Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y31: Polymer Melts and Solutions |
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Sponsoring Units: DPOLY Chair: Howard Wang, Michigan Tech University Room: LACC 503 |
Friday, March 25, 2005 11:15AM - 11:27AM |
Y31.00001: Shear SANS Study of Entangled Polymer Solutions Howard Wang, Lionel Porcar, Derek L. Ho, Prashant Tapadia, Shi-Qing Wang, Michael Olechnowicz, Roderic Quirk In situ small angle neutron scattering and rheological measurements have been carried out on entangled polybutadiene solutions in simple shear flow in a Couette shear cell (SANS) and cone/plate cell (rheology). Stable shear flow has been achieved over a range of shear rates covering the stress plateau, at which polymer chains are expected to partially disentangle [1]. Shear-induced structure was recorded in both the flow-vorticity and gradient-vorticity planes. The data appeared to imply the absence of ensemble averaged large chain deformation in steady state, whereas geometrical scaling analysis suggested that chain stretching could occur locally. [1] Tapadia, P.; Wang, S. Q. Macromolecules, 37, 9083 (2004). [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y31.00002: Molecular Weight Dependence of the Viscosity of Polyethylene Macrocycles Jian Wang, Gregory McKenna, Diego Benitez, Irina Gorodetskaya, Robert Grubbs The Grubbs group at Caltech is using a new method of synthesis for the production of macrocycles that avoids the problems of sample contamination with long chain linear molecules. They have produced macrocyclic polyethylenes having molecular weights as high as 400,000 g/mole. At Texas Tech University rheological measurements are being made using both the Plazek Magnetic Bearing Torsional Creep Apparatus and a Paar Physica controlled stress rheometer. Because the samples are polydisperse, with M$_{w}$/M$_{n}$ values of 1.6-1-9, we here report only the results of the viscosity as a function of molecular weight. Surprisingly, unlike reports in the literature for polystyrene rings or polybutadiene rings, the viscosities of the polyethylene rings are larger than those for equivalent molecular weight linear polyethylenes and the viscosity increases approximately exponentially with weight average molecular weight. Further results will be reported at the meeting. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y31.00003: Thermorheological Complexity in Polystyrene Melt Yn-Hwang Lin Using the successful description of creep compliance$ J(t)$ of nearly monodisperse polystyrene melts$^{1}$ in terms of the extended reptation theory$^{2}$ (ERT) in the rubber(like)-fluid region as the\textit{ reference frame} in time, the analysis of the glassy-relaxation process$ A_{G}u_{G}(t)$ that occurs in the short-time region of $J(t)$ in terms of a stretched exponential form incorporated into ERT reveals that the temperature dependence of the $A_{G}u_{G}(t)$ process being stronger in a simple manner than that of the \textit{entropy-derived} ERT processes accounts fully for the uneven thermorheological complexity in the $J(t)$. The results being displayed in the modulus $G(t)$ form, it is shown that at $T_{g}$, the contribution from$ A_{G}u_{G}(t)$ to $G(t)$ at the time scale corresponding to the highest Rouse-Mooney normal mode greatly exceeds that derived from entropy, indicating vitrification at the Rouse-segmental level. At the same time the Rouse-Mooney normal modes provide an internal yardstick for estimating the length scale of the polymer at $T_{g}$, giving 3 nm for polystyrene. Based on the obtained results, the basic mechanism for the thermorheological complexity is analysed, showing that the break-down of Stoke-Einstein relation in glass-forming liquids, such as OTP, should occur for a similar reason. Ref: (1) D. J. Plazek, \textit{J. Phys. Chem.} \textbf{1965}, $69$, 3480; \textit{J. Polym. Sci. A}-2 \textbf{1968}, $6$, 621. (2) Y.-H. Lin, \textit{Macromolecules} \textbf{1984}, $17$, 2846; \textbf{1986}, $19$, 159; 168; \textbf{1987}, $20$, 885; \textbf{1999}, $32$, 181; \textit{Polymer Viscoelasticity}:\textit{ Basics, Molecular Theories and Experiments}; World Scientific: Singapore, 2003. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y31.00004: Nonlinear hydrodynamic description of non-Newtonian fluids Harald Pleiner, Mario Liu, Helmut R. Brand We review conventional constitutive equations for non-Newtonian fluids from a hydrodynamic point of view. Using general thermodynamic and symmetry arguments and applying valid physical principles we describe viscoelasticity by setting up nonlinear dynamic equations either for a relaxing (Eulerian) strain tensor or for a transient orientational order parameter tensor. This covers the usual non-Newtonian effects, like shear thinning, strain hardening, stress overshoot, normal stress differences and non exponential stress relaxation. In both cases an effective dynamic equation for the stress tensor can be derived approximately and compared with conventional non-Newtonian rheological models. It is more general in structure than those, comprises most, restricts some, and discards a few of them. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y31.00005: Mechanical Hole Burning Spectroscopy of Branched and Linear Polymers Xiangfu Shi, Gregory McKenna We have developed a mechanical spectral hole burning (MSHB) scheme that is analogous to non-resonant dielectric spectral hole burning (DSHB). DSHB experiments have been performed close to the glass temperature and interpreted in terms of dynamic heterogeneity. Here we find that holes are burned far above the glass temperature and in the terminal regimes for a branched polymer melt and a polymer solution. The results suggest that MSHB is a potentially powerful tool with which to examine dynamics of complex fluids. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y31.00006: Stress relaxation of polymer networks containing low concentrations of dangling chains and star shaped polymers Daniel A. Vega, Leopoldo R. G\'omez, Marcelo A. Villar, Enrique M. Vall\'{e}s We analyze the influence of low concentrations of star and dangling polymer chains on the stress relaxation process of model polymer networks. Model PDMS networks with well defined structure were obtained by the hydrosylilation reaction, based on the addition of hydrogen silanes from a trifunctional cross- linker to end vinyl groups of $\alpha - \omega$ polydimethylsiloxane chains. Rheological characterization was carried out in a rotational rheometer by dynamic and stress relaxation tests. Viscoelastic properties of the networks depend strongly on the molecular weight of the stars or pendant chains. It was found that a modified Pearson-Helfand model provides a very good fit to the behavior of these networks. This model incorporates the effect of higher Rouse modes on the arm retraction [Milner and McLeish, Macromolecules, 1997] and the potential for arm retraction originally proposed by Doi and Kuzuu. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y31.00007: Stress Relaxation of 1,4-polyisoprene T- and Y-shaped Star Polymers Jung Hun Lee, Lynden A. Archer We discuss the stress relaxation of 1,4-polyisoprene T- and Y-shaped star polymers in order to identify the effect of branch-point on the relaxation dynamics. The effects of arm length asymmetry and the position of branch-point on the stress relaxation are investigated by varying molecular architecture from T-shaped to Y-shaped. We observed the measured loss modulus more sensitively reflects a transition in the governing dynamics from star-like to linear-like according to the given architectures. This finding suggests a possibility to describe the dynamic transition by the constituent structural terms. A simple relationship between the branch-point motion and the given architecture is discussed based on the widely used microscopic model frame for the branched polymers. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y31.00008: When does a molecule become a polymer? Yifu Ding, Alexander Kisliuk, Vladimir Novikov, Alexei Sokolov The molecular weight (MW) dependences of chain, segmental and fast dynamics have been studied for a few polymers. These properties (except chain dynamics) appear to have similar MW dependence that they all saturate when chain approaches Gaussian coil behavior. Chain dynamics in PDMS reaches asymptotic Rouse prediction at the same MW. We demonstrate that the difference in the MW dependence for various polymers does not correlate to either the difference in the Kuhn length or Me. We introduce an additional parameter, mR (MW for each random step) that might be important for characterizing chain statistics and the MW dependence of many physical properties. The most intriguing result is that the MW dependence of the fast dynamics, elastic property and fragility observed in PS is opposite to the one observed in PIB. We speculate that the difference in symmetry of the structural unit is responsible for the opposite behavior. Based on this idea, predictions for the dependence of the fast dynamics and fragility in polymers on MW and tacticity are formulated. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y31.00009: Pressure effects on Solutions of Diblock Copolymers: Small Angle Neutron Study Dvora Perahia, Gang Cheng The effects of pressure on the shape of the micelles of Polystyrene-b-Polyisoprene (PS-PI) in decane were studied as a function of temperature and concentration using small angle neutron scattering (SANS). Decane is a preferential solvent for the PI block, thus driving the association of the polystyrene groups. These diblocks self-assemble in decane to form star-like spherical micelles with a swollen polystyrene core and a diffuse polyisoprene corona. All the solutions studied exhibit a critical micelle temperature, i.e. temperatures under which most of the polymer molecules are within the aggregates. In solutions consisting of none-interacting micelles, with increasing pressure, increasing the pressure up to 2000 PSI, results in changes of the shape of the micelles from spherical to elongated objects. At higher concentrations, the inter-micellar interactions increase significantly. Changing the pressure affects the miscibility of the different components and as such the structure of the basic aggregates. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y31.00010: Brownian Dynamics Study on the Dynamics of Asymmetric and Symmetric Star-Branched Polymers in Dilute Solutions Yong Lak Joo, Yongmin Lee It has been known that the dynamical behavior of branched polymers can significantly be different from linear polymers even in dilute solutions. The goal of this investigation is to understand detailed configuration and stretching dynamics of polymers with complex branching topologies in dilute solutions under various flows. In particular, we are interested in the transient stress response and conformational hysteresis of star-branched polymers in shear and extensional flows. Using Brownian dynamics simulations of bead-spring models, systems such as dilute solutions of Y-branched and star-branched polymers under flows are investigated. Studies on polymers with asymmetric arms reveal that the initial transient response is governed by both the number of arms and the shortest arm, whereas the stress response at intermediate strains becomes dominated by the longest arm. The results obtained from stress-conformation hysteresis simulations indicate that rapid extension followed by retarded relaxation of short arms in star-branched molecules. Finally, excluded-volume interactions are incorporated through the Lennard-Jones potential, and are applied to the study of the dynamics of branched polymers under confinement. [Preview Abstract] |
Friday, March 25, 2005 1:15PM - 1:27PM |
Y31.00011: Diffusive Transport in Hydroxypropylcellulose:Water Kiril A. Streletzky, George D.J. Phillies, Robert O'Connell, Paul Whitford, Helen Hanson A systematic analysis of mode structure of diffusive relaxations in solutions of neutral polymer of 1MDa hydroxypropylcellulose (HPC) is presented. Experimental techniques that have been applied in our studies include static light scattering, dynamic light scattering, optical probe diffusion, and viscometry. In the optical probe diffusion method we monitor the translational diffusion of dilute monodisperse spheres through aqueous polymer solutions over a range of temperatures, distances, and time scales. Based on the data from these extensive studies we conclude that: 1) HPC solutions have a characteristic length of 50-70nm that matches the polymer’s hydrodynamic radius; 2) the diffusion rate of optical probes through aqueous polymer solutions is not determined by the macroscopic viscosity of the solutions; 3) probe and polymer relaxations are not generally the same; 4) the apparent viscometric crossover near 6g/L is confirmed by the optical probe behavior. [Preview Abstract] |
Friday, March 25, 2005 1:27PM - 1:39PM |
Y31.00012: Multiple Light Scattering Probes of Polyurethane Foam Structure Weijun Zhou, Dwight Latham, Anne Leugers The cellular structure of polyurethane foams is probed quantitatively by investigating the multiple scattering characteristics of foams with near IR wavelength light. In the limit of strong multiple scattering regime, the propagation of light can be approximated as a diffusion process. Static diffusive transmission (T{\%}) of foams showed a rather interesting relationship with foam thickness (L) as T $\sim $ 1/L. This is consistent with what one expects from diffusion theory. The transport mean free path of light diffusion was determined to correlate directly with foam cell size. But surprisingly, the transport mean free path of light within polyurethane foam was found to be smaller than cell size, suggesting a strong intra-cellular scattering mechanism. Applications of multiple light scattering techniques to dynamic foaming process will also be discussed. [Preview Abstract] |
Friday, March 25, 2005 1:39PM - 1:51PM |
Y31.00013: Scaling laws for polymer chains using mesoscopic simulations Vasileios Symeonidis, Bruce Caswell, George Karniadakis Several types of bead-spring representations of chains are used to model simulations of dilute polymer solutions by dissipative particle dynamics (DPD). The static exponent scaling law is used to detect the prevalence of phantom collisions. For the models used, values range between 1/2 and 3/5, the Flory value for self-avoiding walks. Chain behavior under steady shear shows no similar scaling law, but different stretching response for finitely and infinitely extensible springs. The effect of temperature is also investigated for a chain of five, ten and twenty monomers. [Preview Abstract] |
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