Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F19: Focus Session: Theory and Simulations of Macromolecules IV |
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Sponsoring Units: DPOLY Chair: Anupriya Agarwal, Clemson University Room: 404 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F19.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F19.00002: Twinkling Fractal Analysis of PolyVinyl Acetate (PVAc) Yutao Zhang, Richard P. Wool In amorphous polystyrene melts we have shown by Atomic Force Microscopy (Height and Phase) that dynamic rigid fractal clusters form in equilibrium with the fractal liquid and their relaxation behavior determines the kinetic nature of T$_{\mathrm{g}}$ [J. Non Cryst Solids 357(2): 311-319 2011]. The fractal clusters of size R $\sim$ 1-100 nm have relaxation times $\tau $ $\sim$ R$^{1.8}$ (solid-to-liquid) where the exponent is related to the Fractal dimension D$_{\mathrm{f}}$ and Fracton dimension d$_{\mathrm{f}}$ via Df/d$_{\mathrm{f}} =$1.8. Israeloff et al (2006) showed nanoscale spatio-temporal thermo fluctuations in PVAc using a non-contact Dielectric Force Microscopy technique; PVAC shows similar dynamic clustering using both phase and height tapping AFM modes. The dynamic clusters are clearly evident in the range 1-700 nm. The cluster relaxation behavior was explored in both height and phase modes and found to be different. The fractal clusters have a TFT vibrational density of states G(w) $\sim$ w$^{\mathrm{df-1}}$ with eigenvalues (frequencies) and eigenvectors (displacements) and these are expected to manifest differently in these AFM studies on PVAc thin films. We examine the cluster relaxation functions C(t)$\sim$ t$^{\mathrm{-4/3}}$ predicted by the TFT and look for the presence of highly mobile layers near surfaces and holes in nanothin films. These results are in accord with computer simulations of anharmonically interacting particles and the recent observation of ``Dancing Molecules'' in strained ceramic glass (Huang et al, Science Oct 2013), as predicted by the TFT. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F19.00003: Viscoelasticity of crosslinked epoxy networks under extreme conditions from molecular dynamics simulation Timothy Sirk, Mir Karim, Ketan Khare, Rajesh Khare, Jan Andzelm Understanding viscoelastic behavior at ballistic conditions is critical for the design of new polymeric-based protective materials in civilian and military applications. The relaxation mechanisms available to polymer networks at ballistic conditions (strain rate \textgreater 10$^{\mathrm{5}}$ s$^{\mathrm{-1}})$ include both segmental and chain relaxations, and thus cannot be understood as a simple superposition of the relaxations acting at the much lower strain rates typically considered in experiments. We present viscoelastic properties of polymer networks found from atomistic molecular dynamics simulation, where we consider an epoxy monomer, di-glycidyl ether of bisphenol A, reacted with binary amine mixtures, 4,4'-methylenebis(cyclohexylamine) and poly(oxypropylene) diamine. Our results show that (1) oscillatory strain simulations similar to experimental dynamic mechanical analysis are capable of predicting the complex modulus at high frequencies, (2) the maximum of the loss modulus can be expected to occur well-above the glass transition temperature predicted by simulated volumetric data, and (3) the molecular weight between crosslinks strongly influences the thermodynamic state required for ideal energy dissipation. These results from oscillatory strain will also be compared with other methods of evaluating linear viscoelasticity from molecular dynamics simulation. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F19.00004: Optimization of constant pH replica exchange molecular dynamics method Danial Sabri Dashti, Adrian Roitberg Improvement in sampling of configuration space enhances sampling in protonation space. Recently a constant pH replica exchange (PHREM) method has been developed by Itoh et al to improve the coupling between conformational and protonation sampling. We present a technique for estimating the exchange acceptance ratio (EAR) between two arbitrary replicas (i.e., with distinct pHs) in PHREM. Moreover, we designed a scoring function to optimize the position of each replica on pH ladder. Maximizing the scoring function results in equal EAR between all neighbor pairs, which increases the efficiency of PHREM. We have tested our method on erabutoxin and hen egg white lysozyme (HEWL). We found that the estimations of pKa values in the optimized set of replicas converge faster respect to equally spaced set. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F19.00005: Evaluating the Applicability of the Fokker-Planck Equation for Polymer Translocation James Polson, Taylor Dunn Computer simulation methods are used to study the dynamics of polymer translocation through a nanopore for a coarse-grained model. The variation of the translocation time distributions with nanopore friction strength is examined using Brownian dynamics simulations. The distributions are analyzed using the Fokker-Planck (FP) formalism together with free energy functions explicitly calculated using Monte Carlo simulations. When the pore friction is weak, translocation is rapid and the polymer is not conformationally relaxed. In this regime, the FP equation yields quantitatively poor predictions. By contrast, in the limit of sufficiently strong pore friction, the translocation is slow and the polymer maintains a state of conformational quasi-equilibrium. In this regime, the theoretical predictions for both driven and non-driven translocation are in good agreement with the simulation results. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F19.00006: Ab initio calculations of the atomic and electronic structure of crystalline PEO$_{3}$:LiCF$_{3}$SO$_{3}$ electrolytes Sha Xue, Yingdi Liu, Hongli Dang, Dale Teeters, Daniel Crunkleton, Sanwu Wang With the advent of high conductivity polymer batteries, a great deal of research interest has been generated in the study of PEO:LiCF$_{3}$SO$_{3}$ polymer electrolyte, because of its enhanced stability at the lithium/polymer interface. Experimental studies have concluded that both the PEO$_{3}$:LiCF$_{3}$SO$_{3}$ crystalline complex and the PEO$_{3}$:LiCF$_{3}$SO$_{3}$ amorphous phase are both present when PEO/Li ratio is greater than 3. However, most theoretical investigations to date are concerned about the short chain amorphous PEO:LiCF$_{3}$SO$_{3}$ system. We report first-principles-density-functional-theory calculations of crystalline PEO$_{3}$:LiCF$_{3}$SO$_{3}$. In particular, we provide the atomic-scale characteristics and electronic structures. The calculated results about the bonding configuration, electronic structures, and conductivity properties are in good agreement with the experimental measurements. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F19.00007: Cellulose microfibril formation within a coarse grained molecular dynamics Abdolmadjid Nili, Oleg Shklyaev, Vincent Crespi, Zhen Zhao, Linghao Zhong Cellulose in biomass is mostly in the form of crystalline microfibrils composed of 18 to 36 parallel chains of polymerized glucose monomers. A single chain is produced by cellular machinery (CesA) located on the preliminary cell wall membrane. Information about the nucleation stage can address important questions about intermediate region between cell wall and the fully formed crystalline microfibrils. Very little is known about the transition from isolated chains to protofibrils up to a full microfibril, in contrast to a large body of studies on both CesA and the final crystalline microfibril. In addition to major experimental challenges in studying this transient regime, the length and time scales of microfibril nucleation are inaccessible to atomistic molecular dynamics. We have developed a novel coarse grained model for cellulose microfibrils which accounts for anisotropic interchain interactions. The model allows us to study nucleation, kinetics, and growth of cellulose chains/protofibrils/microfibrils. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F19.00008: Novel and Efficient Methods for Calculating Pressure in Polymer Lattice Models Pengfei Zhang, Qiang Wang Pressure calculation in polymer lattice models is an important but nontrivial subject. The three existing methods -- thermodynamic integration, repulsive wall, and sedimentation equilibrium methods -- all have their limitations and cannot be used to \textit{accurately} calculate the pressure at \textit{all} polymer volume fractions $\varphi $. Here we propose two novel methods. In the first method, we combine Monte Carlo simulation in an expanded grand-canonical ensemble with the Wang-Landau -- Optimized Ensemble (WL-OE) simulation to calculate the pressure as a function of polymer volume fraction, which is very efficient at low to intermediate $\varphi $ and exhibits negligible finite-size effects. In the second method, we introduce a repulsive plane with bridging bonds, which is similar to the repulsive wall method but eliminates its confinement effects, and estimate the two-dimensional density of states (in terms of the number of bridging bonds and the contact number) using the 1/$t$ version of Wang-Landau algorithm. This works well at all $\varphi $, especially at \textit{high} $\varphi $ where all the methods involving chain insertion trial moves fail. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F19.00009: Statistical Behavior of Polymer Chains in Curved Space Jianfeng Li, An-Chang Shi, Hongdong Zhang, Feng Qiu, Yuliang Yang Recently, we have derived the modified diffusion equation of the propagator (the end-segment distribution function) in general curved space for both Gaussian and wormlike chains. Mathematically, a Gaussian-curvature term appears as an extra external field in the diffusion equation of Gaussian chains while there is an additional normal-curvature term for the case of wormlike chains. The basic statistical behavior of polymer chains in curved space can be also extracted by examining these newly derived diffusion equations revealing that Gaussian chains are aware of the intrinsic curvature of the space but are blind to the external curvature while wormlike chains can feel both. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F19.00010: Formation and structural properties of multi-block copolymer vesicles Rong Wang, Shiying Ma Due to the unique structure, vesicles have attracted considerable attention for their potential applications, such as gene and drug delivery, microcapsules, nanoreactors, cell membrane mimetic, synthetic organelles, \textit{etc}. By using dissipative particle dynamics, we studied the self-assembly of amphiphilic multi-block copolymer. The phase diagram was constructed by varying the interaction parameters and the composition of the block copolymers. The results show that the vesicles are stable in a large region which is different from the diblock copolymer or triblock copolymer. The structural properties of vesicles can be controlled by varying the interaction parameters and the length of the hydrophobic block. The relationship between the hydrophilic and hydrophobic block length \textit{vs} the aqueous cavity size and vesicle size are revealed. The copolymers with shorter hydrophobic blocks length or the higher hydrophilicity are more likely to form vesicles with larger aqueous cavity size and vesicle size as well as thinner wall thickness. However, the increase in hydrophobic-block length results to form vesicles with smaller aqueous cavity size and larger vesicle size. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F19.00011: Diamond-Forming Block Copolymers and Diamond-like Morphologies: a New Route towards efficient Block Copolymer Membranes? Igor Erukhimovich, Yury Kriksin Formation of ordered (microphase separated) block copolymer nanostructures is a promising route towards creating isoporous membranes suitable for technological applications. We propose a new route to achieve this target: to choose such block copolymer architectures, which would provide a practically isotropic permeability both in the bulk and in thin films. Basing both on the weak segregation theory extension into the thin films and the self-consistent field theory numerical procedure we present the results concerning the effects of the wall confinement both with neutral, selective and patterned walls on the structure and stability of the block copolymer ordered films. The diamond-like morphology is found to be the most promising one as to optimizing the permeability of thin films. A new effect of the diamond morphology stability enhancement in the presence of a properly designed lamellar-like wall pattern is discovered and the corresponding phase diagram demonstrating the effect of the pattern scale and film width on the diamond morphology stability is presented. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F19.00012: The influence of chain length polydispersity of of ABA triblock copolymers on bicontinuous network structures Zhong-Yuan Lu, Yue Li, Hu-Jun Qian, An-Chang Shi We study the polydispersity effect on microphase separation of ABA triblock copolymers using dissipative particle dynamics simulations, focusing on the formation of bicontinuous structures. The composition window for observing the bicontinuous network structures can be controled by designing polydispersity distributions of ABA triblock copolymers. We find that increasing polydispersity in both A and B blocks can significantly enhance the composition window for observing bicontinuous network structures. The network structures possess good continuity throughout the material, implying possible applications in photovoltaic devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F19.00013: Phase Behavior of Semiflexible Block Copolymer droplets in isotropic homopolymer matrix Ping Tang, Jie Gao, Jianfeng Li, Yuliang Yang We investigate the phase behavior of semiflexible-coil block copolymer droplets in the matrix of isotropic homopolymers by using an efficient pseudo spectral method to solve self-consistent field theory (SCFT) equations. The semiflexible blocks are described with worm-like chain model and the Maier-Saupe orientational interactions are included to deal with semiflexible chains. We will interested that the influence of microphase separation of semiflexible-coil block copolymers coupling with liquid crystalline behavior of rod blocks on the interface and droplets shapes. [Preview Abstract] |
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