Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y25: Theory and Simulations III |
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Sponsoring Units: DPOLY Chair: Hank Ashbaugh, Tulane University Room: Morial Convention Center 217 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y25.00001: Early Stage Crystallization in Isotactic Polypropylene: Influence of Nanofillers Rahmi Ozisik, Xiaofeng Chen, Sanat Kumar, Phillip Choi Formation of helices in isotactic polypropylene was studied using on-lattice, coarse-grained, Metropolis Monte Carlo simulations. Influence of polymer-particle interaction and particle size on polymer crystallization was studied by inserting isotropic particles into neat iPP melt. Results indicated that the surface of isotropic particle exerts a strong orientation effect on helices and their ordering. In addition, isotropic particle shows a length scale effect on the formation of long helical structures at low temperatures, i.e., below melting temperature. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y25.00002: Early Stage Crystallization in Isotactic Polypropylene: Influence of Substrate-Polymer Interaction and Confinement Xiaofeng Chen, Rahmi Ozisik, Sanat Kumar, Phillip Choi Formation of helices in isotactic polypropylene was studied using on-lattice, coarse-grained, Metropolis Monte Carlo simulations. Influence of polymer-substrate interaction on polymer crystallization was studied by placing iPP chains on a flat surface. Results indicated that attractive interaction between polymer and particle plays a dominant role in the formation of helical structures. Repulsive interaction excludes polymer chains from the neighborhood of the surface and triggers crystallization transition earlier (at higher temperatures). Irrespective of the energy potential used, flat surface always influences the orientation of the helices to be parallel to the surface. Confinement effect was also investigated by changing the gallery spacing between two flat surfaces. Confinement significantly prohibits the growth of long helical structures but has no effect on the overall helicity as well as the ordering of helices. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y25.00003: Growth, non-coalescence and assembly of water drops that form ordered arrays over evaporating polymer solutions Vivek Sharma, Mohan Srinivasarao Breath figures are patterns formed, when cold solid or liquid substrates contact humid air. Typically, the condensed water drops exhibit a range of sizes, and their self-similar growth is marked by coalescence in late stages. But in the breath figures formed on evaporating polymer solutions exposed to the blast of humid air, non-coalescent drops grow and self-assemble into close packed arrays of nearly monodisperse drops. These drops evaporate away leaving an ordered array of air bubbles in polymer film. In this study, we elucidate the physics that drives nucleation, growth, non-coalescence and assembly of drops. We compute the growth kinetics of a droplet population under the mass and heat transport of water vapor that are intimately coupled with the corresponding fluxes of the evaporating solvent. We elucidate the role of solvent, polymer and air flow conditions and determine why the drops are non-sticky and why drops and pores are monodisperse. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y25.00004: Theory of competitive counterion adsorption on flexible polyelectrolytes: Divalent salts Arindam Kundagrami, M. Muthukumar Counterion distribution around an isolated flexible polyelectrolyte in the presence of a divalent salt is evaluated using the adsorption model [M. Muthukumar, J. Chem. Phys. {\bf 120}, 9343 (2004)] that considers temperature, salt concentration, and local dielectric heterogeneity as physical variables in the system. Self consistent calculations of effective charge and size of polymer show that divalent counterions replace condensed monovalent counterions in competitive adsorption. The theory further predicts that at modest physical conditions, polymer charge is compensated and reversed with increasing divalent salt. Consequently, the polyelectrolyte collapses and reswells, respectively. Lower temperatures and higher degrees of dielectric heterogeneity enhance condensation of all species of ions. Complete diagram of states for the effective charge is calculated as functions of temperature and salt concentration. A simple theory of ion-bridging is also presented which predicts a first-order collapse of polyelectrolytes. The theoretical predictions are in agreement with generic results from experiments and simulations. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y25.00005: Confinement free energy of flexible polyelectrolytes in spherical cavities Rajeev Kumar, M. Muthukumar A single flexible polyelectrolyte chain in a spherical cavity is analyzed using self-consistent field theory (SCFT) in the presence of solvent molecules and salt ions. It is found that the confinement of the chain leads to creation of a charge density wave along with the development of a potential difference across the centre of cavity and the surface. We have computed different energetic and entropic contributions to the free energy of the system. In particular, the role of wall-segment repulsive interactions and concentration fluctuations (at one loop level) in free energy has been explored. Results for the finite size corrections to free energy and osmotic pressure will be presented. Predictions about the effects of salt concentration, chain length, radius of the cavity, electrostatic interaction strength, degree of ionization and solvent quality will also be presented. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y25.00006: Simulation study of proton transport in ionomers Philip Taylor, Elshad Allahyarov Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion-like ionomer by the imposition of a strong electric field. We observe that proton transport through this polymer electrolyte membrane is accompanied by morphological changes that include the formation of structures aligned along the direction of the applied field. The polar head groups of the ionomer side chains assemble into clusters, which then form rod-like formations, and these cylindrical structures then assemble into a hexagonally ordered array aligned with the direction of current flow. For dry ionomers, at current densities in excess of 1 A/cm$^2$ these rod-like clusters undergo an inner micro-phase separation, in which distinct wire-like lines of sulfonate head groups are accompanied by similar wire-like alignments of bound protons. The clusters appear to be of two types. If there are two, four, or five lines of sulfonates then there is an equal number of lines of protons, but if there are three lines of sulfonates then they are accompanied by four lines of protons. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexagonal array of rod-like structures remains, but the microphase separation disappears below the threshold current of 1 A/cm$^2$. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y25.00007: Polymer capture by electro-osmotic flow of oppositely charged nanopores Chiu Tai Andrew Wong, M. Muthukumar We have addressed theoretically the hydrodynamic effect on the translocation of DNA through nanopores. We consider the cases of nanopore surface charge being opposite to the charge of the translocating polymer. We show that, because of the high electric field across the nanopore in DNA translocation experiments, electro-osmotic flow is able to create an absorbing region comparable to the size of the polymer around the nanopore. Within this capturing region, the velocity gradient of the fluid flow is high enough for the polymer to undergo coil-stretch transition. The stretched conformation reduces the entropic barrier of translocation. The diffusion limited translocation rate is found to be proportional to the applied voltage. In our theory, many experimental variables (electric field, surface potential, pore radius, dielectric constant, temperature, and salt concentration) appear through a single universal parameter. We have made quantitative predictions on the size of the adsorption region near the pore for the polymer and on the rate of translocation. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y25.00008: Depletion interaction and effect of polydispersity in non-adsorbing polymer solutions Dadong Yan, Shuang Yang, C.C. Han, An-Chang Shi The depletion effect between two spherical colloidal particles in non-adsorbing polymer solutions is investigated using the self-consistent field theory. The density distributions of polymer segments, the depleted amount and depletion potential are calculated numerically in bi-spherical coordinates. The effects of chain length, bulk concentration, and solvency are also investigated for the dilute regime, semidilute regime and high concentration. Also, the effect of polymer polydispersity on the depletion interaction between two plates immersed in a non-adsorbing polymer solution with Schulz molecular weight distribution is studied within self-consistent-field theory. For the case of two large spheres the Derjaguin approximation is used to study the effect of polydispersity. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y25.00009: Unimolecular spreading of a molecular brush on adsorbing surface. Ekaterina Zhulina, Sergey Panyukov, Michael Rubinstein Using scaling concepts and the analytical self-consistent field theory we explore different conformations of a molecular brush on a planar substrate in nonsolvent environment (air-solid interface). The relationship between architecture and stress in adsorbed macromolecule is determined in terms of spreading parameter, grafting density and degree of polymerization of the side chains. A novel tentlike shape of molecular cross-section as well as rectangular and combined (tentlike + rectangular) conformations are predicted and examined. We demonstrate that strong adsorption of densely branched macromolecules on a planar substrate can lead to stress in molecular backbone sufficient to break covalent bonds. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y25.00010: Effect of chain stiffness on structural and thermodynamic properties of polymer melts Jutta Luettmer-Strathmann Static and dynamic properties of polymers are affected by the stiffness of the chains. In this work, we investigate structural and thermodynamic properties of a lattice model for semiflexible polymer chains. The model is an extension of Shaffer's bond- fluctuation model [1] and includes attractive interactions between monomers and an adjustable bending penalty that determines the Kuhn segment length. For isolated chains, a competition between monomer-monomer interactions and bending penalties determines the chain conformations at low temperatures. For dense melts, packing effects play an important role in the structure and thermodynamics of the polymeric liquid. In order to investigate static properties as a function of temperature and chain stiffness, we perform Wang-Landau type simulations and construct densities of states over the two- dimensional state space of monomer-monomer and bending contributions to the internal energy. In addition, we present first results from an algorithm for equation-of-state effects in lattice models. \newline [1] J. S. Shaffer, J. Chem. Phys. 101, 4205 (1994). [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y25.00011: Ameba-like diffusion in two-dimensional polymer melts: how critical exponents determine the structural relaxation Torsten Kreer, Hendrik Meyer, Joerg Baschnagel By means of numerical investigations we demonstrate that the structural relaxation of linear polymers in two dimensional (space-filling) melts is characterized by ameba-like diffusion, where the chains relax via frictional dissipation at their interfacial contact lines. The perimeter length of the contact line determines a new length scale, which does not exist in three dimensions. We show how this length scale follows from the critical exponents, which hence characterize not only the static but also the dynamic properties of the melt. Our data is in agreement with recent theoretical predictions, concerning the time-dependence of single-monomer mean-square displacements and the scaling of concomitant relaxation times with the degree of polymerization. For the latter we demonstrate a density crossover-scaling as an additional test for ameba-like relaxation. We compare our results to the conceptually different Rouse model, which predicts numerically close exponents. Our data can clearly rule out the classical picture as the relevant relaxation mechanism in two-dimensional polymer melts. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y25.00012: Connections between static and dynamic properties of athermal polymer melts: a Monte Carlo simulation study Nenad Stojilovic, Jutta Luettmer-Strathmann The motion of individual chains in polymer melts and blends is governed by local friction and entanglement effects. In simulations and experiments it can be difficult to separate these effects since both local friction coefficients and entanglement lengths depend on the thermodynamic state and the chain structure and since many systems display neither ideal Rouse nor fully entangled dynamics. In this work, we investigate local and chain dynamics of athermal polymer melts with Monte Carlo simulations of two versions of Shaffer's bond- fluctuation model [J. S. Shaffer, J. Chem. Phys. 101, 4205 (1994)]. In the first version, bonds are allowed to cross each other with the result that the chains do not entangle; in the second, bond crossings are prohibited and entanglement effects become apparent. Since both versions of the model have very similar static properties, local friction and entanglement effects can be separated. With simulations for a range of densities and chain lengths, we investigate connections between static and dynamic properties, in particular, scaling with the packing length and the size of the moving segment responsible for local friction. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y25.00013: Thermodynamic modeling of melt deformation. J.P. Ibar In current tube models, the chain is the focus of interest; it is \textit{the} statistical system. For entangled chains, part of the chain (of molecular weight Me) becomes the system. In the statistical model of this paper, systems are sets of conformers coherently interactive, where interactive coupling is defined with respect to 2 types of interaction between conformers, covalent or intermolecular. The duality is described by a new statistics, a crossed-statistics, which calculates the conformational state of all conformers, not just whether they are cis, gauche or trans, but also whether they are either covalently or inter-molecularly bonded. Entanglements manifestation result from a disturbance of the crossed-statistics by the increase of the number of covalently bonded conformers resulting in thermodynamically stable dual phases. The deformation of a statistical system results from a change of the conformation population between the flexed and trans conformations in the direction of the imposed macroscopic field vector. Shear or elongational flow mechanisms differ for the amount played by diffusion in feeding the deformed systems with undeformed (or relaxed) conformers to minimize the total energy required to accommodate the macroscopic deformation. Strain, strain rate and temperature determine how many systems are deformed and to what extent. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y25.00014: Can a material clock based model describe highly non-linear creep? Grigori Medvedev, James Caruthers Most constitutive models developed to date assume that the non-linear behavior observed in glassy polymers it is due to a deformation dependent material clock, where the challenge is to find the correct functional form of how the rate of relaxation depends upon structural variables. A two order-of-magnitude change in mobility upon deformation has been observed experimentally [1], confirming that a material clock is a necessary component of any constitutive description. A non-linear viscoelastic constitutive theory [2], where the clock is configurational energy based, captures with a single parameter set a wide variety of phenomena, including yield, stress/ volume/ enthalpy relaxation, and physical aging. The model is also successful in describing linear creep and recovery; however, it fails dramatically in case of highly non-linear creep in the glass transition region. No change in the functional form of the material clock can significantly improve the prediction of the model for non-linear creep. We postulate that spatial dynamic heterogeneity of glassy materials, which is well established experimentally, must be incorporated into the constitutive model in order to describe non-linear creep. [1] Ediger, et al., APS Meeting - Denver, CO March, 2007. [2] Caruthers, et al., Polymer, 45, 4577-4597, 2004. [Preview Abstract] |
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