Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A29: Charged and Ion-Containing Polymers I |
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Sponsoring Units: DPOLY Chair: Andrey Dobrynin, University of Connecticut Room: LACC 504 |
Monday, March 21, 2005 8:00AM - 8:12AM |
A29.00001: Complexation between flexible polyelectrolytes and oppositely charged particles M. Muthukumar, C.Y. Kong, Abhijit Sarkar The recent theory[M. Muthukumar, J. Chem. Phys. 120, 9343 (2004)] of counterion adsorption on flexible polyelectolyte molecules is extended to complexation of polyelectrolytes and oppositely charged entities such as proteins. The modification of the adsorption isotherm under tension of the polyelectrolyte will also be presented. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A29.00002: Solvent effects on the dynamics of polyelectrolyte chains near a charged wall: Molecular dynamics simulations with explicit solvent Govardhan Reddy, Rakwoo Chang, Arun Yethiraj The effect of solvent quality on the behavior of salt-free dilute and semi-dilute polyelectrolyte chains near a charged wall is studied using molecular dynamics simulation. The polyions are modeled as a chain of charged spherical beads, counter ions to the polyion and the surface are charged spheres, and solvent molecules are uncharged spheres. The wall is atomically smooth with a uniform charge density. For dilute solutions, the chain radius of gyration decreases monotonically with decreasing solvent quality but shows a non-monotonic dependence on surface charge density. As the surface charge density is increased, the polyions orient in a direction parallel to the surface. The diffusion constant of the centre of mass and the characteristic rotational relaxation time of the polyions show explicit dependence on the solvent quality: The diffusion constant increases and the rotational relaxation time decreases as solvent quality is decreased. As the surface charge density is increased, the rotational correlation function does not decay in good solvents, contrary to what is seen in poor solvents. In semidilute solutions the thickness of the adsorbed layer increases as solvent quality is decreased suggesting that solvent effects play an important role in polyelectrolyte adsorption. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A29.00003: Self-Consistent Field Calculations of Polyelectrolytes on Flat Surfaces Qiang Wang, Glenn Fredrickson We apply a self-consistent field (SCF) theory to study the behavior of polyelectrolytes (PE) on flat surfaces either electrically neutral or carrying opposite charges to the PE. In the former case, PE are less depleted from a non-adsorbing surface than neutral polymers, due to the presence of counterions near the surface. In the latter case, PE form an adsorption layer near the surface (with no salt added). We then compare the results from full SCF calculations with those obtained under the ground state dominance approximation (GSDA); our results show that the GSDA gives in most cases good quantitative description for PE adsorption on oppositely charged surfaces. Finally, we examine in detail the effects of various parameters on PE adsorption and surface charge compensation by the adsorbed PE, including the charge distribution and degree of ionization of PE, surface charge density, short-range interactions between the surface and polymer, solvent quality, bulk polymer concentration and salt concentration. Our results show that, for PE on oppositely charged surfaces, Coulombic interactions dominate in most cases with no added salt. In such cases, adsorbed PE almost exactly compensate the surface charge; the bulk polymer concentration, solvent quality, and short-range interactions between the surface and polymer have little effects on the amount of adsorbed PE. The added salt plays an important role by screening Coulombic interactions. Further work exploring the importance of fluctuations in the system is undergoing. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A29.00004: Forces of Interaction between Polyelectrolyte Brushes in the Presence of Multivalent Ions and Cationic Surfactant Matthew Tirrell, Feng Li, Akira Ishikubo The surface forces apparatus has been used to measure the forces between polystyrene sulfonate (PSS) brushes, tethered to mica surfaces via hydrophobic poly-t-butylstyrene anchoring blocks, immersed in media containing various concentrations of di- and tri-valent salt, as well as in micellar solutions of the cationic surfactant, CTAB. All of these media produce strong attractive forces between the PSS brushes under some conditions, but with different patterns for the appearance of these attractions as functions of the concentrations of the added ions. These patterns have been studied in detail experimentally. Generally, attractions appear at low concentrations of added salt or surfactant and can be made to disappear at higher concentrations. The interplay between salt and CTAB has some interesting feature in the case of added cationic surfactant. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A29.00005: Ion Sensors based on Polyelectrolyte Hydrogels David Hoagland, Douglas W. Howie, Jr. Attaching a thin polyelectrolyte hydrogel layer to a quartz crystal microbalance (QCM) potentially enables a new class of online ion-sensing devices. The device's operating principal would be conceptually straightforward: ions of a liquid under examination, upon ion exchange with counterions of the hydrogel layer, alter the layer's mass and thus the QCM's resonant frequency. To examine device feasibility, we have grafted thin, chemically crosslinked poly(allylamine) chloride hydrogels to the gold electrodes of a thickness-shear-mode QCM. In otherwise pure water, the resulting devices are easily able to detect nitrate ions at the hundreds of ppm level, displaying sensitivity suitable for drinking water evaluation. The physics of these devices, however, are not well understood. Questions regarding ion selectivity as well as optimal layer thickness and stiffness must be addressed before routine use is considered. Polyelectrolyte and viscoelastic aspects of these questions will be addressed in this presentation. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A29.00006: Time Resolved Studies of Bundle Formation in Rod-Like Polyelectrolytes John C. Butler, Tommy Angelini, Gerard C. L. Wong It is known that multivalent ions can generate attractions between like-charged polyelectrolytes in a wide range of systems. We find that sparteine, a chiral divalent cation, can condense polyelectrolytes in a temperature dependent manner. The condensation behavior of the fd virus, a negatively charged polyelectrolyte, can be modulated by tuning the sparteine mediated attraction via temperature. Moreover, we use real-time fluorescence microscopy imaging to study the temporal evolution of bundle formation. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A29.00007: Computer Simulations of Aggregate Formation and Dynamics in Ionomers Monojoy Goswami, Sanat Kumar, Gerassimos Orkoulas, Aniket Bhattacharya We investigate the structure and the dynamics of aggregate formation in ionomers, through the device of Monte Carlo (MC) and Brownian Dynamics (BD) simulations in the canonical ensemble. We carried out several computer experiments for different temperatures, different chain length and also for different charge states of counterions. Our result shows the formation of aggregates in telechelic ionomers. Pair distribution function for counterions clearly shows cluster formation at low temperatures. At high temperature, for longer chain lengths BD shows clear spherical structure which qualitatively matches with the experimental findings. For longer chain lengths, vesicular structure has been seen which is consistent with experimental findings. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A29.00008: Langevin Dynamics Simulations of Counterion-mediated Complexation of Polyelectrolytes Zhaoyang Ou, M. Muthukumar We have used the Langevin dynamics simulation to study the complexation of oppositely charged flexible polyelectrolytes in salt free solutions. Two uniformly charged chains with condensed counterions are separated in space initially, and allowed to move and interact with each other. The chain size, Coulomb energy and the release of counterions are monitored. The simulations are carried out at different temperatures and electrostatic interaction strengths. We have found in simulations that while the release of the counterions dominates at lower temperatures, the electrostatic attraction between two chains takes over at higher temperatures. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A29.00009: Ionic Conductivity at the Ordinary-Extraordinary Transition in Polyelectrolyte Solutions Ryan Murphy, Murugappan Muthukumar Experimental results on the individual contributions of the polyion, counterion, and added salt to the bulk conductivity of polyelectrolyte solutions will be presented. Concentration dependence of sodium polystyrenesulfonate (NaPSS) and potassium chloride (KCl) as the added salt was studied with respect to conductivity, viscosity, and dynamic light scattering (DLS) measurements. The bulk conductivity of NaPSS with KCl remains independent of polymer concentration up to the ``ordinary-extraordinary'' transition (C$_{s}$/C$_{p}\sim $1), at which there is a split in the diffusion coefficient into slow and fast modes. As the polymer concentration increases (C$_{s}$/C$_{p} \quad <$1), there is a strong positive dependence of conductivity on polymer concentration. However, the severity of this positive dependence decreases with increasing salt concentration, thus pointing to a shift in the dominating conductive contribution from the counter-ions/added salt to the polyion chain at this transition region. With viscosity and diffusion coefficient measurements backing up conductivity data, it is the first time such a systematic experimental investigation has been done with regard to this issue. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A29.00010: Comments on Electrostatic Persistence Length Andrey Dobrynin I have shown that the quadratic dependence of the electrostatic persistence length on the Debye screening length obtained in the classical Odijk-Skolnick-Fixman (OSF) theory is a result of incorrect assumption made about the energetic penalty for chain deformation. By including chain elasticity the linear dependence of the electrostatic persistence length on the Debye screening length is obtained. This result is derived by applying simple scaling analysis of the angle fluctuations and Gaussian variational principle to the system of strongly and weakly charged polymer chains. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A29.00011: Influence of charge density and backbone rigidity on the structure and properties of polyelectrolyte solutions S.I. Yun, Y.B. Melnichenko, G.D. Wignall, K. Hong, J. Mays, R.M. Briber The influence of chain stiffness on the structure and thermodynamic properties of \textit{neutral polymer solutions }is significant in both the good and poor solvent domains. The conformational properties of a dissolved polymer can undergo large dimensional changes depending on the value of the stiffness parameter ($\eta )$, polymer concentration and temperature. At the same time, the collective behavior can develop long-ranged correlations as the phase boundary is approached. Recent theoretical considerations have suggested that the chain stiffness should also play a key role in determining the structure and thermodynamic properties of polyelectrolyte solutions, where a complex crossover from the second order critical phenomena ($\eta \to $0) to the first order isotropic -- nematic transition ($\eta \to $1) may be observed. The key experimental variable of our research is backbone stiffness and the charge density of polyelectrolyte. For a ionizable group (sulfonate), the effect of chain stiffness can be elucidated through studies of poly(styrene sulfonate) (PSS) as a flexible molecule and poly(cyclohexadiene sulfonate) (PCHDS), with a ``semiflexible'' backbone. We also varied the degree of sulfonation to couple the effect of hydrophobic interactions with backbone stiffness. Small angle neutron scattering (SANS) and zero averaged contrast methods were used to characterize the PCHD backbone and PCHDS polyelectrolyte in solutions. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A29.00012: Liquid structure of flexible polyelectrolyte solutions James Donley, David Heine MD simulations and the recently developed RORPA theory are used to examine the liquid structure of flexible polyelectrolyte solutions. Quantitative comparison is made with experiments that show an invariance of the structure factor peak height and wavevector with chain charge fraction f. This invariance has important implications for the design of polyelectrolyte-based materials. Previous theoretical results for rod polymers show that proper inclusion of polymer-polymer density correlations is sufficient to yield this invariance. However, for flexible polyelectrolytes other aspects such as counterion condensation or an explicit solvent seem to be at least partially necessary to produce the experimental trends. Possible causes of the anomalously large low wavevector scattering, i.e., slow mode, seen in many experiments will also be discussed. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A29.00013: Solid-State NMR Investigations of a Perfluorinated Ionomer (Nafion) Qiang Chen, Klaus Schmidt-Rohr The chain dynamics and supramolecular structure of Nafion$^{\mbox{{\textregistered}}}$, a perfluorinated ionomer which is widely used as a hydrophilic permselective membrane in fuel cells and chloralkali electrolysis, have been studied by solid-state NMR. With 1D and 2D NMR under 30-kHz magic-angle spinning (MAS), the $^{19}$F and $^{13}$C NMR peak widths and positions are determined, which corrects several previous assignments. The peak widths reveal static disorder around the branch point, increasing mobility towards the side group end, and a conformationally ordered backbone, which is essentially polytetrafluoroethylene (PTFE). Fast rotations of the helical backbone segments around their axis are confirmed in PTFE and observed similar in Nafion. The equal $^{19}$F chemical shifts within parallel packed rotating chains in PTFE crystals result in slow $^{19}$F spin diffusion between differently oriented chains. This spin diffusion is observed very fast for a majority of backbone segments in Nafion and the orientational correlations of the remainder backbones are weak. The typical diameter of backbone ``clusters'' in Nafion was 1 - 3 nm estimated by $^{19}$F spin diffusion. Relatively fast $^{19}$F spin exchange from any site in the side group to the backbone is observed. Absorbed water increases the side-group dynamics and conformational averaging, but not the segments near the branch point. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A29.00014: Small Angle Neutron Scattering (SANS) Study of Perfluorinated Ionomer Membrane under In-situ Vapor Sorption Man-Ho Kim, Charles J. Glinka SANS measurements were made on both solvent cast and extruded perfluorinated ionomer membranes as a function of humidity. The specially designed in-situ vapor sorption apparatus changes the relative humidity rapidly, allowing SANS measurements to follow initial structural changes with sorption time. The two types of membranes show significant structural differences in the position and breadth of their characteristic SANS features even for the same equivalent weight (EW=1100). The structure and diffusion of water vapor into the membrane at room temperature will be discussed. [Preview Abstract] |
Monday, March 21, 2005 10:48AM - 11:00AM |
A29.00015: Structure of highly rigid ionic polymers from single molecules to membranes Lilin He, Dvora Perahia, Christopher J. Cornelius The structure of ionic polymers in solutions and in their solid state is governed by the segregation to hydrophilic and hydrophobic regions. Very rigid backbones with persistence lengths much larger than the size of the monomer limit the segregation affecting the resulting structure and dynamics of the polymers. Using a newly synthesized \textit{para }phenylene based sulfonated polymer with a potential to serve as a polymeric electrolytic membrane for fuel cell applications, we followed the structure of highly rigid ionic polymers from a single molecule to a water swollen membranes using small angle neutron scattering and AFM/TEM techniques. AFM and TEM images show that the dry membranes have domains with a diameter from 30 nm to 70nm. Small angle neutron scattering probes the smaller structure in the membranes from dry to swollen states. Fitting to Teubner-Strey model of SANS data indicates the bi-continuous phases were formed with water and ethanol despite the rigidity of the backbone. [Preview Abstract] |
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