Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U25: Theory and Simulation II |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Marina Guenza, University of Oregon Room: Morial Convention Center 217 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U25.00001: Monte Carlo simulations of a coarse-grain model for block-copolymer melts: method and application Francois Detcheverry, Darin Pike, Paul Nealey, Juan de Pablo, Marcus Mueller A new Monte Carlo based approach has been developed for simulation of polymeric systems, including block copolymers. The approach represents the system at the level of a coarse-grain Hamiltonian, akin to that employed in widely used self-consistent field theoretic (SCFT) treatments. In contrast to traditional implementations of SCFT, however, molecules are treated explicitly and fluctuations are taken into account. We present two distinct implementations of the method; the first relies on a grid, and the second does not. While the grid-based method is highly computationally efficient, the gridless implementation permits simulations in arbitrary ensembles, including the grand-canonical and Gibbs ensembles, thereby facilitating study of phase transitions. The gridless implementation also gives access to the local mechanical properties. The performance of the two implementations is discussed in the context of several applications, including the directed assembly of multi-block copolymer thin films on patterned substrates, either chemical or topographical. In both cases we examine the ordering of the material and the effect of pattern or surface roughness. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U25.00002: Interaction between Polymer Grafted Particles: Self-Consistent-Field Study Jaeup Kim, Mark Matsen Recently, there has been an ongoing debate regarding a possible attraction between two polymer grafted nanoparticles. Using numerical self-consistent field theory (SCFT) we investigate the inter-particle potential, showing that only a monotonically increasing repulsive force is expected between the two particles regardless of the particle size and brush thickness. We also compared the exact mean-field result to approximate solutions using the Derjaguin approximation. The previously reported attraction is thought to be an anomaly caused by the use of bispherical coordinates. We avoid this problem by developing a new SCFT scheme using two separate spherical coordinate systems centered on each particle. The idea of using multiple coordinate systems is applicable for many polymeric problems involving complicated geometries. In this scheme, two or more coordinate systems share an overlapping volume. Then the statistics of the polymers belonging to a certain coordinate system are solved in trial fields, and the resulting polymer concentration is shared by all coordinate systems to find a self-consistent solution. This method has been tried for other problems such as the behavior of a Janus nanoparticle (solved in spherical coordinates) in block copolymer lamellar phase (solved in cylindrical coordinates). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U25.00003: Discovering Ordered Phases of Block Copolymers: A New Fourier-space Approach Feng Qiu, An-Chang Shi, Zuojun Guo, Hongdong Zhang, Yuliang Yang A new method to solve the self-consistent field theory of block copolymers is developed. This method is based on the fact that, for any computational boxes with periodic boundary conditions, all spatially varying functions are spanned by the Fourier series determined by the size and shape of the box. This method is well suited for the discovery of ordered structures of block copolymer systems. The symmetry of the ordered structures emerges from the minimization of the free energy density. Application of the technique to diblock copolymers recovers all the previously known ordered structures plus a few new metastable ones. As an example of application, the method is used to construct a phase diagram for a model of frustrated triblock copolymers. A variety of stable or metastable three-dimensional ordered structures are discovered. Furthermore, the capability of the method to reproduce experimentally observed structures is demonstrated by the knitting pattern in triblock copolymers. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U25.00004: Cylindrical phase of diblock copolymers in thin films Marianne Breuer, Barbara Drossel We investigate the microphases of diblock copolymers confined in a thin film with walls attracting one of the monomer types.We focus on the possible structures of copolymers that form cylindrical phases in the bulk. We employ both self-consistent field theory and strong segregation theory to obtain the concentration profile minimizing the free energy of the system and to compare the free energy of possible morphologies. We present a phase diagram showing the possible microphases for a diblock copolymer with fixed volume fraction and fixed segregation parameter in dependence of the film thickness and the affinity of the walls. We study the effects of numerical inaccuracies on the appearance of different morphologies and their free energies. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U25.00005: Cubic Micellar Crystals of $A_nB_mA_n$ Block Copolymers from MD Chris Lorenz, Joshua Anderson, Alex Travesset Amphiphilic block copolymers exhibit a wide variety of phases in solution. One common phase of $A_nB_mA_n$ polymers is made up of spherical micelles with hydrophobic (B) cores and hydrophilic (A) coronas. At high enough concentrations, these micelles order on a lattice forming a micellar crystal. The dynamics during the formation of this phase are fascinating, being controlled almost entirely by the polymer transfer between micelles, as shown in Molecular Dynamics~simulations. Application of a standard nucleation and growth analysis shows that the micellar crystals grow extremely rapidly and are probably aided by the periodic box in the simulation. A study of the dynamics at equilibrium shows that polymer transfer is still a continuing process and can be understood in the context of transition state theory. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U25.00006: Brownian Dynamics Simulation of Kinetics of HEX Cylinders to FCC Spheres Transition in ABA Triblock Copolymer in Selective Solvent Minghai Li, Rama Bansil We report Brownian Dynamics simulations on 400 bead-spring chains of triblock copolymer, A$_{10}$B$_{10}$A$_{10}$, in a selective solvent for the A block using a FENE potential together with Lennard-Jones (LJ) for B-B attraction and a Weeks-Chandler-Anderson potential for A-B and A-A repulsions. On varying volume fraction and temperature T (in units of $\varepsilon $/k$_{B}$, where $\varepsilon $ is the well depth of the LJ interaction and k$_{B}$ the Boltzmann constant) we observe spheres in cubic phases, HEX cylinders, worm-like and disordered micelles. The time evolution following a quench from T= 0.8 to 0.5 shows a nucleation and growth mechanism where one cylinder breaks into spheres and induces neighboring cylinders to break into spheres. This observation is confirmed by calculating the density profile of each cylinder and Fourier transform of the density distribution. We also performed jumps at constant T = 0.8 by changing the LJ well depth ($\varepsilon )$ from 1 to various higher values. We found that for $\varepsilon \quad >$ 4 the cylinders are kinetically trapped, and the transition is fastest for $\varepsilon $ = 1.5. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U25.00007: Thermal and Mechanical Properties of Polymer Nanofibers from Molecular Simulations Sezen Curgul, Krystyn J. Van Vliet, Gregory C. Rutledge Polymer nanofibers exhibit new, emergent behavior as the diameter of the fibers are decreased from macroscopic to nanometer length scales. Since individual nanofibers are challenging to characterize experimentally due to their small size, computer simulations can be helpful in predicting the properties. We present the results of molecular dynamics (MD) simulations of polymer nanofibers to study their size-dependent properties. The fibers mimic the prototypical polymer polyethylene and have diameters in the range 2.0 to 23.0 nm. The fibers have been analyzed size dependent behavior in their thermal and mechanical properties. The glass transition temperature (Tg) of these amorphous nanofibers decreases with decreasing fiber diameter, and is independent of molecular weight over the range considered. Application of a volume averaged layer model for Tg shows that the cooperativity length scale compares well with previous estimates for polyethylene. Young's moduli of these nanofibers also decrease with decreasing fiber diameter, in agreement with T$_{g}$ depression. There is a significant decrease in modulus when the temperature increases above the glass transition temperature of the surface layer. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U25.00008: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U25.00009: Predicting glass transition temperatures from simulation studies Solomon Duki, Philip Taylor We have been seeking techniques by means of which the glass transition temperature $T_g$ of a polymer can be predicted with minimal computational effort. With this goal in mind, the glass transition in syndiotactic poly(methyl methacrylate) was studied through atomistic molecular-dynamics simulations performed at temperatures in the range from 320 K to 700 K. The mean squared deviations of atoms, monomers, and molecules from their initial positions were analyzed by several different techniques. The most direct method looks at the long-time diffusive motion, and detects a characteristic change in the diffusion constant at $T_g$. This approach required lengthy computer runs to achieve meaningful results. Other techniques study the velocity correlation functions and the short-time vibrational motion. All three yield identical values for the glass transition temperature, but it is found that the method that is most economical of computing resources is the analysis of the short-time departure from ballistic behavior. The apparent softening of the ``cage'' in which a monomer or chain segment oscillates coincides with the onset of diffusive motion. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U25.00010: Band Structure Controlled by Chiral Imprinting Adrian Reyes Cervantes, P. Castro-Garay, Ruben Ramos-Garcia Using the configuration of an imprinted cholesteric elastomer immersed in a racemic solvent, we find the solution of the boundary--value problem for the reflection and transmission of incident optical waves due to the elastomer. We show a significant width reduction of the reflection band for certain values of nematic penetration depth, which depends on the volume fraction of molecules from the solvent, whose handedness is preferably absorbed. The appearance of nested bandgaps of both handednesses during the sorting mixed chiral process is also obtained. This suggests the design of chemically controlled optical filters and optically monitored chiral pumps. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U25.00011: Coarse-graining and Multiscale Modeling of Polymeric Materials Ivan Lyubimov, Marina Guenza Dynamics of macromolecules are characterized by the presence of several length scales in which relevant phenomena take place. Theoretical models play a pivotal role in building the infrastructure that allows one to model multiscale properties. Starting from the Ornstein- Zernike equation we derive analytical methods that coarse-grain the structure of polymeric liquids (homopolymer melts, diblock copolymers, and polymer mixtures) at different length scales of interest. These methods provide effective potentials input to mesoscale simulations. Information obtained from simulations, performed at the united-atom and at the coarse-grained scales, is combined in an original multiscale modeling procedure, resulting in the complete physical picture of the system across the many length scales of interest. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U25.00012: Static properties of equilibrium polymers confined in ultrathin films Anna Cavallo, Joachim P. Wittmer, Albert Johner, Joerg Baschnagel The static properties of equilibrium polymer melts confined in ultrathin films are studied by means of Monte Carlo simulations of a lattice model: the bond fluctuation model. In this work we focus on the effects of ultrathin film confinement between two parallel and neutral walls on chain size and molecular weight distribution. We compare our numerical results to analytical calculations by Semenov and Johner [Eur. Phy. J. E, 12, 469 (2003)] who predicted for ultrathin films, logarithmic corrections to the leading mean-field behavior. Our simulation data are compatible with the theoretical results. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U25.00013: Promotion of the Polyfluorene Beta-Phase: A First Principles Study Elizabeth M. Lupton, Feng Liu, David G. Prendergast, Jeffrey B. Neaton Two configurations of polyfluorenes - potentially important for their blue emission properties in organic devices - have been identified in single molecule spectroscopy experiments: a glassy phase with random torsional angles between fluorene units, and a planar beta-phase. The twisted conformtaion is known to be the lowest energy structure, and the factors which promote the stabilization of the beta-phase are unclear. We present a density functional theory study of ways in which polyfluorene molecules could be manipulated to favor the formation of the photophysically more stable beta-phase. Extension along the molecular axis, which increases the stability of the planar conformation relative to the glassy phase, and the role of side groups are examined in fluorene oligomers and a polyfluorene infinite in the molecular axis. Implications for excited state properties, including fluorescence, will be discussed in the context of these results. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U25.00014: Atomic structures and electronic properties of poly(3-hexyl thiophene) on ZnO(110-1) surface. Sefa Dag, Lin-Wang Wang The atomic structures of adsorbed poly(3-hexyl thiophene) (P3HT) polymers on the non-polar ZnO surface (110-1) are studied with molecular dynamics using ab initio adjusted atomic force fields, and the electronic structures of the resulting systems arc studied with direct ab initio calculations. We investigated different P3HT attachment orientations on the ZnO surface. We also studied the influence of the crystallization among the P3HT polymers to the polymer - ZnO attachment. We found that the strength of the attachment depends strongly on the P3HT crystal orientation, and to the partial charge of the surface Zn, O atoms and the end atoms of the P3HT. We studied the temperature dependence of the attachment, and the effects of the details of the attachment atomic structures to the electronic properties of the interface. \newline \newline This work is supported by U.S. Department of Energy, BES, under contract No. DE-AC02-05CH11231 and it used the resource of the National Energy Research Scientific Computing Center. \newline [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U25.00015: Ab initio study of a promizing class of copolymers for application to high-efficiency photovoltaics Jean Fr\'ed\'eric Laprade, Michel C\^ot\'e In order to achieve high power conversion efficiency in bulk-heterojunction solar cells using PCBM as electron acceptor, it is essential to identify an electron donor polymer which i) harvests the largest part of the solar spectrum and ii) shows an electronic structure appropriate to PCBM. In the last few years, different groups synthetized copolymers based on either fluorene$^{i}$, carbazole$^{ii}$ or dibenzosilole$^{iii}$ with interesting results. This presentation will report the results of density-functional theory (DFT) and time-dependant density-functional theory (TDDFT) calculations on those copolymers and their units in order to better assess the impact of changing the fluorene's 9-atom on the electronic properties. We will focus our discussion on the interplay of the counits on the energy levels and on the oscillator strength of the oligomers. $^{i}$ O. Ingan\"{a}s {\&} \textit{al}. \textit{Appl. Phys. A}, \textbf{2004}, 79, 31 $^{ii}$ N. Blouin {\&} \textit{al}., \textit{Adv. Mater.}, \textbf{2007}, 19, 2295 $^{iii}$ P.L.T. Boudreault {\&} \textit{al}, Macromol. Rapid Commun., \textbf{2007}, 28, 2176 [Preview Abstract] |
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