Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z42: Theory and Modeling of Polymer Nanocomposites, Interfaces, and Surfaces |
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Sponsoring Units: DPOLY Chair: Ting Ge, University of North Carolina Room: 214B |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z42.00001: Physical gelation in polymer-nanofiller systems Di Xu, Dilip Gersappe Polymer gelation by physically crosslinking to sheet-like nanofillers was studied by Molecular Dynamics simulation. Nanofillers were modeled as rigid bodies of disk-like shapes and crosslinks were simulated by introducing a short-range attraction between the nanofillers and polymer chain ends. The structure, dynamics and mechanics of this polymer gel was studied as function of nanofiller volume fraction. Micelle like clusters were formed by polymers wrapping around nanofillers as its cores. These structures grow, with increased filler fraction, into fibrous structures. We observe the formation of a percolated nework of these fibrous structures, with ordered local structure but disordered globally, as we increase the filler fraction. The dynamics of polymers showed significant caging at intermediate time, in gel state, while the polymers move heterogeneously. Stress autocorrelation and elongation results were analyzed as a function of the nano-filler concentration. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z42.00002: Polymer Crowding and Depletion-Induced Interactions in Polymer-Nanoparticle Mixtures Wei Kang Lim, Alan Denton Macromolecules in crowded environments, such as biopolymers (DNA, RNA, proteins) in biological cells or synthetic polymers in nanocomposite materials, adopt conformations that can differ substantially from those in unconfined spaces. In mixtures of nanoparticles and nonadsorbing (free) polymers, depletion of polymers induces effective interactions between the nanoparticles. Depletion-induced interactions in turn affect the structure and thermodynamic phase behavior of polymer-nanoparticle mixtures. Such interactions can drive bulk demixing and may be involved in compartmentalization of macromolecules in the cell nucleus. Within a coarse-grained model of hard-sphere nanoparticles and ellipsoidal polymer, we perform Monte Carlo simulations to compute polymer shape distributions*, depletion-induced pair potentials, and pair distribution functions, and to explore demixing of polymer-nanoparticle mixtures in the protein limit. We compare our results with theoretical predictions and available experimental data. \\[1ex] *W. K. Lim and A. R. Denton, J. Chem. Phys. 141, 114909 (2014). [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z42.00003: Depletion potential between nanoparticles: From small molecule liquids to dense polymer melts Debapriya Banerjee, Kenneth Schweizer An entropic depletion attraction generically exists between two hard spheres dissolved in a nonadsorbing fluid. PRISM integral equation theory is used to study this problem over an exceptionally wide range of polymer-particle size ratio and chain length (N) including the monomer limit. To mimic constant atmospheric pressure conditions, the dimensionless melt compressibility is fixed at realistic values and polymer density varied with N accordingly. At constant polymer size, the attractive contact minimum of the particle potential of mean force (PMF) scales roughly as particle radius. At fixed particle size, this contact minimum deepens logarithmically with N before generically saturating beyond a crossover N~150. The equilibrium aggregation behavior is dominated by this local feature. However, the PMF beyond contact has features (including repulsive barriers) of a spatial range and amplitude that vary non-monotonically with N which are most pronounced when the particle radius is of order the polymer radius of gyration, Rg. At fixed particle size, this implies a value of N exists that maximizes kinetic stabilization. A weak but long range (Rg-scale) component of the PMF is also found when the radius of gyration is smaller than, or comparable to, the particle radius. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z42.00004: Self-assembled chains of polymer-grafted nanorods in homopolymer films Christina Ting, Boris Rasin, Russell Composto, Amalie Frischknecht An understanding of the self-assembly of nanoparticles in a polymer matrix is needed to utilize their tunable optical and electrical properties. In particular, for anisotropic nanoparticles, the inter-particle distance and orientation are important variables to consider. Using self-consistent field theory (SCFT), we study the self-assembly of polymer-grafted nanorods in homopolymer melts of the same chemistry. The theoretical calculations are performed over a range of parameters for an experimental system of CdSe/CdS nanorods grafted with polystyrene brushes of varying molecular weights. Previously, we have shown that polymer-grafted nanorods were found to transition from dispersed to aligned (side by side) as the matrix chain lengths were increased, depending also on the grafting density and the dimensions of the nanorod. Here, we explore the parameters required for end to end linking, where it has been shown that coupling of localized surface plasmon resonances in a chain of end-linked nanorods can result in a periodic array of enhanced electric fields (hot spots).\\[4pt] Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000 [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z42.00005: Field-theoretic study on polymer-depletion interaction between colloids in solution Wei Li, Kris Delaney, Glenn Fredrickson Using field-theoretic simulations, we study the depletion interaction between colloidal particles in a solution of free block copolymers. Our system consists of two solid non-adsorbing plates and polymer solution in between. A field theory model formulated in the grand canonical ensemble is used for our simulations, and the potential of mean force between two colloids is computed by means of Derjaguin approximation. We begin the investigation with solvent that is neutral to the copolymer blocks. Several effects on the depletion interaction are considered, including surface affinity of the plates to the copolymer, and block copolymer architecture and composition. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z42.00006: Field Theoretic Simulations of Polymer Nanocomposites in the Presence of Adsorbing Block Copolymers Jason Koski, Robert Riggleman The immersion of nanoparticles in a polymer matrix has given rise to improved mechanical, electrical, and optical properties of polymer-based materials. Understanding the phase behavior and controlling the spatial distribution of nanoparticles in these systems plays a critical role in controlling the resulting material properties. Polymer field theory continues to play an important role in our understand of polymeric materials, and recently we have extended the field theoretic framework to incorporate nanoparticles with arbitrary shape and surface grafting. In this talk, I will demonstrate some more recent extensions that incorporate local surface attractions either between nanoparticles or between the particles and a strongly adsorbing polymer, which could be important for systems where one block of the polymer has strong preferential interactions with the particle surface. Our approach enables the study of systems where many-body particle effects could become important and systems that can macro- or microphase separate while fully incorporating fluctuation effects. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z42.00007: Theory and Simulation Studies of Effect of Entropic and Enthalpic Driving Forces on Morphology in Polymer Grafted Particle Filled Nanocomposites Tyler Martin, Arthi Jayaraman Polymer nanocomposites are a class of materials that consist of a polymer matrix embedded with nanoscale fillers or additives that enhance the inherent properties of the matrix polymer. To engineer polymer nanocomposites for specific applications it is important to have design rules that relate molecular features to morphologies of the composite. Using theory and simulation, we previously studied polymer nanocomposites with homopolymer grafted particles in a homopolymer matrix with chemically identical graft and matrix polymers. Specifically, we found that increasing the polydispersity in grafted chain lengths or decreasing the graft and matrix chain flexibility stabilizes the dispersed phase of polymer nanocomposites, due to increased wetting of the grafted layer by matrix chains. We now explore composites with chemically different graft and matrix polymers, that allow us to tune in enthalpic driving forces in addition to the entropic driving forces for particle dispersion/aggregation. We vary the grafting density, composition of the graft and matrix polymers, and strength of the attractive interactions between the grafts and matrix monomers, to study their impact on the phase behavior and structure of polymer grafted particles in a polymer matrix. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z42.00008: Disentanglement in polymer-star mixtures Hendrik Meyer Recent molecular dynamics simulations provide new insights to entangled polymer melts and mixtures with compact stars as a model system of nanocomposites without polymer-particle adsorption. The particle size is in the order of the tube diameter, the particles remain well dispersed over the whole concentration range and the stars are sufficiently compact that the pure system is jammed. For this system, we observe a weak compression of the matrix chains with increasing volume fraction of stars. Short (unentangled) matrix chains get slowed down by adding particles to the system. When the matrix chains become significantly longer than the entanglement length, this trend is inversed and the matrix chains become faster because the particles dilute the entanglement network. The center-of mass (CM) dynamics exhibits regimes of anomalous diffusion in accordance with viscoelastic hydrodynamic interactions (VHI) [1]. At low and intermediate star-particles concentration, the particles themselves vary little in mobility, only at high concentration (above percolation), they become slowed down because of colloidal packing. [1] J. Farago et al. PRL 107, 178301 (2011); PRE 85, 051807 (2012). [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z42.00009: Strong Selective Adsorption of Polymers Ting Ge, Michael Rubinstein A scaling theory is developed for selective adsorption of polymers induced by the strong binding between specific (sticky) monomers and complementary surface adsorption sites. We demonstrate that, in addition to the expected dependence on the polymer volume fraction in the bulk solution, selective adsorption strongly depends on the ratio between two characteristic length scales, the root-mean-square distance l between neighboring sticky monomers along the polymer and the average distance d between neighboring surface adsorption sites. The role of the ratio l/d arises from the fact that a polymer needs to deform to enable the spatial commensurability between its sticky monomers and the surface adsorption sites for selective adsorption. The competition between the entropic penalty associated with the deformation of chains and the energetic gain from binding to adsorption sites determines the optimized structure of adsorbed polymers. We study strong selective adsorption of both telechelic polymers with two end monomers being sticky and multi-sticker polymers with many sticky monomers between sticky ends. We have constructed diagrams illustrating different adsorption regimes as a function of l/d and the bulk volume fraction of polymers for telechelic and multi-sticker polymers. For each regime, the conformations of adsorbed chains have been determined, and the thickness of the adsorption layer and the adsorbed amount of polymers in the layer have been calculated. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z42.00010: Detailed atomistic simulations of functionalized graphene/polymer systems Petra Bacova, Anastassia Rissanou, Vagelis Harmandaris Graphene structures produced by the reduction of the graphene oxide contain some oxygen percentage coming mainly from the carboxyl groups remained on the edges of the graphene. With the increasing importance of the graphene in the material science, here we draw our attention to the effect of these groups on the properties of the graphene-based materials. Molecular simulations can be a valuable tool for the study of such complex materials at the molecular level. We have performed detailed atomistic simulations of hybrid nanostructured polymer/graphene materials for different polymer matrices. We study the behaviour of polymer nanocomposites with three types of dispersed graphene: (a) the pure non-functionalized sheet, (b) graphene with hydrogens grafted on the edges and (c) carboxyl-functionalized graphene. Data concerning the structural and dynamical properties of the polymer chains are presented. In addition, we compute the dynamic properties of the particular graphene sheets and we discuss in detail the importance of the strong electrostatic interactions present in the systems. The information obtained on the molecular scale in our work contributes to the understanding of the miscibility and the mechanical properties of the graphene/polymer nanocomposites. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z42.00011: Surface Tension of Nano-Confined Lattice Polymers Pengfei Zhang, Qiang Wang Surface tension at solid/liquid interface is a key concept in understanding many important surface and interfacial phenomena such as wetting and capillarity. It is, however, not trivial to accurately calculate surface tension in lattice Monte Carlo (LMC) simulations, which are much faster than simulations in continuum. Here we propose a novel, efficient, and accurate method for calculating the surface tension of polymers confined between two parallel and impenetrable surfaces in LMC simulations, and examine how surface tension varies with the degree of confinement (i.e., separation distance between the two surfaces). Direct comparisons between our LMC results and the corresponding lattice self-consistent field (LSCF) calculations also unambiguously and quantitatively reveal the fluctuation/correlation effects on surface tension neglected in LSCF theory. Keywords: Surface tension, lattice polymers, Monte Carlo simulations [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z42.00012: Polymer adsorption transition: Applications of the Wang-Landau and partition function zeros methods Mark Taylor, Samip Basnet, Jutta Luettmer-Strathmann The Wang-Landau (WL) algorithm is a Monte Carlo simulation technique providing a direct computation of the density of states (and thus the partition function) of a many-body system. The partition function encodes all thermodynamic information about a system, and thus, its construction allows for an efficient determination of phase behavior. Here we describe the application of the WL approach to the adsorption transition for both lattice [1] and continuum chains tethered to an attractive surface. We compute the canonical partition function for chains up to length N=1536 and analyze the zeros of these function in the complex inverse-temperature plane. These zeros define a nearly closed circular region, centered on the origin, intersected near the positive real axis by two flaring tails. With increasing chain length the intersection point pinches down towards the positive real axis, dividing the real axis into two distinct regions or phases in accord with Yang-Lee theory. We apply finite size scaling theory for the leading partition function zeros to locate the adsorption transition in the thermodynamic limit and obtain values for the polymer crossover, order parameter, and specific heat exponents. [1] M.P. Taylor and J. Luettmer-Strathmann, J. Chem. Phys., in press, (2014). [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z42.00013: Molecular Simulation studies of adsorption of polymers on non-planar surfaces: Influence of surface characteristics Abishek Venkatakrishnan, Anne Shim, Aquil Frost, John Lewnard, Vikram Kuppa Molecular simulations are employed to investigate the adsorption of freely rotating polymer chains adsorbing on to non-planar surfaces. Adsorption studies on planar surfaces have been studied extensively and fairly well understood. However, in reality, surfaces are non-planar and cannot be represented using smooth surface models. We investigate the effect of surface characteristics on adsorption via molecular dynamics and Monte Carlo molecular simulations in the NVT ensemble. Both regular (uniform) and irregular (self-affine) roughness parameters are studied. The adsorbed polymer chains are characterized by density and orientation profiles, adsorbed fraction and chain topologies. Our results elucidate the extent to which surface roughness influences adsorption, in competition with other factors such as chain length and monomer-surface interaction. We also demonstrate how both adsorption and desorption can be controlled solely by tuning surface inhomogeneities. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z42.00014: Molecular structure of poly(methyl methacrylate) surface: Combination of interface-sensitive infrared-visible sum frequency generation, molecular dynamics simulations, and ab initio calculations He Zhu, Kshitij C. Jha, Ram S. Bhatta, Mesfin Tsige, Ali Dhinojwala The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interface-sensitive sum frequency generation spectroscopy (SFG), all-atom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the $\alpha$-methyl groups of syndiotactic-PMMA are ordered and tilted towards the surface normal. For the isotactic-PMMA, the $\alpha$-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and on average tilt along the surface plane, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. [Preview Abstract] |
Friday, March 6, 2015 2:03PM - 2:15PM |
Z42.00015: Molecular-dynamics study of the Case-II diffusion of methanol in PMMA Jiayuan Miao, Mesfin Tsige, Philip Taylor In Case-II diffusion, a sharp diffusion front moves at a nearly constant speed. In the widely accepted Thomas-Windle model, swelling caused by the entry of solvents into the polymer matrix plays a central role in determining the speed of the diffusion front. The principal difficulty encountered in modeling this process is the large mismatch between the time scale of atomic motion, which is measured in femtoseconds, and the time scale of diffusion in a macroscopic sample, which is measured in millions of seconds. In the work reported here, we approach the problem as one in which the diffusivity D has a strong dependence on the concentration of the penetrant, and then assume that a numerical solution of the appropriate non-linear diffusion equation will yield an accurate portrayal of the characteristics of the diffusion process. In the calculation of D, we use a theoretical model to relate D to the mean squared deviations calculated in the atomistic molecular-dynamics simulations. The advantage of this technique is that the large mismatch in time scales can be bridged in a self-consistent approach to the Case-II diffusion problem that includes the effects of swelling. [Preview Abstract] |
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