Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y14: Invited Session: Dynamics of Polymer Nanocomposites |
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Sponsoring Units: DPOLY DBIO Chair: Russell Composto, University of Pennsylvania Room: 301-303 |
Friday, March 7, 2014 8:00AM - 8:36AM |
Y14.00001: Dynamics of nanoparticles in models of soft and hard porous media Invited Speaker: Jacinta Conrad The transport properties of nanoparticles in complex porous media impact the processing of polymer and hydrogel nanocomposites. In the limit of strong confinement, in which the size of nanoparticles is comparable to typical length scales within the complex confining medium, the local mechanisms that influence nanoparticle transport remain poorly understood. I will describe experiments in which we use optical microscopy to probe the diffusive and transport properties of particles of size 200-400 nm in models of hard and soft porous media. As models of hard porous media, we fabricate arrays of nanoposts that are arranged in a square lattice; as models of soft porous media we formulate aqueous soultions of hydrolyzed polyacrylade over a wide range of dilute and semi-dilute concentrations. In both quiescent diffusion and in flow-driven transport through hard media, we generally find that the dynamics of the nanoparticles become increasingly slowed and stretched as the particles are more strongly confined. Strong confinement leads to deviations from the dispersion behaviors expected for small solute molecules. In soft media, we find that the local viscosity experienced by the nanoparticles systematically underestimates the zero-shear rate viscosity measured using bulk rheology, which we attribute to coupling between particle and polymer dynamics. I will discuss these results and their implications for nanocomposite processing as well as for other applications that require confined transport of nanomaterials in complex media. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y14.00002: X-ray photon correlation spectroscopy studies of nanoparticle motion in glassy polymer melts and entangled polymer solutions Invited Speaker: Robert Leheny Microrheology, in which colloids suspended in a complex fluid probe the 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) studies tracking the nanometer-scale motion of dilute suspensions of gold nanoparticles in low-molecular-weight polystyrene melts and in high-molecular-weight polystyrene solutions. In the melts, the high-temperature nanoparticle dynamics are diffusive with a rate that tracks the melt viscosity. Close to the glass transition, a hyper-diffusive process that we identify with heterogeneous strain in the melts supersedes the diffusion. Following a quench, the hyper-diffusive dynamics display characteristics of aging. Similar slow, heterogeneous strain has been observed in a range of soft glassy materials such as colloidal gels and emulsions. The apparently universal nature of the phenomenon hence provides a link between the microscopic processes of aging in hard and soft glassy systems. In contrast, the nanoparticle motion in the high-molecular-weight solutions reveals qualitatively different behavior. Over displacements from nanometers to tens of nanometers, the particles undergo anomalous subdiffusion in which the particle mean-squared displacement grows as a power law in time with power-law exponent in the range 0.3 to 0.5 depending on solution conditions. Scaling behavior of the nanoparticle mobility with respect to temperature and polymer concentration and molecular weight indicates that the subdiffusive motion results from the temporal evolution of the entanglement mesh in the immediate vicinity of the particles. The results thus provide novel microscopic characterizations of the structural dynamics in the melts and entangled solutions and more broadly demonstrate the ability of XPCS-based microrheology to interrogate the nanoscale mechanical behavior of polymer materials. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y14.00003: Microscopic Theories of Diffusion, Tube Localization and Slow Relaxation in Polymer Nanocomposites Invited Speaker: Kenneth Schweizer Dynamics in polymer nanocomposites is rich and complex but poorly understood due to the presence of multiple length scales, excluded volume effects and other factors. We have developed new statistical mechanical theories at the level of forces for particle and polymer motion in flexible and rigid polymers. This talk presents an overview, including quantitative comparisons to simulations and experiments. First, by combining Brownian motion, polymer physics and mode coupling ideas, a self-consistent theory for the non-hydrodynamic diffusion of a spherical nanoparticle in melts has been constructed. Three competing mechanisms are predicted: sieving-like diffusion through unentangled regions, reptation-driven constraint release in entangled melts, and activated hopping through entanglement meshes. The controlling mechanism depends on particle size, tube diameter and entanglement density. The approach can also treat soft fillers, nonspherical particles, adsorption, solutions and networks. Second, a self-consistent microscopic theory for the slow dynamics of a needle fluid in a matrix of static spheres has been developed which exactly enforces inter-needle topological uncrossability and needle- sphere impenetrability constraints at the two-body level. The rich dependences of the effective tube diameter and anisotropic diffusion constants on filler-needle aspect ratio, polymer concentration and particle volume fraction has been established. Due to steric blocking of longitudinal motion by obstacles, a literal localization transition is predicted that is controlled by the particle to tube diameter ratio. For a restricted window of parameter space, needles are predicted to diffuse via a ``renormalized'' reptation dynamics where compression of the tube and suppression of longitudinal diffusivity enter in a manner that depends on all system variables. Generalization of the approach to treat mobile fillers, flexible chains and nonrandom microstructure is possible. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y14.00004: Rotational diffusion in polymer nanocomposites as probed by anisotropic particles Invited Speaker: Laura Clarke Metal nanoparticles strongly absorb specific wavelengths of light with no (or only a very weak) radiative relaxation by which to release this energy. As a result, the absorbed energy is efficiently converted to local heat (a photothermal effect). With an effective cross-section of up to 10 times its physical size, each particle acts as a ``super-sized'' absorber even when embedded within a transparent material environment such as a polymer, resulting in dramatic heating originating at the particles. Thus, with spatially-uniform illumination, one can metaphorically reach inside a polymer nanocomposite and apply heat to pre-selected subsets (e.g., causing them to dramatically change properties due to actuation, cross-linking, crystallization, or chemical reaction) without heating the sample surface or strongly affecting the remainder of the material. By utilizing optically-accessible additives including the particles themselves, the thermal gradient from the particle outward can be experimentally determined. In particular, rotational diffusion of anisotropic particles can be used to measure the temperature at the nanoparticle, which is the warmest point in a polymeric film or nanofiber under photothermal heating. Conversely, the same technique can be utilized to measure polymer dynamics in nanocomposites in the immediate vicinity of the particle.\\[4pt] [1] S. Maity et al., \textit{Polymer} \textbf{52}, 1674 (2011).\\[0pt] [2] S. Maity et al., \textit{Adv. Funct. Mater} \textbf{22}, 5259 (2012).\\[0pt] [3] S. Maity et al., \textit{Part. \& Part. Sys. Char} \textbf{30}, 193 (2013). [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y14.00005: Polymer Diffusion in the Presence of Nanoparticles Invited Speaker: Karen Winey The center-of-mass diffusion of polymers within a polymer melt proceeds by the mechanism of reptation wherein the polymer is confined to a tube that is defined by neighboring entanglements and moves along its contour. Polymer diffusion is perturbed when the melt contains nanoparticles that are comparable in size to the radius of gyration (Rg) of the polymers. Within this talk, we will present tracer diffusion coefficients (D) results for three types of nanocomposite: spherical nanoparticles with surface functionalization, spherical nanoparticles with brushes, and cylindrical nanoparticles (aspect ratio $=$ 5 to 50). When functionalized spherical nanoparticles have neutral or attractive interactions with the polymer matrix, a monotonic decrease in the diffusion coefficient is observed across a wide range of polymer molecular weight, nanoparticle size, and nanoparticle concentration. These data collapse onto a master curve when plotted as D normalized by the diffusion coefficient into a neat homopolymer (D/Do) versus our confinement parameter defined as the interparticle distance divided by 2Rg (ID/2Rg). Polymer diffusion in systems with grafted spherical nanoparticles exhibit the same D/Do versus ID/2Rg, when ID accounts for the extent to which the tracer polymer penetrates the polymer brush. For various cylindrical nanoparticles D/Do versus nanoparticle concentration exhibits a minimum when 2Rg is both larger than the nanoparticle diameter and smaller than the nanoparticle length. Complimentary molecular dynamics simulations and neutron scattering results will also be presented. [Preview Abstract] |
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