Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S17: Modeling of Polymers: Blocks, Networks and Solutions |
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Sponsoring Units: DPOLY DCOMP Chair: Venkat Ganesan, University of Texas at Austin Room: Colorado Convention Center 102 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S17.00001: Single-Chain in Mean-Fied simulations for Block Copolymer/Nanoparticle Composites Francois Detcheverry, Yioryos Papakonstantopoulos, Huiman Kang, Paul Nealey, Juan De Pablo, Kostas Daoulas, Marcus Mueller Incorporating nanoparticles into self-assembling copolymers is a promising route towards creation of structures tailored at the nanometer scale and for design of new functional materials. However, predicting the behavior of nanoparticles dispersed in diblock copolymers remains a theoretical challenge. We have developed a single-chain in mean-field simulation technique that permits study of copolymer/nanoparticle composites in two limits, including hard and soft nanoparticles. The models proposed in this work are capable of describing the morphological changes induced by adding nanoparticles to block copolymers, and the distribution of nanoparticles in block copolymer thin films on patterned substrates. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S17.00002: Multiscale Simulations of Pluronic Micelles Grant Smith, Dmitry Bedrov Poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymers (Pluronics{\textregistered}) self-assemble in aqueous solution to form (roughly) spherical micelles. With increasing polymer concentration and temperature, these nanoscale micellar polymer building blocks begin to interact resulting in formation of particle gels and micellar crystalline phases. The structure of Pluronic solutions has been probed extensively via small angle neutron scattering (SANS). Interpretation of SANS measurements relies on models of various degrees of sophistication for both the single micelle form factor F(q) and the micelle-micelle structure factor S(q). Information regarding single micelle structure and micelle-micelle interactions gleaned from SANS measurements depends sensitively on the model used. A key assumption in modeling of SANS data for these solutions is that the micelles are perfectly spherical, allowing for representation of the q-dependent scattering intensity as a product of F(q) and S(q). We have carried out multiscale molecular simulation studies of Pluronic micelle solutions in order to better understand the structure of these important nanoscale polymer particles, their interaction in aqueous solution, and the validity of the various models utilized in interpreting SANS measurements. Our simulations reveal that the micelles exhibit significant anisotropic character that strongly influences their interaction and the structure of the micellar solution. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S17.00003: Spinodal Decomposition of Polydispersed ABA' Triblock Copolymers Determined from the Random Phase Approximation T.W. Capehart, Armand Soldera Triblock copolymers produced by free radical polymerization are typically characterized by each of the blocks having a broad distribution of molecular weight. To investigate the effect of this polydispersity on the stability of the homogenous phase of a triblock copolymer, the spinodal decomposition of an ABA' copolymer consisting of ideal Gaussian chains was determined using the random phase approximation (RPA), with each block length characterized by a Zimm-Schulz chain length distribution. The spinodal stability and scattering behavior resulting from microphase separation were determined for volume fractions 0.1 $\le \quad \phi _{B} \quad \le $ 0.9 and polydispersity indices 1.67 $\le $ PI $\le $ 100. Consistent with the reported behavior of fully dispersed multiblock copolymers and diblock copolymers having a single polydispersed block, polydispersity in symmetric ABA' triblocks decreases the stability of the homogenous phase and lowers the value of the Flory-Huggins mixing parameter $\chi _{HF}$ required for microphase separation at the Lifshitz point by more than a factor of two. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S17.00004: Architecture phase diagram for branched block copolymers: Scott Sides, Bobby Sumpter Self-consistent field theory (SCFT) for dense polymer melts has been highly successful in describing complex morphologies in block copolymers. Field-theoretic simulations such as these are able to access large length and time scales that are difficult or impossible for particle-based simulations such as molecular dynamics, while still incorporating more realistic polymer models than many macroscopic, continuum simulations. Using block copolymers as mesoscale templates has potential applications for improved photovoltaic devices and fuel-cells. Many of these applications require control over the domain size of the phase-separated regions. One possible method is changing the architecture of branched copolymers. In this talk I will outline the SCFT method, discuss some efficient methods of numerically solving the SCFT equations and present results for modeling PI-PS block copolymers. The results will be compared to experimental data examining the influence (structure and mechanical properties) of adding more branches of PS along a PI backbone. These copolymer configurations include one PS branch at each graft point on PI, 2 PS's, and up to 4 PS's branches with varying number of branch points on the PI backbone. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S17.00005: Theoretical Investigation of Hydrogen Bonding Networks in Cellulose I$\alpha $ and I$\beta$ Xianghong Qian The cellodextrins in native crystalline cellulose I$\alpha $ and I$\beta $ are unusually stable compared to other polysaccharides, not easily prone to hydrolysis even with chemical or biological catalysts. The stability of crystalline celluloses is most likely due to theirs highly enhanced hydrogen-bonding (HB) networks. We carried out \textit{ab initio} calculations to determine the atomic and conformational structures of native crystalline celluloses I$\alpha $ and I$\beta $. The differences in their HB networks will be discussed and compared with available experimental data. A theoretical model based on competition between hydrogen bonding energy and electronic energy was constructed to explain the size of native crystalline celluloses. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S17.00006: Percolation and Diffusivity of Ideal Polymer Networks Yong Wu, Beate Schmittmann, Royce Zia We study the properties of ideal polymer networks both near and far from the percolation threshold. The polymers are modeled by non-interacting random walks on the bonds of a two-dimensional square lattice. We use numerical techniques to measure the percolation threshold and critical exponents of polymer networks for various polymer lengths. Further, we allow particles to diffuse by hopping over this quenched network of polymers. In particular, we measure the particle current in response to an externally imposed concentration gradient. When the system is far from percolation, we use the effective medium theory to predict its diffusivity and compare the results to the numerical simulation. An application of this study is the investigation of transport properties of gas molecules through thin polycarbonate films [Macromolecules 36, 8673, (2003)]. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S17.00007: Random Networks of Semiflexible Polymers Panayotis Benetatos, Annette Zippelius We present a semimicroscopic replica field theory of the formation of a random network built from wormlike chains. We consider permanent cross-links which fix the orientations of the corresponding filaments to be locally parallel, and we treat them as quenched disorder. We show that, upon increasing the cross-links in the fluid, an isotropic amorphous solid phase emerges, in which the orientations of the chains are frozen in random directions. A different transition to an orientationally ordered (nematic) phase is also possible. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S17.00008: Collapse transition of a chain in the bulk and next to adsorbing surfaces I.A. Bitsanis, A.N. Rissanou, S.H. Anastasiadis We performed lattice MC simulations of single, flexible, self-avoiding chains in bulk solution$^{\ast }$, or adsorbed onto a surface, under poor solvent conditions. Our simulations spanned a wide range of chain lengths (N=20-10000) and cohesive energies. The chain length dependence of the chain size in poor solvents was characterized by a wide plateau of almost null growth. This plateau was related with the development of the incipient constant density core. The ``volume approximation'' regime and genuine power law dependence (1/3) was not reached even for the longest chains and poorest solvents studied. Sufficiently long chains became more but not fully spherical and underwent a 2$^{nd}$ order phase transition. Conformations of the adsorbed chains onto attractive surfaces are not controlled by the bulk $\Theta $- temperature, but by a new temperature $\Theta $' which depends strongly on the interactions with the surface. The adsorption-desorption transition width is determined by the N-dependence of the bulk radius of gyration, for every solvent quality. In poor solvents and strongly attractive surfaces, the coil-to-globule transition turns into a coil to ``pancake'' transition. $^{\ast}$ Rissanou \textit{et al. J. Polymer Sci.~:Part B~: Polymer Phys}., \textbf{44}, p.3651 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S17.00009: Polymer relaxation in flow: dynamical slowdown around the coil-stretch transition D. Vincenzi, E. Bodenschatz, A. Puliafito, A. Celani We investigate polymer relaxation dynamics both in extensional and random flows. We show a significant slowdown of dynamics in the vicinity of the coil-stretch transition. The time needed for the probability density function of polymer extension to relax to the equilibrium distribution is much larger than the Zimm time scale. In other words, the effective Weissenberg number differs considerably from the ``bare'' one. For the elongational flow, this effect is related to conformation hysteresis. For random flows, we show that hysteresis is not present. Nonetheless, the amplification of the equilibration time persists, albeit to a lesser extent, due to the large heterogeneity of polymer configurations around the coil-stretch transition. In both cases, the dependence of the drag force on the polymer configuration plays a prominent role. This effect may be relevant for drag-reducing turbulent flows, where the strain rate often fluctuates around values typical of the coil-stretch transition. Our conclusions thus suggest that the conformation-dependent drag should be included as a basic ingredient of continuum models of polymer solutions. The problem is solved analytically in terms of the Fokker-Planck equation for the distribution of the extension of the polymer. The computation of the equilibration time of the polymer in the flow is recast as a central two-point connection problem for a generalized spheroidal wave equation. The results are confirmed by Brownian Dynamics simulations and by experiments in a random flow generated by elastic turbulence (S. Gerashchenko \& V. Steinberg, private communication). [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S17.00010: Solvation potentials for polymer chains in solution Mark Taylor The conformation of a polymer chain in solution is intrinsically coupled to properties of the solvent. In much of the theoretical work on polymers in dilute solution the effects of solvent are treated in an implicit fashion: thus one studies an isolated chain interacting via an effective site-site potential. Although a formally exact mapping is possible between the chain-in-solvent system and a corresponding isolated effective-potential-chain, this mapping involves a many-site solvation potential which is not practical to compute. Thus, one generally resorts to a two-site potential approximation. Here we first demonstrate that the two-site approximation for flexible interaction-site chain-in-solvent systems is rigorously valid for short chains by computing ``exact'' solvation potentials for these chains. We then combine these exact short chain results with the potential of mean force of the pure solvent to construct approximate two-site solvation potentials for long chains. Monte Carlo simulations have been performed for both the isolated effective-potential chains and the full chain-in- solvent systems. These simulations show that our solvation potentials provide a quantitatively accurate description of the conformation of a chain in explicit solvent. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S17.00011: Model-specific features of random walk polymers beyond the mean field limit Kirill Titievsky Much of the theory of block copolymers and polymer interfaces is based on infinite molecular weight (mean field) limit of random walk models. In this limit, specific assumptions about an individual monomer -- its length distribution and contribution to the density fields -- become immaterial, leading to universal behavior. Unfortunately, this assumption is unrealistic for many common systems. Recent field theoretic and explicit chain simulation methods promise to address this problem, but raise an even more fundamental one. With finite chains, we may no longer assume universal behavior and must explicitly analyze the effect of monomer-level representation of a chin on the physical meaning of parameters and global predictions of a random walk model. In this talk, we present key results quantifying the balance between universal and model-specific behavior of common models. The discussion of the fundamental uncertainty of experimental methods used to measure Flory $\chi$ parameters will interest experimentalist. Polymer theorists interested in fluctuations corrections to mean field theories will find our results immediately applicable to their work. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S17.00012: Amorphous and crystalline states of ultrasoft colloids: A Molecular Dynamics study A.N. Rissanou, M. Yiannourakou, I.G. Economou, D. Vlassopoulos, I.A. Bitsanis In dense suspensions of multi-arm star polymers a ``\textit{reversible thermal vitrification}'' was observed experimentally under ``marginal'' solvent conditions. We have investigated the origin of this phenomenon via MD simulations at the \textit{mesoscopic scale}$^{1,2,}.$ We reported the emergence of an amorphous solid state, upon heating of the ``soft spheres''. This transient glassy state resulted from star swelling, ``free volume'' deprivation and ``dynamical arrest'' of ``soft-spheres''. We monitored the ageing of the amorphous stage towards more crystalline FCC structures. The effects of size-dispersity and arm MW on crystallization were studied qualitatively. The overall picture revealed the existence of new ``dynamically arrested'' states, all of which could be termed ``crystalline'' but differed as to the ``degree of crystallinity''. Quantitative analysis of particle trajectories supplied mean square displacement curves which at the higher temperatures are typical of ``delayed'' Fickian diffusion. Even the aged crystalline states exhibited weak diffusion in contrast with the null diffusion of the crystals resulting from a FCC initial configuration. $^{1}$ Rissanou et al., \textit{Phys. Rev. E } \textbf{71} 011402-1~:12 (2005) $^{2}$ Rissanou et al., \textit{J. Chem. Phys. }\textbf{124} 044905-1~:11 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S17.00013: Polymer Statics and Dynamics Under Box Confinement Joshua Kalb, Bulbul Chakraborty Current work on biological systems and glass forming polymers (JCP 106, 6176 (1997)) has led to an interest in the study of single polymer systems. The main questions concern relaxation phenomena and the shape adopted by single polymers under hard and soft boundaries. We are concerned with whether or not there is a critical length scale for a confined polymer system. Both structure and relaxation can be described using scaling arguments and tested with Monte Carlo simulations using the bond-fluctuation algorithm (Macromolecules 21,2819 (1988)), which uses a lattice representation of the polymer chain with excluded volume effects. We look at the effects of confinement on a single polymer chain confined to a box by measuring dynamical quantities such as the end-to-end vector and single monomer positions (JACS 124, 20 (2004)). A primary question is how spatial correlations between monomers, `blob's, influence the dynamics. Understanding how these quantities change with various confining geometries will lead to a deeper understanding of biological structures and glass formation. Work supported by NSF-DMR 0403997. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S17.00014: Polymer Translocation in Crowded Environments Ajay Gopinathan, Yong Woon Kim Polymer translocation is an important biological process that involves the transport of biopolymers across a membrane, through a pore, into a different environment. However the influence of the crowded nature of the cellular cytoplasm on translocation dynamics has received little attention. We systematically treat the entropic penalty due to the crowded environment by modeling the crowding effect as arising from the excluded volume due to hard spherical obstacles that could be static or free to diffuse. Using a Fokker-Planck description of the translocation dynamics we find novel exponents describing the scaling of the translocation time with polymer length. We also explicitly consider situations where both sides of the membrane are crowded and where the translocation is driven by a chemical potential gradient. In both cases we observe new and qualitatively different translocation regimes as a function of crowding, transmembrane chemical potential asymmetry and polymer length. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S17.00015: Models of polymers subject to a force Gerasim Iliev Atomic force microscopy (AFM) and optical tweezer techniques allow individual polymer molecules to be micromanipulated. For instance, an adsorbed polymer can be pulled off a surface. We consider simple, exactly solvable models of this effect. In addition, we consider models of random copolymers adsorbed at an impenetrable surface and localized at an interface and investigate their response to an elongational force. This is related to a model of unzipping duplex DNA. [Preview Abstract] |
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