Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session V21: Charged and Ion-Containing Polymers I |
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Sponsoring Units: DPOLY Chair: Jodie Lutkenhaus, Yale University Room: Morial Convention Center 213 |
Thursday, March 13, 2008 11:15AM - 11:27AM |
V21.00001: Nano-Patterns in Gels of Charged Chains with Self-Attracting Interactions Monica Olvera de la Cruz, Juan J. de Pablo Gels of weakly charged chains have large swelling capabilities that depend on pH and/or salt concentration. In the presence of attractions among elastic units, such as poor-solvent conditions, stable nano-patterns are observed. These systems are ideal actuators, since they undergo large volume changes at the nanophase transition. We find that the nanophases are due to the entropy of the counterions, and are stabilized as the monomer density decreases and as the salt concentration increases by hard-core interactions and network heterogeneities. Our model is constructed with results from a non-linear Poisson Boltzmann approach in the limits when it is applicable (low ionic densities) and a grand canonical Monte Carlo method simulation with Donnan partitioning of counterions and co-ions between the gel and reservoir phases. Our theory and simulations are in close agreement with experiments. [Preview Abstract] |
Thursday, March 13, 2008 11:27AM - 11:39AM |
V21.00002: Interactions in Ion-containing Polymers Probed by ab initio Methods Wenjuan Liu, Ralph Colby, Michael Janik We use ab initio methods to estimate dipole moments and interaction energetics in ion-containing polymers. Our calculation quantitatively includes electrostatic interactions (using both permanent and induced dipoles) and effectively estimates solvation energetics for ions interacting with various functional groups. Interactions are reported for various small cations with common functional groups on polymers and carboxylate, sulfonate and phosphonate groups that can be present in anionic ionomers. We demonstrate how these interaction energies can be utilized to design polymer membranes with facile ion transport. [Preview Abstract] |
Thursday, March 13, 2008 11:39AM - 11:51AM |
V21.00003: Conformation transition and counterion distribution of single polyelectrolyte chains in aqueous solution Jiang Zhao, Shengqin Wang We adopted single fluorescence techniques to study the conformation transition of poly 2-vinylpyridine in its aqueous solution. The first-order conformation transition from extended coil to collapsed globule was observed as the pH value in the solution was raised. The critical pH value was shifted higher largely upon the addition of salt in the solution. The study shows a difference of proton concentration at the chain to that in the solution (a difference of three orders of magnitude), and the addition of salt in the solution increased the local proton concentration at the chain and therefore shift the transition point. [Preview Abstract] |
Thursday, March 13, 2008 11:51AM - 12:03PM |
V21.00004: Unifying Self-Consistent Field Theory for Weak Polyelectrolytes Kevin Witte, You-Yeon Won A self-consistent field (SCF) theory for weak polyelectrolytes has been derived from a grand canonical partition function. The formalism accounts for the location and mixing of the charged and uncharged polymer species, treating the local (spatially dependent) charge fraction as a field variable with which to minimize the total free energy. This method of the derivation gives the resulting equations, especially those governing the local charge fraction, that are identical to the results obtained by Szleifer and coworkers (J. Polym. Sci. B Polym. Phys., 2006) who built upon the mean-field ``annealed'' free energy expression proposed by Raphael and Joanny (Europhys. Lett., 1990). However, we show that these results are further identical to the ``two-state'' model of Borukhov, Andelman and Orland (Eur. Phys. J. B, 1998), namely, the potential field due to the polymer charges with which the chains interact and the local charge fraction are shown to be exactly equal. This annealed model is derived by averaging the partition function with regard to the monomer charges. The charged and uncharged states are weighted by their probabilities which is, in our notation, the bulk charge fraction and one minus the bulk charge fraction, respectively. The utility of this theory is demonstrated by comparing its predictions against various experimental results from bulk potentiometric measurements and also from polyelectrolyte brush compression studies. [Preview Abstract] |
Thursday, March 13, 2008 12:03PM - 12:15PM |
V21.00005: Composition and Structure Changes of the Ionic Aggregates with Acid Content and Neutralization Level in Poly (styrene-\textit{co}-methacrylic acid) Ionomers Wenqin Wang, Tsung-Ta Chan, Andrew Perkowski, Shulamith Schlick, Karen I. Winey The morphology of poly(styrene-\textit{co}-methacrylic acid) (SMAA) copolymers neutralized with copper(II), and the corresponding local structures and compositions of ionic aggregates were investigated as a function of acid content and level of neutralization. Using X-ray scattering and scanning transmission electron microscopy, the sizes of ionic aggregates in Cu(II)-neutralized SMAA were found to be independent of acid content and neutralization level. The number density of ionic aggregates increased with acid content and neutralization level, but the increase is significantly less than expected for a fixed ionic aggregate composition. Electron spin resonance spectroscopy indicates \textbf{three} types of cation sites with corresponding relative population changing with acid content, which further indicates a compositional variation of ionic aggregates with neutralization. The correlation between morphology and compositional evolution of the ionic aggregates will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 12:15PM - 12:27PM |
V21.00006: Multiple Nanoscale Morphologies of Poly(Ethylene-\textit{co}-Acrylic Acid) Ionomers Christopher D. Chan, Travis W. Baughman, Kathleen L. Opper, Kenneth B. Wagener, Karen I. Winey We have synthesized linear poly(ethylene-co-acrylic acid) (EAA) copolymers with precisely placed acid groups using ADMET (acyclic diene metathesis). In the acid form, the EAA copolymers with precisely placed acid groups exhibit the typical orthorhombic PE crystal structure along with a new layered structure. The layered structures coexist with the PE crystals and have spacings consistent with the separation between acid groups; at 9.5mol{\%} acid the layer-to-layer spacing is 2.53 nm. When these linear EAA copolymers are neutralized with zinc acetate in solution, high angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the Zn-rich ionic aggregates decrease in size as the level of neutralization increases from 25{\%} to 100{\%}. X-ray scattering indicates that the cations decorate the acid-acid layered structure at low neutralization level, but eventually disrupt the layers at higher neutralization levels. [Preview Abstract] |
Thursday, March 13, 2008 12:27PM - 12:39PM |
V21.00007: Conduction, Ion Association and Dynamics in Polyethylene Oxide-based Polyester Ionomers Daniel Fragiadakis, Shichen Dou, Ralph Colby, James Runt A series of single-ion conducting PEO-based polyester copolymers is synthesized, containing different amounts of ionic sulfonate groups covalently attached to the polymer chains. The dynamics of the polymer chains and the mobile lithium cations is investigated using dielectric relaxation spectroscopy. A physical model of electrode polarization is employed to decompose dc conductivity into the contributions of mobile ion concentration and ion mobility, and the physical meaning of these parameters and relation to literature results on similar systems is discussed. The segmental and local dynamics of the polymer chains is studied. We investigate both the modification of the dynamics due to the presence of the ionic groups, as well as the relation of chain motion to ion transport. [Preview Abstract] |
Thursday, March 13, 2008 12:39PM - 12:51PM |
V21.00008: Correlation between structure and conductivity in stretched Nafion Elshad Allahyarov, Philip Taylor We have used coarse-grained simulation methods to investigate the effect of stretching-induced structure orientation on the proton conductivity of Nafion-like polyelectrolyte membranes. Recent experimental data on the morphology of ionomers describe Nafion as an aggregation of polymeric backbone chains forming elongated objects embedded in a continuous ionic medium. Uniaxial stretching of a recast Nafion film causes a preferential orientation of these objects in the direction of stretching. Our simulations of humid Nafion show that this has a strong effect on the proton conductivity, which is enhanced along the stretching direction, while the conductivity perpendicular to the stretched polymer backbone is strongly reduced. Stretching also causes the perfluorinated side chains to orient perpendicular to the stretching axis. The sulphonate multiplets shrink in diameter as the stretching is increased and show a spatially periodic ordering in their distribution. This in turn affects the distribution of contained water at low water contents. The water forms a continuous network with narrow bridges between small water clusters absorbed in head-group multiplets. We find the morphological changes in the stretched Nafion to be retained upon removal of the uniaxial stress. [Preview Abstract] |
Thursday, March 13, 2008 12:51PM - 1:03PM |
V21.00009: Morphology study in block copolymer electrolytes Scott Mullin, Nisita Wanakule, Nitash Balsara Poly(styrene-b-ethylene oxide)/lithium bis(trifluoromethane)sulfonimide (SEO/LiTFSI) is of interest in battery applications since the doped PEO phase can conduct ions and the glassy PS phase can prevent dendrite growth upon recharging. It is believed that the LiTFSI molecules are localized in the PEO microphases. Previous studies have shown that highly conducting electrolytes can be made from symmetric SEO copolymers. The purpose of this study is to explore the conductivity of asymmetric SEO copolymer systems doped with LiTFSI. Our studies encompass both neat asymmetric SEO copolymers and SEO copolymers blended with PS homopolymers to separate the effects of architecture of the copolymer molecules and morphology adopted by the system in the melt state. Conductivity is measured by AC impedance, morphology is determined by small angle X-ray scattering, and crystallinity of the PEO chains is determined by differential scanning calorimetry. All samples were prepared in hermetically sealed sample cells in an Argon glovebox. [Preview Abstract] |
Thursday, March 13, 2008 1:03PM - 1:15PM |
V21.00010: Increased Water Retention in Polymer Electrolyte Membranes Assisted by Capillary Condensation Moon Jeong Park, Nitash P. Balsara We establish a new systematic methodology for controlling the water retention of polymer electrolyte membranes. We show that block copolymer membranes with well-defined hydrophilic channels in the 2 to 5 nm range remain moist in a relatively dry environment (relative humidity = 50 \%) up to temperatures as high as 90 $^{o}$C. This retention of water leads to an increase in the overall conductivity with increasing temperature. Simple calculations suggest that capillary condensation is important at these length scales. The morphology of the hydrated membranes is determined by a combination of in-situ neutron scattering and cryogenic electron microscopy. [Preview Abstract] |
Thursday, March 13, 2008 1:15PM - 1:27PM |
V21.00011: Engineering polyelectrolyte multilayer structure at the nanometer length scale by tuning polymer solution conformation. Soheil Boddohi, Christopher Killingsworth, Matt Kipper Chitosan (a weak polycation) and heparin (a strong polyanion) are used to make polyelectrolyte multilayers (PEM). PEM thickness and composition are determined as a function of solution pH (4.6 to 5.8) and ionic strength (0.1 to 0.5 M). Over this range, increasing pH increases the PEM thickness; however, the sensitivity to changes in pH is a strong function of ionic strength. The PEM thickness data are correlated to the polymer conformation in solution. Polyelectrolyte conformation in solution is characterized by gel permeation chromatography (GPC). The highest sensitivity of PEM structure to pH is obtained at intermediate ionic strength. Different interactions govern the conformation and adsorption phenomena at low and high ionic strength, leading to reduced sensitivity to solution pH at extreme ionic strengths. The correspondence between PEM thickness and polymer solution conformation offers opportunities to tune polymer thin film structure at the nanometer length scale by controlling simple, reproducible processing conditions. [Preview Abstract] |
Thursday, March 13, 2008 1:27PM - 1:39PM |
V21.00012: Layer Thickness and Charge Compensation of Polyelectrolyte Multilayers Qiang Wang Using a continuum self-consistent field theory, we have modeled the sequential process of layer-by-layer assembly of flexible polyelectrolytes on flat surfaces as a series of kinetically trapped states. Up to 60 depositions of oppositely charged polyelectrolytes (A and B) are performed, each followed by a washing step. Here we focus on the effects of polymer charge fractions, bulk salt concentrations, solvent qualities for A and B, and their incompatibility on the layer thickness and charge compensation of the polyelectrolyte multilayer. We also compare our modeling with available experimental measurements. [Preview Abstract] |
Thursday, March 13, 2008 1:39PM - 1:51PM |
V21.00013: pH-Induced Release of Polyanions from Multilayer Films Svetlana Sukhishvili, Eugenia Kharlampieva, John Ankner, Michael Rubinstein Many studies deal with polymer chains irreversibly bound within electrostatically assembled layer-by-layer (LbL) films. Here we present new insights on the desorption of polymer chains assembled within layered polymer films, triggered by pH variation. Specifically, we report that in the case of a polycation assembled with poly(methacrylic acid) (PMAA) at low pH, the films selectively release the polyacid in response to an increase of external pH. By applying \textit{in} \textit{situ} Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), we find that pH-induced film response is controlled by charge imbalance within the film. Studies of the kinetics of PMAA release reveal that the characteristic time of PMAA chain release, $\tau $, scales with molar mass, $M_{w}$, as $M_{w}^{1.1\pm 0.1}$. We present a theoretical model of the ``sticky gel electrophoresis'' of polyacids with excess charge which predicts a release time proportional to the molecular weight of released polyions and to the film thickness, in agreement with experimental results. Finally, neutron reflectivity studies show explicitly that PMAA release results in disordering of the film structure. [Preview Abstract] |
Thursday, March 13, 2008 1:51PM - 2:03PM |
V21.00014: Properties of Polyelectrolytes in an Ionic Liquid John Harner, David Hoagland In solvents such as water, polyelectrolyte properties depend strongly on ionic strength, reflecting the ability of free ions to screen electrostatic interactions. At high ionic strength, polyelectrolytes remaining soluble behave similarly to neutral polymers. What happens to polyelectrolyte properties in an ionic liquid? A series of polyelectrolytes were dissolved in [EMIM][EtSO4] (ethyl-methyl-imidazolium ethylsulfate) and studied by viscometry as well as static and dynamic light scattering. Both scattering approaches show that sodium polystyrene sulfonate is more swollen in aqueous 0.1M NaBr than in [EMIM][EtSO4]. Furthermore, classical polyelectrolyte effects (fast and slow mode, increased reduced viscosity with dilution) are absent in the ionic liquid. Lastly, variably quaternized polyvinylpyridine exhibits no evidence of coil expansion as charge density increases. We conclude macroion charges are fully screened in ionic liquids. [Preview Abstract] |
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