Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y4: Polymer Dynamics and Rheology |
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Sponsoring Units: DPOLY Chair: Kalman Migler, NIST Room: LACC 515 A |
Friday, March 25, 2005 11:15AM - 11:51AM |
Y4.00001: Nonlinear Flow Behavior of Model Branched Polymers Invited Speaker: Stress relaxation dynamics of entangled long-chain branched (LCB) polymers are investigated using model symmetric stars, asymmetric stars, and multiarm (pom-pom), polymer liquids subject to small-amplitude oscillatory shear and nonlinear step and steady shear deformations. This talk focuses on the effect of molecular architecture on chain stretching, tube dilation, convective constraint release, and interfacial slip processes. It is shown that even small degrees of arm length asymmetry leads to large differences in the nonlinear relaxation dynamics of branched molecules. Specifically, while symmetric stars manifest greater strain softening characteristics and much lower degrees of chain extension than entangled linear chains, asymmetric stars and pom-poms with even small levels of arm length asymmetry display nonlinear rheological properties that are in most respects identical to those of entangled linear polymers with comparable terminal times. Consequences of these observations on tube-based constitutive models for branched polymers are discussed. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:27PM |
Y4.00002: How Nanoparticles Impact Phase Evolution in Polymer Blend Films Invited Speaker: Polymer films containing nanoparticles (NP) are of technological interest because their unique mechanical, electrical and optical properties. Here, we present a systematic study of phase separation dynamics in polymer blend films containing mobile NP. Films (650nm) of PMMA:SAN (50:50) containing methyl-terminated silica NP provide a model system to investigate wetting and phase separation dynamics. Concurrent with PMMA wetting, NP segregate to the air and substrate interfaces. The NP also preferentially partition into the PMMA rich domains. During the intermediate stage, the correlation length between domains, $\xi $, scales as $t^{1/3}$ for neat blends as well as those with 2 and 5 wt.{\%} NP. As NP concentration increases, $\xi $ increases more slowly, consistent with a coalescence model that reflects the increase in PMMA viscosity due to the NP. The effect of PMMA-grafted silica NP on phase separation and wetting depends on the length of the grafted chain. In some cases, the early stage morphology is preserved for long-times and film rupture is prevented. Selected studies of Ag-nanoparticle formation in homopolymer films will also be presented. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 1:03PM |
Y4.00003: Time-dependent structure of polymer brushes Invited Speaker: The time dependence of polymer brush structure may be viewed at a number of levels: One level is the construction time for a brush from solution of polymers that self-assemble on a surface by attachment at one end only to form the brush; another concerns the relaxation of brushes towards equilibrium after they have been sheared, a property of importance for lubricating brushes. The talk will consider progress at both of these levels, especially the effect on the first level of varying the brush-end sticking energy, and the effect on the second level of shear of mutually-compressed sliding brushes. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:39PM |
Y4.00004: Phonons in Soft Microstructures Invited Speaker: George Fytas Among the inelastic scattering methods only Brillouin light scattering spectroscopy possesses sufficient resolution to record the rich phonon spectrum S(q,\"{y}) of soft microstructures. Ordered systems with spacing qd=0(1) (q being the scattering photon wavevector) and elastic constants varying in space control the propagation of several high frequency excitations. Besides the acoustic phonon of the effective medium, ``Bragg'' modes due to the interaction of q with the phonon wavevector k and the reciprocal lattice vector G, mixed phonons of the phononic band structure and vibrational eigenfrequencies can contribute to the S(q,\"{y}) of ordered polymer and colloidal systems. We present rich phonon dispersion relations in polystyrene opals, ordered diblock copolymers and interference lithography patterned single crystalline polymer films. Theoretical calculations of dispersion relations were performed to explain the nature of the observed propagation modes. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 2:15PM |
Y4.00005: Turbulent drag reduction by additives Invited Speaker: Turbulent drag reduction by additives Daniel Bonn ENS/LPS Paris, France and WZI Amsterdam, The netherlands A spectacular reduction of energy losses in turbulent flows can be achieved by the addition of small amounts of certain polymers or surfactants. I will discuss a few of our recent experiments that shed some light on the different possible mechanisms of drag reduction. Polymer drag reduction is believed to be due to the large elongational viscosity of the polymer solution; this stabilizes the turbulent boundary layer, leading to less turbulent energy generation, and hence less dissipation. For flexible polymers, indeed a correlation between drag reduction and elongational viscosity can be found. We show however that solutions of rigid polymers that exhibit no measurable elastic effects, drag reduction is as efficient as for flexible polymer solutions with an elevated elongational viscosity. This drag reduction can be attributed to the existence of a spatial viscosity gradient in the boundary layer. For surfactants, we show that drag reduction is due to an effective wall slip. Due to the wall slip, the bulk of the turbulent system does not experience a large wall friction. Since it is the wall friction that generates the turbulence, this naturally leads to a smaller dissipation. [Preview Abstract] |
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