Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V17: Focus Session: Dynamics of Polymers and Complex Fluids II |
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Sponsoring Units: DPOLY Chair: Ron Larson, University of Michigan Room: B116 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V17.00001: Diffusion of Particles in Polymer Solutions Liheng Cai, Michael Rubinstein We use scaling theory to derive the time dependence of the
mean-square-displacement $\langle\Delta r^2\rangle$ of a probe
particle of size $d$ in an entangled semidilute polymer solution.
Particles with size smaller than solution correlation length
$\xi$ undergo ordinary diffusion ($\langle\Delta r^2 (t)\rangle
\sim t$) with diffusion coefficient determined by the solvent
viscosity. The motion of particles with intermediate sizes
($\xi |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V17.00002: Diffusion of Small Penetrants in Polybutadienes Ahmed E. Ismail, Gary S. Grest, Flint Pierce, Mathew C. Celina Polybutadiene is a high-volume synthetic rubber with a low glass transition temperature that can be made in a wide range of forms from pure cis to pure trans conformations. In spite of its industrial importance, there is significant variance in the observed diffusivity of small penetrants through polybutadiene for both experiment and simulation. We use explicit-atom molecular dynamics simulations to model the diffusivity of the penetrants water, oxygen, and methanol in three polybutadiene systems, all {\em cis}-1,4, all {\em trans}-1,4 and a microstructure of 40\% {\em cis}-1,4, 50\% {\em trans}-1,4, and 10\% vinyl monomeric units. In addition to varying the monomeric composition, we also study the effects of varying the chain length of the polybutadiene molecules, temperature and penetrant concentration. The trajectories of individual molecules are analyzed to determine if the ``caging'' and ``hopping'' phenomena observed by M\"uller-Plathe for methane in polyethylene occurs. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V17.00003: Application of the discrete slip-link model to bidisperse linear systems Renat Khaliullin, Jay Schieber It has been shown that tube models can predict linear viscoelasticity (LVE) of monodisperse linear polymers well. The LVE predictions of bidisperse systems by these models are less good, since tube models essentially predict double reptation for blends. However, better agreement with data is obtained by using a phenomenological exponent of 2.2, which was proposed by Marrucci in 1985 and also recommended by Ruymbeke et al. in 2002. The exponent is hypothesized to be an effect of a new physics that was missing in the monodisperse case, either from non-binary entanglements or tube dilation. In previous works we showed that, although the LVE predictions of monodisperse systems by the discrete slip-link model (DSM) are at least as good as those made by tube models, there are significant differences in contributions to relaxation from polymer chain dynamics and environment dynamics between DSM and tube models. This observation suggests that tube models and DSM might yield different predictions for the observable relaxation modulus of bidisperse blends. In this work we show that DSM with only binary entanglements predicts bidisperse LVE at least as well as double reptation with the phenomenological exponent. Since no additional physics are required the parameters fit to monodisperse LVE may be used to predict polydisperse systems. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V17.00004: Relaxation dynamics in chain fluids Joanne Budzien We performed molecular dynamics simulations of coarse-grained bead-spring models for a variety of state points to investigate relaxation properties. Varying the interactions with the chains (e.g., adding angle and torsion constraints) affects the specific dynamics of the systems, but not the form of their relationship with thermostatic quantities. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V17.00005: X-ray Intensity Fluctuation Spectroscopy Studies of Dynamics of Block Copolymers in Selective Solvents Yongsheng Liu, Julian Spring, Karl Ludwig, Rama Bansil We investigated the dynamics of a block copolymer SEBS (triblock of styrene (S) and ethylene-butylene, EB) in selective solvents using X-ray Intensity Fluctuation Spectroscopy (XIFS). We examined the temperature dependence of dynamics in cylindrical, spherical and lamellar phases as well as in coexisting phases through the transitions. We were able to observe dynamics in hexagonally-packed-cylinders (HEX) and body-centered-cubic (BCC) phases in 45{\%} SEBS in mineral oil as well as HEX and lamellar (LAM) phases in 40{\%} SEBS in dibutyl phthalate (selective to S). We observed two exponentially decaying dynamic modes in coexisting HEX and BCC phases, and in coexisting LAM and HEX phases. While the slower mode has a q-independent decay rate, the faster mode's decay rate decreases with increasing q, consistent with the well-known deGennes narrowing as q varies through a structural peak [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V17.00006: Modeling dynamic mechanical response in polydomain nematic elastomers Badel Mbanga, Fangfu Ye, Jonathan Selinger, Robin Selinger When a polydomain nematic elastomer is subject to a uniaxial strain, it may undergo a transition to a monodomain texture. We model the resulting microstructural evolution and dynamic mechanical response using a 3-d finite element elastodynamics approach. We show that the nature of the transition is governed by the thermomechanical history of the sample. In particular, polydomain samples crosslinked in the nematic phase (N-PNE) show a pronounced ``crosslink memory'' effect, in which the local preferred nematic director orientation is imprinted in the polymer network upon crosslinking. By contrast, the cross-link memory effect is at least an order of magnitude smaller in samples crosslinked in the isotropic phase (I-PNE). Our simulation results are in good qualitative agreement with observed differences in the stress-strain behaviors of N-PNE and I-PNE materials, as seen in recent experiments [K. Urayama, Macromolecules 2009]. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V17.00007: Pressure effects on the dynamics of semiflexible polymer melts Jutta Luettmer-Strathmann, Kiran Khanal An increase in pressure increases the density in the local environment of chain segments and therefore reduces segmental mobility. In melts of flexible polymer chains, chain conformations are little affected by an increase in pressure and, thus, the chain mobility decreases with the segmental mobility. In melts of semiflexible chains, on the other hand, an increase in pressure affects both the local environment and the chain conformations since packing effects lead to an increase in orientational order. In this work, we investigate the effect of pressure on the segmental and chain dynamics of semiflexible polymer melts with Monte Carlo simulations of a bond-fluctuation lattice model. Simulation results for short chains (to avoid entanglement effects) for a range of temperature and pressures are presented and analyzed to reveal the correlation between conformational and dynamic properties of the chains. [Preview Abstract] |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V17.00008: Irreversible Flow-Induced Structure Transition in Rodlike Micelle Solutions M. Vasudevan, E. Buse, D. Lu, A. Shen, B. Khomami, R. Sureshkumar It is well known that translucent solutions containing rodlike surfactant micelles can form flow-induced structures. To date all reported FIS transitions are reversible, i.e., the gel disintegrates after flow stoppage. We show that in microfluidic devices that allow for the generation of extension rates much greater than those realized in conventional rheometers, irreversible FIS transitions can occur. Cryo-TEM analysis of the gel reveals a partially aligned micelle network. The critical flow rate for gel formation is consistent with a mechanism based on the fusion by collision of flow-aligned micelles proposed by Turner and Cates (J. Phys: Condens. Matter, 4, 3719 (1992)). [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V17.00009: Rheology and structure of concentrated multi-walled carbon nanotube dispersions under shear flow Wesley Burghardt, Saswati Pujari, Sameer Rahatekar, Jeffrey Gilman, Krzysztof Koziol, Alan Windle We describe measurements of flow-induced orientation in concentrated multi- walled carbon nanotube (MWNT) suspensions using x-ray scattering techniques, and its manifestation in bulk rheology. Measurements were conducted on a sample with 2wt\% MWNTs dispersed in a Newtonian uncured epoxy matrix. Steady shear rheology exhibited shear thinning behavior, with a viscosity close to that of the matrix at the highest shear rates. This is accompanied by increased MWNT orientation towards the flow direction in the 1-2 and 1-3 planes of shear flow. Transient rheology during step down tracked the dynamics of the increase in viscosity upon decreasing shear rate. MWNT orientation in both planes decreased during step down at time scales similar to the rheological measurements. The transient behavior is attributed to shear induced break up/ reformation of a spanning MWNT network. Cessation experiments showed slow, prolonged growth of solid modulus at rest, accompanied by rapid partial recovery of anisotropy in structure. This is attributed to fast, elastic recoil of MWNT clusters at short times, leading the drop in anisotropy, followed by slow ``sticking'' of MWNT clusters over prolonged time, leading to network reformation. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V17.00010: Logarithmic Relaxation in Supercooled Liquids Jacob Eapen Slowly relaxing materials are classified as either strong or fragile with the latter displaying a pronounced non-linear variation in viscosity with temperature. While viscosities are easily measured in an experiment, the attendant relaxations of stresses are not easily accessible. This paper describes a Molecular Dynamics (MD) investigation of the stress relaxations in a supercooled system. The stress correlator in the model fragile system portrays three characteristic variations as a function of temperature with three identifiable cross-over temperatures -- To, Tx and Tc. The stress correlator transitions from a power law behavior at high temperatures to an incipient logarithmic variation at a cross-over temperature To. Upon decreasing the temperature, the logarithmic relaxation grows in time and finally breaks down at a second cross-over temperature Tx. In the temperature range between To and Tx, the stress correlator relaxes logarithmically through a dynamically homogeneous crowding mechanism. Tx marks the transition from a logarithmic to a stretched exponential relaxation which coincides with the emergence of dynamical heterogeneity characterized by relatively fast relaxations through strings and small cluster movements. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V17.00011: Decoupling of Relaxation Processes in Dynamics of Macromolecules Alexander Agapov, Alexei Sokolov The presented research is focused on studies of the microscopic parameters that control different relaxation phenomena in polymer melts and protein solutions. We analyze the influence of intermolecular interactions on the structural and chain relaxations and on the ionic conductivity in polymers. It was found that the increase in polarity of the monomeric unit leads to an increase in both glass transition temperature and fragility, but only weakly affects the temperature dependence of chain motion and conductivity. This indicates that while interaction strength between segments is important on the lengthscale of structural dynamics, for chain motion and ionic transport such local factor is largely averaged out. It was also found that the temperature dependence of protein dynamics in glycerol-water solvent decouples from the viscosity similar to chain dynamics decoupling from segmental dynamics in polymers This indicates that the same approach may be used to describe these phenomena. [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V17.00012: All-atom molecular dynamics simulations of amorphous, crosslinked PDMS Philip T. Shemella, Teodoro Laino, Oliver Fritz, Alessandro Curioni The structure, dynamics, and self-diffusion properties of an amorphous, binary mixture of large poly(dimethylsiloxane) (PDMS) and small poly(hydromethylsiloxane) (PHMS) are characterized with atomic-level molecular dynamics simulations. Molar masses and mixing properties are comparable and relevant to commerical products, and simulations require massively parallel molecular dynamics simulations. The system consists of 100 vinyl-terminated PDMS molecules ($\sim$72,500 g/mol) and 200 crosslinker PHMS molecules ($\sim$1960 g/mol), for a total of more than 1 million atoms. Molecular diffusion is studied as a function of temperature, electric field, and end-crosslinking density. Simulation results provide an atomic-level description for molecular motion and an understanding of the local structure that contributes to molecular self-diffusion. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V17.00013: Dynamics of Hydrogen-Bonded Supramolecular Polymers Eric Buhler, Jean Candau, Elena Kolomiets, Jean-Marie Lehn Supramolecular polymers formed from molecular recognition directed association between monomers bearing complementary hydrogen bonding groups were studied by rheology, small-angle neutron and light scattering experiments. The semiflexible fibers consist of few aggregated monomolecular wires. At T= 25\r{ }C the formation of branched aggregates occurs around the crossover concentration, C$^{\ast }$, between the dilute and semi-dilute regimes, whereas the classical behaviour of equilibrium polymers is observed at T=65\r{ }C. For semi-dilute solutions the steady-state flow curves showed a shear banding type instability, namely the occurrence of a stress plateau $\sigma _{p }$above a critical shear rate $\mathop \gamma \limits^\bullet _c$. The values of $\sigma _{p}$ and $\mathop \gamma \limits^\bullet _c $ were found to be of the same order of magnitude as those of the elastic plateau modulus and the inverse stress relaxation time, respectively. The above features are in agreement with the theoretical predictions based on the reptation model. Dynamic light scattering experiments showed the presence in the autocorrelation function of the concentration fluctuations of a slow viscoelastic relaxation process that is likely to be of Rouse type. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V17.00014: Dynamics of associative polymer solutions: Capillary break-up, jetting and rheology Vivek Sharma, James G. Serdy, Phil Threfall-Holmes, Gareth H. McKinley Associative polymer solutions are used in extensively in the formulations for water-borne paints, food, inks, cosmetics, etc to control the rheology and processing behavior of multi-component dispersions. The commercially relevant formulations use dilute solutions of associative polymers, which have low viscosity and short relaxation times, and hence their non-Newtonian response is not apparent in a conventional rheometer. In this talk, we explore several methods for systematically exploring the linear and nonlinear solution rheology of associative polymer dispersions, including: high frequency oscillatory tests at frequencies up to 10 kHz, microfluidic shear rheometry at deformation rates up to 10$^6$ s$^{-1}$ and the influence of transientextensional rheology in the jet breakup. The presence of inertial, elastic and viscous effects typically leads to complex dynamics in a necking fluid thread. We show that by carefully controlling the excitation frequency, it is possible to drive the break-up in a particularly simple and symmetric mode, which can be used to extract extensional viscosity information using capillary thinning analysis. [Preview Abstract] |
Thursday, March 18, 2010 10:48AM - 11:00AM |
V17.00015: Shear banding in a simulated telechelic polymeric gel Joris Billen, Joris Stegen, Mark Wilson, Avinoam Rabinovitch, Arlette R.C. Baljon We report on the rheological properties of telechelic polymers through computer simulations.\footnote{Baljon et al., J. Chem. Phys. 126, 044907 (2007).} Such polymers show Newtonian behavior at low shear rate, followed by a non-Newtonian shear thinning regime. In this non-Newtonian regime, experiments as well as simulations show shear banding; two bands with different shear rate exist. The stress versus shear rate shows a plateau. When looking at the stress versus time, rheochaotical fluctuations are observed. These fluctuations are believed to be caused by microscopic shear-induced structural changes.\footnote{Sprakel et al., Phys. Rev. E. 79, 056306 (2009).} We report on these microstructural changes and on how they correlate with stress fluctuations. [Preview Abstract] |
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