Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J31: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinement: Friction and Adhesion |
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Sponsoring Units: DPOLY Chair: Connie Roth, Emory University Room: 339 |
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J31.00001: DILLON MEDAL BREAK
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Tuesday, March 19, 2013 3:06PM - 3:18PM |
J31.00002: Viscous Friction of Polymer Brushes Aykut Erbas, Michael Rubinstein Polymer brushes are unique soft structures that can exhibit solid-like behaviors, i.e., if they are deformed by an external force, they can relax and take their original conformations when the external force is removed. Despite their solid-like character, tribological behavior of polymer brushes exhibits fluid-like properties: For instance, friction force exerted on two interdigitated brushes sheared in opposite directions goes to zero linearly as the shear velocity vanishes, i.e., no static friction occurs, which is a property observed mostly for fluidic friction. In this talk, we present our simulation result and scaling arguments on the friction of planar brush-on-brush systems. Our theoretical approach and simulation regimes encompass both linear and non-linear regimes. We show that individual brush ends move on well-defined average trajectories. The dissipation in the system can be related to these average trajectories for a wide range of shear velocities. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J31.00003: Stick-Slip Dynamics Using Velcro as Model System Lisa Mariani, Cara Esposito, Paul Angiolillo Described by Galileo and further developed phenomenologically by Amontons and Coulomb, friction remains to be poorly understood especially with respect to its transition from the static to the kinetic regimes. In particular, the dynamics and control thereof of systems exhibiting stick-slip motion continues to be an area of fascination. The dry sliding behavior of the hook-and-loop system evinced by common Velcro captures many of the hallmarks of stick-slip motion typically manifested in systems at very small and very large length scales in addition to satisfying some of the classical laws as put forth by Amontons and Coulomb. Specifically, the kinetic frictional force is independent of driving velocity over nearly three orders of magnitude. In stark contrast to classical behavior, both the maximum static and the kinetic frictional forces reveal a linear dependence on the ``area of contact'' or more appropriately, hook number. Moreover, the frictional force (static and kinetic) exhibits a power law dependence on load with an exponent of approximately 0.25 similar to behavior seen in AFM, the implication being non-constant coefficients of static and kinetic friction. Statistical analysis shows that the fluctuations of stick-slip events follow a power law behavior with an exponent of approximately 0.5. Interestingly, this relatively simple system demonstrates evidence of precursor events prior to the onset of motion and may provide insight to the nucleation and transition from static to kinetic friction. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J31.00004: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J31.00005: Structure and dynamics of hyperbranched polymers in bulk and under nanoscopic confinement S. H. Anastasiadis, K. Chrissopoulou, K. Karatasos, S. Fotiadou, C. Karageorgaki, I. Tanis, D. Tragoudaras, B. Frick The structure and dynamics of a hyperbranched polyesteramide (Hybrane S 1200) and its nanocomposites with natural montmorillonite (Na+-MMT) are investigated. In bulk, the behavior is probed by QENS with MD simulations employed for a deeper insight into the relevant relaxation processes. The energy-resolved elastically scattered intensity from the polymer relaxes with two steps, one below and one above the polymer Tg. The QENS spectra are consistent with the elastic measurements and can be correlated to the results emerging from the detailed description afforded by the atomistic simulations, which cover a broad time range and predict the existence of three relaxation processes. The nanocomposites are investigated by XRD, DSC and QENS. XRD reveals an intercalated nanocomposite structure. The polymer chains confined within the galleries show similarities in the dynamic behavior with that of the bulk polymer for temperatures below the bulk polymer Tg, whereas they exhibit frozen dynamics under confinement at temperatures higher than that. Sponsored by the Greek GSRT ($\Sigma\Upsilon$NEP$\Gamma$A$\Sigma$IA 09$\Sigma\Upsilon$N-42-580). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J31.00006: Contact Mechanics of Nanoparticles J.-M.Y. Carrillo, A.V. Dobrynin We perform molecular dynamics simulations of the detachment of nanoparticles from a substrate. The critical detachment force, $f$*, is obtained as a function of the nanoparticle radius, $R_{p}$, shear modulus, $G$, surface energy, $\gamma_{p}$, and work of adhesion, $W$. The magnitude of the detachment force is shown to increase from $\pi $\textit{WR}$_{p}$ to 2.2$\pi $\textit{WR}$_{p}$ with increasing nanoparticle shear modulus and nanoparticle size. This variation of the detachment force is a manifestation of a neck formation upon nanoparticle detachment. Using scaling analysis, we show that the magnitude of the detachment force is controlled by the balance of the nanoparticle elastic energy, surface energy of the neck, and nanoparticle adhesion energy to a substrate. It is a function of the dimensionless parameter $\delta \propto \gamma_{p} (GR_{p} )^{-1/3}W^{-2/3}$which is proportional to the ratio of the surface energy of a neck and the elastic energy of deformed nanoparticle. In the case of small values of the parameter $\delta $ \textless \textless 1, the critical detachment force approaches a critical Johnson, Kendall and Roberts force, $f\ast \approx 1.5\pi WR_{p} $, as is usually the case for strongly crosslinked large nanoparticles. However, in the opposite limit, corresponding to soft small nanoparticles, for which $\delta $\textgreater \textgreater 1, the critical detachment force, $f$*, scales as $f\ast \propto \gamma_{p}^{3/2} R_{p}^{1/2} G^{-1/2}$. Simulation data are described by a scaling function $f\ast \propto \gamma_{p}^{3/2} R_{p}^{1/2} G^{-1/2}\delta^{-1.89}$ . [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J31.00007: Probing gradient of dynamics in confined polymers with nanoparticles Sivasurender Chandran, Nafisa Begam, Jaydeep Basu, Mrinmay Mukhopadhyay We report [1] the evidence of gradient in dynamics by probing the diffusion coefficient of polymer grafted nanoparticles (PGNP) in polymer thin films of different thickness (2.5 R$_{g}$ and 8R$_{g\, }$of the matrix). Using surface x-ray scattering, we observe a systematic vertical dispersion of PGNP from a pinned in substrate interface layer to the surface on thermal annealing. Even after annealing at high temperature (T\textgreater \textgreater T$_{g})$ and longer times, a fraction of PGNP pertain to stay at the substrate forming a stable interface layer. This hints about the low mobility of particles at the substrate interface and also emphasizes the presence of high viscous/gel-like interfacial layer. Real space microscopic images show the formation of lateral domains of the particles at air surface suggesting the higher surface mobility. In addition, it is also observed that the fraction of particles in the air surface is more in annealed thinner films compared to the thicker ones. Thus, we have correlated the observed lateral and vertical dispersion and its evolution with annealing, to the gradient in dynamics along the thickness of the thin films. \textit{[1] Sivasurender Chandran, J. K. Basu and M. K. Mukhopadhyay, in communication} [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J31.00008: Confinement of conjugated polymers into soft nanoparticles: molecular dynamics simulations Sidath Wijesinghe, Dvora Perahia, Gary S. Grest The structure and dynamics of conjugated polymers confined into soft nanoparticles (SNPs) have been studies by molecular dynamic simulations. This new class of tunable luminescent SNPs exhibits an immense potential as bio-markers as well as targeted drug delivery agents where tethering specific groups to the surface particles offers a means to target specific applications. Of particular interest are SNPs that consist of non- crosslinked polymers, decorated with polar groups. These SNPs are potentially tunable through the dynamics of the polymer chains, whereas the polar entity serves as internal stabilizer and surface encore. Confinement of a polymer whose inherent conformation is extended impacts not only their dynamics and as a result their optical properties. Here we will present insight into the structure and dynamics of dialkyl poly \textit{para} phenylene ethynylene (PPE), decorated by a carboxylate groups, confined into a soft particle. The conformation and dynamics of polymer within SNP will be discussed and compared with that of the linear chain in solution. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J31.00009: Forces between nanoparticles grafted with rigid polymers: a pathway for tunable hybrids Sabina Maskey, Dvora Perahia, J. Matthew D. Lane, Gary S. Grest The forces between the nanoparticles hybrids that consist of para dialkyl phenyleneethynylenes (PPEs) grafted to a silica nanoparticle have been studied using molecular dynamic simulations. PPEs are rigid polymers whose conformation determines their degree of conjugation and their assembly mode which in turn affects the electro-optical response of the nanoparticle-polymer complexes. When confined to a nanoparticle surface, the PPE chains are fully extended but cluster as the quality of the solvents is reduced. Tuning the degree of clustering by tuning the solvent-polymer interaction is expected to direct the assembly of the particles. Results for the forces between two nanoparticles functionalized with rigid polymers as a function of solvent quality, velocities and distances will be presented. These simulations will provide for the first time insight to the interactions of the nanoparticles grafted with rigid polymer, which in turn, results in formation of tunable hybrids. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J31.00010: Polymer Film Surface Fluctuation Dynamics in the Limit of Very Dense Branching Mark Foster, Boxi Liu, Suresh Narayanan, David T. Wu The surface height fluctuations of melt films of densely branched comb polystyrenes of thicknesses greater than 55nm and at temperatures more than 23 C above the $T_{g,bulk}$ can be rationalized using the hydrodynamic continuum theory (HCT) known to describe melts of unentangled linear and cyclic chains. Film viscosities ($\eta_{XPCS})$ for three combs inferred from fits of the HCT to X-ray Photon Correlation Spectroscopy (XPCS) data are the same as bulk viscosities ($\eta_{bulk})$ measured with rheometry. For the comb most like a star polymer and the comb closest to showing bulk entanglement behavior, $\eta_{XPCS}$ is greater than $\eta_{bulk}$. However, the values of $\eta_{XPCS}-\eta_{bulk}$ are much smaller than those seen for less densely branched polystyrenes. We conjecture that the smaller magnitude of $\eta_{XPCS}-\eta_{bulk}$ for the densely grafted combs is due to a lack of interpenetration of the side chains when branching is very dense. While data of relaxation time versus $T$ for cyclic chains virtually collapse to a single curve when $T_{g,bulk}$ is accounted for, that is not the case for combs. $T_{g,bulk}$ and specific chain architecture both play important roles in determining the surface fluctuations. Acknowledgements: NSF CBET 0730692, CBET-0731319, DURIP W911NF-09-1-0122. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J31.00011: Axial and radial nanostructures in electrospun polymer fibers Israel Greenfeld, Andrea Camposeo, Francesco Tantussi, Stefano Pagliara, Francesco Fuso, Maria Allegrini, Dario Pisignano, Eyal Zussman The high tensional stresses during electrospinning of semidilute polymer solutions affect the dynamic conformation of the polymer network within the liquid jet, leaving a distinctive trace in the molecular structure after solidification. We investigated such effects in electrospun nanofibers made of conjugated polymers. Modeling the polymer network evolution during electrospinning showed that as the network stretches axially, it contracts towards the jet core. The model represents the semi-flexible conjugated polymer chains as flexible freely-jointed chains, whose joints are bonding defects. Using the conjugated polymer MEH-PPV dissolved in a mixture of THF and DMF solvents, and taking advantage of its unique photophysical characteristics, we investigated optically the variations in the density and orientation of the polymer macromolecules in electrospun nanofibers. In agreement with our model, we found higher density and axial orientation at the fiber core, while lower density and radial orientation closer to the fiber surface. The non-uniformity of the resulting molecular structure can be tuned and exploited in diverse optical and structural applications. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J31.00012: Correlation between the interfacial bond orientational order and the shift in T$_{g}$ upon confinement Simone Napolitano The two-order-parameter (TOP) model rationalizes the interfacial slower dynamics in terms of enrichment in bond orientational order (BOO), near the wall [1]. Recently, we verified that the dielectric strength, $\Delta\epsilon$, is a robust parameter for measuring the BOO, as $\Delta\epsilon$ = g$<$$\mu$$^{2}$$>$/k$_{B}$T, where $\mu$ is the dipole moment, and g accounts for the correlation among neighbor dipole moments. We obtained interfacial values of the dielectric strength, $\Delta\epsilon$$_{int}$, analyzing the thickness dependence of all the polymers for which $\Delta\epsilon$ was measured upon confinement in ultrathin films. Although for all the investigated systems $\Delta\epsilon$ decreases in proximity of a solid interface due to the reduction in $<$$\mu$$^{2}$$>$ upon adsorption [3], we identified a striking correlation between $\Delta\epsilon$$_{int}$ and the shift in T$_{g}$ upon confinement. Increases in T$_{g}$ were univocally correlated to nonzero positive values of $\Delta\epsilon$$_{int}$, implying a larger BOO near the wall, in line with the predictions of the TOP model. [1] Watanabe, Kawasaki, Tanaka, Nature Materials 2011, 10, 512 [2] Capponi, Napolitano, Wuebbenhorst, Nature Comm. 2012, accepted [3] Napolitano, Wuebbenhorst, Nature Comm. 2011, 2, 260 [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J31.00013: Conformational Relaxation of Polystyrene at Substrate Interface Hirofumi Tsuruta, Yoshihisa Fujii, Hiroshi Morita, Keiji Tanaka The local conformation of polymer chains in a film at a substrate interface was examined by sum-frequency generation spectroscopy. When a polystyrene (PS) film was prepared on a quartz substrate by a spin-coating method, the chains were aligned in the interfacial plane of the substrate. A dissipative particle dynamics simulation revealed that a spinning torque induced the chain orientation during the film preparation process and the extent of the orientation was a function of the distance from the interface. This interfacial orientation of chains was not observed for a PS film prepared by a solvent-casting method. Interestingly, the local conformation of chains at the substrate interface was unchanged even at a temperature that was 80 K higher than the bulk glass transition temperature. This observation means that polymer chains at the substrate interface can be only partially relaxed under conditions where the bulk chains are fully relaxed. On the other hand, interfacial chains could be easily relaxed by solvent annealing. [Preview Abstract] |
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