Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q45: Transport in Charged and Ion-Containing Polymers |
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Sponsoring Units: DPOLY Chair: Christopher Soles, National Institute of Standards and Technology Room: 216AB |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q45.00001: Molecular Dynamics Simulations of Ion Transport and Mechanisms in Polymer Nanocomposites Santosh Mogurampelly, Venkat Ganesan Using all atom molecular dynamics and trajectory-extending kinetic Monte Carlo simulations, we study the influence of $Al_2O_3$ nanoparticles on the transport properties of $Li^+$ ions in polymer electrolytes consisting of polyethylene oxide (PEO) melt solvated with $LiBF_4$ salt. We observe that the nanoparticles have a strong influence on polymer segmental dynamics which in turn correlates with the mobility of $Li^+$ ions. Explicitly, polymer segmental relaxation times and $Li^+$ ion residence times around polymer were found to increase with the addition of nanoparticles. We also observe that increasing short range repulsive interactions between nanoparticles and polymer membrane leads to increasing polymer dynamics and ion mobility. Overall, our simulation results suggest that nanoparticle induced changes in conformational and dynamic properties of the polymer influences the ion mobilities in polymer electrolytes and suggests possible directions for using such findings to improve the polymer matrix conductivity. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q45.00002: Viscoelastic Nanomechanics of Ionically Cross-linked Polyelectrolyte Networks Biao Han, Daeyeon Lee, Lin Han Understanding the mechanics of ionic polyelectrolyte networks is critical for applications where nm-to-um mechanics is the key to success. This study aims to reveal the roles of ionic cross-links and fixed charges in the viscoelasticity of layer-by-layer poly(allylamine hydrochloride)/poly(acrylic acid) microfilms, PAH/PAA, a complex held by pH-sensitive amine-carboxyl links. AFM-nanoindentation and force relaxation (tip R$=$12.5um) was performed at ionic strength(IS)$=$0.01-1.0M, pH$=$5.5-2.0 (pKa of PAA$=$2.3). When pH changes from 5.5 to 2.0, the films swell for 4x from densely linked, net neutral state to loosely linked, positively charged one. A \textgreater 100x reduction in indentation modulus was observed at all IS, suggesting the dominance of decrease in cross-link density. In most states, more than 90{\%} force relaxation was observed, where cross-link breaking/reformation likely dominates viscoelasticity. However, at pH$=$2.5 and IS$=$0.01M, when electrical double layer repulsion is important (Debye length$=$3nm), relaxation was about 60{\%}, highlighting the contribution of fixed charges. In summary, this study revealed unique viscoelastic behaviors of PAH/PAA due to the pH- and IS-dependent cross-link and charge densities. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q45.00003: Charge transport and glassy dynamics of poly(ethylene oxide)-based single-ion conductors under geometrical confinement James Runt, Ciprian Iacob Segmental and local dynamics as well as charge transport are investigated in a series of poly(ethylene oxide)-based single-ion conductors (ionomers) with varying counterions (Li$+$, Na$+)$ confined in uni-directional nanoporous silica membranes. The dynamics are explored over a wide frequency and temperature range by broadband dielectric relaxation spectroscopy. Slowing of segmental dynamics and a decrease in dc conductivity (strongly coupled with segmental relaxation) of the confined ionomers are associated with surface effects -- resulting from interfacial hydrogen bonding between the host nanoporous silica membrane and the guest ionomers. These effects are significantly reduced or eliminated upon pore surface modification through silanization. The primary transport properties for the confined ionomers decrease by about one decade compared to the bulk ionomer. A model assuming reduced mobility of an adsorbed layer at the pore wall/ionomer interface is shown to provide a quantitative explanation for the decrease in effective transport quantities in non-silanized porous silica membranes. Additionally, the effect of confinement on ion aggregation in ionomers by using X-ray scattering will also be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q45.00004: High Efficiency Conduction at High Ion Contents in Ionomeric Electrolytes Keran Lu, Janna Maranas, Scott Milner High conductivity solid polymer electrolytes (SPEs) can open the door to safer batteries with greater capacity. Current SPEs have low conductivity, which in part is due to collective motion losses from ions diffusing as clusters. Charge is ``carried'' by neutral ion clusters (i.e. pairs). Using an ion-only coarse-grained molecular dynamics simulation, we show that a high ion content ionomeric electrolyte shows negligible collective motion losses due to the passing nature of ion transport. Compared to carrying, passing randomizes cation-anion motion beyond their coordination distance, resulting in greater conduction efficiency in agreement with experiments. These results suggest that well designed ion networks at higher ion contents could potentially produce highly-conductive SPEs. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q45.00005: Effect of molecular weight on ion diffusion and transference number in poly(ethylene oxide) Ksenia Timachova, Nitash Balsara Solid polymer electrolytes are of great interest for their potential use in high specific energy, solid-state batteries, however, salt transport properties in polymer electrolytes have not been comprehensively addressed over a wide range of molecular weights. Poly(ethylene oxide) (PEO) has been the most widely studied polymer electrolyte due to its high solvation of lithium salts and low glass transition temperature. This study presents measurements of the transport properties of lithium bis(trifluoromethanesulfone)imide (LiTFSI) in PEO at both the high concentration present in functional electrolytes and in the dilute limit for a large range of PEO molecular weights. Individual diffusion coefficients of the Li$+$ and TFSI- ions were measured using pulsed-field gradient nuclear magnetic resonance and the cation transference number was calculated. The diffusion coefficients, transference number, and conductivity as a function of molecular weight and salt concentration provide a complete set of transport properties for PEO. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q45.00006: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q45.00007: Conductivity Scaling Relationships in Nanostructured Membranes based on Protic Polymerized Ionic Liquids Gabriel Sanoja, Nathaniel Lynd, Rachel Segalman Nanostructured membranes based on protic polymerized ionic liquids are of great interest for a variety of electrochemical applications. Understanding the relationship between composition, structure, and ionic conductivity for these materials is essential for designing novel membranes with improved properties. In this work, we explore the effect of volume fraction of ionic liquid on conductivity, $\sigma $ using a model system composed of poly[isoprene-\textit{block}-(ethylene oxide\textit{-stat}-histamine glycidyl ether) diblock copolymers [PI-$b-$P(EO\textit{-stat-}HGE)] and the resulting [PI-$b-$P(EO\textit{-stat-}IL)] obtained after treatment with trifluoroacetic acid. These materials self-assemble into lamellar structures with volume fractions of ionic liquid ranging from 0.50 to 0.90 as demonstrated by SAXS. PI-$b-$P(EO\textit{-stat-}IL) membranes exhibit conductivities up to 4 x 10$^{\mathrm{-3}}$ S/cm at room temperature. In addition, PI-$b-$P(EO\textit{-stat-}IL) based membranes have lower water uptake ($\lambda \quad =$ 8-10) in comparison with most proton conducting membranes reported elsewhere. The low $\lambda $ in these membranes might translate into a stronger effect of morphology on transport properties. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q45.00008: Electrochemical Doping of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] using Polymerizable Ionic Liquids Layla Masri, Janelle Leger A number of emerging organic electronic technologies utilize the mixed ionic/electronic conducting character of conjugated polymeric materials. We have developed a process by which fixed doping can be achieved in conjugated polymers through the formation of covalent bonds by replacing the salt used in traditional devices with polymerizable ionic liquids (PILs). It has previously been shown that poly[2-methoxy-5-(3',7'-dimethyl-octyloxy)-p-phenylenevinylene (MDMO-PPV) doped with allyltrioctylammonium allylsulfonate (ATOAAS) will produce a fixed junction light-emitting electrochemical cell due to the dissociation and subsequent immobilization of ATOAAS. We will discuss electrochemical doping of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) films with ATOAAS. We characterize these films via UV-Vis and cyclic voltammetry with emphasis on studying the formation of new mid-band gap energies associated with the color change observed when the film is electrochemically doped. We will discuss the mechanism for the formation of these mid-band gap energies and the applications of these films to developing technologies. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q45.00009: Structure and dynamics of single-ion conducting P(STFSILi)-ran-P(EGMA) copolymer electrolytes Jennifer Schaefer, Christopher Soles Recently, PEO-based copolymers containing the lithiated STFSI monomer have been investigated for use as single-ion conducting electrolytes in lithium batteries. Single-ion conducting electrolytes eliminate ion concentration gradients that diminish cell performance. The low ionic conductivity of these electrolytes has limited their applicability thus far, but electrolytes based on the STFSI monomer have been shown to have sufficient conductivity to support cell operation at moderate temperatures. We will report on the characterization of the morphology and dynamics of P(STFSI)-ran-P(EGMA) copolymer electrolytes as a function of the monomer ratio (ion loading) and length of the polyethylene glycol comb. Copolymers containing sufficiently short PEG combs remain amorphous at ambient temperatures over a range of STFSI content. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q45.00010: Tuning the ionic conductivity in protic polymerized ionic liquid homo, random, and block copolymers Christopher Evans, Rachel Segalman Proton conducting membranes are of interest for a number of energy applications including use in fuel cells and artificial photosynthesis systems. We have synthesized a new class of protic polymerized ionic liquids (PILs) based on imidazolium cations which exhibit high conductivities in the solid state. In contrast to previous imidazolium based PILs, the ionic liquid moiety is attached via a carbon on the imidazole thus leaving the two nitrogens available to act as a proton donor/acceptor. The conductivies of these protic PILs, measured by dielectric spectroscopy, are orders of magnitude higher than the analogous non-protic PILs at a given distance above (Tg). These high conductivities are the result of a strong contribution from proton motion. A series of random and block copolymers containing the polymerized ionic liquid monomer and a non-ionic comonomer were also investigated to determine the role of comonomer on the conductivity of these materials. It was found that methyl acrylate, which has a low glass transition temperature and high dielectric constant, can result in improvements of ionic conductivity. Studies using solid state NMR are underway to understand the role of protons and mobile anions in controlling the overall conductivity of these materials. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q45.00011: Morphology-Conductivity Relationship in Salt-containing Diblock Copolymer/Homopolymer Mixtures Matthew Irwin, Robert Hickey, Timothy P. Lodge In order to unravel how ion conductivity is affected by material morphology, a model system of polystyrene (PS), poly(ethylene oxide) (PEO), PS-\textit{block}-PEO, and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) was fabricated and characterized. These pseudo-ternary polymer blends, in which the lithium salt associates nearly exclusively with the ethylene oxide, have the potential to form a variety of morphologies such as lamellae and the three-dimensionally interpenetrating bicontinuous microemulsion by simply changing blend composition. Similar to what has been observed in salt-containing diblock copolymers, both the order-disorder transition (ODT) temperature and the ODT temperature window of these blends increase sharply with salt loading. By modulating the relative volume fraction of the homopolymers in the blends, it was shown that, although less than order-of-magnitude changes in the domain spacing do not appreciably affect ion conductivity, some morphologies can result in significantly better conductivity than others. These results outline what factors matter most when designing polymer electrolytes for applications such as rechargeable lithium metal batteries and proton exchange membranes. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q45.00012: The Effect of Structural Modifications on Ionic Conductivity in Newly-Designed Polyester Electrolytes Danielle Pesko, Yuki Jung, Geoff Coates, Nitash Balsara Gaining a fundamental understanding of the relationship between molecular structure and ionic conductivity of polymer electrolytes is an essential step toward designing next generation materials for battery applications. In this study, we use a systematic set of newly-designed polyesters with varying side-chain lengths and oxygen functional groups to elucidate the effects of structural modifications on the conductive properties of the corresponding electrolytes. Mixtures of polyesters and lithium bis(trifluromethanesulfonyl)imide (LiTFSI) were characterized using ac impedance spectroscopy to measure the ionic conductivity at various temperatures and salt concentrations. The relative conductivities of these electrolytes in the dilute limit are directly comparable to results of molecular dynamics simulations performed using the same polymers. The simulations correspond well with the experimental results, and provide molecular level insight about the solvation environment of the lithium ions and how the ions transport through these polyesters. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q45.00013: Membranes with artificial free-volume enabled by block copolymer self-assembly Nikos Petzetakis, Nitash Balsara There has been considerable success towards the development of polymeric porous materials with pore sizes in the meso- or macro-scale regime. However, manipulation of polymer porosity in the micro-scale (pore diameter $<$ 2nm) remains challenging. Previous studies relied on changes on the chemical composition and structure of the polymeric material in order to achieve the formation of larger fractional free volume. In the present report we demonstrate a methodology with which we can force a polymeric material away from structural equilibrium and then kinetically arrest it at this -out of equilibrium- state, ultimately, enabling the creation of a polymeric material with artificial free volume. Our methodology is based on block copolymer/homopolymer binary blend self-assembly where the membranes are made by first creating a heterogeneous film of a ABA type triblock copolymer containing a soluble homopolymer, B. Then in a second washing step the soluble homopolymer chains are dissolved away. The volume fraction of the composite membrane occupied initially by chains of homopolymer B is now converted to extra free volume in the microphase of block B. Key role of block A is to kinetically arrest the structure of the polymer during and after the washing step. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q45.00014: Noise and Ionic Conductivity in Glass Nanochannels Benjamin Wiener, Alessandro Siria, Lyd\'eric Bocquet, Derek Stein Ion transport in nanochannels is relevant to processes in biology and has technological applications like batteries, fuel cells, and water desalination. We report experimental studies of the ionic conductance and noise characteristics of pulled glass capillaries with openings on the order of 200 nanometers. We employed an AC measurement technique to probe very low frequency fluctuations in the conductivity and to test a theory attributing these to chemical fluctuations in the surface charge density of the glass. We also investigate Hooge's empirical description of the noise power spectrum and its relationship to current rectification observed in nanochannels in the surface dominated ``Dukhin'' regime. Finally, we test the effects of anion and cation mobility on the direction and magnitude of the observed rectification. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q45.00015: Structure of Nafion Thin Films on Gold Adam Weber, Ahmet Kusoglu, Alexander Hexemer Nafion is the prototypical ionomer in electrochemical energy devices due to its good ionic conductivity and permselectivity. In most devices, ionomers are in contact with precious metal catalysts. When confined to nanometer-thick ``thin'' films (10 to 100 nm), Nafion's morphology and associated transport properties deviate from the bulk. These changes are a function of the substrate and film thickness. In this talk, results from a systematic study of Nafion thin-film morphology on gold substrate using Grazing-incidence X-Ray Scattering (GISAXS) will be presented. GISAXS experiments carried out for a range of incident angles combined with the simulations of the electron density are used to demonstrate that the collected patterns are real and show an anisotropic long-range structural order that is strongest when the film thickness is around 50 nm and weakens for thicker and thinner films. Such ordering is not readily discernible on other substrates like carbon, nor with non-phase separated polymers like polystyrene. Results presented herein provide new insights into the key role of substrate/film interactions in inducing ordered structure in Nafion, which has implications for understanding ionomers interacting with various organic and inorganic materials in electrochemical devices. [Preview Abstract] |
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