Session S20: Invited Session: Ion-Containing Polymers

Thermal and pH Transitions in Polyelectrolyte Complexes and Multilayers

When oppositely charged polymers are mixed in water, they form a polyelectrolyte complex. Analogously at a surface, oppositely charged polymers can be assembled to form a polyelectrolyte multilayer. Complexation is entropically driven, as it results in the release of small counter ions and water molecules into the surrounding media. First, the effect of time and temperature on the formation of polyelectrolyte complexes containing model polyelectrolytes poly(diallyldimethyl ammonium chloride) and poly(styrene sulfonate) is presented. We show that complexation is a time-dependent phenomena, which is consistent with complexes existing in a kinetically trapped state, rather than a thermodynamic equilibrium. Upon heating, a glassy-viscous transition that shows features of an LCST is demonstrated. Second, the effect of pH on polyelectrolyte multilayer microtubes is presented. The microtubes are made of poly(allylamine) and poly(acrylic acid), both of which are weak polyelectrolytes. Modulating the pH induces a nanoporous transition that results in nanoporous microtubes. These results, in summary, show that noncovalent interactions are very sensitive to external stimuli such as temperature and pH.   

Multicomponent transport in membranes for redox flow batteries

Redox flow batteries (RFBs) incorporate separator membranes, which ideally prevent mixing of electrochemically active species while permitting crossover of inactive supporting ions. Understanding crossover and membrane selectivity may require multicomponent transport models that account for solute/solute interactions within the membrane, as well as solute/membrane interactions. Application of the Onsager--Stefan--Maxwell formalism allows one to account for all the dissipative phenomena that may accompany component fluxes through RFB membranes. The magnitudes of dissipative interactions (diffusional drag forces) are quantified by matching experimentally established concentration transients with theory. Such transients can be measured non-invasively using DC conductometry, but the accuracy of this method requires precise characterization of the bulk RFB electrolytes. Aqueous solutions containing both vanadyl sulfate (VOSO$_{\mathrm{4}})$ and sulfuric acid (H$_{\mathrm{2}}$SO$_{\mathrm{4}})$ are relevant to RFB technology. One of the first precise characterizations of aqueous vanadyl sulfate has been implemented and will be reported. To assess the viability of a separator for vanadium RFB applications with cell-level simulations, it is critical to understand the tendencies of various classes of membranes to absorb (uptake) active species, and to know the relative rates of active-species and supporting-electrolyte diffusion. It is also of practical interest to investigate the simultaneous diffusion of active species and supports, because interactions between solutes may ultimately affect the charge efficiency and power efficiency of the RFB system as a whole. A novel implementation of Barnes's classical model of dialysis-cell diffusion [\textit{Physics} \textbf{5}:1 (1934) 4-8] is developed to measure the binary diffusion coefficients and sorption equilibria for single solutes (VOSO$_{\mathrm{4}}$ or H$_{\mathrm{2}}$SO$_{\mathrm{4}})$ in porous membranes and cation-exchange membranes. With the binary diffusion and uptake measurement in hand, a computer simulation that extends the approach of Heintz, Wiedemann and Ziegler [\textit{J. Membrane Science} \textbf{137}:1-2 (1997) 121-132] is used to establish Onsager resistances that describe the drag forces VOSO$_{\mathrm{4}}$ and H$_{\mathrm{2}}$SO$_{\mathrm{4}}$ exert on each other as they interdiffuse. The ramifications of these interactions for different classes of membranes -- and for RFB applications -- will be discussed.   

Morphology control in solid polymer electrolytes

Solid polymer electrolytes (SPEs) with high ionic conductivity are important for energy-related applications, such as solid state batteries and fuel cells. In this talk, I will discuss how nanoscale morphology affects the properties of SPEs. In the first part of the talk, I will show quantitatively that the effect of polymer crystallization on ion transport is twofold: structural (tortuosity) and dynamic (tethered chain confinement). We decouple these two effects by designing and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity, and orientation. Ion conduction is confined within the chain fold region and guided by the crystalline lamellae. We show that, at low ion content, due to the tortuosity effect, the in-plane conductivity is 2000 times greater than through-plane one. Contradictory to the general view, the dynamic effect is negligible at moderate ion contents. Our results suggest that semicrystalline polymer is a valid system for practical polymer electrolytes design. In the second part of the talk, I will discuss how to use holographic photopolymerization (HP) to fabricate long-range, defect-free, ordered SPEs with tunable ion conducting pathways. By incorporating polymer electrolytes into the carefully selected HP system, electrolyte layers/ion channels with length scales of a few tens of nanometers to micrometers can be formed. Confinement effects on ion transport will be reported.   

Morphology and Ionic Conductivity of Block Copolymer Electrolytes Containing Ionic Liquids

The global energy crisis and an increase in environmental pollution in the recent years have drawn the attention of the scientific community towards the development of efficient electrochemical devices. Polymers containing charged species have the potential to serve as electrolytes in next-generation devices and achieving high ion transport properties in these electrolytes is the key to improving their efficiency. Although the synthesis and characterization of a wide variety of ion-containing polymers have been extensively reported over the last decade, quantitative understanding of the factors governing the ion transport properties of these materials is in its infancy. In this talk, I will present the current understanding of the diverse factors affecting the thermodynamics, morphologies and ion transport of ion-containing polymers by focusing on the use of ionic liquids (ILs). Various strategies for accessing improved transport properties of IL-containing polymers are elucidated by focusing on the role of IL-polymer interactions. The major accomplishment of obtaining well-defined morphologies for these IL-containing polymers by the use of block copolymer is particularly emphasized as a novel means of controlling the transport properties. The application of IL-incorporated polymer electrolytes in high temperature fuel cells and electro-active actuators is also enclosed.   

Ionomer Dynamics: Insights from Broadband Dielectric Spectroscopy*

Ionomers (polymers containing ionic functionality) have been traditionally used as packaging materials and in molding applications, and are now of increasing interest as candidate single ion conductors for energy storage devices, in energy conversion, and for other electroactive materials applications. The focus of this presentation is on the insight that broadband dielectric (impedance) spectroscopy brings to our understanding of ion and polymer dynamics of this family of materials. As an example of our recent work on relatively conductive ionomers, the first portion of the presentation will focus on anion conducting polyphosphazene ionomers, in which polymer bound cations are quaternized with either short alkyl or short ether oxygen chains. The low Tg, amorphous nature, and cation-solvating backbone distinguish polyphosphazenes as promising materials for ion conduction, the iodide variants being of particular interest in solar cells. In the second part of this overview, the first findings on the molecular dynamics of linear precise polyethylene-based ionomers containing 1-methylimidazolium bromide pendants on exactly every 9th, 15th, or 21st carbon atom will be summarized. In order to develop a robust interpretation of the dynamics of these materials, it is imperative to develop a thorough understanding of microphase separation (e.g. ion aggregation), and each of the above studies is complimented by multiangle X-ray scattering experiments.