Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C10: Polymer Nanocomposites - Dynamics From Segmental to Chain ScaleFocus
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Sponsoring Units: DPOLY Chair: Robert Riggleman, University of Pennsylvania Room: 269 |
Monday, March 13, 2017 2:30PM - 2:42PM |
C10.00001: Observation of Small Particle Driven Chain Disentanglements in Polymer Nanocomposites Erkan Senses, Siyam Ansar, Christopher Kitchens, Suresh Narayanan, Yimin Mao, Antonio Faraone We apply neutron spin-echo spectroscopy on athermal and isotopically labeled PEO-gold nanoparticle composites to evaluate the role of particle size on single chain dynamics at space-time resolution relevant to the segmental and the collective polymer motion in melt. We found a $\approx $ 25 percent increase of the reptation tube diameter with addition of nanoparticles smaller than entanglement mesh size ($\approx $ 5 nm), at a volume fraction of 20 percent. No significant effect, however, is observed on the local segmental relaxation. The effects of decreasing topological confinement on the bulk viscoelastic behavior of the nanocomposites are discussed along-with the rheological trends. These results provide the first direct experimental observation of particle size driven disentanglements that can cause non-Einstein-like viscosity trends often observed in polymer nanocomposites. [Preview Abstract] |
Monday, March 13, 2017 2:42PM - 2:54PM |
C10.00002: Unexpected Thermal Annealing Effect on the Zero-shear Viscosity of Polymer Nanocomposites. Fei Chen, Koshu Takatsuji, Dan Zhao, Xuanji Yu, Sanat Kumar, Ophelia Tsui A comprehensive study was performed on the effect of thermal annealing above the glass transition temperature on the zero-shear viscosity, $\eta $, of polymer nanocomposites (PNCs) and their host polymers. For all the specimens studied, including neat and 4 wt{\%} DOP plasticized PS and PMMA as well as PNCs containing bare and grafted silica nanoparticles (NP, core radii, $r_{\mathrm{c}} \quad =$ 7 and 13.3 nm), $\eta $ initially increased with annealing time then approached a steady-state value after \textasciitilde 100 to \textasciitilde 200 h. We found that this phenomenon held true regardless of the solvent used to prepare the sample, including THF and toluene for PS and chloroform for PMMA. Moreover, the PNCs without DOP showed larger $\delta \eta $/$\eta $ than their host polymers while the plasticized ones showed $\delta \eta $/$\eta \quad \approx $ 0. By correlating the viscosity measurements with the evolution of the solvent content and average NP aggregate size in the samples, we infer that the viscosity evolutions may originate from an out-of-equilibrium chain conformation that got locked in the samples upon preparation and relaxed only after extensive annealing. [Preview Abstract] |
Monday, March 13, 2017 2:54PM - 3:06PM |
C10.00003: Modeling Hairy Nanoparticle Assemblies: Effect of Surface Adsorption Strength on Canopy Entanglements and Dynamics Jeffrey Ethier, Lisa M. Hall Neat hairy nanoparticles (polymer-grafted nanoparticles) exhibit unique mechanical, dielectric, and optical properties while avoiding dispersion challenges found in traditional polymer nanocomposites. On surfaces, such HNPs can form ordered arrays with controlled interparticle spacing whose properties depend on interpenetration or entanglement of the polymer canopies. We use coarse-grained molecular dynamics (MD) simulations to model individual or small groups of HNPs with various grafting densities, graft lengths, and surface adsorption strengths. Specifically, particles and monomers are modeled as spherical beads which can adsorb on a flat surface; the particle diameter is ten times that of the monomer. We validate our model by comparing our height profiles to those from recent experiments as a function of bead-surface adsorption strength. As expected, the canopy height decreases with increasing surface adsorption strength, and the polymer spreads out to maximize its contact with the surface. For various grafting densities and graft lengths, we analyze the polymer dynamics as a function of distance from the substrate and NP surface. We also examine the entanglement density between neighboring adsorbed HNPs. [Preview Abstract] |
Monday, March 13, 2017 3:06PM - 3:18PM |
C10.00004: Multi-Scale Polymer Conformations Between Confining Nanoparticles Eric Bailey, Robert A. Riggleman, Karen I. Winey The addition of nanoparticles (NPs) to a polymer matrix is known to significantly enhance the mechanical and functional properties of the polymer. Despite recent research efforts, several structural and dynamic properties remain mechanistically and comprehensively unknown. This is especially true in the confined regime, when the interparticle distance (ID) is on the order of the size of the polymer chain (2Rg). Here, coarse-grained molecular dynamics simulations are used to systematically study the effect of ID on multi-scale polymer conformations as a function of ID and chain length (spanning unentangled and entangled chains). A monolayer of NPs are hexagonally packed and placed within bulk polymer, revealing the length scale and magnitude of perturbations caused by two or more confining NPs. When ID/2Rg\textgreater 1, chains are flattened within Rg of the NP surface and bulk conformations are retained even between NPs. This is consistent with extrapolations from previous simulations using a single isolated particle. For ID/2Rg\textless 1, preliminary results suggest that the perturbed chain profiles interact unexpectedly and chain-scale conformations may not be appropriate to describe chain distortions. These conformational changes and perturbations are expected to influence dynamic properties. [Preview Abstract] |
Monday, March 13, 2017 3:18PM - 3:30PM |
C10.00005: Enthalpic and Entropic Competition in Blends of Self-Suspended Hairy Nanoparticles Snehashis Choudhury, Akanksha Agrawal, Lynden Archer Self-suspended hairy nanoparticles, where polymer chains are grafted onto nanoparticles, have attracted significant recent attention. These materials have been reported to manifest several interesting phenomena like thermal jamming, slowing-down of polymer chain dynamics, as well as small-strain stress overshoots during start-up of steady shear. The entropic penalty on tethered polymers produced by the requirement that they fill the space between the nanoparticle cores explain most of these behaviors. Here, we show that the entropic attraction between tethered polymer chains can be manipulated in mixtures of hairy nanoparticles using different polymer chemistry to design materials with unusual characteristics. Specifically, the degree of interpenetration of polymer chains can be controlled by tuning their interaction parameter ($\chi )$. For SiO$_{\mathrm{2}}$-PEG/SiO$_{\mathrm{2}}$-PMMA blends, oscillatory rheological measurements show that the plateau modulus and yielding energy are significantly increased, while an opposite effect is seen with SiO$_{\mathrm{2}}$-PEG/SiO$_{\mathrm{2}}$-PI blends. More subtle effects of this enthalpy-entropy competition are well captured in Dielectric Spectroscopy measurements and SAXS experiments that can be used to quantify the degree of stretch and interdigitation of polymer chains. [Preview Abstract] |
Monday, March 13, 2017 3:30PM - 3:42PM |
C10.00006: Molecular Dynamics Simulation on Rheological and Dyanmics Properties of Polymer Nanocomposite System With Thermal Stiffening Behaviors Wei Peng, Rahmi Ozisik, Pawel Keblinski The rheological and dynamic properties of polymer nanocomposite systems were studied via Molecular Dynamics (MD) simulations. The current study was inspired by Senses et al. (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682), where a reversible thermal stiffening behavior was observed in nanocomposite polymers, in which the matrix chains and surfactant polymer chains on the nanofillers showed 200 ºC difference in their glass transition temperatures (Tgs). The simulated systems contained nanoparticles that were grafted with high-Tg chains mixed within a matrix of low-Tg flexible chains. The rheological and dynamic properties of this polymer nanocomposite system with two distinct morphologies, “stretched” and “collapsed”, were studied. In “stretched” morphology, the grafted chains were well dispersed within the matrix, whereas in the “collapsed” morphology, the grafted and the matrix chains were phase separated. Non-equilibrium MD simulation results showed that “stretched” systems had significantly greater storage modulus than “collapsed” ones. The effects of chain length and volume fraction of grafted chains were further investigated to study the mechanism of the stiffening observed in the “stretched” nanocomposite. [Preview Abstract] |
Monday, March 13, 2017 3:42PM - 4:18PM |
C10.00007: Structure and Dynamics of Polymer/Polymer grafted nanoparticle composite Invited Speaker: Lynden Archer Addition of nanoparticles to polymers is a well-practiced methodology for augmenting various properties of the polymer host, including mechanical strength, thermal stability, barrier properties, dimensional stability and wear resistance. Many of these property changes are known to arise from nanoparticle-induced modification of polymer structure and chain dynamics, which are strong functions of the dispersion state of the nanoparticles' and on their relative size ($D)$ to polymer chain dimensions (e.g. Random coil radius $R_{g}$ or entanglement mesh size $a)$. This talk will discuss polymer nanocomposites (PNCs) comprised of Polyethylene Glycol (PEG) tethered silica nanoparticles (SiO$_{\mathrm{2}}$-PEG) dispersed in polymers as model systems for investigating phase stability and dynamics of PNCs. On the basis of small-angle X-ray Scattering, it will be shown that favorable enthalpic interactions between particle-tethered chains and a polymer host provides an important mechanism for creating PNCs in which particle aggregation is avoided. The talk will report on polymer and particle scale dynamics in these materials and will show that grafted nanoparticles well dispersed in a polymer host strongly influence the host polymer relaxation dynamics on all timescales and the polymers in turn produce dramatic changes in the nature (from diffusive to hyperdiffusive) and speed of nano particle decorrelation dynamics at the polymer entanglement threshold. A local viscosity model capable of explaining these observations is discussed and the results compared with scaling theories for NP motions in polymers [Preview Abstract] |
Monday, March 13, 2017 4:18PM - 4:30PM |
C10.00008: Understanding the interfacial chain dynamics of fiber-reinforced polymer composite Monojoy Goswami, Jan-Michael Carrillo, Amit Naskar, Bobby Sumpter The polymer-fiber interface plays a major role in determining the structural and dynamical properties of fiber reinforced composite materials. We utilized LAMMPS MD package to understand the interfacial properties at the nanoscale. Coarse-grained flexible polymer chains are introduced to compare the various structures and dynamics of the polymer chains. Our preliminary simulation study shows that the rigidity of the polymer chain affects the interfacial morphology and dynamics of the chain on a flat surface. In this work, we identified the `immobile inter-phase' morphology and relate it to rheological properties. We calculated the viscoelastic properties, e.g., shear modulus and storage modulus, which are compared with experiments. MD simulations are used to show the variation of viscoelastic properties with polymer volume fraction. The nanoscale segmental and chain relaxation are calculated from the MD simulations and compared to the experimental data. These observations will be able to identify the fundamental physics behind the effect of the polymer-fiber interactions and orientation of the fiber to the overall rheological properties of the fiber reinforced polymer matrix. [Preview Abstract] |
Monday, March 13, 2017 4:30PM - 4:42PM |
C10.00009: Unexpected Decoupling of Translational and Reorientational Segmental Dynamics in Attractive Polymer Nanocomposites Philip Griffin, Eric Bailey, Madhusudan Tyagi, Karen Winey Polymer dynamics in the vicinity of an enthalpically attractive surface can be significantly modified relative to the bulk state depending on the length scale being probed. We characterize the local segmental relaxations in attractive polymer nanocomposites comprising 25 nm diameter silica nanoparticles (NPs) and lightly entangled poly(2-vinyl pyridine) over a wide range of NP concentrations (up to 50 vol{\%}) using quasielastic neutron scattering and broadband dielectric spectroscopy. We find that the average translational diffusion coefficient of polymer segments is strongly suppressed in these attractive PNCs (by up to a factor of five), while the average reorientational segmental relaxation times are comparatively unaffected. Our results demonstrate that even on local intermolecular length scales, physisorbed polymer chains contain a large population of segments that are effectively translationally ``bound'' while still undergoing a dynamically decoupled, and relatively unperturbed, reorientational segmental relaxation process. [Preview Abstract] |
Monday, March 13, 2017 4:42PM - 4:54PM |
C10.00010: Nanostructures and nanosecond dynamics at the polymer/filler interface Tad Koga, Deborah Barkley, Maya Endoh, Tomomi Masui, Hiroyuki Kishimoto, Michihiro Nagao, Takashi Taniguchi We report in-situ nanostructures and nanosecond dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called ``bound polymer layer (BPL)'') in polymer solutions (from dilute to concentrated solutions). The BPL on the CB fillers were extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene (a good solvent) to label the BPL for ``contrast-matching'' small-angle neutron scattering (SANS) and neutron spin echo (NSE) techniques. The SANS results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of $\approx $ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the NSE results show that the dynamics of the swollen bound chains in the polymer solutions can be explained by the collective dynamics, the so-called ``breathing mode''. Intriguingly, it was also indicative that the collective dynamics is independent of the polymer concentrations and is much faster than that predicted from the solution viscosity. We will discuss the mechanism at the bound polymer-free polymer interface at the nanometer scale. [Preview Abstract] |
Monday, March 13, 2017 4:54PM - 5:06PM |
C10.00011: Linking structure and dynamics in polymer-graphene oxide nanocomposites Michael Weir, Stephen Boothroyd, David Johnson, Rana Ashkar, Erkan Senses, Steven Parnell, Madhu Sudan Tyagi, Antonio Faraone, Richard Thompson, Karl Coleman, Nigel Clarke Graphene and related two-dimensional materials are excellent candidates as fillers in polymer nanocomposites due to their extraordinary physical properties and high aspect ratio. The effect of the added nanoparticle upon the structure and dynamics of the polymeric host material has a critical influence upon the final performance and properties of the composite material. High specific surface area nanomaterials in the graphene family alter the properties of the bulk polymer at very low concentrations. Our previous structural measurements on poly(methyl methacrylate)-graphene oxide (PMMA-GO) nanocomposites have shown reductions in polymer chain dimensions in the presence of graphene oxide and associated reductions in chain entanglement at around 0.5 per cent by volume of GO. We now extend our studies to the dynamics of the PMMA using quasi-elastic neutron scattering and neutron spin echo spectroscopy. We discover changes in PMMA dynamics that do not vary linearly with GO concentration. We observe an increase in the segmental relaxation time at low GO concentration and a decrease at high GO concentration, reflecting the non-monotonic trends seen in the structural data. We now explore simple scaling arguments to link the structural and dynamical interpretations of our system. [Preview Abstract] |
Monday, March 13, 2017 5:06PM - 5:18PM |
C10.00012: Surface Dynamics in Polymer Nanocomposite Films Kyle Johnson, Emmanouil Glynos, Suresh Narayanan, Georgios Sakellariou, Peter Green The dynamics of supported polymer nanocomposite thin films were studied using x-ray photon correlation spectroscopy. Measurements were performed on 50 and 200 nm films of unentangled poly(2-vinyl pyridine) (P2VP) grafted gold nanoparticles in an unentangled P2VP host of smaller molecular weight at temperatures above the glass transition. The grafted and host chain degree of polymerization were $N=$96 and $P=$44 respectively. The presence of the nanoparticles and their interactions with the polymer host are shown to slow down the dynamics of the film at large thicknesses. As the film thickness $h$ approaches the interparticle spacing $l_{D}$, the dynamics become much slower due to confinement of the nanoparticles. [Preview Abstract] |
Monday, March 13, 2017 5:18PM - 5:30PM |
C10.00013: Interfacial Dynamics of Polymers and Influence on Viscosity in Polymer Nanocomposites Gerald Schneider Nanocomposites based on polymer melts and nanoparticles are important for applications, such as enhanced barrier properties. Certainly, novel effects may emerge due to the high specific surface area. If polymers can adsorb the polymer dynamics may be significantly changed. Mostly these are associated with a slowing down of the polymer motion or a layer where the segments are immobilized. We used neutron spin echo and time-of-flight spectroscopy to track these changes. We found the segments are highly mobile, but additionally identified considerable changes depending on the polymer, the constituents and the interaction strength. The talk reports recent results and identifies potential impact to macroscopic properties, such as the viscosity measured by rheology. [Preview Abstract] |
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