Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H42: Focus Session: Polymers for Energy Storage and Conversion -- Physics of Ion Conductivity in Polymers |
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Sponsoring Units: DMP DPOLY GERA Chair: Jodie Lutkenhaus, Texas A&M University Room: A302/303 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H42.00001: Polymer Physics Prize Break |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H42.00002: Broadband Dielectric Spectroscopy and Quasi-Elastic Neutron Scattering on Single-Ion Polymer Conductors Christopher Soles, Hua-Gen Peng, Kirt Page, Chad Snyder, Ashoutosh Pandy, Youmi Jeong, James Runt The application of solid polymer electrolytes in rechargeable batteries has not been fully realized after decades of research due to its low conductivity. Dramatic increases of the ion conductivity are needed and this progress requires the understanding of conduction mechanism. We address this topic in two fronts, namely, the effect of plasticizer additives and geometric confinement on the charge transfer mechanism. To this end, we combine broadband dielectric spectroscopy (BDS) to characterize the ion mobility and quasi-elastic neutron scattering (QENS) to quantify segmental motion on a single-ion model polymer electrolyte. Deuterated small molecules were used as plasticizers so that the segmental motion of the polymer electrolyte could be monitored by QENS to understand the mechanism behind the increased conductivity. Anodic aluminum oxide (AAO) membranes with well defined channel sizes are used as the matrix to study the transport of ions solvated in a 1D polymer electrolyte. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H42.00003: A Quasi Elastic Neutron Scattering study of polymer dynamics in PEO based sulfonate ionomers: Effect of ion content and ion identity Kokonad Sinha, Janna Maranas We present Quasi Elastic Neutron Scattering (QENS) data for characterizing dynamics in ion containing polymers (ionomers) with varying ion content (cation to ether oxygen ratio) and ion identity. To remove electrode reverse polarization, the anion is immobilized by covalently bonding it to the PEO backbone through an `ionizable' isophthalate co-monomer unit and only the cation contributes to the conductivity. We vary the ion content in two ways: changing the ratio of neutral to ionized co-monomer units, and changing the length of the PEO spacer separating the co-monomer units. In neutral ionomers, we observe two segmental processes; PEO segments in the spacer midpoint are one order of magnitude faster than those near the isophthalate groups. In ionized samples, cross-linking between ionic groups considerably slows the dynamics of PEO segments near the isophthalate group. The extent of cross linking depends on the ion content and spacer length. This effect is also ion dependent, which indicates that cations have different binding capacities and formation of this complex controls the availability of free cations for conduction. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H42.00004: Structure and Dynamics of Proton-Conducting Azoles Confined within Metal-Organic Frameworks Jamie Ford, Jason Simmons, Taner Yildirim Efficient polymer electrolyte membrane (PEM) fuel cells are one of the most promising candidates to power our vehicles of the future. Hydrated sulfonated polymers are currently the preferred membrane material because of their excellent conductivity and gas diffusion characteristics. The intrinsic water dependence in these systems limits the operating temperature to 100 C, leading to reduced electrode kinetics and increased CO poisoning. If water can be replaced by a small molecule with a higher boiling point, the overall efficiency of the system can be improved. To this end, we have investigated a set of new host/guest materials based on metal-organic frameworks (MOFs) loaded with a variety of azoles. The thermally and chemically stable frameworks provide a well-defined porous structure that accommodates the proton conduction pathways formed by the azole networks. We will present the structure of the azole networks as well as insight into the proton motion dynamics as a result of a variety of neutron scattering experiments. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:48AM |
H42.00005: Ion solvation thermodynamics in polymer blends and block copolymers Invited Speaker: There is much current interest in ion-containing polymers as materials for energy applications. For example, a promising system for rechargeable battery applications consists of diblock copolymers of an ion-dissolving block, typically polyethylene oxide (PEO) and a nonconducting block such as polystyrene. The addition of lithium salts has been shown to significantly alter the order-order and order-disorder transition temperatures, which reflects a change in the miscibility between the two polymer blocks. In this talk, I discuss some simple theoretical ideas for explaining and predicting the change in polymer miscibility due to the addition of salt ions for both polymer blends and block copolymers. A key effect is the solvation energy of the ions by the polymers, which we approximate using the Born solvation model. The difference in the Born energy of the ions between different polymers provides a driving force towards phase separation, whereas the translational entropy of the ions favors keeping the polymers mixed. In the case of lithium salts added to systems containing PEO, we develop a complexation model in which the lithium ions are tightly bound to the oxygen groups in the EO monomers, while the anions can either be free or form ion pairs with the lithium. For PEO-PS blends or block copolymers, we show that adding lithium salts leads to significant increase in the effective $\chi$ parameter between the two polymers. Our theory predicts that the effect should weaken with increasing radius of the anion, in agreement with available experimental data. Furthermore, we show that the domain spacing in microphase separated block copolymers should increase, also in agreement with experiments. We also examine the issue of ion distribution using self-consistent field theory. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H42.00006: Understanding Ion Transport in Plasticized Polymer Electrolytes using Dielectric Spectroscopy U. Hyeok Choi, Siwei Liang, James Runt, Ralph Colby A challenge facing the development of new renewable energy storage materials is the low ionic conductivity within polymer matrices. Most materials development must overcome two main hurdles: Increase the ionic mobility and maximize the conducting ion concentration. The main role of small molecule plasticizers is not only to improve flexibility and segmental motion, which consequently lowers the T$_{g}$ and increase ion mobility, but also to solvate the counterion through some specific interaction, which increase the conducting ion content. In this study, we add plasticizers to polysiloxane-based ionomers that have anions covalently attached to the polymer chain, with Li$^{+}$ counterions. Using the 1953 Macdonald model it is possible to separate the conductivity of plasticized ionomers into the number density of conducting ions and their mobility, allowing us to quantify these vital quantities as functions of plasticizer content and temperature. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H42.00007: Ionic conductivity of mesoporous block copolymer membranes in liquid electrolyte as a function of copolymer and homopolymer molecular weight David Wong, Scott Mullin, Greg Stone, Vincent Battaglia, Nitash Balsara Mesoporous block copolymer membranes have been synthesized using poly(styrene-block-ethylene-block-polystyrene) (SES). A series of symmetric SES copolymers and PS homopolymers have been studied at different blending fractions. Ionic conductivities of the porous films in a liquid electrolyte, 1.0 M LiPF$_{6}$ in ethylene carbonate/diethyl carbonate, compare favorably to conventional battery separators and generally increase with internal surface area, as measured by nitrogen adsorption. Characterization of the effects of pore structure and SES morphology on conductivity will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H42.00008: Cation/Anion Associations and Transport in Ionic Polymer Membranes Louis Madsen, Jianbo Hou, Zhiyang Zhang, Jing Li Ionic polymer membranes and ionic liquids (ILs) find fruitful applications in a range of ion conduction applications, from electromechanical ``artificial muscles'' to organic batteries. Various intermolecular interactions determine local structure and dynamics in these ion-dense media. In particular, ion aggregation can drastically affect ion transport, especially since neutral species (dipoles, quadrupoles...) will not be driven by electric fields. We are investigating mixtures of different ILs, ILs with water, and ILs swollen into ionomer membranes, using pulsed-gradient NMR to probe diffusion and electrophoretic mobility. We observe strong dependencies of the cation/anion diffusion coefficient ratio (ranging from 3X to 0.25X) on mixture and membrane properties, which we relate to ion association phenomena. We will further discuss NMR for transport and dynamics studies, especially regarding chemically resolved transport of various mobile species, probing a range of length and time scales, and quantifying ion aggregation. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H42.00009: Correlation between cation conduction and ionic morphology in a PEO-based single ion conductor Kan-Ju Lin, Janna Maranas We use molecular dynamics simulation to study ion transport and backbone mobility of a PEO-based single ion conductor. Ion mobility depends on the chemical structure and the local environment of the ions, which consequently impact ionic conductivity. We characterize the aggregation state of the ions, and assess the role of ion complexes in ionomer dynamics. In addition to solvated cations and pairs, higher order ion clusters are found. Most of the ion clusters are in string-like structure and cross-link two or more different ionomer chains through ionic binding. Ionic crosslinks decrease mobility at the ionic co-monomer; hence the mobility of the adjacent PEO segment is influenced. Na ions show slow mobility when they are inside large clusters. The hopping timescale for Na varies from 20 ns to 200. A correlation is found between Na mobility and the number of hops from one coordination site to another. Besides ether oxygens, Na ions in the ionomer also use the anion and the edge of the cluster as hopping sites. The string-like structure of clusters provide less stable sites at the two ends thus ions are more mobile in those regions. We observed Grotthus like mechanism in our ionomer, in which the positive charge migrates within the string-like cluster without the cations actually moving. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H42.00010: Decoupling Between Ionic Conductivity and Segmental Dynamics in Polymers Alexander Agapov, Alexei Sokolov The idea of solid polymer-based electrolytes (SPE) with high ionic conductivity at room temperature is known in scientific community for more than three decades. The interest is caused by unique advantages these materials may offer: mechanical flexibility, high power density, enhanced environmental and operational safety, etc. However, even after several decades of studies, the main challenge remains -- there is no ``dry'' SPE with conductivity of $\approx $ 10$^{-2}$ -- 10$^{-3}$ S/cm at room temperature. Ionic conductivity is controlled by two parameters, number of ions and their diffusion. Traditional views relate the diffusion of ions in a polymer to the segmental relaxation. Thus, when segmental dynamics freeze the ion motion halts, leading to low conductivity in solid state. A very good example of a material with such behavior is poly(ethylene oxide). In this work we demonstrate that the temperature dependence of ionic conductivity and segmental relaxation can be decoupled in a material specific way. Degree of the observed decoupling exhibits strong correlation with the steepness of the temperature dependence of structural relaxation (fragility). We predict that more fragile materials can have higher ionic conductivity in the solid state than the strong polymers (e.g. PEO). [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H42.00011: Molecular dynamics simulations of ionic aggregates in a coarse-grained ionomer melt Lisa Hall, Mark Stevens, Amalie Frischknecht Ionomers (polymers with a small fraction of covalently bound ionic groups) have potential application as solid battery electrolytes. Understanding ion transport is essential for such applications. A key question is how molecular properties affect ionic aggregation and counterion dynamics. Recent experimental advances allowed synthesis and extensive characterization of ionomers with a precise spacing of charged groups, which is ideal for comparison with simulations. We use coarse-grained molecular dynamics to simulate ionomers with charged beads placed periodically either in the polymer backbone or pendant to the backbone. The polymers, along with counterions, are simulated at melt densities. Pendant ions at low dielectric form roughly spherical aggregates with liquidlike interaggregate order, qualitatively different from the aggregate morphology of analogous linear ionomers. The effects of dielectric constant and backbone spacing of charged beads on ionic structure and diffusion will be discussed. [Preview Abstract] |
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