Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Z17: Charged and Ion-Containing Polymers II |
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Sponsoring Units: DPOLY Chair: Jodie Lutkenhaus, Yale University Room: B116 |
Friday, March 19, 2010 11:15AM - 11:27AM |
Z17.00001: Segmental dynamics and cross-linking in ion containing polymers Kokonad Sinha, Janna Maranas We present Quasi Elastic Neutron Scattering (QENS) data for characterizing proton dynamics in ion containing polymers (ionomers) with varying ion content and ion identity. 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, thereby isolating cation-polymer interaction for study. The ion content is varied 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. This effect is ion dependent, which indicates that cations have different binding capacities and formation of this complex determines the availability of free cations for conduction. [Preview Abstract] |
Friday, March 19, 2010 11:27AM - 11:39AM |
Z17.00002: The interplay of ion crosslinking, free ion content, and polymer mobility in PEO-based single-ion conductors Kan-Ju Lin, Janna Maranas We use molecular dynamics simulation to study ion clustering and dynamics in ion containing polymers. This PEO based single-ion conducting ionomer serves as a model system for understanding cation transport in solid state polymer electrolytes (SPEs). Although small-angle x-ray scattering does not show an ionomer peak, we observer various cation-anion complexes in the simulation, suggesting ionomer backbones are crosslinked through ion complexes. These crosslinks reduce the adjacent PEO mobility resulting in a symmetric mobility gradient along the PEO chain. We vary the cation-anion interaction in the simulation to observe the interplay of cation-anion association, polymer mobility and cation motion. Cation-anion association controls the number of free ions, which is important in ionic conductivity when these materials are used as SPEs. Polymer mobility controls how fast the free ions are able to move through the SPE. High conductivity requires both a high free ion content and fast polymer motion. To understand the connection between the two, we ``tune'' the force field in order to manipulate the free ion content and observe the influence on PEO dynamics. [Preview Abstract] |
Friday, March 19, 2010 11:39AM - 11:51AM |
Z17.00003: Conductivity and Water Content in Asymmetrical Sulfonated Block Copolymers Xin Wang, Nitash P. Balsara, Keith M. Beers, Moon J. Park We have determined the morphology, proton conductivity and water uptake of asymmetric sulfonated poly(styrene-$b$-methylbutylene) (PSS-PMB) membranes equilibrated with 98{\%} relative humidity (RH) air. To our surprise we found that the conductivity of low molecular weight PSS-PMB samples decreased slowly and irreversibly when the temperature of the membrane (and air) was increased. In contrast, high molecular weight PSS-PMB samples with the same asymmetry decreased more rapidly in response to a temperature change. In addition the factor by which the conductivity decreased was significantly higher in the case of the low molecular weight PSS-PMB. This puzzle was resolved by in-situ small angle neutron scattering which enabled determination of the morphological response of the samples to changes in temperature at RH=98{\%}. The morphology-conductivity relationship in the equilibrated state gives insight into factors that govern charge transport in these systems. [Preview Abstract] |
Friday, March 19, 2010 11:51AM - 12:03PM |
Z17.00004: Ionic Conductivity Trends with Molecular Weight in PEO and PEO-Based Solid Polymer Electrolytes Alexander Teran, Scott Mullin, Nisita Wanakule, Ashoutosh Panday, Nitash Balsara Poly(ethylene oxide) based polymer electrolytes with lithium bis(trifluoromethane)sulfonamide (LiTFSI) salt remain one of the most promising class of solid polymer electrolyte for rechargeable lithium metal batteries. Among those, poly(styrene-b-ethyleneoxide) (SEO) doped with LiTFSI has been shown to exhibit acceptable levels of conductivity while possessing a sufficiently high modulus to suppress the growth of dendrites. The purpose of this study is to explore the molecular weight dependence on conductivity for the PEO/LiTFSI system to which previous studies have alluded, but never quantified, and contrast this with the observed molecular weight dependence of SEO reported in previous work. Conductivities were measured using AC impedance spectroscopy over a broad range of temperatures and molecular weights beyond those reported in the literature. [Preview Abstract] |
Friday, March 19, 2010 12:03PM - 12:15PM |
Z17.00005: Electrospinning of an Alkaline Polymer Electrolyte Supacharee Roddecha, Zexuan Dong, Yiquan Wu, Mitchell Anthamatten The polymer electrolyte membrane is a key component of the low temperature fuel cell to block fuel and electron crossover, while enabling ions to pass and complete the half-cell reactions. Proton exchange membranes (PEMs) are anion-containing polymers, such as Nafion, which offer proton conduction pathways. Alkaline polymer electrolytes utilize hydroxyl anions as charge carriers and are currently being researched as an alternative to PEMs because they may offer the use of inexpensive metal catalysts. However, hydroxyl anion in an alkaline electrolyte has relatively low mobility compared to that of protons in an acid electrolyte; hence a high concentration of OH$^{-}$ is required to obtain high ionic conductivity. Here, we report the use of an electrospinning process to prepare nonwoven membranes. Polysulfones are first functionalized with varied ionic content of quaternary ammonium functional groups and then are electrospun to get alkaline electrolyte mat. The morphology at various ionic content, mechanical property, and in-plane conductivity of resulting films will be discussed and compared to solvent-cast films of the same material. [Preview Abstract] |
Friday, March 19, 2010 12:15PM - 12:27PM |
Z17.00006: Thermal Properties of Poly(allylamine hydrochloride)/Poly(acrylic acid) Layer-by-Layer Assemblies Jodie Lutkenhaus, Lin Shao Layer-by-layer (LbL) assemblies are promising for global energy and health applications, but their materials properties are not well understood. LbL assemblies are created from the alternate adsorption of oppositely charged species from solution to a substrate. Particularly, little is known about the thermal properties of LbL assemblies because the supporting substrate impedes characterization. It is not initially clear if electrostatic LbL assemblies possess a glass transition temperature, if they are rubbery or glassy, or if their heat capacity is comparable to their homopolymer constituents. Here, we isolate large areas of LbL assemblies from a low-energy substrate, which facilitates thermal characterization via modulated differential scanning calorimetry (MDSC) and thermal gravimetric analysis (TGA). LbL assemblies of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were deposited onto hydrophobic substrates, and subsequently isolated. Results highlight that PAH/PAA LbL films are glassy, and have low mobility because of the high density of ion pair crosslinks. The techniques presented here are general, and can be applied to any LbL film. [Preview Abstract] |
Friday, March 19, 2010 12:27PM - 12:39PM |
Z17.00007: Mean Field Theory of Aggregation and Symmetry-Breaking in Ionomer Melts Erica Saltzman, Sanat Kumar, Igal Szleifer Solutions and melts of charged polymers are studied with Single Chain Mean Field Theory, which preserves intramolecular correlations. System parameters include temperature, chain length, and monomer density. The thermodynamics of the equilibrium system are calculated, including electrostatic energy and polymer and counterion entropy. Phenomenology of interest include the association-dissociation transition of counterions and cooperative conformational and morphological transitions which are expected to dominate the macroscale temperature variations. Theoretical results are compared to simulation findings of low temperature condensation of chains to form ordered sheets. This study provides a basis for future stochastic models of the temporal evolution of such large-scale structures, with immediate relevance to measurable dynamic properties. [Preview Abstract] |
Friday, March 19, 2010 12:39PM - 12:51PM |
Z17.00008: Hierarchical Structure of Poly(ethylene) Based Ionomers Michelle E. Seitz, Christopher D. Chan, Kathleen L. Opper, Travis W. Baughman, Kenneth B. Wagener, Karen I. Winey The effect of chain architecture (linear vs branched), acid placement (precise vs random), acid content, neutralization extent, and crystallinity on the hierarchical structure of poly(ethylene-acrylic acid) ionomers was investigated via X-ray scattering and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). HAADF STEM reveals randomly dispersed, spherical ionic aggregates in all materials. At temperatures where the ionomers are fully amorphous, the scattering at intermediate angle arises from interaggregate interference and can be described by the Kinning-Thomas model. If the acid groups are placed every 21$^{st}$ carbon, the materials are semicrystalline at room temperature and contributions from acid layers associated with crystallites are convoluted with interaggregate scattering. The ionic aggregates have diameters of $\sim $ 1 nm for all samples; however, the number density of aggregates is strongly dependent on the acid content but weakly dependent on the extent of neutralization. [Preview Abstract] |
Friday, March 19, 2010 12:51PM - 1:03PM |
Z17.00009: Interdiffusion of long alcohols into thin ionomer films; In situ Neutron Reflectivity study Thusitha Etampawala, Dilru Ratnaweera, Umesh Shrestha, Dvora Perahia, Christopher Cornelius, Jaroslaw Majewski Transport of solvents and ions within ionic polymers controls their many current and potential applications from energy related to drug delivery systems. The transport is determined by the phase structure and the interaction of the diffusing species with the polymers, coupled with interfacial effects. The current work presents the kinetics of penetration of long chain alcohols diffusing into rigid ionomer thin films formed by a rigid polyphenylene sulfonated ionomer, using \textit{in situ} neutron reflectivity. The penetration of deuterated n-octanol and n-hexanol into $\sim $20nm thick films was followed as a function of time for different sulfonation levels of the polymer. As for shorter molecules, the diffusion process consists of two stages, a relatively fast one in which the film thickness increases linearly with time followed by a slow phase in which structural changes take place. With increasing sulfonation levels, the diffusion first increases and then decreases; a trend that is attributed to hydrophilic/hydrophobic balance. [Preview Abstract] |
Friday, March 19, 2010 1:03PM - 1:15PM |
Z17.00010: Anisotropic Proton Conduction in Aligned Block Copolymer Electrolyte Membranes at Equilibrium with Humid Air Moon Park, Nitash Balsara The effect of alignment of proton-conducting domains in hydrated poly(styrenesulfonate-b-methylbutylene) copolymer films on conductivity was studied by impedance spectroscopy. Pressing isotropic samples obtained by casting results in lamellae aligned in the plane of the film. Application of electric fields and flow fields on the isotropic samples results in lamellae aligned perpendicular to the plane of the film. The alignment of lamellae, quantified by a combination of 2D SAXS, birefringence, and TEM, was much better in the pressed samples than in the field-aligned samples. Conductivity was measured in the plane of the film and normal to the plane of the film. Only the pressed sample showed highly anisotropic proton conduction with the ratio of 75. In this case, the parallel conductivity increased by 30\% after alignment, relative to that obtained from the as-cast samples. The conductivity ratio obtained from after electric field and shear field alignment were 1.2 and 1.4, respectively, in spite of partial alignment of the domains, and the increase in the perpendicular conductivity after alignment was less than 20 percent. [Preview Abstract] |
Friday, March 19, 2010 1:15PM - 1:27PM |
Z17.00011: Ab-initio study of polypyrrole as a pervaporation membrane Andre Botelho, Xi Lin The affinities between polypyrrole, water, ethanol and a sulfonate-carrying ion were calculated from first principles. All interactions were demonstrably hydrogen bonds between the oxygen from the sulfonate groups and the hydrogen in the hydroxyl (for water and ethanol) and amine groups (for polypyrrole). Each sulfonate group was shown to form three hydrogen bonds, with any of the three other types of molecules, allowing the ion complex to bind to multiple polypyrrole chains, water, and ethanol molecules simultaneously. The energies indicated a higher affinity between the ion and poypyrrole, second highest between water and the ion, and the lowest for ethanol and the ion. A high affinity of the ion to the polymer backbone is desirable to prevent leeching. The ion was found to have a higher affinity to water molecules than to ethanol, confirming the system's selectivity in separating water from ethanol. [Preview Abstract] |
Friday, March 19, 2010 1:27PM - 1:39PM |
Z17.00012: Dynamics of Sulfonated Polystyrene Ionomers by Dielectric Relaxation Spectroscopy Alicia Castagna, Wenqin Wang, Karen Winey, James Runt Broadband dielectric spectroscopy was used to investigate the dynamics of sulfonated polystyrene (SPS) ionomers, in both the acid and neutralized form. This study seeks to elucidate the role of counter ion type (Zn, Na, and Cs), degree of sulfonation (9 and 6{\%}), and ion cluster morphology on the relaxation phenomena of SPS. Degree of neutralization and ion type have been found to significantly impact the breadth and time scale of the segmental relaxation process. High temperature relaxation processes, tentatively proposed to arise from Maxwell-Wagner-Sillars interfacial polarization and a hydrogen bonding relaxation, have also been identified. Bands in the sulfonate stretching region of FTIR spectra reveal information about ion coordination in the local aggregate environment. A combination of scanning transmission electron microscopy imaging and X-ray scattering confirmed the presence of homogeneously distributed, nearly monodisperse spherical ionic aggregates in the polymer matrix. [Preview Abstract] |
Friday, March 19, 2010 1:39PM - 1:51PM |
Z17.00013: A Polarizable Potential for Poly(ethylene oxide) ~in Aqueous Solution Oleg Starovoytov, Oleg Borodin, Dmitry Bedrov, Grant Smith We have developed a quantum chemistry-based polarizable potential for poly(ethylene oxide) (PEO) in aqueous solution based on the APPLE{\&}P{\textregistered} polarizable ether and SWM4-DP polarizable water model. Ether-water interactions were parameterized to reproduce the binding energy of water with 1,2-dimethoxyethane (DME) determined from high-level quantum chemistry calculations. Simulations of DME/water and PEO/water solutions at room temperature using the new polarizable potential yielded thermodynamic and transport properties in better agreement with experiment than previously published polarizable and non-polarizable potentials. The predicted miscibility of PEO and water as a function of temperature was found to be strongly correlated with the predicted free energy of solvation of DME in water for the various force fields investigated. Simulations of PEO/water solutions confirm the ability of the new potential to capture, at least qualitatively, the LCST behavior of these solutions [Preview Abstract] |
Friday, March 19, 2010 1:51PM - 2:03PM |
Z17.00014: Charging and Screening in Nonpolar Solutions of Nonionizable Surfactants Sven Behrens Nonpolar liquids do not easily accommodate electric charges, but surfactant additives are often found to dramatically increase the solution conductivity and promote surface charging of suspended colloid particles. Such surfactant-mediated electrostatic effects have been associated with equilibrium charge fluctuations among reverse surfactant micelles and in some cases with the statistically rare ionization of individual surfactant molecules. Here we present experimental evidence that even surfactants without any ionizable group can mediate charging and charge screening in nonpolar oils, and that they can do so at surfactant concentrations well below the critical micelle concentration (cmc). Precision conductometry, light scattering, and Karl-Fischer titration of sorbitan oleate solutions in hexane, paired with electrophoretic mobility measurements on suspended polymer particles, reveal a distinctly electrostatic action of the surfactant. We interpret our observations in terms of a charge fluctuation model and argue that the observed charging processes are likely facilitated, but not limited, by the presence of ionizable impurities. [Preview Abstract] |
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