Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K10: Ion Containing Polymers - The Role of Structure and Dynamics IIFocus
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Sponsoring Units: DPOLY GSNP Chair: Christopher Soles, NIST Room: 269 |
Wednesday, March 15, 2017 8:00AM - 8:36AM |
K10.00001: Structure and Transport in Ion-containing Polymers under Confinement: Nafion Thin Films Invited Speaker: Christopher Stafford Our research program has investigated the effect of confinement on the structure and transport properties of Nafion, the most widely used ion exchange membrane material for fuel cell applications. A consistent picture is emerging from a diverse set of measurement platforms, indicating that the morphology of Nafion is dramatically altered upon confinement. As expected, these morphological changes result in a remarkable decrease in the transport of water in these films, which is an indicator of ion transport. Additionally, we show that the processing history of these thin Nafion films has a profound impact on the observed properties. Specifically, thermal annealing and humidity ageing leads to decreased water transport in thin Nafion films, the origins of are still yet not completely understood. This talk will highlight the results of our measurements as well as others that help paint a more complete physical picture of the structure and resulting transport in confined Nafion films. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K10.00002: Water transport, free volume, and polymer dynamics in crosslinked polymer networks Bradley Frieberg, Christopher Soles Many technologies rely on amorphous polymer membranes that selectively transport small molecules or ions, which has led to a significant scientific interest in elucidating the mechanisms of transport. A recurring theme among several different materials systems is that free volume and polymer chain dynamics facilitate transport. In order to understand the interplay between free volume, transport and polymer dynamics we quantify these properties for a model epoxy network. The epoxy chemistry allows for systematically varying both the structural rigidity of the network as well as the cross-link density. We performed positron annihilation lifetime spectroscopy measurements to characterize the unoccupied volume and correlated the unoccupied volume to the equilibrium moisture uptake and effective diffusion coefficient. We have recently extended this work to include polymer dynamics measured by quasi-elastic neutron scattering on the NIST High Flux Backscatter Spectrometer. These measurements reveal a strong correlation between the MSD and the transport kinetics, which was even stronger than the correlation previously observed between free volume and water diffusion. These observations challenge previous theories that suggest free volume governs transport. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K10.00003: A Comparison of Water Diffusion in Polymer Based Fuel Cell and Reverse Osmosis Membrane Materials Christopher Soles, Bradley Frieberg, Jacob Tarver, Madhusudan Tyagi, Cheol Jeong, Edwin Chan, Christopher Stafford Hydrated polymer membranes are critical in both fuel cells and water filtration and desalination. In both of these applications the membrane function (selectively transporting or separating ions) is coupled with the transport of water through the membrane. There is a significant need to understand the nature by which the water and ions distribute and move through these membranes. This presentation compares the transport mechanisms in in an ion containing block copolymer alkaline fuel cell membrane with that of a polyamide membrane that is used as the active layer in a reverse osmosis water desalination membrane. Small angle neutron scattering measurements are used to locally probe how water swells the different materials and quantitatively describe the distribution of water within the membrane microstructures. Quasielastic neutron scattering measurements are then used to separate the polymer dynamics of the host membranes from the dynamics of the water inside the membranes. This reveals that water moves at least an order of magnitude slower through the ion containing fuel cell membrane materials, consistent with a solution-diffusion model, while the water in the polyamide membranes moves faster, consistent with a pore-flow diffusion mechanism. These insights will be discussed in terms of a coupling of the water and polymer dynamics and design cues for high performance membrane materials. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K10.00004: Charge transport and glassy dynamics in poly(ethylene oxide) based phosphonium ionomers. Ciprian Iacob, Hsin-Jung Yu, Victoria Lumsargis, Jing-Han Helen Wang, Quan Chen, Ralph Colby, James Runt We use broadband dielectric spectroscopy to investigate ionic conduction and dielectric response, and X-ray scattering to investigate morphology of poly(ethylene oxide) based phosphonium ionomers with varying ion content. Two models of electrode polarization are used to separate ionic conductivity of the ionomers into number density of conducting ions and their mobility and calculate the dc conductivity from components of electrode polarization. Ion mobility is coupled to polymer segmental motion ($\alpha $-relaxation), as these are observed to share similar Vogel temperatures. Ionomers with higher ion content impart higher static dielectric constant than those with lower ion content. From temperature activated plots of static dielectric constant, there is more ionic aggregation in ionomers with higher ion content (affecting dc conductivity), consistent with X-ray scattering, which shows much stronger ionic aggregate peaks for the ionomers with higher ion content. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K10.00005: Solvent Tuning of Cluster Morphology in Ionic Block Copolymer Solutions Manjula Senanayake, Sidath Wijesinghe, Anuradhi Wickramasinghe, Supun Mohottalalage, Dvora Perahia, Lilin He, Carl Willis, Marc Charendoff Ionic block copolymers find broad uses in transport-controlled applications. Tuning ionic clusters formed in solutions that can propagate into membranes offers a path to tailor performance. Here micellar structures of a symmetric ABCBA co-polymer resolved by SANS, as the polarity of the solvent is changed by adding propanol to cyclohexane, are reported. C is a randomly sulfonated polystyrene, B is hydrogenated polyisoprene, and A is poly (t-butyl styrene). Core-shell aggregates dominate the solutions of the pure solvents with the ionic blocks residing in the core of the micelle. The packing of the ionizable segments is significantly tighter in a hydrophobic environment than in a hydrophilic one. The corona however is more compact in the hydrophilic environment. A transitional region is observed as the fully miscible solvents are mixed with elongated structures dominating the solution. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K10.00006: Electrostatic effects on clustering and ion dynamics in ionomer melts Boran Ma, Trung Nguyen, Victor Pryamitsyn, Monica Olvera de la Cruz An understanding of the relationships between ionomer chain morphology, dynamics and counter-ion mobility is a key factor in the design of ion conducting membranes for battery applications. In this study, we investigate the influence of electrostatic coupling between randomly charged copolymers (ionomers) and counter ions on the structural and dynamic features of a model system of ionomer melts. Using coarse-grained molecular dynamics (CGMD) simulations, we found that variations in electrostatic coupling strength ($\Gamma$) remarkably affect the formation of ion-counter ion clusters, ion mobility, and polymer dynamics for a range of charged monomer fractions. Specifically, an increase in $\Gamma$ leads to larger ionic cluster sizes and reduced polymer and ion mobility. Analysis of the distribution of the radius of gyration of the clusters further reveals that the fractal dimension of the ion clusters is nearly independent from $\Gamma$ for all the cases studied. Finally, at sufficiently high values of $\Gamma$, we observed arrested heterogeneous ions mobility, which is correlated with an increase in ion cluster size. These findings provide insight into the role of electrostatics in governing the nanostructures formed by ionomers. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K10.00007: Effect of Confinement in PFSA Ionomer Thin Films Peter Dudenas, Meron Tesfaye, Adam Weber, Ahmet Kusoglu Ion-conducting polymer (ionomer) behavior in catalyst layers of electrochemical devices is fairly unknown and presents challenges for their characterization in this heterogeneous environment. In order to enable next-generation electrochemical devices, structure-property relationships of these ionomers must be elucidated under confinement. In an effort to do so, model systems of ionomer thin films of varying thicknesses (10 to 200 nm) are utilized to understand effect of confinement on crystallinity, swelling and mechanical properties. Grazing incidence wide-angle x-ray scattering (GIWAXS) is used to determine the relative degree of crystallinity (rDoC) as a function of thickness, which is then correlated to mechanical properties and swelling of thin films, as measured by a cantilever bending method and ellipsometry, respectively. Moreover, factors controlling the thin-film behavior, such as thermal transitions and substrate effects, are explored further through temperature-dependent ellipsometry and GIWAXS. Multiple perfluorinated sulfonic acid (PFSA) ionomer chemistries are used to demonstrate the effect of side-chain length and density on the interplay between the confinement effects and structure-property relationships. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K10.00008: Quantifying Ion Transport in Polymers Using Electrochemical Quartz Crystal Microbalance with Dissipation Jodie Lutkenhaus, Shaoyang Wang For polymers in energy systems, one of the most common means of quantifying ion transport is that of electrochemical impedance spectroscopy, in which an alternating electric field is applied and the resultant impedance response is recorded. While useful, this approach misses subtle details in transient film swelling, effects of hydration or solvent shells around the transporting ion, and changes in mechanical properties of the polymer. Here we present electrochemical quartz crystal microbalance with dissipation (EQCMD) monitoring as a means to quantify ion transport, dynamic swelling, and mechanical properties of polymers during electrochemical interrogation. We focus upon EQCMD characterization of the redox-active nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA). Upon oxidation, PTMA becomes positively charged, which requires the transport of a complementary anion into the polymer for electroneutrality. By EQCMD, we quantify anion transport and resultant swelling upon oxidation, as well as decoupling of contributions attributed to the ion and the solvent. We explore the effect of different lithium electrolyte salts in which each salt gives different charge storage and mass transport behavior. This is attributed to varied polymer-dopant and dopant-solvent interactions. The work was supported by the grant DE-SC0014006 funded by the U.S. Department of Energy, Office of Science. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K10.00009: Clustering Effects on Dynamics in Ionomer Solutions: A Neutron Spin Echo Insight Dvora Perahia, Sidath Wijesinghe, Manjula Senanayake, Anuradhi Wickramasinghe, Supun S Mohottalalage, Michael Ohl Ionizable blocks in ionomers associate into aggregates serving as physical cross-links and concurrently form transport pathways. The dynamics of ionomers underline their functionality. Incorporating small numbers of ionic groups into polymers significantly constraint their dynamics. Recent computational studies demonstrated a direct correlation between ionic cluster morphology and polymer dynamics.$^{\mathrm{1}}$ Here using neutron spin echo, we probe the segmental dynamics of polystyrene sulfonate (PSS) as the degree of sulfonation of the PSS and the solution dielectrics are varied. Specifically, 20Wt{\%} PSS of 11,000 g/mol with polydispersity of 1.02 with 3{\%} and 9{\%} sulfonation were studies in toluene (dielectric constant $\varepsilon \quad =$2.8), a good solvent for polystyrene, and with 5Wt{\%} of ethanol ($\varepsilon \quad =$24.3l) added. The dynamic structure factor S(q,t) was analyzed with a single exponential except for a limited q range where two time constants associated with constraint and mobile segments were detected. S(q,t) exhibits several distinctive time and length scales for the dynamics with a crossover appearing at the length scale of the ionic clusters. 1)Agrawal et. al Phys. Rev. Lett.~\textbf{116}, 158001 [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K10.00010: Superfast Ionic Polymer Actuators Based on Single-Ion Conducting Block Copolymers Onnuri Kim, Moon Jeong Park Ionic polymer actuators have attracted enormous attention for a variety of biomimetic applications owing to many beneficial characteristics of low-driving voltages, softness, and lightness. Although ionic polymer actuators have demonstrated large bending strains analogous to human muscles, major drawbacks are the slow response time and back-relaxation. In the present study, we have investigated a new actuator based on cation-conducting block copolymers that show a superfast response time of several tens of milliseconds. Key ingredients are imidazole-doped self-assembled sulfonated block copolymers containing zwitterions. The introduction of zwitterions into nanoscale ionic PSS phases of block copolymers has offered remarkably increased dielectric constant, and accordingly increased the charge density and ionic conductivity while the single-ion conducting nature was preserved. The actuators containing zwitterions were able to move a few millimeters within tens of milliseconds under 1V-operation and no back relaxation was detected, which marked a significant progress as compared to the conventional ionic polymer actuators. It has been further revealed that type of zwitterions play an important role in determining the electromechanical properties of the actuators. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K10.00011: Lithium ion migration via ion concentration gradient in bilayer polymer electrolyte membranes Camilo Piedrahita, Jinwei Cao, Thein Kyu Ion concentration gradient across the cell membrane plays a key role in neuron cells to produce and transmit electrical signals throughout our body. Different ion populations across the cell membranes along with specific channels, gates and pumps promote ion movement in/out of the cell generating electrical signal. The ion transport was mimicked in the solid bilayer PEM consisted of PEGDA/LiTFSI/SCN with different Li-ion populations; one (PEM1) high in Li$^{\mathrm{+}}$ concentration and another (PEM2) low in ion concentration (or 0 wt{\%}). When these two membranes are stacked together, Li$^{\mathrm{+}}$ can diffuse due to the ion concentration gradient. Cyclic voltammetry (CV) was conducted to probe the ion displacement from solid PEM1 to PEM2 in a coin cell (SS/PEM2/PEM1/Li) during charge/discharge cycling and recovery of Li$^{\mathrm{+}}$ concentration during resting. Anodic peaks emerged in the respective CV spectra as Li$^{\mathrm{+}}$ migrates from high to low ion concentration PEM. FTIR analysis on bilayered PEM surfaces further provides evidence of Li$^{\mathrm{+}}$ migration via trans-gauche transformation of ether linkages. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K10.00012: Kinetics of Swelling and Collapse in Polyelectrolyte Systems Arindam Kundagrami, Soumik Mitra, Swati Sen Equilibrium phase behaviors of polyelectrolyte systems have been well-studied over past several decades. Our group has been working on to explore the kinetic processes which take the system from non-equilibrium to such equilibrium configurations. Specifically, we will present recent results on the kinetics of swelling and collapse of a single, isolated, and flexible polyelectrolyte chain, which is described by a theoretical model considering charge-regularization, in both salt-free and salty conditions. The equation of motion of the kinetics is developed from the osmotic and viscous forces, and the numerical solutions provide the temporal profiles for size and charge of the chain for swelling and de-swelling in good solvent, and collapse in poor solvent. Analytical expressions are explored in suitable limits. The results obtained for the single chain are put in perspective to those obtained for polyelectrolyte gels. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K10.00013: Modeling pH-Responsive Adsorption of Polyelectrolytes at Oil-Water Interfaces Shiyi Qin, Xin Yong We use dissipative particle dynamics (DPD) to discover the interfacial adsorption of pH-responsive polyelectrolytes in oil-water binary systems under different pH values. The electrostatic interactions between charged beads and the dielectric discontinuity across the interface are modeled by exploiting a modified Particle-Particle-Particle-Mesh (PPPM) method, which uses an iterative method to solve the Poisson equation on a uniform grid. We first model the adsorption behavior of a single linear polyelectrolyte from the aqueous phase. The Henderson-Hasselbalch equation describes the relation between pH and the degree of ionization of the modeled polyelectrolytes. Through changing the degree of ionization, we explore the influence of pH on the adsorption behavior and show that the electrostatic interactions significantly modulate the adsorption. Time evolutions of the position and conformation of the polyelectrolytes and the variation in the oil-water surface tension will be measured to characterize the adsorption behavior. Furthermore, we model the pH-dependent adsorption behavior of polyelectrolytes with more complicated structures, namely, branched polyelectrolytes with hydrophobic backbones and hydrophilic side chains. We also find that the addition of salts in the medium and the lengths of the backbone and ionized side chain affect the adsorption. [Preview Abstract] |
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