Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B42: Polyelectrolytes, Conformations, Assembly, and Dynamics |
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Sponsoring Units: DPOLY Chair: Venkat Ganesan, University of Texas at Austin Room: A302/303 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B42.00001: Scaling behavior of single chain dimension of polystyrene sulfonate Qingbo Yang, Jiang Zhao Scaling behavior of single chain of polystyrene sulfonate (PSS-) has been studied by fluorescence correlation spectroscopy. The scaling power index of the hydrodynamic radius of the PSS- single chain in aqueous solutions was found to depend on the salt condition in the solution and a systematic investigation on salt concentration and salt valency has been conducted. The results clearly demonstrate the change in conformation of PSS- chain due to its interaction with the counterions. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B42.00002: Bridging induced by multivalent counterions in polyelectrolyte brush Nicolas Laugel, Robert Farina, Philip Pincus, Matthew Tirrell When the counterions of a polyelectrolyte brush are multivalent, significant changes in its behavior are observed. One example is its shrinkage at values of ionic strengths where osmotic pressure would be expected to keep chains extended. This effect could be explained through the existence of a bridging phenomenon, with the multivalency of each counterion enabling attractive interactions with more than one polymeric charge at a time. Here we present a variational and phenomenological free energy model in a description of the charged brush as an homogeneous layer subject to classical energy contributions related to both chains and counterions. Two mean-field order parameters are introduced to describe counterion condensation and bridging. The model predicts an abrupt collapse of the brush height upon very fine changes in values of the parameters value which is reminiscent of the experimentally observed behavior. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B42.00003: Collapse of single polyelectrolytes in a.c.\,electric fields Chunda Zhou, Robert Riehn Experimental and theoretical studies of single polyelectrolyte molecules under alternating electric fields have concluded that stretching is the near-universal response. We confined fluorescently stained $\lambda$-DNA (48.5 kbp, $\approx$16 $\mu$m contour length) in TBE buffer solution in 500 nm x 10 $\mu$m microchannels and applied alternating electric fields ranging from 0 kV/cm to about 2 kV/cm. We observed that DNA molecules collapsed under these conditions, in contrast to the literature reports. We observed single molecules with a fluorescence microscope, and analyzed the radius of gyration of each molecule in each frame. The threshold of the electric field at which DNA molecules start to collapse depends on both the concentration of TBE buffer solutions and the frequencies of the alternating electric fields. In particular, the critical electric field for collapse increases as the frequency increases. In our experiment, DNA molecules were suspended in 0.25x TBE, 0.5x TBE, 1x TBE and 2x TBE buffer solution, and the frequency was 100 Hz, 200 Hz, 300 Hz, 450 Hz, 675 Hz or 800 Hz. The critical electric field ranged from 0.5 kV/cm to 1.5 kV/cm. We believe the phenomenon is due to aggregation of density fluctuations within the polymer coil, which is not described in traditional homogeneous coil models. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B42.00004: Conformation of Randomly Sulfonated Pentablock Ionomers in Dilute Solution: Molecular Dynamic Simulation Study Dipak Aryal, Dvora Perahia, Gary S. Grest As part of our efforts to define the factors that control the structure and dynamics of structures ionic polymers, the conformation of a pentablock copolymer that consists of randomly sulfonated polystyrene, an ionomeric block, bound to poly-ethylene-r-propylene end caped by poly-t-butylstyrene has been studied in dilute solutions using molecular dynamic simulations. Multi-block copolymers offer a means to tailor several properties into one molecule, taking advantage of their rich phase diagram together with unique properties of specific blocks. We varied the solvent quality for the different blocks and followed the changes in conformation. The spatial configuration of the pentablock as well as the dynamics of the polymer was studied. We find that, independent on the solvent, the higher the sulfonation level, the lower R$_{g}$. The static and dynamic structure factors were calculated and compared in an implicit poor solvent, water and a common solvent. These data are compared with results obtained from neutron scattering. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B42.00005: Polyelectrolyte Dendrimer Conformations from Mean Field Theory Thomas Lewis, Venkat Ganesan The unique architecture of dendrimers has led to research in a wide array of applications including drug delivery. It is widely accepted that non-charged dendrimers exhibit a dense-core radial density profile in order to balance entropic and excluded volume forces. The use of polyelectrolyte dendrimers in drug delivery has been suggested as a way to attain internal cavities within the dendrimer, which can be tuned by varying salt concentration and pH of the solution. In order to gain insight into the equilibrium behavior of both annealed and quenched polyelectrolyte dendrimers, we have developed and numerically solved a Self-Consistent Field Theory approach for charged dendrimer molecules in an implicit solvent. We then use this method to examine the effects of pH, salt concentration, and generation number upon the conformations of these molecules. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B42.00006: Controlling the swelling and wettability of weak polyelectrolyte brushes Richard Gurtowski, Benxin Jing, Elaine Zhu Weak polyelectrolytes (PE) of tunable ionization shows great potential as ``smart'' polymer materials for diverse applications from drug delivery to energy storage. However, the conformational dynamics of surfaced-tethered weak PE chains remain inadequately understood due to the complexity of their dynamic charge states in response to solvation and surface immobilization conditions. In this work, we investigate the wetting and swelling characteristics of poly(2-vinyl pyridine) (P2VP) brushes grafted to a gold substrate by AFM and water contact angle measurements. We observe the collapse of P2VP brushes, accompanied with increased surface hydrophobicity, as increasing solution pH across a critical transition pH, which is considerably lower than the pKa of free P2VP chains in bulk solution. Surprisingly, the broadness of the transition pH range shows a strong dependence with brush thickness, but not grafting density, suggesting a distribution of chain ionization along grafted P2VP brushes. We further manipulate P2VP brush structures by applying ac-electric fields across the brushes to make tunable and switchable polymer surfaces. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B42.00007: Light- and pH Switchable Supramolecular Nanoparticles through Electrostatic Self-Assembly Franziska Groehn, Immanuel Willerich Supramolecular structures that can respond to external triggers are of high interest for example for nanotechnology or drug delivery. Recently we have introduced an approach to electrostatic self-assembly for the formation of supramolecular particles in solution: polyelectrolytes and multivalent stiff organic counterions build well-defined and stable nano-objects. In addition to electrostatics, secondary interactions between counterions such as pi-pi stacking directs the association. Aggregates with narrow size distribution and varying shape such as spheres, cylinders, vesicles and networks result. PH-responsive assemblies can be repeatedly switched ``on'' and ``off'' through pH. Furthermore, light is an elegant, non-invasive stimulus offering possibilities for new functional nanostructures. By electrostatic self-assembly, supramolecular particles can be built the size of which can be triggered by light. For example, assemblies of dendrimer macroions and divalent azobenzene counterions can respond to light with a size increase from 30 nm to 165 nm radius. Detailed characterization by static and dynamic light scattering, AFM, SANS and zeta-potential measurements as well as thermodynamic studies yield insight into driving forces and structural control in the self-assembly process. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B42.00008: Tunable morphologies from charged diblock copolymers Monojoy Goswami, Bobby Sumpter Molecular Dynamics (MD) simulations are carried out to understand the physical aspects of different bulk morphologies formed in charged diblock copolymers. It has been seen that the bulk morphologies formed by charged block copolymers, 75 vol \% fluorinated polyisoprene (FPI) - 25 vol\% sulfonated polystyrene (PSS) with 50\% sulfonation are substantially different from their diblock counterparts. In this study we show how the bulk morphologies change from the uncharged diblock counterparts and also how morphology can be tuned with volume fraction of the charged block and with a change in dielectric constant. A physical understanding based on the underlying strong electrostatic interactions between the charged block and counterions is obtained. The 75/25 diblock shows hexagonal morphologies with the minority blocks (PSS) forming the continuous phase due to charge percolation and the FPI blocks arranged in hexagonal cylinders. Some long-range order can be sustained even by changing the dielectric of the medium. Diverse and atypical morphologies are readily accessible by simply changing the number distribution of the charges on PSS block. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B42.00009: Finite size effects in polyelectrolyte adsorption: A simulation study Maria Sammalkorpi, Paul R. Van Tassel In recent experiments, we have uncovered conditions where polyelectrolyte adsorption to a conducting surface may become continuous in the sense of scaling linearly with time over hours [1]. This discovery of continuous layer growth offers an enticing possibility of nanoscale thin film growth in a single step process, but also brings forth questions of the underlying mechanisms. Here, we present a molecular Monte Carlo simulation study aimed at understanding mechanistically the continuous adsorption process and, more broadly, polyelectrolyte adsorption in general. Our system consists of two parallel polymer chains composed of charged tangent spheres above a surface of variable dielectric discontinuity between the substrate and the solution, and spherical counterions and salt ions. We find that counter ion correlations act to enable the formation of stable polymer-polymer binding and aggregation. We discuss the sensitivity of the attractive regime to a Coulombic coupling parameter and to finite ion size, and the implications of finite size effects and charge distribution both in the polyelectrolytes and in the ions, and implications to experimental observations. [1] A. P. Ngankam and P. R. Van Tassel, Proc. Nac. Acad. Sci. 104, 1140-1145 (2007); C. Olsen and P. R. Van Tassel, J. Colloid and Interface Science 329, 222-227 (2009). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B42.00010: Thermal Properties of Linearly and Exponentially Growing Layer-by-Layer Assemblies Ajay Vidyasagar, Jodie Lutkenhaus Polyelectrolyte multilayer thin films have received significant attention for assembling various nanostructured coatings, but their thermochemical properties are challenging to measure. Here, we present results regarding the thermochemical properties of two different ``model'' layer-by-layer (LbL) assemblies. The LbL process involves alternate deposition of positively and negatively charged polymers resulting in interpenetrating networks of layers with fine structural control. Films may grow linearly or exponentially, and each type of growth is expected to give varied internal structure. Poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) multilayers assembled without (or with) added salt are selected as the linear (or exponential) ``models.'' Other systems explored include hydrogen bonding and PAH/poly(acrylic acid) multilayers. In general, linear growth takes place due to charge overcompensation leading to thinner films than exponential growth, where interdiffusion of polyelectrolytes is a major driving force forming much thicker films. Calorimetry and ellipsometry were used to determine glass transition and crosslinking temperatures. A standing hypothesis is that linear (or exponential) growth is observed for glassy (or rubbery) multilayers. The aim of this work is to understand the origin of linear versus exponential growth in polyelectrolytes with respect to their thermal properties. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B42.00011: Polyelectrolyte gel dynamics during volume phase transitions Mithun Mitra, Jing Hua, Murugappan Muthukumar We will address the dynamics of the elastic modes of a polyelectrolyte gel near the first-order volume phase transition. The role of the neutralizing plasma on the modulus of the polyelectrolyte gel will be discussed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B42.00012: Field theoretic simulations of the interfacial properties of complex coacervates Robert Riggleman, Glenn Fredrickson Many biological processes and emerging technologies, such as wet adhesives and biosensors, rely on the association between oppositely charged polyelectrolytes. Such association is driven not only by the electrostatic interactions between the polyelectrolytes, but there is also a substantial entropy gain associated with counterion release upon complexation. In some cases, the association between oppositely charged polymers can lead to a solid precipitate while others can result in a fluid phase rich in polyelectrolytes (coacervate phase) coexisting with a polyelectrolyte-dilute solvent phase. For many of the applications seeking to exploit coacervation, characterization of the interface between the solvent phase and the coacervate is of paramount importance. In this talk, we will present the results of field-theoretic simulations for a coarse-grained polyelectrolyte model that exhibits complex coacervation. Our simulations sample the fully-fluctuating fields in three-dimensions and provide a detailed characterization of the interface between the solvent and the coacervate phase for symmetric polyelectrolytes (where both the polycations and the polyanions carry identical charge densities) as a function of salt concentration and strength of the electrostatic fields. Finally, we characterize the interfacial properties for a select set of asymmetric conditions. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B42.00013: Electrohydrodynamics of polyelectrolytes using Lattice-Boltzmann simulations without electrostatics Owen A. Hickey, James L. Harden, Christian Holm, Gary W. Slater In computer simulations of polyelectrolyte electrophoresis, the effects of long-ranged hydrodynamics are often ignored due to the high computational cost. However, the hydrodynamic interactions often play a key role in the physics and can lead to some surprising phenomena. We present hybrid Molecular Dynamics simulation methods to study the electrohydrodynamics of polyelectrolytes using a Lattice-Boltzmann (LB) fluid. By applying a local slip between the monomer beads and the LB fluid we are able to reproduce realistic dynamics for free solution electrophoresis as well as the correct stall force for a polyelectrolyte subject to an electric field. Simulations also demonstrate how a net-neutral object, such as a block polyelectrolyte, can have a non-zero net force due to hydrodynamic interactions and that the force can even be perpendicular to the applied electric field. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B42.00014: Thermophoresis of a polyelectrolyte Jennifer Kreft Pearce, Audrey Hammack, Andrew Laster, James Lee, Seth Norman Thermophoresis, the migration of a species due to a temperature gradient, has been shown to be a possible mechanism for manipulating molecules in microfluidic devices. The mechanism governing thermophoresis is complex making a molecule's Soret coefficient (S$_{T})$ and its dependence on different physical factors hard to predict. We experimentally investigate thermophoresis of a polyelectrolyte. For sufficiently high average temperatures, two forms of the molecule are present. We measure the Soret coefficient of both and find that one has positive S$_{T}$ and the other negative. We also investigate the dependence of S$_{T}$ on co-dissolved ionic species, specifically NaOH and NaCl. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B42.00015: Charge transport in confined ionic liquids Joshua Sangoro, Ciprian Iacob, Wycliffe Kipnusu, Friedrich Kremer Charge transport and glassy dynamics in neat and polymerized ionic liquids confined in nanoporous silica are investigated in a wide frequency and temperature ranges by a combination of Broadband Dielectric Spectroscopy and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). By applying the Einstein-Smoluchowski relations to the dielectric spectra, diffusion coefficients are obtained in quantitative agreement with independent PFG NMR. The impact of geometrical confinement as well as the pore wall-ionic liquid interactions on the overall ionic mobility is explored for diverse categories of ionic liquids. The results are discussed within the framework of dynamic glass transition assisted charge transport in ionic liquids. [Preview Abstract] |
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