Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S14: Invited Session: Dynamics of Polymers at Interfaces and in Confinement |
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Sponsoring Units: DPOLY DFD Chair: Oliver Baeumchen, Max Planck Institute for Dynamics and Self-Organization Room: 301-303 |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S14.00001: Polymer dynamics and stress transmission at polymer interfaces Invited Speaker: Frederic Restagno End tethered polymer chains have been recognized to be excellent adhesion promoters at polymer or are known to be able to modify the friction at interfaces. In this talk, we will present 3 related sets of experiments on the dynamics of polymer close to an interface. We will present results of polymer chains in contact with a melt, in contact with an elastomer and in contact with a solvent. In a first part, we will present neutron reflectivity experiments characterizing the density profiles and dynamics of interdigitation between H-PS brushes, in contact with d-PS thick layers, heated above their glass transition temperature. This interdigitated brushes will be shared and disentangled from the melt leading to a large slip at the interface. The density profiles of the sheared brushed will be presented. In a second part we will present some friction measurement at a PDMS chains -- PDMS elastomer interface and we will show that the grafted chains penetration and dynamics allows to control the friction at such elastomer interfaces. Finally we shall present some measurements of the mechanical response of the swollen PDMS chains using nanorheology experiments and we will evidence the role of the confining surfaces in the measurements. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S14.00002: Welding and healing of polymer interfaces: Connecting structure, dynamics and strength Invited Speaker: Mark Robbins Applying heat to polymer interfaces is a common means of welding polymer components or healing cracks in polymers. Once chains have diffused by their radius of gyration, the properties of the interface should be indistinguishable from those of the bulk. In practice, welds can achieve bulk strength at much shorter times. The mechanism of strength growth is difficult to determine with experiments, because they cannot directly access the evolution of molecular configurations and entanglements. Large-scale simulations were used to follow the dynamics of interdiffusion at welds and cracks and the associated changes in density and molecular conformations.\footnote{T. Ge, G. S. Grest and M. O. Robbins, ACS Macro Letters 2, 882-886 (2013) T. Ge, F. Pierce, D. Perahia, G. S. Grest and M. O. Robbins, Phys. Rev. Lett. 110, 098301 (2013).} The evolution of entanglements was tracked using Primitive Path Analysis and shown to be directly related to the mechanical strength under shear and tensile loading. As in experiment, the maximum shear strength $\sigma_{max}$ of a homopolymer interface rises as a power of welding time $t$ and then saturates at the bulk value. Simulations show that $\sigma_{max}$ is proportional to the areal density of interfacial entanglements at short times and saturates when chains have formed 2-3 entanglements across the interface. Enthalpy limits interdiffusion across heteropolymer interfaces, and there is a corresponding reduction in interfacial entanglements and mechanical strength. A minimum loop length of order the entanglement length must diffuse across the interface to form entanglements. Cracks are more complicated because of the presence of short segments produced during fracture. Segments that are too strong to confer bulk strength, but longer than the entanglement length, remain near the interface for long time intervals. This leads to a plateau in strength that is below the bulk value. Crazes form under tensile loading. A low interfacial entanglement density can stabilize craze formation and significantly enhance the fracture energy, but the bulk fracture energy is recovered at about the same time as bulk shear strength. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S14.00003: Submicron flow of polymer solutions: slippage reduction due to confinement Invited Speaker: Hugues Bodiguel Managing flows of polymer solutions through micro- and nanochannels is important for many applications in various fields, including energy conversion processes, nanotechnologies and flows in porous media. Even above the micron scale, violation of the no-slip boundary condition has been recognized to be very important, especially for high molecular weight polymers for which slip length up to tens of micrometers have been reported. As a consequence, pressure-driven flow of polymer solutions in channels of the order of one micrometer and less should be dominated by slippage. Using micro- and nanofabrication methods to control the geometry and fluorescence photobleaching based velocimetry technique, we developed an experimental approach to investigate flows of polymer solutions in slits of thickness as low a 150 nanometers. In non-adsorbing conditions, and for various polymer solutions (polyacrylamide or polystyrene at different concentrations), our results unambiguously show that the resistance to the flow decreases when the channel height decreases below the micron scale. Besides, the apparent slippage of the solutions is characterized macroscopically on similar surfaces. Though slippage can explain qualitatively the effective viscosity reduction, quantitative comparison fails. The reduction of the effective viscosity is significantly smaller than one could have expected knowing the slippage properties. This shows that the slip length is reduced below the micron scale. This effect is stronger when decreasing the polymer length or when lowering the concentration. Interpretations of these results are based on chain migration due to hydrodynamics interactions, which would explain on the one hand the large macroscopic slip lengths and on the other hand their reduction in confinement. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S14.00004: The instabilities of a polymer sheet floating at a fluid interface Invited Speaker: Narayanan Menon The beautiful patterns seen on thin floating polymer sheets have led to a new and broadened understanding of the instabilities of an elastic sheet under tension. I will briefly review this progress, which includes identification of a dimensionless number -- the bendability -- that demarcates regimes in which the wrinkling instability of the sheet may either be successfully described by conventional post-buckling theory or requires an entirely different scheme of calculation in which the bending energy is negligible. This new understanding throws into relief new puzzles associated with the dynamics of the pattern growth, and with the transition from the wrinkled state to a crumpled state. I will also describe the new opportunities opened up by phenomena at high bendability. These include measurements of surface energies and contact angles on a deformable substrate, a new method for studying the modulus and extensional rheology of a thin polymer film, and techniques for modification of surface properties of a fluid interface. I thank NSF DMR 12-0778 and the NSFon Polymers at UMass Amherst DMR 08-20506 My thanks to J. Huang, H. King, K.B. Toga, T.P. Russell for collaborations on the experiments and to B. Davidovitch, E. Cerda and R. Schroll for theoretical collaborations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S14.00005: Nanoscale Confinement in Single-Layer and Multilayer Supported Polymer Films: Effects on Glass Transition Temperature and Surface Capillary Wave Dynamics near the Glass Transition Invited Speaker: John Torkelson A number of studies have reported major differences in the effects of confinement on the glass transition temperature, Tg, of polymers as determined by (pseudo-)thermodynamic methods and on cooperative segmental dynamics as probed by techniques such as dielectric spectroscopy. While substantial Tg-confinement effects are often observed, the effects on cooperative mobility are often muted or absent. Here, we describe studies employing single-layer films and multilayer films of immiscible polymers in which both Tg and dynamics, related to surface capillary wave relaxation characterized by x-ray photon correlation spectroscopy, are strongly affected by confinement and neighboring polymer layer species. Regarding Tg, we show that a key parameter governing the effect of confinement is polymer fragility -- that of the polymer being characterized for Tg in single-layer films and that of the neighboring layer for multilayer films. Similarly, at temperature near Tg, surface capillary wave dynamics of a top layer of a bilayer film can be strongly affected by the neighboring underlayer, with underlayer modulus and confinement itself being important factors governing the dynamics. Both factors are negligible at Tg $+$ 40 K in the case of polystyrene top layers, demonstrating the importance of temperature in tuning the effects of confinement and substrates on dynamics. [Preview Abstract] |
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