Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D19: Focus Session: Theory and Simulations of Macromolecules III - Ionic Polymers |
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Sponsoring Units: DPOLY Chair: Amalie Frischknecht, Sandia National Laboratories Room: 404 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D19.00001: Conformation of Ionic Conjugated Polymers: Molecular Dynamic Simulations Sidath Wijesinghe, Dvora Perahia, Gary Grest The structure and dynamics of poly para phenylene ethynylene (PPE) with substituted carboxylate side groups have been studied using molecular dynamics simulations. These polymers consist of two highly interacting segments, conjugated groups with are luminescent and ionic groups that add functionality either for tethering bio compatible groups or ionic transport ones. Here we investigate the conformation of these polymers which is a delicate balance between the conjugation length, and electrostatic interactions. Specifically we resolved the structure of carboxylate substituted PPE chains in three different solvents, toluene, water, and vacuum. Toluene acts as a good solvent for the backbones of PPEs, water which is a good solvent for the side groups and vacuum which is a poor solvent for the entire molecule. We found out that conformation of the backbone depends on both the presence of ionic groups and the specific interactions with the solvent. As the number of ionic groups along the backbone increases the conformation of the polymer is strongly impacted by formation of ionic clusters. The study shows that proper tuning the degree of ionic substitution PPEs we can either maintain long extended chains or folded. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D19.00002: Chain Shapes and Ordering of Conjugated Polymers from Atomstic Simulations Wenlin Zhang, Enrique Gomez, Scott Milner Conjugated block copolymers, such as P3HT-b-PFTBT, are candidates for optimizing the efficiency of OPVs due to their self-assembly on different length scales. With microphase separated domains, and sharp interfaces between donor and acceptor blocks, transfer of excitons and free charge carriers is enhanced and charge recombination is reduced. To better understand mesocopic and interfacial packing and ordering of these materials, homopolymers are first investigated via atomistic simulations. We proposed a numerical averaging method and an analytical approach to estimate single chain dimensions of semiflexible polymers based on DFT computed dihedral potentials. Estimations are compared to our MD simulation results of polymer melts. Shorter persistence lengths from MD simulations indicate side groups and inter-chain interactions bring flexibility to polymer backbones. By assuming molten polymers as persistent chains of rods, nematic phase transitions and orderings of these materials are also discussed. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D19.00003: Atomistic molecular dynamics simulations of the structure of symmetric Polyelectrolyte block copolymer micelle in salt-free aqueous solution Rajalakshmi Chockalingam, Upendra Natarajan The structure of a symmetric polystyrene-$b-$poly(acrylic acid) (PS-$b-$PAA) micelle in salt-free aqueous solution as a function of degree-of-neutralization (or ionization, $f$) of the PAA is studied via explicit-atom-ion MD simulations, for the first time for a polyelectrolyte block copolymer in a polar solvent. Micelle size increases with $f $in agreement with experimental observations in literature, due to extension of PAA at higher ionization. Pair RDF's with respect to water oxygens show that corona-water interaction becomes stronger with $f$ due to an increase in number density of carboxylate (COO$^{-}$) groups on the chain. Water-PAA coordination (carboxylate O's) increases with ionization. H-bonding between PAA and water increases with $f$ due to greater extent of corona-water affinity. With increase in $f$, atom and counter-ion $\rho $ profiles confirm extension of corona blocks and micelle existing in the ``osmotic regime,'' and a decrease in scattering peak intensity, in agreement with neutron scattering experiments and mean-field theory in literature. Inter-chain distance in PS core is found to decrease with ionization. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D19.00004: Coarse-Grained Modeling of Polyelectrolyte Solutions Alan R. Denton, Sylvio May Ionic mixtures, such as electrolyte and polyelectrolyte solutions, have attracted much attention recently for their rich and challenging combination of electrostatic and non-electrostatic interparticle forces and their practical importance, from battery technologies to biological systems. Hydration of ions in aqueous solutions is known to entail ion-specific effects, including variable solubility of organic molecules, as manifested in the classic Hofmeister series for salting-in and salting-out of proteins. The physical mechanism by which the solvent (water) mediates effective interactions between ions, however, is still poorly understood. Starting from a microscopic model of a polyelectrolyte solution, we apply a perturbation theory to derive a coarse-grained model of ions interacting through both long-range electrostatic and short-range solvent-induced pair potentials. Taking these effective interactions as input to molecular dynamics simulations, we calculate structural and thermodynamic properties of aqueous ionic solutions. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D19.00005: Polyelectrolyte complexes and salt: a computational study Hanne Antila, Paul Van Tassel, Maria Sammalkorpi Charged polymers, polyelectrolytes (PEs), are versatile materials with applications ranging from tissue engineering to sensing elements. In aqueous solutions, oppositely charged PEs form complexes which are known to be sensitive to added salt with responses including shrinking, flocculation or swelling, and at higher concentrations loosening and destabilization of the complex. However, the role of electrostatics, charge correlations, hydration, and ion specific interactions remain unclear. In this work, we use all-atom molecular dynamics with explicit water and ions to probe the effect of excess salt to DNA-polylysine complex formation and stability, and demonstrate the mechanism of PE and ion species specific salt-driven dissociation [1]. The dissociation occurs accompanied by charge reversal in which charge correlations and ion binding chemistry play a role. Our results agree with experimental work on complex dissociation but in addition show the underlying microstructural correlations driving the behavior. We expand the full atomic level detail and dynamics results with theoretical and computational work describing the PE complex as oppositely charged rods to provide a more complete understanding of PE interactions in salt. [1] H. Antila and M. Sammalkorpi, submitted (2013) [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D19.00006: Theoretical Study of Polyelectrolyte/homopolymer blends Youhai Sun, Ashkan Dehghan, An-Chang Shi The phase behaviour of polyelectrolyte/homopolymer blends is studied using self-consistent field theory (SCFT). The blends are composed of charged and neutral polymers plus counter ions dissociated from the polyelectrolytes. The phase diagram of the system is constructed as a function of blend composition, charge density and polymer-polymer interactions. Besides the usual macrophage separation behaviour, the SCFT predicts that under appropriate conditions the system undergoes microphase separation, forming various ordered morphologies similar to block copolymers. The formation of ordered phases in the system is due to the competition between the polymer-polymer interaction, electrostatics and the mixing entropy of the counter ions. In particular, the length-scale of the ordered phases is not limited by the polymer size, thus opening the door for the engineering of microphases with large domain spacings. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D19.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 3:54PM - 4:06PM |
D19.00008: The Swelling of Olympic Gels Michael Lang, Jakob Fischer, Marco Werner, Jens-Uwe Sommer The swelling equilibrium of Olympic gels is studied by Monte Carlo Simulations. We observe that gels consisting of flexible cyclic molecules of a higher degree of polymerization $N$ show a smaller equilibrium swelling degree $Q\propto N^{-0.28}\phi_{0}^{-0.72}$ for the same monomer volume fraction $\phi_{0}$ at network preparation. This observation is explained by a disinterpenetration process of overlapping non-concatenated polymers upon swelling. In the limit of a sufficiently large number of concatenations per cyclic molecule we expect that the equilibrium degree of swelling becomes proportional to $\phi_{0}^{-1/2}$ independent of $N$. Our results challenge current textbook models for the equilibrium degree of swelling of entangled polymer networks. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D19.00009: Highly-correlated Charges in Block Copolyelectrolytes: Charge as a Tool for Morphology Manipulation Charles Sing, Jos Zwanikken, Monica Olvera de la Cruz Block copolymers that include at least one charged block have been of great technological interest due to their use in materials for battery membranes. These materials are difficult to understand theoretically, however, due to the disparate length scale effects of charge correlation and chain conformation driving the microphase separation of these systems. Using a new theoretical approach that can account for both of these effects that is based of hybrid liquid state integral equation-self consistent field theory (LS-SCFT) calculations, we elucidate the fundamental physics underpinning the thermodynamics of these materials. In particular, we demonstrate four main effects that drive the phase behavior of block copolyelectrolytes: Coulombic cohesion, counterion entropy, excluded volume, and ion self energy effects. Tuning parameters such as charge fraction and dielectric constant can be used to explore different microphase-separated morphologies on an axis orthogonal to traditional routes of manipulating block copolymers (i.e. $\chi$ N and block fraction). This expands the palette of tools that can be used to tune this important class of polymeric materials. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D19.00010: Investigation of the structure of levan polysaccharide chains in water via molecular dynamics simulations Deniz Turgut, Binnaz Coskunkan, Gulcin Cem, Deniz Rende, K. Yalcin Arga, Seyda Bucak, Nihat Baysal, Ebru Toksoy-Oner, Rahmi Ozisik Levan is a biopolymer consisting of $\beta $-D-fructofuranose units with $\beta $ (2-6) linkages between fructose rings. Investigation of the structure and behavior of levan in aqeous environments is necessary to understand its biological activity and its potential use in various applications such as carbohydrate-derived drug release. The use of different \textit{in vivo} and \textit{in vitro} bioactivity assays fail to relate the chemical structure and conformation to the observed biological activity. Therefore, considerable research has been directed on elucidating the biological activity mechanisms of polysaccharides by structure-function analysis. To overcome the inherent difficulties of experiments, molecular dynamics (MD) simulations have been used to retrieve comprehensive information regarding the conformations of polysaccharides and their dynamic properties. In the current study, the structure of levan is investigated in aqueous medium and in saline solutions via fully atomistic MD simulations at 298 and 310 K, representing room temperature and physiological temperatures, respectively. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D19.00011: Modeling the Transport Properties of CO$_{2}$/Polyamine Reactive Mixtures at Multiple Length-Scales Salomon Turgman-Cohen, Fernando Escobedo Polyfunctional amine oligomers have been utilized as phase changing sorbents for carbon dioxide capture applications. Knowledge of the dynamic properties of these mixtures is essential to the design of efficient separation processes. The reactive character of polyamine/CO$_{2}$ blends and the severe variation in their transport properties as a function of CO$_{2}$ concentration renders these mixtures challenging to probe experimentally. We implement a multiscale modeling strategy in which polyamine/CO$_{2}$ mixtures are approximated by an ionic speciation model. Molecular dynamics (MD) simulations of such models are used to probe the diffusion coefficient and viscosities at various concentrations of ionic species and absorbed CO$_{2}$. The results of MD simulations are applied to a simple mass transfer model to predict optimal thicknesses of amine films and the kinetics of the absorption process. The latter is compared to experimental thermogravimetric results. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D19.00012: Simulation of a Small Molecule Analogue of a Lithium Ionomer in an External Electric Field John McCoy, Sara Waters, Amalie Frischknecht, Jonathan Brown Ion dynamics were studied in lithium-neutralized 2-pentylheptanoic acid, a small molecule analogue of a precise poly(ethylene-co-acrylic acid) lithium ionomer. Atomistic molecular dynamics simulations were performed in an external electric field. The electric field causes alignment of the ionic aggregates along the field direction. The energetic response of the system to an imposed oscillating electric field for a wide range of frequencies was tracked by monitoring the coulombic contribution to the energy. The susceptibility found in this manner is a component of the dielectric susceptibility typically measured experimentally. A dynamic transition is found and the frequency associated with this transition varies with temperature in an Arrhenius manner. The transition is observed to be associated with rearrangements of the ionic aggregates. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D19.00013: A simulation study of poly(ethylene glycol) in ionic liquids using a physically motivated ab initio force-field Eunsong Choi, Jesse G. McDaniel, J.R. Schmidt, Arun Yethiraj The behavior of poly(ethylene glycol) (PEG) in imidazolium-based ionic liquids (ILs) is studied from molecular dynamics simulations using a new physically motivated force-field. The new force-field accounts for various fundamental intermolecular interactions such as electrostatics, induction, exchange, and dispersion in separate terms where the parameters are derived from ab initio, symmetry adapted perturbation theory (SAPT). The crucial point about the new force-field when compared to other existing force-fields is that it is developed free from empirical parameterization; this is a great advantage particularly for the systems like polymer/IL solutions where experimental data are scarce. We first validate the force-field for neat ILs and neat PEG. Then the force-field is applied to the mixture of the two and the final results are compared with available experiments and simulation results using the OPLS-AA force-field. [Preview Abstract] |
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