Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T19: Theory and Simulation II |
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Sponsoring Units: DPOLY Chair: Amalie Frischknecht, Sandia National Laboratories Room: 320 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T19.00001: Theory of polyzwitterionic solutions Rajeev Kumar, Glenn Fredrickson Conformations of polyzwitterionic molecules in aqueous solutions are investigated using the variational method. We have carried out self-consistent calculations for the degree of counterion adsorption on the zwitterionic sites and the size of a single polyzwitterionic chain. These calculations are used to analyze the solubility of these molecules in water. Results for the effect of an asymmetric counterion adsorption, electrostatic interaction strength, salt concentration, solvent quality, specificity of the zwitterionic monomeric units and the added salt on the conformations of the polyzwitterionic chain will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T19.00002: Charge transport in conjugated polymers: a multiscale picture Victor Ruehle, James Kirkpatrick, Kurt Kremer, Denis Andrienko A framework to study charge transport in conjugated polymers using realistic morphologies is developed. First, the atomistic force field is refined using first-principles calculations. Systematic coarse graining is then performed to extend simulation times and system sizes accessible to molecular dynamics simulations. Material morphologies are generated using the coarse grained and atomistic models. Finally, the charge mobility is obtained using temperature activated hopping picture for charge transport [1]. The framework is tested on neutral and oxidized polypyrrole with different structural ordering [2]. \\[4pt] [1] J. Kirkpatrick, V. Marcon, J. Nelson, K. Kremer, D. Andrienko, Phys. Rev. Lett. 98, 227402 (2007)\\[0pt] [2] V. Ruehle, J. Kirkpatrick, K. Kremer, D. Andrienko, Phys. Stat. Solidi B, 245, 844 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T19.00003: First principles electronic properties investigation of polythienoacene and its derivatives Simon Pesant, Paul Boulanger, Guillaume Dumont, Michel C\^ot\'e The electronic properties of ladder-type polythiophene (polythienoacene) and its derivatives are studied using density functional theory. Upon an analysis of the variation of the band gap when comparing the non-ladder and the ladder-type polymers, a discrepancy is found between the thiophene and the pyrrole(nitrogen-substituted thiophene) polymer families. The polythienoacene has a larger band gap than the polythiophene whereas the opposite is found for the pyrrole polymers. Also, it is found that a simple alternation of the sulfur atom in polythienoacene structure by nitrogen or boron atoms can lead to small band gap polymers. The excitations of these polythienoacene's derivatives are investigated using time-dependent density functional theory. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T19.00004: Electronic structure and carrier transport in disordered conjugated polymers Nenad Vukmirovic, Lin-Wang Wang Thin films of realistic conjugated polymer materials contain both crystalline and amorphous regions, where the latter ones are less understood. This study was therefore focused on electronic structure and carrier transport in amorphous regions of polythiophene (PT) and poly(3-hexylthiophene) (P3HT). Atomic structures were obtained from classical molecular dynamics using a simulating annealing procedure and the charge patching method [1] was used to calculate the electronic structure. It was found that disorder in the electronic structure of P3HT comes from disorder in the conformation of individual chains, while in the case of PT there is an additional contribution due to disorder in electronic coupling among the chains [2]. The electron-phonon coupling matrix elements in P3HT were also calculated and carrier mobility due to phonon-assisted hopping was estimated. \\[0pt] [1] N. Vukmirovic and L.W. Wang, J. Chem. Phys. 128 121102 (2008).\\[0pt] [2] N. Vukmirovic and L.W. Wang, J. Phys. Chem. B, submitted. [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T19.00005: \emph{Ab initio} Study of Diketo-Pyrrolo-Pyrrole Polymers for Photovolta\"ic Applications Simon L\'evesque, Jean Fr\'ed\'eric Laprade, Michel C\^ot\'e Using density functional theory with the hybrid functional B3LYP, we investigate the electrical and optical properties of polymers made with diketo-pyrrolo-pyrrole. It is found that the value of the band gap can be tuned by varying the number of thiophene units within the polymer. Band structure and time-dependent density functional theory results will also be presented. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T19.00006: First-principles investigation of PVDF and its copolymers V. Ranjan, Liping Yu, Marco Buongiorno Nardelli, J. Bernholc Recently, PVDF and its copolymers have generated significant interest due to their electroactive properties [1] and potential for ultra-high energy-storage applications [2]. In this talk, we present the results of first-principles calculations of stable phases and dielectric properties of different copolymers and terpolymers of PVDF at varying concentrations. Our results show that at very high concentrations of Chloro-trifluoroethylene (CTFE), PVDF/CTFE displays sharp transitions between non-polar ($\alpha$) and polar ($\beta$) phases. On the contrary, the same transitions in copolymers with trifluoroethylene (TrFE) and tetrafluoroethylene (TeFE) are not sharp and happen at lower concentrations. We discuss the interplay of copolymer admixture on the dielectric properties of PVDF and discuss the suitability of copolymers for energy storage and electroactive applications. [1] S. G. Lu et al., App. Phys. Lett. 93, 042905 (2008). [2] V. Ranjan et al., Phys. Rev. Lett. 99, 047801 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T19.00007: Adaptive Tempering Monte Carlo Study of Dense Polypyrrole Systems Yafei Dai, Estela Blaisten-Barojas A modified rigid-ion polarizable model potential of polypyrrole is developed with parameters fitted on multiple points of the electronic energy surface of pyrrole oligomers (n-Py) of different sizes calculated with a hybrid density functional approach [1]. Using this potential, systems containing 192 chains (4-Py) and 64 chains (12-Py) were structurally optimized with the Adaptive Tempering Monte Carlo algorithm [2]. Energetics and structure of these systems were studied as a function of density. Both systems have characteristics of a liquid for densities in the range 0.66 -- 1.09 g/cm$^3$ at T=300 K. The oligomer radius of gyration is insensitive to density changes. However, an orientational order parameter shows a sharp increase as a function of density indicating a tendency of the chains to stack forming regions of aligned chains for the denser systems. [1] Y. Dai, E. Blaisten-Barojas, J. Chem. Phys. 129, 164903 (2008); [2] X. Dong, E. Blaisten-Barojas, J. Comp. \& Theo. Nanoscience 3, 118 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T19.00008: Effect of the chain length and segment size of the free polymer on the interaction between two grafted monolayers in a good solvent Walter Chapman, Shekhar Jain, Valeriy Ginzburg, Prasanna Jog, Jeffrey Weinhold, Rakesh Srivastava The interaction between two polymer grafted surfaces is important in many applications, like nanocomposites, colloid- stabilization, and polymer alloys. In our previous work [Jain et. al., J. Chem. Phys. 128, 154910 (2008)], we showed that interfacial statistical associating fluid theory (iSAFT) density functional theory (DFT) successfully calculates the structure of the grafted polymer chains in the absence/presence of free polyatomic solvent. In the current work, we have applied iSAFT to calculate the force of interaction between two such grafted monolayers in implicit good solvent conditions. In particular, we have considered the case where the segment sizes of the free ($\sigma _{f})$ and grafted ($\sigma _{g})$ polymers are different. The interactions between the two monolayers in the absence of the free polymer is always repulsive. However, in the presence of free polymer, the force can be either purely repulsive or can have an attractive minimum depending upon the relative chain lengths of the free (N$_{f}$) and grafted polymers (N$_{g})$. The attractive minimum is observed only when the ratio, N$_{f} $/N$_{g}$, is greater than a critical value. We propose a scaling relation for this case, in agreement with self consistent field theory for ($\sigma_{f} = \sigma_{g}$). [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T19.00009: Interpolation between State Points in the Simulated Perturbation Contributions of Polymer Solutions Amir Vahid, Neil H. Gray, J. Richard Elliott Polymeric mixtures of hydrocarbons, alcohols have been simulated with discontinuous potential models to characterize the Helmholtz energy of the repulsive reference fluids (A0) along with the first and second order perturbation contributions (A1, A2) as functions of density and composition. Taken together, these terms generate a complete equation of state for the mixture, including temperature effects as well as density and composition. The specific hydrocarbons studied were methane, ethane, propane, n-butane, n-hexane, n-heptane, n-decane, and benzene. The specific alcohols were water, methanol, ethanol, n-propanol, and n-octanol. Unfortunately, a slight inconsistency was encountered when the trend observed for these small molecules was extrapolated to the long chain limit. Therefore, we extend the analysis to mixtures of n-alkanes, branched hydrocarbons, and aromatics with polymeric molecules of: n-alkanes, ethyl-styrenes, ethyl-propylenes, and isoprenes. The perturbation contributions can be accurately characterized by van der Waals mixing rules and compared with the MCSL SAFT and Guggenheim-Staverman theories. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T19.00010: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T19.00011: Simulation of Microheterogeneous Networks and Extraction of Segment Orientation Behavior from D-NMR Spectra Bernardo Aguilera-Mercado, Claude Cohen, Fernando Escobedo The degree of heterogeneity in the microstructure of end-linked elastomer networks has been shown to have a very strong impact on the network mechanical and elastic properties such as: ultimate strain, modulus, and toughness. Networks with crosslinks and chains inhomogeneously distributed are expected to exhibit heterogeneous segment orientation responses. The global segment orientation of systems with frozen inhomogeneities, and a significant amount of highly stretched chains at the unperturbed state, cannot be captured by measurements of the deuterium NMR spectra splits solely. Spectrum frequency splits quantify the segment orientation due to local excluded volume interactions only and do not account for the contributions arising from large end-to-end chain deformations. Long wings of the spectrum reflect the presence of strongly aligned segments ignored when one considers only the split. A new methodology based on the Maximum-Entropy method is proposed to find the probability density of an order parameter that describes the network segment orientation from which the global orientation behavior can be completely characterized. The methodology is validated with both molecular simulation and experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T19.00012: Cooperative dynamics in polymer melts: a comparison of theoretical predictions with Neutron Spin Echo experiments Marina Guenza We present a comparison between theoretical predictions of the Generalized Langevin Equation for Cooperative Dynamics (CDGLE) and Neutron Spin Echo data of dynamics structure factors for polyethylene melts. Experiments, peformed by Zamponi end coowrkers, cover an extended range of length- and time-scales providing a compelling test for the theoretical approach. Samples investigated include chains with increasing molecular weights, undergoing dynamics across the unentangled to entangled transition. Measured center-of- mass mean-square displacements display a crossover from subdiffusive to diffusive dynamics. The Generalized Langevin Equation for Cooperative Dynamics relates this anomalous diffusion to the presence of the interpolymer potential, which correlates the dynamics of a group of slowly diffusing molecules in a dynamically heterogeneous liquid. Theoretical predictions of the subdiffusive behavior, its crossover to free diffusion, and of the number of macromolecules undergoing cooperative motion are in quantitative agreement with experiments. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T19.00013: Equilibrium Partitioning of Polymers between Bulk Dilute Solution and Confining Pores Yanwei Wang, Flemming Y. Hansen, Guenther H. Peters, Ole Hassager We have developed a novel framework [1] for the description of the steric hindrance effect on polymers that are subject to confining geometries. The two main ingredients are (i) a new computational method, the Confinement Analysis from Bulk Structures (CABS) approach, which enables calculation of the equilibrium partition coefficient (pore-to-bulk concentration ratio) as a function of the confinement size solely based on snapshots of polymer configurations in bulk, and (ii) the definition of a new molecular size parameter, the steric exclusion radius, which permits collapsing all partition coefficient data for different polymers in the weak confinement regime onto a universal curve. Our latest development in extending the CABS method to cylindrical and spherical pores will be presented.\\[0pt][1] Wang et al. J. Chem. Phys. 128, 124904 (2008); 129, 074904 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T19.00014: Multivalent Nanoparticles: adsorption and organization of bidisperse polymer chains onto a solid interface Folusho Oyerokun, Richard Vaia, John Maguire, Barry Farmer Multivalent nanoparticles, i.e. nanoparticles with two or more ligands attached to their surfaces, are used in a variety of scientific and technological applications. The most common protocols for synthesizing these multivalent nanoparticles involves immersion of the particles into a solution containing the various ligands or into a solution containing an excess of one ligand to drive a partial (solvent mediated) exchange reaction with a previously bound ligand. Despite intense experimental activities, the dependence of the surface coverage on free ligand concentration and solvent quality is still poorly understood. This study addresses the thermodynamics of adsorption of bidisperse end-functionalized polymer chains in a good solvent onto a flat surface. At equilibrium, the absorbed chains form a bidisperse polymer brush in contact with the solution. The role of the degree of bidispersity, adsorption energy, solvent quality on monomer concentration profile, brush height and degree of penetration of free short and long chains into the brush layer will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T19.00015: Molecular simulation of crystal nucleation of n-alkane melts Peng Yi, Gregory Rutledge The homogeneous nucleation of a crystal phase is one of the most interesting phenomena of molecular fluids, yet the microscopic mechanism of which still remains poorly understood. It is even more a mystery in chain molecule systems because the chain connectivity could produce very different crystal nucleus conformations, which are important factors in determining the subsequent crystal growth process and the properties of the final product. In this work we report the results of molecular simulations of crystal nucleation of n-alkanes from the melt. A realistic united atom force field was employed. The crystal phase and melting behavior were first determined by ramping temperature in a set of molecular dynamics simulations. The adiabatic nucleation trajectory was then sampled using the Monte Carlo umbrella sampling technique. The surface energy of the crystal nuclei was calculated assuming a spherical nucleus model and compared with previous studies to validate our numerical definition of a crystal nucleus. We were also able to calculate the end and side surface free energies of a cylinder nucleus model from the simulation data without making further assumption. This method can be extended to study longer n-alkane molecules and the change of nucleus conformation as n increases. [Preview Abstract] |
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