Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W29: Polymer Theory and Simulation II: Interfaces and Confinement |
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Sponsoring Units: DPOLY Chair: John McCoy, NMT Room: LACC 504 |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W29.00001: Apex Exponents for Polymer-Probe Interactions Roya Zandi, Michael Slutsky, Mehran Kardar, Yacov Kantor We consider self-avoiding polymers attached to the tip of an impenetrable probe. The scaling exponents $\gamma_1$ and $\gamma_2$, characterizing the number of configurations for the attachment of the polymer by one end, or at its midpoint, vary continuously with the tip's angle. These apex exponents are calculated analytically by $\epsilon$-expansion, and numerically by simulations in three dimensions. We find that when the polymer can move through the attachment point, it typically slides to one end; the apex exponents quantify the entropic barrier to threading the eye of the probe. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W29.00002: Charge Induced Pattern Formation on Surfaces Sharon M. Loverde, Yury Velichko, Monica Olvera de la Cruz Amphiphilic molecules self-assemble into a wide variety of biological structures--micelles, vesicles, membranes, etc. Recent observations support the existence of phase separation on the surface of charged nano-aggregates. In this case, short range van der Waals and long range electrostatic interactions induce microphase separation and determine the resulting equilibrium morphology on the surface. An ideal system of immiscible cationic and anionic components, confined to a two dimensional plane, is analyzed with MD simulation using a $1/r$ Coulomb potential. Implicit counterion effects on structural properties are also considered by introducing a screened Debye-H\"{u}ckel potential to represent the effects of including monovalent salt in the system. Resulting structural and thermodynamic properties are compared with those predicted by theory. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W29.00003: RIS-Monte Carlo Based Molecular Modeling of Elasticity and Photoelasticity of SBS Thermoplastic Elastomer Kapileswar Nayak Phase morphology and molecular conformation play an important role in dictating the physical and chemical properties of a material. SBS (Styrene-Butadiene-Styrene) thermoplastic elastomers (TPEs) exhibit two-phase morphology, and if the polystyrene content is in the range 20-30{\%}, are modeled as polystyrene (PS) spheres in amorphous polybutadiene (PB) domain. The PB block bridges two spheres of the impenetrable PS block, where these PS spheres act as network crosslinks. The PB chain can assume a number of conformational arrangements, due to rotations about single bonds contained in the chain backbone. These conformational arrangements yield distributions of chain vector length and polarizability anisotropy, and have been determined via Rotational Isomeric States Monte Carlo (RIS-MC) simulations. The triblock nature of the SBS chains as well as the impenetrability of the PS domains have been incorporated into the RIS-MC simulations, thus incorporating the relevant primary structural aspects into the framework. The development has been extended to yield stress-elongation and birefringence-elongation relationship for TPE networks. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W29.00004: The Colloidal Force of Bead-Spring Chains in a Good Solvent John McCoy, John Curro A recently developed Density Functional Theory (DFT) for tethered Bead-Spring chains is used to investigate colloidal forces for the good solvent case. A planer surface of tethered chains is opposed to a bare, hard wall, and the force exerted on the bare wall by the tethered chains is calculated by way of the contact density. In this manner, the surface force is found as a function of separation. The resulting force function is analyzed for varying surface coverages, and chain lengths. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W29.00005: Molecular dynamics simulations of a polymer brush-melt interface under shear Claudio Pastorino, Marcus Müller, Kurt Binder Molecular dynamics simulations of a polymer melt between two brush-covered surfaces under shear have been performed. The end-grafted polymers and the free chains of the melt have identical properties. The grafting density of the brush-layer provides a way of changing the surface behavior without altering the molecular interactions. The deformability and wettability of the brush layer, including the dewetting of a melt on the top of a dense brush of identical chains (autophobicity), are studied as functions of the grafting density. The chains are described by a coarse-grained bead spring model. We perform equilibrium and non-equilibrium simulations at constant temperature and volume using the Dissipative Particle Dynamics thermostat to duly account for hydrodynamic correlations. The equilibrium properties and behavior under shear are studied, as well as, the interdigitation of the melt into the brush, the polymer orientation on different length scales (bond vectors, radius of gyration, and end-to-end vector) of free and grafted chains, and velocity profiles. The viscosity and slippage at the interface are also calculated as functions of the grafting density and shear velocities. This work is a first step towards studying droplet spreading on top of a brush. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W29.00006: Capillary Waves, Chain Conformations, and Viscoelasticity at Sheared Blend Interfaces: DSCF - MD Comparison. Tak Lo, Maja Mihaljovic, Yitzhak Shndiman, Wentao Li, Dilip Gersappe We have recently improved a previously proposed dynamic self-consistent field (DSCF) lattice theory of polymer fluids by accounting for transient viscoelastic effects in time evolution. We have applied the DSCF theory to study both the time evolution and the steady state chain conformation and rheology at interfaces of sheared, immiscible polymer blends. While DSCF allows direct determination of interfacial morphology and rheology evolution, chain conformation statistics was sampled with a Monte Carlo process using the probability distributions and transition rates generated by the DSCF equations. The results were compared to molecular dynamics (MD) simulations. We found that discrepancy with MD results is minimized if broadening of the DSCF interface by capillary wave fluctuations is accounted for. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W29.00007: Instability of Polymer Films by Complete Dispersion Forces Heping Zhao, Yong Jian Wang, Ophelia K.C. Tsui Many recent experiments showed that some polymer film systems with film thickness, $h$, in the nanometer range are unstable and may rupture spontaneously upon heating by the spinodal mechanism. The very criterion for this instability is the system free energy, $G(h)$, possessing a negative second derivative. In apolar liquid films with $h \quad \sim $10 nm, the van de Waals interactions (vdW) comprise a major contribution to $G(h)$ (besides surface tension) for which the approximate form, $G(h) \quad \sim \quad A$/12$\pi h^{2}$ (where $A$ is the Hamaker constant), neglecting retardation effects, has been widely adopted. In this work, we calculated the exact solution for the complete vdW interactions based on the theory of Dzyaloshinskii, Lifshitz and Pitaevskii for the four-layer system, air/polystyrene/SiO$_{2}$/Si. We found that even when the thickness of the polymer and the SiO$_{2}$ layer are only 5 nm, retardation effects produce significant modifications to $G(h)$, contrary to conventional expectations. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W29.00008: Molecular dynamics simulations of electrostatic layer-by-layer assembly of polyelectrolytes near charged planar surface Pritesh Patel, Junhwan Jeon, Patrick Mather, Andrey Dobrynin We performed molecular dynamics simulations of multilayer assembly of flexible polyelectrolytes at a charged planar surface from dilute polyelectrolyte solutions. We have studied the effects of fraction of charged monomers on polymer backbone and the chain degree of polymerization on multilayer formation and film structure. Our simulations show that multilayer growth proceeds through surface overcharging, chain intermixing and linear increase in polymer surface coverage at each deposition step. There are almost perfect periodic oscillations of density difference between positively and negatively charged polymers in the adsorbed film despite of chain's intermixing. We have established that it usually requires more than one deposition step to complete a single layer and the ion pair formation plays an important role in multilayer stabilization. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W29.00009: Dynamical properties of DNA under confinement Satheesh Kumar, Wokyung Sung The recent advances in experimental techniques of single molecule manipulation have attracted remarkable interests in the structure and dynamics of polymers under confinement. In this work we develop a computational scheme, based on Brownian dynamics, to systematically incorporate the effect of the confining geometry on hydrodynamic interactions between the polymer segments. In this scheme the surface of the confining geometry is treated as a collection of beads in the same way as the well-known bead-spring chain represents a polymer molecule so that we can consider the hydrodynamic interactions among the segments of the polymer as well as between the polymer and wall in a unified manner. One of significant advantages of this method is the possibility of considering fluctuating boundaries which may be relevant in many biological situations. The dynamical properties such as relaxation time of DNA confined within a channel are computed and compared with the existing results. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W29.00010: Modelling polymer-obstacle collisions: Molecular Dynamics simulations and theory Martin Kenward, Gary W. Slater We present results from Molecular Dynamics (MD) simulations that explicitly include a Lennard--Jones solvent and therefore hydrodynamic interactions. We utilize these MD simulations to explore the collision of single polymers with obstacles in three specific cases: i) a polymer in the presence of an external force colliding with a fixed obstacle, ii) a polymer in a laminar flow colliding with a fixed obstacle and iii) a polymer in the presence of an external force colliding with a free obstcale. We focus on the limit where the applied force, during a collision, is able to fully elongate the polymer (in the direction of the force) to a length on the order of its contour length. To complement the MD simulations we present a general set of equations which describe these polymer-obstacle collisions. In certain cases of interest these equations yield analytical results for the observed escape dynamics. The data describing the escape of the molecules as a function of chain length, can be collapsed onto a single universal curve. We also show that the molecules undergo a non--neglible compression during the final stages of the escape process which modifies the resulting dynamics. In the case of the inclusion of a laminar flow we also examine qualitatively how the presence of the both the obstacle and the polymer modify the resulting flow profile and the forces dictating the escape of the molecules. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W29.00011: Monte Carlo simulation and self-consistent integral equation theory for polymers in quenched random media Bong June Sung, Arun Yethiraj The structure of polymers in quenched random media is studied using self-consistent integral equation theory and Monte Carlo (MC) simulations. The theory combines field theoretic methods with the replica symmetric polymer reference interaction site model (RSP) theory of Yethiraj. Self-consistent RSP theory is in good agreement with MC simulations for the pair correlation functions at high polymer densities but the theoretical predictions are not as good when the polymer density is low. The size of a chain, characterized by the rootmean square radius of gyration, $R_g$, is predicted to be a strong function of the size of the matrix particles. When the size of the matrix particles is comparable to the chain monomer size, $R_g$ is a non-monotonic function of polymer volume fraction. On the other hand, when the size of the matrix particles is comparable to $R_g$, the chain size is a monotonically decreasing function of the polymer volume fraction. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W29.00012: The Equilibrium Partitioning of Block Copolymer at Critical Condition Yongmei Wang, Shazia Khan, Wenhua Jiang The partitioning of diblock (AB) and triblock copolymers (ABA and BAB) into a slit pore with the B block set at the critical condition were investigated with lattice Monte Carlo simulations and were compared with the partitioning of a homopolymer (A) with the same length of the A block. The study aims to understand the effect of block copolymers architecture on the elution pattern of these copolymers in chromatography study with one of the blocks being made ``invisible''. The partition coefficient of a diblock copolymer chain (AB) is found to be larger than that of the homopolymer (A) and the difference is more significant when the visible A block length is much shorter than the invisible B block length. This would suggest that the diblock copolymer (AB) with the B block at the critical condition would elute later than the corresponding homopolymer A would. As a result, the estimation of the molecular weight of A block in AB copolymer based on the elution time of the copolymer at the critical condition would be underestimated. For the triblock copolymers, the partition coefficient for ABA was found to be smaller than those for AB and BAB. Hence ABA would elute earlier than AB and BAB. The simulation results are in good agreement with experimental results of elution times of copolymers at the critical conditions in liquid chromatography. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W29.00013: What is the critical condition for equilibrium partitioning of SAW chains into pores? Wenhua Jiang, Scott Orelli, Yongmei Wang The critical condition of polymer chains partitioning into pores refers to the point at which the entropy loss is compensated by enthalpy gain. This is thought to occur at the critical adsorption point (CAP) of the chain above a solid planar surface and at this point the equilibrium partition coefficient $K$ =1 and is independent of chain length. We investigated this issue by examining the equilibrium partitioning of a SAW chain into a square channel, which mimics the microporous media better than a slit. The partition coefficient of a SAW chain at CAP determined earlier was found to vary dramatically with the chain length in a narrow square channel. As a result, the critical condition point relevant to the experiments can not be defined as the critical adsorption point. Instead, the critical condition point can be identified clearly from the plot of root-mean square deviation in ln$K$ for a given range of chain length against the surface/bead interaction energy \textit{$\varepsilon $}$_{w. }$The critical condition point thus found, \textit{$\varepsilon $}$_{w}^{cc}$, was more attractive than the critical adsorption point. The narrower the channel is, the more attractive the surface interaction would be at the critical condition point. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W29.00014: Thermodynamics of Heteropolymers in Confinement: A Wang-Landau Monte Carlo Study Yelena Sliozberg, Cameron Abrams We investigate the thermodynamic behavior of a generic, flexible, off-lattice A$_9$-(BA$_9$)$_{10}$ heteropolymer, for which BB interactions are strongly attractive, confined in spherical cavities. A modified Wang-Landau-type Monte-Carlo algorithm was used to compute densities-of-state, $\Omega(E)$, for various cavity sizes, $R$. Under no confinement, the heteropolymer considered displays a first order transition from an open coil to a partially collapsed structure with a core of 8 of the 10 B-type monomers, and a weaker second-order transition from the semi-collapsed state to the final structure with a core of all 10 B's and an A-type corona. Under moderate confinement, for which $R$ is somewhat larger than the polymer's unperturbed mean radius of gyration in bulk, the critical temperature of the first-order transition increases significantly. For strong confinement, we observe an even greater increase in the critical temperature. A detailed analysis of the accessible conformations at each energy level indicate that the mechanism responsible is an increase in stability of the {\em partially} collapsed structure relative to the open coil with increasing confinement. These results should contribute to the ongoing discussion of how crowding and confinement play roles in the thermodynamics and kinetics of heteropolymer intramolecular self-assembly. [Preview Abstract] |
Thursday, March 24, 2005 5:18PM - 5:30PM |
W29.00015: Shape Templating Effects Among Growing Anisotropic Particles Ashoutosh Panday, Samuel Gido This study illustrates an orientational templating mechanism by which anisotropic grain shape coupled with sporadic nucleation generates orientational correlations. A modeling and simulation study indicated that despite the randomness of nucleation and growth, such particles tend to align laterally to each other which leads to an azimuthal inter-grain orientational correlation among such particles. The simulation results agree with crystallization studies on small molecule crystals as well as experimental observation of inter-lamellar correlations in block copolymers. [Preview Abstract] |
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