Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S45: Focus Session: Polymer Nanocomposites: Dynamics |
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Sponsoring Units: DPOLY Chair: Laura Clarke, North Carolina State University Room: 216AB |
Thursday, March 5, 2015 8:00AM - 8:36AM |
S45.00001: Dynamics in Polymer Nanocomposites Invited Speaker: Nigel Clarke Since nanoparticles are increasingly being added to polymers to impart mechanical and functional properties, we are exploring how nanoparticles impact polymer dynamics with a focus on the diffusion coefficients. In high molecular weight polymer melts, chain diffusion is well described by the reptation model. Motion proceeds as a snake-like diffusion of the chain as a whole, along the contour of a tube that mimics the role of physical entanglements, or topological constraints, with other chains. In polymer nanocomposites there are additional constraints due to the dispersed nanoparticles in the polymer matrix. Chain motion can be altered by nanoparticle size, shape , aspect ratio, surface area, loading and the nature of the interactions between the nanoparticles and the polymer matrix. We have observed a minimum in the diffusion coefficient as a function of nanoparticle concentration when the nanoparticles are rod-like and a collapse of the diffusion coefficient onto a master curve when the nanoparticles are spherical. We are simulating the dynamics using molecular and dissipative particle simulations in order to provide physical insight into the local structure and dynamics, and have also carried out highly coarse grained Monte Carlo simulations of entangled polymers to explore how reptation is affected by the presence of larger scale obstacles. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S45.00002: Determination of the Tracer Diffusion Coefficient of Soft Polystyrene Nanoparticles using Neutron Reflectivity Adam Imel, Brad Miller, Wade Holley, Durairaj Baskaran, Jimmy Mays, Mark Dadmun The diffusion properties of nanoparticles in polymer nanocomposites are largely unknown and depend intimately on the dispersion of the nanoparticles. We examine the diffusion of soft, organic nanoparticles, which disperse in a polymer matrix due to the interpenetration of polymer chains and particles and the reduction in the depletion of entropy in the system. The impact of the presence of soft nanoparticles on the diffusion coefficient of polystyrene chains has recently been determined with neutron reflectivity. This was completed by monitoring the interdiffusion of deuterated and protonated polystyrene nanocomposite bilayers with and without the soft nanoparticles dispersed throughout both layers and extracting the diffusion coefficient from the one-dimensional solution to Fick's second law of diffusion. In this work, we extend this method to bilayer systems with only the soft nanoparticles as one of the layers and a linear deuterated polystyrene as an adjacent layer. The development of this method allows us to determine the tracer diffusion coefficient of the soft polystyrene nanoparticles for the first time by analyzing the mutual diffusion coefficient from Fick's second law and the fast and slow modes theories for diffusion. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S45.00003: Viscosity of Polymer Nanocomposite with Athermal Hairy Nanoparticles Fei Chen, Ophelia Tsui We studied the zero shear viscosity of polymer nanocomposites (PNC) containing silica nanoparticles grafted with polystyrene ligands blended with polystyrene homopolymer. As the ratio of the molecular weight of the homopolymer, P, to that of the ligands, N was increased from about 0.01, we observed a transition from viscosity enforcement to viscosity reduction near P/N = 1. Interestingly, many of the samples exhibiting viscosity reduction have the dry diameter of the particles exceeding the radius of gyration of the homopolymer (i.e., $2r > R_g$), making them exceptional cases according to the viscosity phase diagram published by Kalathi et al. (Phys. Rev. Lett. 109, 198301 (2012)). We discuss whether hydrodynamic effect and plasticizer effect might have caused our observations. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S45.00004: Probing the interfacial region in polymer-graphene oxide nanocomposites Michael Weir, Stephen Boothroyd, David Johnson, Richard Thompson, Nigel Clarke, Karl Coleman Graphene and related two-dimensional materials are excellent candidates as filler materials in nanocomposites due to their extraordinary physical properties and high aspect ratio. We are studying graphene oxide (GO), a highly functionalized form of graphene, due to its relative ease of dispersion within polymer matrices. Interruptions to the pristine two-dimensional carbon network by oxygen-containing groups, which provide functionality, also make GO rather flexible. In this paper we show that GO is wrinkled and rough over a hierarchy of length scales from a few nanometers to a few microns, when it is incorporated in composites with poly(methyl methacrylate) (PMMA) and polystyrene (PS). Small-angle neutron scattering measurements, highlighting individual polymer chains, show a decrease in polymer radius of gyration with increasing GO concentration in PMMA/GO nanocomposites. The decrease is consistent with models of a solid interface in a polymer melt. The interface influences the polymer matrix within an interfacial volume stretching on the order of one polymer radius of gyration from the surface. This work is a direct measurement of the effect of the nanofiller upon the polymer matrix and progresses our understanding of interfacial interactions within nanocomposites. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S45.00005: Polymer dynamics in PMMA-carbon nanocomposites Rana Ashkar, Mansour Abdulbaki, Christopher Bertrand, Madhusudan Tyagi, Antonio Faraone, Paul Butler, Ramanan Krishnamoorti Particle-polymer attractions in nanocomposites can lead to significant heterogeneities in the polymer dynamics and remarkably impact the material properties. While dynamical perturbations are expected to be limited to interfacial polymer segments, for nanoparticle concentrations above percolation, however, the interfacial regions overlap. The impact of interfacial-polymer network results in a complex relaxation behavior of the polymer, that is unanticipated from dilute nanoparticle dispersions in polymer matrices. Neutron spectroscopy on C60 and SWNT composites reveals that dynamical perturbations can extend to non-interfacial polymer segments and significantly influence their local mobility and their meso-scale cooperative relaxations. In this case of attractive polymer-particle interactions, a gradual decrease in the polymer mean-square displacement is observed with increasing nanoparticle loadings below the percolation threshold. However, once the nanoparticles are percolated -- be it C60 or SWNT -- the mean-square displacement seizes to change with increasing loading, indicating kinetic arrest of the polymer. Interestingly, upon percolation, the composites experience an order of magnitude slowdown in the structural relaxations relative to the pure matrix. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S45.00006: Macromolecular Diffusion in Dynamic Polymer Nanocomposite Chia-Chun Lin, Matteo Cargnello, Nigel Clarke, Karen Winey, Russell Composto We consider diffusion of tracer polymers in the presence of mobile nanoparticles in polymer nanocomposites (PNCs). These nanoparticles are mobile on the time scale of polymer diffusion and have dimensions less than the entanglement mesh size (i.e., tube diameter). The PNC consists of titanium dioxide nanorods (NR, diameter=4.5nm; length=30.1nm) grafted with phenyl groups uniformly dispersed in a polystyrene (P=650kg/mol; tube diameter=8nm) matrix up to 10 volume percent. Three deuterated polystyrenes (dPS; M=800, 1800 and 3200 kg/mol) are chosen because their diffusion relative to NR allows for investigating fixed and mobile NR by simply changing M. For all M, the reduced tracer diffusivities are observed to decrease monotonically as NR loading increases. However, the reduced diffusivity of dPS (3200 kg/mol) is faster than expected compared to the fixed NR case. These findings suggest that mobile NR do not effectively slow down tracer diffusion relative to fixed particles. To test this hypothesis, dPS diffusion is investigated in a high molecular weight matrix PS (2000 kg/mol) in order to slow down NR diffusion relative to dPS (3200 kg/mol). New models are needed to incorporate these mobility dependent entanglements into a comprehensive understanding of dynamics in PNCs. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S45.00007: Dynamics of Brush-grafted Nanoparticles in Polymer Melts Russell Composto, Chia-Chun Lin, Kohji Ohno, Michael Hore, Jeffrey Meth, Nigel Clarke, Karen Winey Grafting a polymer brush to nanoparticles is an effective approach to achieve a good dispersion of nanoparticles in a polymer melt and the structure of this dispersion has been well studied. However, the interplay between the structure of the brush and nanoparticle diffusion is lacking. Here, we study the diffusion of poly (methyl methacrylate), or PMMA,-grafted iron oxide nanoparticles (core diameter=5nm) in PMMA melts. Different brush architectures are obtained by tuning brush molecular weight (16 and 21kg/mol), brush grafting density (0.17, 0.33 and 0.55 chains/nm2) and PMMA matrix molecular weight (4-70kg/mol). Preliminary results show that the diffusion of nanoparticles is slowed down relative to predictions of the classic Stokes-Einstein relation applied to a 5nm particle suggesting that the interpenetration between the brush and matrix influences nanoparticle mobility. Self-consistent field theory is performed to predict the structure of brush and matrix in the vicinity of the particle to quantify the effect of brush-matrix interpenetration. These experiments demonstrate that the structure of the brush could affect nanoparticle center of mass diffusion and the brush-nanoparticle interpenetration should be considered. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S45.00008: Unexpected Molecular Weight Dependence of Dynamics in Polymer Nanocomposites Adam Holt, Shiwang Cheng, Vera Bocharova, Philip Griffin, Adam Imel, Mark Dadmun, Alexei Sokolov The impact of nanoscale confinement in polymer nanocomposites has invoked tremendous interest ever since the reports of dramatic changes in polymer dynamics with relatively low nanoparticle concentrations. It has been shown that the segmental dynamics at an attractive polymer/nanoparticle interface slows down significantly due to formation of an interfacial layer. The general expectation is that the length scale of this layer and its influence will grow with increasing polymer molecular weight (MW). Our measurements reveal an opposite trend: the magnitude of the effect on segmental dynamics decreases with increasing MW. Based on detailed analysis of dielectric spectroscopy and small angle x-ray scattering measurements we provide a qualitative explanation of the unexpected observation. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S45.00009: Role of the Interfacial Interactions from an Adjacent Wall on Polymer Diffusion Jihoon Choi, Nigel Clarke, Karen Winey, Jeffrey Meth, Russell Composto The tracer diffusion of deuterated polystyrene (dPS; $M_{\mathrm{n}} = $23 - 1866 kg/mol) with a thickness (l) is measured diffusing away from hydroxyl-terminated, phenyl-terminate, and PS-grafted silicon substrates. For a hydroxyl-functionalized substrate, short polymer chains ($M_{\mathrm{n}} = $23 and 49 kg/mol; $l$ \textgreater \textgreater $R_{g})$ exhibit a diffusion coefficient that is comparable to bulk PS, whereas long polymer chains ($M_{\mathrm{n}} = $532 and 1866 kg/mol; $l$ \textless $R_{g})$ are significantly slower than the bulk case. This slowing down is consistent with the observation by Zheng et al. [1]. In particular, bimodal diffusion coefficient was observed for intermediate molecular weight ($M_{\mathrm{n}} = $168 kg/mol;$ l$ $\sim$ $R_{g})$. For phenyl-functionalized and PS-grafted substrates, no significant differences in the diffusion coefficients are observed although long polymer chains showed a moderate slowing down. These experiments demonstrate that the polymer diffusion of thin, confined films ($\sim$ $R_{\mathrm{g}})$ away from the substrate is determined by the friction due to surface-monomer contacts, and is sensitive to the chemical state of the substrate, providing a new insight into a role of the interfacial interactions on polymer dynamics. \\[4pt] [1] X. Zheng, et al., \textit{Phys. Rev. Lett}., 74, 407 (1995). [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S45.00010: Length-Scale Dependent Viscosity in Semidilute Polyelectrolyte Solutions Ryan Poling-Skutvik, Ramanan Krishnamoorti, Jacinta Conrad Using optical microscopy and particle tracking algorithms, we measured the mean-squared displacements (MSDs) of fluorescent polystyrene particles with diameters ranging from 300 nm to 2 $\mu$m suspended in semidilute solutions of high molecular weight partially hydrolyzed polyacrylamide. The solutions had polymer concentrations ranging from 0.67 to 67c$^*$, where c$^*$ is the overlap concentration, and estimated correlation lengths of $\sim$ 100 to 900 nm. At short times, the particles exhibited subdiffusive behavior characterized by MSD $\sim t^\alpha$ with $\alpha < 1$. On long time scales, the particles transitioned to Fickian diffusion $( \alpha = 1 )$ and their diffusivity was calculated from the slope of the MSD. Whereas the large particles agreed with predictions using the Stokes-Einstein equation and bulk zero-shear viscosity, the smaller particles diffused much faster than predicted. The relative diffusivities do not collapse onto a single curve, but rather form a continuum that varies with particle size. This indicates that the particles experience a size-dependent effective viscosity mediated by the ratio of particle diameter to characteristic length scales in the polymer solution. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S45.00011: Temperature Dependence of Rheology and Polymer Diffusion in Silica/Polystyrene Nanocomposites Wei-Shao Tung, Nigel Clarke, Russell Composto, Jeffrey Meth, Karen Winey Time-temperature superposition using the WLF equation is well-established for both the zero shear viscosity and the polymer diffusion coefficient in homopolymer melts. This talk will present the temperature-dependence of polymer dynamics in polymer nanocomposites comprised of polystyrene and phenyl-capped silica nanoparticles (0 -- 50 vol{\%}). The WLF equation fits the temperature dependence of the tracer polymer diffusion coefficient and the fitting parameter (B/fo) decreases smoothly with nanoparticle concentration suggesting an increase in the thermal expansion coefficient for the free volume. The WLF equation also fits the temperature dependence of the zero shear viscosity from oscillatory shear experiments, although the fitting parameter (B/fo) increases substantially with nanoparticle concentration. This discrepancy between the diffusion and rheology will be discussed with respect to the reptation model, which predicts that the temperature dependence of polymer diffusion depends predominately on the temperature dependence of local viscosity, and the elastic response in nanocomposites. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S45.00012: Anomalous Drag Reduction and Hydrodynamic Interactions of Nanoparticles in Polymer Nanocomposite Thin Films Jaydeep Basu, Nafisa Begam, Sivasurender Chandran, Michael Sprung One of the central dogma of fluid physics is the no-slip boundary condition whose validity has come under intense scrutiny, especially in the fields of micro and nanofluidics. Although various studies show the violation of the no-slip condition its effect on flow of colloidal particles in viscous media has been rarely explored. Here we report unusually large reduction of effective drag experienced by polymer grafted nanoparticles moving through a highly viscous film of polymer, well above its glass transition temperature. The extent of drag reduction increases with decreasing temperature and polymer film thickness. We also observe apparent divergence of the wave vector dependent hydrodynamic interaction function of these nanoparticles with an anomalous power law exponent of $\sim$ 2 at the lowest temperatures and film thickness. Such strong hydrodynamic interactions are not expected in polymer melts where these interactions are known to be screened to molecular dimensions. We provide evidence for the presence of large hydrodynamic slip at the nanoparticle-polymer interface and demonstrate its tunability with temperature and confinement. Our study suggests novel physics emerging in dynamics nanoparticles due to confinement and interface wettability in thin films of polymer nanocomposites. [Preview Abstract] |
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