer diffusion is accounted for by the truncation of the partition function for a random walk in a cylinder. For low loadings of spherical particles in nanocomposites, we show that this theory results in a master curve for the reduced diffusion coefficient. The theory, with no adjustable parameters, is in agreement with recent data for tracer diffusion measurements in polymer nanocomposites at low loading. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M32.00005: Diffusivity and Transient Localization of Filler Particles in Polymer Melts and Crosslinked Systems Zachary E. Dell, Kenneth S. Schweizer Building on recent progress in describing the microscopic equilibrium structure of polymer nanocomposites (PRISM theory), as well as the na\"{i}ve mode coupling and nonlinear Langevin equation approaches for predicting localization and activated barrier hopping, we have initiated the study of dynamical phenomena in nanocomposites at finite filler loading. A colloidal suspension perspective is adopted whereby the polymer dynamics are assumed to remain unperturbed by fillers. Both entangled polymer melts and crosslinked systems are studied. The long time behavior of a tagged nanoparticle (localization and diffusivity) is calculated for various melt (tube diameter, polymer radius of gyration) and nanoparticle (filler size and volume fraction, polymer-filler attraction strength) parameters. For transiently localized particles, a dynamic free energy is constructed and employed to compute the nanoparticle localization length, mean barrier hopping time, and self-diffusion constant. The influence of filler-filler interactions on the Stokes-Einstein violation phenomenon in entangled melts is established. In addition, the influence of nanocomposite statistical structure (e.g., in the depletion, steric stabilization, or bridging regimes) on slow dynamics and localization is investigated. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M32.00006: Nanoparticle diffusion in dense polymer melts Jagannathan T. Kalathi, Sanat K. Kumar, Gary S. Grest The diffusion of nanoparticles in melts and solutions of polymers facilitates understanding of the viscoelastic behavior of the respective polymers and their composites. It also plays a vital role in determining the equilibrium structure and morphology of polymer nanocomposites and hence, their mechanical properties. In this work, we present the diffusion coefficients of non-sticky smooth spherical particles of different sizes (1-10 $\sigma$) in an athermal mixture of particles and polymers of different chain lengths ($N$ = 20 to 400) using molecular dynamics simulations. The diffusion of nanoparticles of size comparable to the polymer segment size ($\sigma$) is independent of chain length and hence, nanoparticles apparently feel only the local viscosity, as predicted by scaling theories. When the nanoparticle becomes larger than a segment (or alternately the correlation length in the melt), then, the diffusion coefficient decreases. This is due to the fact that the mobility of the particles is retarded either by a chain section of size equivalent to the particle size or by entanglement mesh size depending on the nanoparticle size. We also elucidate the role of chain entanglements on diffusion of nanoparticles. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M32.00007: Entanglement-Controlled Subdiffusion of Nanoparticles within Concentrated Polymer Solutions R.L. Leheny, H. Guo, G. Bourret, R.B. Lennox, M. Sutton, J.L. Harden Microrheology techniques, in which colloids suspended in a complex fluid probe their mechanical environment, can provide unique information on the microscopic length scales characterizing the fluid's hierarchical structure. We describe x-ray photon correlation spectroscopy (XPCS) experiments tracking the motion of colloidal gold nanoparticles in solutions of high-molecular-weight polystyrene. The particle radius is tuned to be comparable to the length scales characterizing the entangled polymer mesh. Over displacements of nanometers to tens of nanometers, the particles undergo subdiffusive motion in which the particle mean-squared displacement grows as a power law in time, with power-law exponent, $\alpha < 1$, that depends on solution conditions. Scaling behavior of the nanoparticle mobility with respect to temperature and to polymer concentration and molecular weight indicates the subdiffusion results from the temporal evolution of the entanglement mesh in the immediate vicinity of the particles. The results thus provide a novel microscopic dynamical characterization of a key structural property of polymers and more broadly demonstrate the capability of XPCS-based microrheology to interrogate heterogeneous mechanical environments in nanostructured soft materials. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M32.00008: Segmental Dynamics of Polymer Nanocomposites by Dielectric Relaxation Spectroscopy Shushan Gong, Quan Chen, Ralph Colby, Joseph Moll, Sanat Kumar The addition of nanoparticles dramatically affects the physical properties of polymer melts. The general agreement on this interaction mechanism is that the polymer-filler interface is the key region for the changes of properties. Previous studies have suggested the existence of a bound polymer layer in this interfacial region by various techniques. Here, we use Dielectric Relaxation Spectroscopy (DRS) to study the segmental relaxation of poly-2-vinylpyridine (P2VP) nanocomposites by the presence of silica nanoparticles (NPs), with sizes ranging from 14nm to 100nm. For nanocomposites with large amounts of surface area per unit volume (i.e., 14 nm NPs at high loadings) the segmental relaxation dispersion is broadened significantly, suggesting that the bound layer of P2VP is slower than the bulk P2VP, which is attributable to a restriction from solid surface of NPs. Additionally, the thickness of the bound polymer layer is estimated from the reduction in the magnitude of the segmental relaxation. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M32.00009: Dissipative Particle Dynamics Simulations of Polymer Nanocomposites Nigel Clarke, Argyrios Karatrantos, Russell Composto, Karen Winey We investigate the topological constraints (entanglements) in polymer - nanorod nanocomposites in comparison to polymer melts using dissipative particle dynamics (DPD) simulations. The nanorods have a radius smaller than the polymer radius of gyration. We observe an increase in the number of entanglements, corresponding to a 50{\%} decrease of the entanglement degree of polymerization in the case of 0.11 volume fraction of nanorods dispersed in the polymer matrix, in the nanocomposites as evidenced by larger contour lengths of the primitive paths. The end-to-end distance is essentially unchanged with the nanorod volume fraction for the range of concentrations that we have studied. An increase of the nanorod radius reduces the polymer - nanorod entanglements while the polymer -- polymer entanglements remain unaffected. Interaction between polymers and nanorods affects the dispersion of nanorods in the nanocomposites and also alters the primitive path. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M32.00010: Polymer Chain Conformation in CNT/Polystyrene Nanocomposites by SANS Wei-Shao Tung, Vikki J. Bird, Nigel Clarke, Russell J. Composto, Karen I. Winey Polymer conformations are a critical factor that affects the performance of polymer nanocomposites. Using small angle neutron scattering, we probed chain conformations and confinement of polymers in both SWCNT/polystyrene ($R_{SWCNT}$ \textless\ $R_{g})$ and MWCNT/polystyrene ($R_{MWCNT}$ $\sim$ $R_{g})$ nanocomposites. Through contrast matching experiments, we optimize the dPS:hPS ratio (0.725:0.275) to minimize the scattering from CNTs. To fit the scattering data, we developed a fitting model that includes scattering from polymer chains, rod networks, and defects. We found that the rod network scattering increases as the CNT concentration increases (0.3wt{\%} - 10wt{\%}) in both SWCNT and MWCNT composites, and the rod network scattering is much higher for SWCNT due to the smaller mesh size. When the CNTs concentration is below 2wt{\%}, there is no significant change in $R_{g}$ for both SWCNT and MWCNT nanocomposites. Above 2wt{\%}, the $R_{g}$ for SWCNT nanocomposites increases monotonically as a function of CNT concentration ($\sim$ 30{\%} increase for 10wt{\%} SWCNT loading), while the $R_{g}$ for MWCNT is not affected. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M32.00011: Microscopic theory for tube confinement and self-diffusivity of entangled needle liquids in presence of hard spherical obstacles Umi Yamamoto, Kenneth Schweizer A microscopic theory for the motion of topologically entangled, non-rotating needles in presence of spatially fixed, hard sphere inclusions has been formulated. Exact two-body dynamical uncrossability constraints are imposed, and an effective Brownian evolution equation at two-needle level is self-consistently constructed. The needle transverse localization length (effective tube diameter) and long-time diffusivity are determined as a function of its length and concentration, the sphere diameter and volume fraction, and needle-sphere liquid pair structure. In contrast to single-component entangled needle liquids, the transverse and longitudinal diffusivity become coupled, and reptation is increasingly suppressed with sphere volume fraction in a manner that depends on the relative sphere-needle size. The slow dynamics also depends on needle concentration, reflecting a competition between inter-needle topological uncrossability constraints and needle-sphere excluded volume interactions. The effective tube diameter is a monotonically decreasing function of the sphere density, consistent with the suppression of polymer translational diffusion. Extension to treat entangled flexible chains, and comparison with recent simulations and experiments, are under study. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M32.00012: Dynamics of nanoparticles in non-Newtonian aqueous dispersions Jacinta Conrad, Firoozeh Babaye Khorasani, Ramanan Krishnamoorti The transport properties of nanoparticles in soft complex media are relevant for polymer and hydrogel nanocomposites but are still poorly understood. We use single-particle tracking to measure the diffusional dynamics of nanoparticles in non-Newtonian aqueous polymer solutions, which also serve as models of viscoelastic porous media. We track the motion of polystyrene nanoparticles of diameter 400 nm in aqueous solutions of hydrolyzed polyacrylamide whose radii of gyration are comparable to the diameter of the nanoparticles over a wide range of dilute and semi-dilute concentrations. At all concentrations, the mean-square displacement (MSD) of nanoparticles at long times is linearly proportional to time, indicating diffusive motion. The viscosity extracted from the MSD systematically varies with polymer concentration but is smaller than the zero shear rate viscosity measured at each polymer concentration using bulk rheometry, indicating that the dynamics cannot be explained in the context of microrheology of viscous solutions. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M32.00013: The impact of fragility on the properties of the glass formation of polymer nanoparticle composites Beatriz A. Pazmino Betancourt, Jack F. Douglas, Francis W. Starr We investigate the effects of nanoparticles on glass formation in a model polymer melt by molecular dynamics simulations. The addition of nanoparticles allows us to change the relaxation time, glass transition temperature $T_g$, the fragility of glass formation in a controlled fashion. We show that the structural relaxation for different temperatures, concentrations, and polymer-NP concentrations can be expressed in terms of a simple universal function of the short-time Debye-Waller factor. We further examine how the stretching exponent $\beta$ and the degree of the breakdown of the Stokes-Einstein relation depend upon fragility, which we relate to the extent of cooperative motion. [Preview Abstract] |
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