Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L45: Focus Session: Polymers in Batteries and Electrochemical Capacitors II |
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Sponsoring Units: DPOLY Chair: Jodie Lutkenhaus, Texas A&M University Room: 216AB |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L45.00001: Ion transport and softening in a polymerized ionic liquid Rajeev Kumar, Vera Bocharova, Evgheni Strelcov, Veronika Strehmel, Joshua Sangoro, Alexei Sokolov, Sergei Kalinin, Bobby Sumpter Polymerized ionic liquids (PolyILs) are promising materials for various solid-state electronic applications such as dye-sensitized solar cells, lithium batteries, actuators, and field-effect transistors. However, fundamental understanding of interconnection between ionic transport and mechanical properties in PolyILs is far from complete. In this work, local charge transport and structural changes in films of a PolyIL are studied using an integrated experiment-theory based approach. Kinetics of charging, steady state current-voltage relations and softening of the PolyIL films beyond certain threshold voltages are studied by applying electric field through a scanning probe microscopy (SPM) tip. All of the experimental data can be explained by a modified Poisson-Nernst-Planck formalism for the charge transport, which takes into account the dissociation of ions under an applied electric field (the Wien effect). [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L45.00002: Formation of ion clusters in the phase separated structures of neutral-charged polymer blends Ha-Kyung Kwon, Monica Olvera de la Cruz Polyelectrolyte blends, consisting of at least one charged species, are promising candidate materials for fuel cell membranes, for their mechanical stability and high selectivity for proton conduction. The phase behavior of the blends is important to understand, as this can significantly affect the performance of the device. The phase behavior is controlled by $\chi$N, the Flory-Huggins parameter multiplied by the number of mers, as well as the electrostatic interactions between the charged backbone and the counterions. It has recently been shown that local ionic correlations, incorporated via liquid state (LS) theory, enhance phase separation of the blend, even in the absence of polymer interactions. In this study, we show phase diagrams of neutral-charged polymer blends including ionic correlations via LS theory. In addition to enhanced phase separation at low $\chi$N, the blends show liquid-liquid phase separation at high electrostatic interaction strengths. Above the critical strength, the charged polymer phase separates into ion-rich and ion-poor regions, resulting in the formation of ion clusters within the charged polymer phase. This can be shown by the appearance of multiple spinodal and critical points, indicating the coexistence of several charge separated phases. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L45.00003: Diffusional Response of Assembled Polyelectrolyte Chains to Salt Annealing Victor Selin, John F. Ankner, Svetlana Sukhishvili We report on the effect of salt on the diffusion of polyelectrolyte (PE) chains within electrostatically assembled polyelectrolyte multilayers. Layer-by-layer (LbL) films were assembled using poly(methacrylic acid) (PMAA) as a polyanion and quaternized poly-2-(dimethylamino)ethyl methacrylate as a polycation. Fluorescence recovery after photobleaching and neutron reflectometry were used to monitor the center-of-mass diffusion of PMAA chains in directions parallel and perpendicular to the substrate (D$_{\mathrm{//}}$ and D$_{\mathrm{\bot }}$, respectively). In both directions, the diffusion coefficient was exponentially dependent on salt concentration, with significantly faster diffusion in the direction parallel to the substrate. At the same time, D$_{\mathrm{//}}$ dramatically decreased with salt annealing time as the films became increasingly intermixed, reflecting strong coupling between internal layering and PE chain dynamics within LbL films. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L45.00004: Characterization of diblock copolymer lamellar structure from neutron scattering measurements and molecular dynamics simulations Cheol Jeong, Jenny Kim, Sangcheol Kim, Tsung-han Tsai, E. Brian Coughlin, Christopher Soles The Nanoscale structure of block copolymers (BCP) with lamellar morphology plays an important role in transport properties for fuel cell and battery application. We develop a paracrystalline model to interpret small angle neutron scattering (SANS) and X-ray scattering (SAXS) data of hydrated amphiphilic BCP lamellar phase in order to elucidate water distribution as well as hydrophilic and hydrophobic domain spacing. We assume Gamma distribution for the fluctuation of lamellar thickness instead of Gaussian. It is observed that BCP can deswell upon hydration along lamellar normal direction due to chain collapse of hydrophobic domain overcoming expansion of hydrophilic domain, which has been compared with coarse-grained molecular dynamics simulation (CGMD). CGMD results show that the variation of interfacial area per chain is strongly correlated to the conformation of hydrophobic chains, domain spacing and water distribution in the hydrophilic domain, compatible with the observation from SANS and SAXS. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L45.00005: Phase separation predicted to induce water-rich channels in fuel cell membranes Daniel Herbst, Thomas Witten, Tsung-Han Tsai, Bryan Coughlin, Ashley Maes, Andrew Herring Fuel cells are a promising alternative energy technology that convert chemical fuel directly into electric power. One important fundamental property is exactly how and where water is absorbed in the polyelectrolyte membrane. Previous theoretical studies have used idealized parameters. In this talk, I show how we made a rigorous connection to experiment to make parameter-free predictions of the water-swelling behavior, using self-consistent field theory. The model block co-polymers we studied form alternating hydrophilic/hydrophobic lamellar domains that absorb water in humid air. I will show how simple measurements of the hydrophilic portion in solution lead to predictions of non-uniform water distribution in the membrane, and compare the results to x-ray scattering. The results suggest locally near-uniform water distributions. In special cases, however, each hydrophilic lamella phase-separates, forming an additional water-rich lamella down the center, a beneficial arrangement for ion conductivity. A small amount of water enhances conductivity most when it is partitioned into such channels, improving fuel-cell performance. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L45.00006: Cation-containing Polymers with Co-continuous Microphase-Separated Morphologies for Rapid Transport Membranes Frederick Beyer, Samuel Price, Alice Savage, Xiaoming Ren, Natalie Pomerantz, Walter Zukas Cation-containing polymer membranes are the subject of renewed research for their potential to enable the use of alkaline fuel cells, and are also of interest for their water vapor transport properties. Charge and water vapor transport are both heavily dependent on membrane morphology and the development of hydrophilic channels throughout the material. Reaction induced phase separation has been shown to create such morphologies when used with uncharged copolymers and crosslinking monomers. Here we have applied this same technique but used ion-containing block copolymers of 4-vinylbenzyltrimethylammonium chloride and styrene to create a cation-containing polymer membrane having a microphase-separated, co-continuous morphology, as characterized by small-angle X-ray scattering (SAXS) and high-angle annular dark field scanning transmission electron microscopy (HAADF STEM). These materials show excellent charge transport behavior and water vapor transport properties, surpassing commercially available materials. These results and efforts to improve other important physical characteristics for membrane applications will be presented. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L45.00007: Morphology and Proton Transport in Porous Block Copolymer Electrolyte Membranes Chelsea Chen, Jeffrey Kortright, David Wong, Nitash Balsara Block copolymer electrolyte membranes consisting of a proton-conducting block and an uncharged structural block are attractive due to their potential in clean energy applications. Herein we demonstrate a novel approach of fabricating block copolymer electrolyte membranes, by inducing pores in the proton-conducting phase. We examine morphology of these membranes with contrast-matched resonant soft X-ray scattering (RSoXS) and electron tomography. Proton conductivity as a function of porosity and water activity is also investigated. By tuning the porosity of the membranes, we are able to adjust the water uptake of the membranes for improved proton conductivities, in both humid air and liquid water. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L45.00008: Structure-morphology-property relationships in polymerized ionic liquids Joshua Sangoro, Maximilian Heres, Joseph Minutolo, Jacob Shamblin, Maik Lang, Stefan Berdzinski, Veronika Strehmel, Stephen Paddison Charge transport and structural dynamics in systematic series of polymerized ammonium- and imidazolium- based ionic liquids are investigated by broadband dielectric spectroscopy, temperature-modulated differential scanning calorimetry, and x-ray as well neutron scattering techniques. Detailed analysis reveal strong decoupling of these processes in the polymerized ionic liquids, implying failure of the classical theories in describing charge transport and molecular dynamics in these systems. In addition, a strong correlation is observed between the ionic conductivity at the respective calorimetric glass transition temperatures and the morphologies revealed by the scattering experiments. In this talk, a physical explanation of the origin of the observed decoupling of ionic conductivity from structural dynamics will be proposed. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L45.00009: Controlling ion aggregation and conduction in PEO-based ionomers. David Caldwell II, Janna Maranas PEO-based ionomers are ideal for reducing concentration polarization found in typical solid polymer electrolytes. This is achieved by binding the anion to the polymer backbone, significantly reducing the anions mobility. Ion aggregation is prevalent in these systems, but their influence on SPE performance is difficult to study experimentally. We present results of molecular dynamics simulations that explore the relationship between ion content and temperature on ion aggregation, polymer motion, and ion conduction. An unforeseen result of ionomers is the creation of string like aggregates that form conduction pathways in the amorphous region. These conduction pathways allow for a partial decoupling of ion conduction with polymer dynamics. The improvement in conductivity through the use of ion aggregates can be quantified by calculating the inverse of the Haven Ratio, dubbed f-value. Typical SPEs have an f-value less than 0.2, while the ionomers of study exhibit f-values near unity or higher. Understanding what properties influence the development and use of these conduction pathways will provide insight for further development of solid polymer electrolytes. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:24AM |
L45.00010: Dynamics of Polymerized Ionic Liquids and their Monomers Invited Speaker: Ralph H. Colby Dielectric spectroscopy determines the static dielectric constants ($\varepsilon_{s} )$ of polymers with imidazolium pendant structures containing a combination of alkylene and ethyleneoxy units as spacers between the backbone and the imidazolium cation. All monomers and their polymers exhibited two dipolar relaxations, assigned to the usual segmental motion ($\alpha )$ associated with the glass transition and a lower frequency stronger relaxation ($\alpha_{2} )$, attributed to ions rearranging. While ion pairs in conventional (smaller) ionic liquids prefer antiparallel alignment (Kirkwood $g\approx 0.1$ with $\varepsilon_{s} $ $\approx $ 15), because their polarizability volumes strongly overlap, ion pair dipoles in the larger ionic liquid monomers display $g$ of order unity and $50\le \varepsilon_{s} \le 110$. Longer spacers lead to higher static dielectric constant, owing to a significant increase of the relaxation strength of the $\alpha_{2} $ process, which is directly reflected through an unanticipated increase of the static dielectric constant with ionic liquid molecular volume. The ionomers consistently exhibit 1.5 - 2.3 times higher static dielectric constants than the monomers from which they were synthesized, suggesting that polymerization encourages the observed synergistic dipole alignment ($g>1)$. Comparison of dielectric and linear viscoelastic responses reveals a strong connection between the time scales of polymer segmental motion ($\alpha )$, ion rearrangements ($\alpha_{2} )$ and the viscoelastic softening associated with the glass transition. For all polymers with imidazolium side chains and a wide variety of counter-anions, there is a strong correlation between glass transition temperature and repeat unit molecular volume. Large side chains have low T$_{\mathrm{g}}$ $\approx $ -50 $^{\mathrm{o}}$C and their ionic conductivity increases as ethylene oxide repeats are incorporated into the side chains. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L45.00011: Morphology and Ionic Conductivity of Oriented Block Copolymer/Ionic Liquid Mixtures Sharon Sharick, Karen I. Winey Ion-containing block copolymers with increased continuity and long-range order of ion-containing microdomains were prepared to probe the impact of grain boundaries and microdomain orientation on ion transport. We studied poly(styrene-$b$-methyl methacrylate) diblock copolymers swollen with 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonylimide) (SbMMA/IL), and characterized the thermal transitions, morphologies, and ionic conductivities by differential scanning calorimetry, small-angle X-ray scattering, and electrochemical impedance spectroscopy over a range of compositions. Two glass transition temperatures ($T_{g}$s) are observed, corresponding to PS and PMMA/IL microdomains, and $T_{g}$,$_{PMMA/IL}$ is modeled well by the Gordon-Taylor expression. SbMMA/IL films prepared by solvent evaporation exhibit strongly microphase-separated lamellar morphology with long-range order. Slower rates of solvent evaporation produce films with lamellae preferentially oriented to be in the plane. In-plane conductivities increase with both increasing ionic liquid content and with better parallel alignment of lamellae. The Sax and Ottino model will be used to compare the conductivity of SbMMA/IL with the homopolymer/IL mixture, PMMA/IL, and to discuss the ion transport mechanism. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L45.00012: Effect of Lithium Ion Concentration of a Single-Ion-Conducting Block Copolymer Electrolyte on the Morphology-Conductivity Relationship Adriana A. Rojas, Sebnem Inceoglu, Nikolaus G. Mackay, Didier Devaux, Greg Stone, Nitash Balsara Single-ion-conducting electrolytes are desirable for lithium metal batteries because they enable the sole conduction of lithium ions, the reacting species in lithium batteries; hence, they avert detrimental battery limitations due to salt concentration gradients. A single-ion-conducting block copolymer electrolyte, poly(ethylene oxide)-b-polystyrenesulfonyllithium (trifluoromethyl sulfonyl) imide (PEO-b-PSLiTFSI), was characterized \textit{in-situ} and \textit{ex-situ} for its ionic conductivity and morphology using AC impedance spectroscopy and small angle x-ray scattering, respectively. This work is the first to elucidate the relationship between the two properties in a single-ion block copolymer electrolyte. The transference number for the copolymers was determined to be greater than or equal to 0.87, indicating that to a good approximation, the block copolymers are single-ion conducting electrolytes. It was found that increasing the molecular weight of the PSLiTFSI block led to an increase in the extent of block copolymer block-mixing and a change in the conductivity profile from discontinuous to continuous. These effects can be attributed to the disruption of PEO crystallization, which was shown to drive microphase separation. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L45.00013: Role of Constituent Hard Polymer in Enhancing Lithium Transference Number of Lithium Salt-Polymer Complexes Gyuha Jo, Moon Jeong Park Lithium polymer batteries have been projected as promising energy storage systems, owing to their unique advantages such as long cycle life, high specific capacity, and high cell potential. While the polymer electrolytes such as poly(ethylene oxide) (PEO) employed in lithium polymer batteries have high ionic conductivity and low volatility, the PEO-lithium salt complexes indicated immense shortcomings of concentration polarization, ascribed to the motion of free anions within PEO. This has limited charge/discharge rate of lithium batteries. In this study, we present a new methodology for improving the ionic conductivity and lithium transference number of PEO, by block copolymerization with a hard polymer, namely poly(dithiooxamide) (PDTOA). Compared to a simple PEO/PDTOA blend, lithium-salt doped PEO-b-PDTOA block copolymers exhibited significantly improved ionic conductivity values. Further, lithium transference numbers as high as 0.66 were observed, which are much higher than the corresponding values for conventional PEO-salt electrolytes ($\sim 0.25$). [Preview Abstract] |
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