Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T32: Focus Session: Charged and Ion Containing Polymers |
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Sponsoring Units: DPOLY Chair: Lilin He, Oak Ridge National Labs Room: 340 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T32.00001: Puzzle of the Electrostatic Persistence Length A.V. Dobrynin, J.-M.Y. Carrillo Electrostatic interactions play an important role in controlling properties of synthetic and biological polyelectrolytes. The change in the ionic environment in such systems can significantly influence their conformational properties. For semiflexible polyelectrolyte chains with ionic groups interacting via the screened Debye-Huckel potential the electrostatic contribution to the chain persistence length scales quadratically with the Debye screening length (OSF model). However, recent computer simulations of flexible polyelectrolyte chains with explicit counterions and salt ions show that in the wide interval of the solution ionic strengths the electrostatic contribution to chain persistence length is proportional to the Debye screening length, $r_{D}$. To understand the crossover between flexible and semiflexible chain behavior and elucidate the effect of explicit ions on chain conformations we performed molecular dynamics of polyelectrolyte chains with degree of polymerization $N=$300 and different values of the chain bending rigidity varying between $K=$1 and $K=$160. Our simulations have shown that the bond-bond correlation function describing chain's orientational memory can be approximated by a sum of two exponential functions manifesting the existence of the two characteristic length scales. One describes the chain's bending rigidity at the distances along the polymer backbone shorter than $r_{D}$ while another controls the long-length scale chain's orientational correlations. The long-length scale bending rigidity is proportional to $r_{D}$ for chains with bending rigidity smaller than a crossover bending rigidity $K$*. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T32.00002: Theory of complexation of polyelectrolytes onto curved surfaces Hamidreza Shojaei, Murugappan Muthukumar We have derived analytically the critical conditions for the complexation of flexible polyelectrolytes onto curved interfaces, in terms of the various experimental variables characterizing the interface, the polymer, and the electrolyte condition of the medium. We have used the WKB method and the calculated results will be compared with the previously known results from the variational method. Although the results from both methods are qualitatively similar, the WKB method avoids ad hoc choice of trial functions for the monomer density profile. Implications of our results in the context of experimental situations will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T32.00003: Complexation Between Weakly Basic Dendrimers and Linear Polyelectrolytes: Effects of Chain Stiffness, Grafts, and pOH Thomas Lewis, Gunja Pandav, Ahmad Omar, Venkat Ganesan The unique architecture and high charge density of dendrimer molecules have attracted interest for their utilization in gene delivery applications. The strong binding affinity of cationic dendrimers to genetic materials make them effective gene delivery vectors not only by shielding the nucleic acid (NA) material from degradative enzymes in the blood stream, but also by reducing the overall negative charge of the dendrimer-NA material complex, which in turn creates more favorable interaction with the anionic cell membrane. However, the high cytotoxicities of cationic dendrimers have motivated the development of polyethylene glycol (PEG) conjugated dendrimer molecules, which have been shown to reduce dendrimer cytotoxicity while still retaining transfection ability. In order to gain insight into how the addition of neutral grafts affects the binding affinity and conformations of dendrimer-NA material complexes, we have developed and numerically solved a Self-Consistent Field Theory approach for both grafted and non-grafted annealed charged dendrimer molecules in the presence of linear polyelectrolyte molecules. Specifically, this work examines the effect of linear polyelectrolyte stiffness, grafting chain length, and solution pOH. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T32.00004: Self-organization of multivalent counterions in polyelectrolyte brushes Invited Speaker: Jianzhong Wu The structure and interfacial properties of a polyelectrolyte brush (PEB) depend on a broad range of parameters such as the polymer charge and grafting density, counterion valence, salt concentration, and solvent conditions. These properties are of fundamental importance in technological applications of PEBs including colloid stabilization, surface modification and lubrication, and in functioning of biological systems such as genome packaging in single-strand DNA/RNA viruses. Despite intensive studies by experiments, molecular simulations, and myriad analytical methods including scaling analyses, self-consistent-field theory, and most recently density functional theory, the behavior of PEBs in the presence of multivalent counterions remains poorly understood. In this talk, I will present a density functional method for polyelectrolyte brushes and discuss self-organization of multivalent counterions within highly charged polyelectrolyte brushes. The counterion-mediated attraction between polyions leads to a first-order phase transition similar to that for a neutral brush in a poor solvent. The self-organization of multivalent counterions results in a wavelike electrostatic potential and charge density that oscillate between positive and negative values. [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T32.00005: Linear Viscoelastic and dielectric behavior of Phosphonium Ionomers Quan Chen, Siwei Liang, U Hyeok Choi, James Runt, Ralph H. Colby Linear viscoelastic (LVE) and dielectric (DRS) responses were examined for polysiloxane-based phosphonium-ionomers with fractions of ionic monomers $f$ $=$ 0 to 0.3; the other monomers have short poly(ethylene oxide) side chains. LVE of these samples shows a glassy relaxation followed by a terminal polymer relaxation that is increasingly delayed with increase of $f$. The glassy relaxation broadens when $f$ \textgreater\ 0.1. DRS of these samples shows a segmental $\alpha $ process associated with motion of monomers, followed by an additional $\sim$ 100X slower $\alpha _{\mathrm{2}}$ process before electrode polarization. A detailed comparison between LVE and DRS reveals that the $\alpha_{\mathrm{2}}$ relaxation in DRS corresponds to a characteristic modulus of $k_{\mathrm{B}}T$ per ionic group in LVE. This result strongly suggests that the molecular origin of the $\alpha_{\mathrm{2}}$ relaxation is the dissociation/association of the ionic groups from/into the ionic clusters, consistent with the observed magnitude of the $\alpha_{\mathrm{2}}$ relaxation increasing with ion content. Based on this molecular view, we can predict the terminal polymer relaxation from the $\alpha_{\mathrm{2}}$ relaxation time obtained in DRS, assuming this is the lifetime of ionic associations in a sticky Rouse model. Meanwhile, the broadening of glassy mode distribution with increasing $f$ \textgreater\ 0.1 is attributed to an enhanced cooperation for motion of glassy segments. This enhancement is possibly due to decrease of distance between the ionic groups with increasing $f$, leading to stronger overlap of polarizability volumes. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T32.00006: Ionic Conductivity of Nanostructured Block Copolymer Electrolytes in the Low Molecular Weight Limit Alexander Teran, Rodger Yuan, Inna Gurevitch, Nitash Balsara Nanostructured block copolymer electrolytes containing an ion-conducting block and a modulus-strengthening block are of interest for applications in solid-state lithium metal batteries. Previous work using symmetric polystyrene-block-poly(ethylene oxide) mixed with a lithium salt has demonstrated that the ionic conductivity increases with increasing molecular weight of the poly(ethylene oxide) block in the high molecular weight regime due to an increase in the width of the conducting channel. Our current study extends the previous work to the low molecular weight limit. Small angle X-ray scattering, differential scanning calorimetry, and ac impedance spectroscopy experiments help identify the opposing forces influencing the conductivity in these materials. We also examine the annealing process for these materials, whose ion transport characteristics are well known to be influenced by sample preparation and thermal history. The conductivity appears to be influenced by the conductive channel width as well as the glass transition temperature of the insulating block, which also plays an important role in the annealing process. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T32.00007: Aggregation Behavior of Charged Surfactants and their Mixtures in Ionic Liquids Lang Chen, Harry Bermudez Room-temperature ionic liquids (ILs) have been recently explored as extraordinary solvent with potential opportunities for numerous applications. We set out to obtain a better understanding of the aggregation behavior of charged surfactants within ILs. From phase diagrams and isotherms in several distinct ILs, a connection between solubility of the surfactant and the physical properties of the underlying ionic liquid was established. We conclude that the interfacial energy is crucial in determining aggregation behavior while electrostatic interactions can be largely ignored. This study was extended to include mixtures of cationic and anionic surfactants where our data further demonstrate near-complete charge screening. Mixtures of charged surfactants in ILs can therefore be considered as nearly ideal, in sharp contrast to aqueous solutions. The results here give insight into the nature of self-assembly of surfactants in ILs and the interaction between solutes and IL solvents. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T32.00008: Morphology and Aggregate Local Structure of Precise Polyolefins with Associating Pendant Groups Francisco Buitrago, Dan Bolintineanu, Mark Stevens, Amalie Frischknecht, Karen Winey Polyolefins containing acid and/or ionic pendant groups have specific interactions that produce complex and hierarchical morphologies providing a remarkable set of properties. Despite the widespread industrial use of such materials, rigorous morphology-property relationships remain elusive due to structural heterogeneities in the available copolymers. Recently, linear polyethylenes with associating pendant groups separated by a precisely controlled number of carbon atoms have been synthesized by acyclic diene metathesis (ADMET) polymerization. At room temperature, X-ray scattering shows that the molecular uniformity of these materials results in periodic morphologies of the microphase separated ionic groups. Above their transition temperatures (T$_{\mathrm{g}}$, T$_{\mathrm{m}})$, loss of the periodic structures occurs due to polyethylene crystals melting. The morphologies of precise ionomers at elevated temperatures were further investigated via atomistic molecular dynamics (MD) simulations. The simulations complement the X-ray scattering experiments by providing a clear picture of the aggregate shape and size as a function of counterion type, neutralization level and spacer length. [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T32.00009: Plasticizer Influence on Ionic Morphology and Transport in PEO Ionomers Michael O'Reilly, Hanqing Masser, Daniel King, Paul Painter, Ralph Colby, James Runt, Karen Winey Sulfonated poly(ethylene oxide) ionomers have been blended with a miscible, oligomeric poly(ethylene glycol) in order to study the effect of plasticizers on ionomer performance. Plasticizers can increase ionic conductivity in ionomers by depressing the glass transition temperature and dissolving ionic aggregates. In this study, the relative volume fractions of ionic aggregates in various blend compositions is investigated by curve fitting the X-ray scattering aggregate peak. Two fitting parameters are utilized to quantify aggregate composition, peak area and peak position. Fitting results conclude that plasticizer content dilutes and dissolves ionic aggregates, providing higher conducting ion density than comparable neat ionomers. Dielectric relaxation spectroscopy data confirms that ionic conductivity improves with plasticizer content. Similar curve fitting methods were executed for FT-IR signals, and quantification of aggregate structure is compared with X-ray scattering. [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T32.00010: Predicting the Solution Morphology of a Sulfonated Pentablock Copolymer in an Arbitrary Solvent Mixture Jamie Ford, William Kyei-Manu, Karen Winey Block copolymers self assemble into a wide array of morphologies in solvents. To predict the solution morphology of the polymer, we assess the interactions between the individual blocks and the solvent or solvents. Here, we use the Hansen solubility parameters to calculate the interactions between a library of solvents and an ABCBA pentablock copolymer with non-polar A and B blocks and a polar, sulfonated C block to predict the expected morphology for a given solvent and compare it to our small-angle X-ray scattering data. In non-polar solvents, we observe micelles with a C core and an A-B corona. We observe inverted micelles in polar solvents -- an A-B core with a C corona. We extended our methodology to mixed polar/non-polar solvent systems to predict the solvent ratios corresponding to the transition from micelles to inverted micelles. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T32.00011: Morphology and Dynamics of Ion Containing Polymers using Coarse Grain Molecular Dynamics Simulation Monojoy Goswami, Bobby Sumpter Ion containing polymers are of particular interest in polymer batteries and membranes for separation chemistry applications. With the increasing interest in this field, novel and modern experimental techniques have been developed to design better materials, however, the fundamental understanding of these polymers, their morphology and ion/counterion dynamics are still not very well understood. We present a coarse grain simulation study to understand the structural detail and physics of ion/counterion dynamics. We do implicit as well as explicit solvent calculation to observe the effect of dielectric constant and temperature on dynamics of polymer chain and ion/counterion. The results are then compared with the small angle neutron scattering experiments. These works will help design better materials for future applications. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T32.00012: Packing of charged chains on toroidal geometries? Zhenwei Yao, Monica Olvera de la Cruz We study sequential Langmuir adsorption of a flexible charged polyelectrolyte chain on tori. In the regime of monomer-monomer electrostatic interaction dominating over thermal fluctuations, it becomes a generalized Thomson problem. Various patterns of adsorbed chain are found including double spirals, disclination-like structures, Janus tori and uniform wrappings, arising from the long-range electrostatic interaction and the toroidal geometry. Their broken mirror symmetry and energetics are analyzed. In particular, we find a power law for the electrostatic energy; the dependence of the power on the geometry of tori implies a geometric origin. Furthermore, in the regime of large thermal fluctuation, we systematically study random walks on tori that generate chain configurations; the features associated with the toroidal geometry are discussed. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T32.00013: Quantum mechanical calculation of ion chains in Poly(ethylene oxide)-based Sulfonate Ionomers Huai-Suen Shiau, Michael Janik, Ralph Colby Ion-containing polymers are of interest as single-ion conductors for use as electrolytes in electrochemical devices, including lithium ion batteries. Current ion conductivities of the best ionomers are roughly 100X too small for practical applications and have a small fraction of their Li$^{+}$ counterions participating in conduction. \textit{Ab initio} methods are used to investigate the dissociation/association of ionic chain aggregates. The binding energy as a function of distance between ions is explored, in which the energy at each separation is optimized with respect to the number and location of solvating ether oxygen moieties. We study the barrier between the solvated and bound states as a function of distance between the ions, including the barrier to break ion chain aggregates in different positions along the chain. This is prerequisite to mesoscale simulations capable of reproducing the equilibrium between various ion chain aggregates, with realistic dynamics, from which conductivity pathways can be investigated. [Preview Abstract] |
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