Session R4: Where Electrostatics Counts: Assembly and Dynamics of Ionic Polymers

Funny and Functional Physics: PEC Nanoparticles

When the tendency of polyelectrolyte complexes to phase separate is judiciously combined with appropriate concepts from the realms of polymer physics and supramolecular chemistry, a wealth of novel self-assembled nanoparticles with original properties can be prepared. This presentation discusses how a high degree of complexity and functionality arises spontaneously, in the form of equilibrium structures, and how these structures can be understood from physical principles. Some promising applications are mentioned as well.   

Quantifying Contributions to Transport in Ionic Polymers Across Multiple Length Scales

Self-organized polymer membranes conduct mobile species (ions, water, alcohols, etc.) according to a hierarchy of structural motifs that span sub-nm to \textgreater 10 $\mu $m in length scale. In order to comprehensively understand such materials, our group combines multiple types of NMR dynamics and transport measurements (spectroscopy, diffusometry, relaxometry, imaging) with structural information from scattering and microscopy as well as with theories of porous media,$^{\mathrm{1}}$ electrolytic transport, and oriented matter.$^{\mathrm{2}}$ In this presentation, I will discuss quantitative separation of the phenomena that govern transport in polymer membranes, from intermolecular interactions ($\le $ 2 nm),$^{\mathrm{3}}$ to locally ordered polymer nanochannels (a few to 10s of nm),$^{\mathrm{2}}$ to larger polymer domain structures (10s of nm and larger).$^{\mathrm{1}}$ Using this multi-scale information, we seek to give informed feedback on the design of polymer membranes for use in, $e.g.$, efficient batteries, fuel cells, and mechanical actuators. \textbf{References:} \begin{enumerate} \item J. Hou, J. Li, D. Mountz, M. Hull, and L. A. Madsen. \textit{Journal of Membrane Science }\textbf{448}, 292-298 \textbf{(2013)}. \item J. Li, J. K. Park, R. B. Moore, and L. A. Madsen. \textit{Nature Materials} \textbf{10}, 507-511 \textbf{(2011)}. \item M. D. Lingwood, Z. Zhang, B. E. Kidd, K. B. McCreary, \quad J. Hou, \quad and L. A. Madsen. \textit{Chemical Communications} \textbf{49}, 4283 - 4285 \textbf{(2013)}. \end{enumerate}   

Nonequilibrium Simulations of Ion Dynamics in Ionomer Melts

Ionomers, polymers containing a small fraction of covalently bound ionic groups, are of interest as possible electrolytes in batteries. However, to date ionomers do not have sufficiently high conductivities for practical application, most likely because the ions tend to form aggregates, leading to slow ion transport. To build a better understanding of the relationships among ionomer chemistry, morphology, and ion transport, we have performed a series of molecular dynamics simulations and connected aspects of these simulations with experiment. In previous work using both atomistic and coarse-grained models, we showed that precise ionomers (with a fixed spacing between ionic groups along the polymer backbone) exhibit a range of ionic aggregate morphologies, from discrete clusters to percolated aggregates. In this talk I will describe recent simulations of our coarse-grained ionomer melts in an applied electric field. From a constant applied field, we are able to extract the ion mobilities and hence conductivities. We find that ionomers with percolated ionic aggregate morphologies have higher ion mobilities and hence higher conductivities. Application of an oscillating electric field enables us to calculate the frequency-dependent conductivity of the model ionomer melts. The real part of the conductivity has a high frequency peak associated with plasma oscillations, and a very broad low frequency peak associated with ion motions in ionic aggregates. I will end with comments on the connections to atomistic simulations and to experimental probes of ion dynamics. \\ Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Electrostatic Assembly of Polymers and Nanoparticles at Liquid-Liquid Interfaces.

The electrostatic attraction between charged solutes on opposite sides of the interface between immiscible liquids offers an efficient route to the self-assembly of two-dimensional films. As implemented by us, a hydrophobic polymer with amine end(s) or block(s) is presented in an oil phase, and a negatively charged nanoparticle is presented in an aqueous phase; both solutes are insoluble in the opposite phase but efficiently driven to the liquid-liquid interface by mutual electrostatically attraction to the solute in the opposite phase. Depending on experimental conditions (salt concentration, pH, solute concentrations, etc.), a continuous, nanoscopically thin composite film builds at the oil-water interface over the timescale of minutes, often accompanied by a dramatic reduction of interfacial tension akin to that observed for a surfactant. Film formation and properties by the new route will be discussed, as principally probed through pendant drop interfacial tensiometry and pendant drop interfacial rheometry. Components of model system are toluene-dissolved amine end-capped polystyrene and water-dispersed acid-treated carbon nanotubes or citrate-treated gold nanospheres. Film structures are complicated, as are crucial electrostatic interactions near the interface. With amine end-capped polystyrene partnered with acid-treated carbon nanotubes, high pH (above 5) and high polystyrene molecular weight (above 5000 g/mol) strongly hinder film formation. These films, which are liquid-like, show two viscoelastic relaxations, a fast relaxation (about 10 s) associated with polystyrene chain rearrangements (slightly impacted by carbon nanotube association) and a slow relaxation (about 20 min) associated with polystyrene adsorption/desorption; at intermediate times (or frequencies), the two-dimensional storage and loss moduli follow approximately the same power law dependences.   

\textbf{Polymerized Ionic Liquids: Promising Class of Polymer Electrolytes }

Use of polymer electrolytes instead of traditional liquid electrolytes offers an elegant solution to many problems in current battery technology. However, a major obstacle in use of polymer electrolytes is their low ionic conductivity and low transference number (percentage of charge transported by the desired ion). Polymerized ionic liquids (PolyILs), a relatively new class of polymer electrolytes, are essentially single ion conductors and provide simple solution for the increase of the transference number. However, their ionic conductivity at ambient conditions remains low. Our earlier studies demonstrated that only \textit{strong decoupling of ionic conductivity from segmental dynamics} can lead to a `superionic' behavior of a polymer and might provide sufficiently high conductivity [1,2]. Based on this concept, we overview recent developments in the field of polymerized ionic liquids, with the emphasis on the polymer specific decoupling of ionic conductivity from segmental dynamics. The latter is well illustrated by the comparison of ionic liquids with their polymerized analogs [3,4]. Ways to further improvement of ionic conductivity in PolyILs, and their possible limitations are discussed at the end. \begin{enumerate} \item Y. Wang, et al., \textbf{Phys. Rev. Letters 108}, 088303 (2012). \item Y. Wang, et al., \textbf{Polymer 55}, 4067 (2014). \item J. R. Sangoro, et al., \textbf{Soft Matter 10}, 3536 (2014). \item F. Fan, et al., \textbf{Macromolecules 48}, 4461 (2015). \end{enumerate}