Session Q19: Structure and Dynamics of Ion-Containing Polymers I

Molecular Dynamics of a Soft-Matter Analog System near Nuclear Densities

For more than a decade, nuclear physicists have been using molecular dynamics simulations to study exotic phases of matter in neutron stars. At high densities, the strong, short-ranged proton-neutron attraction and long-ranged proton-proton repulsion create complex shapes called 'nuclear pasta' analagous to many self assembling systems known in the soft matter literature. We will present some recent simulations and phase diagrams using this model, including some results showing a helicoidal defect in a lamellar phase. With appropriately chosen parameters and units, this model may be useful for simulating soft matter systems such as block copolymers and lipid membranes.   

Ion Transport in Polyacrylonitrile (PAN) Based Electrolytes

Polyacrylonitrile (PAN) is one of the alternative candidate polymer hosts to form solid polymer electrolytes (SPEs) besides the widely used poly(ethylene oxide). In this study, the ion transport mechanism and electrochemical properties of PAN-based SPEs are thoroughly examined. It is found that the ion transport, lithium stability and the oxidative stability of PAN-based SPEs are critically dependent on the film processing conditions, particularly the drying condition. We systematically study four different drying conditions and the resulting residual solvent is quantified by infrared (IR) spectroscopy. We demonstrate that conductivity variation across 5 orders of magnitude can be obtained depending on the drying conditions. Furthermore, the effects of residual solvent on the ion transport mechanism, electrochemical stability against lithium and the oxidative stability are elucidated. This thorough study lays the groundwork for future development of PAN based electrolytes.   

Polymer-ceramic composite electrolyte for high energy lithium batteries

Solid-state electrolytes are on their way to enable the next generation of energy storage architectures with higher energy density and improved safety. However, each major class of solid electrolytes has intrinsic weaknesses. By combining different classes of solid electrolytes, such as a polymer electrolyte and an oxide ceramic electrolyte, the ultimate goal is to overcome the intrinsic weaknesses of each component and develop a composite electrolyte to achieve high ionic conductivity, good mechanical properties, good chemical stability and adhesion with the electrodes, and thin sheet processability.  In this presentation, both fundamental and applied aspects of composite electrolytes will be discussed. Fundamentally, ion mobility and segmental dynamics at the polymer-ceramic interface are examined by a combined quasi-elastic neutron scattering/solid-state nuclear magnetic resonance methodology. The origin of interfacial resistance for ion transport across the polymer-ceramic interface is elucidated. On the practical side, strategies to achieve optimum interfacial structures is discussed. Limitations of each strategy and an outlook of the development of composite electrolyte will be summarized.     

Using in situ SAXS to Investigate Salt Concentration Gradients, Morphological Changes, and Cell Failure Mechanisms in Block Copolymer Electrolytes at the Limiting Current

Solid block copolymer electrolytes with lithium salt are promising materials which could enable practical batteries with highly energy-dense lithium metal anodes. These materials phase separate into ion-conductive domains, enabling ion transport, and mechanically-rigid domains, which enhance safety. In this work, we study a polystyrene-block-poly(ethylene oxide) (PS-b-PEO) / lithium bis-(trifluoromethylsulfonyl)amine (LiTFSI) block copolymer electrolyte which forms hexagonally packed polystyrene cylinders in a PEO / LiTFSI matrix.

 

As electrochemical cells are charged and discharged, salt concentration gradients develop. Characterizing these gradients is valuable, as when the salt concentration reaches zero at any point, the cell fails. While theoretical predictions exist, they are difficult to validate experimentally. In this work, we use in situ x-ray transmission measurements to directly measure salt concentration as a function of both position and time as a cell approaches its limiting current and eventually fails.

 

We find that cells fail at lower currents than predicted by the theory, suggesting that there is a failure mechanism for which current theoretical models do not account. To resolve this, we use in situ small-angle x-ray scattering to investigate block copolymer morphology during polarization. We observe significant structural changes, including transitions between spherical and cylindrical morphologies, as local salt concentration evolves, and quantify the strain exerted by expanding and contracting grains. Based on this, we propose a new, structural mechanism for cell failure and limiting current.   

Polyelectrolyte-Grafted Nanoparticles in Ionic Liquids for Enhanced Ion Transport

Poly(methyl methacrylate) (PMMA) chains grafted on magnetite nanoparticles are copolymerized to form two types of copolymers with different ionic natures. A copolymer with poly(styrene sulfonate) (PSS), and a copolymer with poly(ionic liquid) of 1-hexyl-3-vinylimidazolium bis(triflouromethylsulfonyl)imide (HVImTFSI) are synthesized by RAFT polymerization. The interactions between ionic liquid (HMIM-TFSI) and copolymer electrolytes are investigated by electron microscopy, SAXS and electrochemical impedance spectroscopy. Ionic groups in the polyelectrolyte domains promote ion dissociation differently as well as the ionic conductivity that is governed by the underlying particle assemblies and chain conformations. We will compare these differences and morphologies formed of the two copolymer-grafted systems and will present the electrochemical properties of the particle electrolytes.   

Li-ion transport and morphology study of ion-condensed, liquid-crystalline polymer electrolytes

Metal-ion containing ionomers are well-known for their high strength, high toughness, and slow relaxations due to the physical cross-linking by ionic aggregates. However, recent coarse-grained simulations on melt ionomers by Frischknecht, Winey, and co-workers have shown that there is a possible extremely fast metal-ion transport mechanism that can occur within a percolate grain. In such a mechanism, the metal-ion hops successively between two adjacent sites, decoupled from segmental relaxation. Meanwhile, certain metal oxide glasses present conductivity as high as ~ 10 mS/cm at ambient temperature through a similar hopping mechanism. We have designed and synthesized a series of ion-condensed, anion-tethered, liquid crystalline polymers to study the relationship between transport mechanisms and polymer architecture. In our work, we investigate various backbone, ionic group tethered polymers with different polarities to modulate the ionic phase morphology. By utilizing broadband dielectric spectrometry, we probe ionic motion in various length and time scales to interpret the Li-ion transport mechanism. We correlate the ion dynamics with the morphology of the ionic phase.   

Polyelectrolyte Diffusion in Convex Lens-induced Confinement (CLiC)

Understanding the transport and thermodynamics of polymers in confined spaces is helpful for many separation processes like water purification, drug delivery, and oil recovery. Specifically for water purification, typical membrane foulants are polyelectrolytes, which have been modeled using polyacrylic acid. Uncovering how these polyelectrolytes interact in confinement can reduce the fouling of organic membranes and will lead to better separation processes overall. We have determined the diffusion coefficient, D, of sodium polyacrylic acid (NaPAA) and dextran in Convex Lens-induced Confinement (CLiC) using differential dynamic microscopy (DDM). In this setup, the confinement ranges continuously from 0.085 – 21.7 µm. To increase the contrast, the polymers were fluorescently tagged. It was found that Dextran diffusion become slower in higher confinement, which is consistent with a change in hydrodynamic interactions. On the other hand, the diffusion of NaPAA in a high ionic strength solution (I ~ 160 mM), increased as the confinement became stronger. These findings indicate that polyelectrolyte diffusion changes in confinement and can lead to a better understanding of separation processes.   

Impacts of Anion Structure on Ion Conduction of Li-ion Single-ion Conducting Ionomers

Single-ion conducting ionomers are potential candidates for energy-related applications. However, their conductivity is orders of magnitude lower than the requirement for device operation. Understanding how the chemical structure impacts ion conduction offers new perspectives on designing ionomers with higher conductivity. Here, two different random copolymers containing two types of anions, bis(fluorosulfonyl)imide (TFSI-like) anion and sulfonate anion, copolymerized with poly(ethylene glycol) methacrylate were synthesized using RAFT polymerization. Synthesized polymers with different anion groups had similar glass transitions temperatures (T_g), ion contents, molecular weights, and dispersities. Li-ion conduction was measured using dielectric relaxation spectroscopy (DRS), and X-ray scattering investigated ionomer morphology. DRS reveals orders of magnitude higher conductivity for ionomers with the imide anion than with the sulfonate anion. Ion aggregation causes the lower ionic conductivity of sulfonate lithium ionomer. Our results highlight the significant impacts of anion structure on ionic conductivity. In addition to lowering T_g, weakening the binding energy between the attached anion and the Li counterion promotes ion conduction for single-ion conducting ionomers.   

Coarse-grained Simulations of Ion Transport in Single-ion and Salt-doped Polymer Electrolytes

Solid polymer electrolytes with a high lithium-ion transference number are of interest as they can improve charging rate, reduce ion polarization, and suppress lithium dendrite growth. This transference number, representing the fractional contribution of cations to overall conductivity, is generally low in salt-doped polymer electrolytes. In contrast, if anions are tethered to the polymer backbone to create a single-ion system, the transference number goes to one in the limit of negligible polymer motion. We use coarse-grained molecular dynamics simulations in which the only difference between salt-doped and single-ion systems is the addition of bonds tethering the anions; thus, we show a controlled comparison of the effect of tethering between systems of the same chemistry. We calculate the conductivity, transference number, and ion clustering behavior with an applied electric field in homopolymers and diblock copolymers. We will discuss effects of polymer architecture, ion concentration, and dielectric constant on ion transport in single-ion versus salt-doped polymers.   

Thermal and structural studies of polyzwitterion/salt complexes

The thermal and structural properties of five amorphous polyzwitterion (PZI)/LiCl salt complexes were investigated using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). Poly(sulfobetaine methacrylate), PSBMA, poly(sulfobetaine acrylate), PSBA, poly(ethyl sulfobetaine methacrylate), PESBMA, poly(ethyl sulfobetaine acrylate), PESBA, and poly(sulfobetaine methacylamide), PSBMAm, were synthesized with variations in the sidechain and backbone. Here, we investigate the effects of LiCl salt addition to PZI solution cast films; LiCl composition in the complexes varied from about 0.06 to 1.65 mol.%. In PZI without added salt, intra- and inter-molecular crosslinks are formed between zwitterionic moieties, leading to an increase in the glass transition temperature (T_g) and a decrease in the heat capacity increment at T_g. As the LiCl content increases, crosslinking is disrupted leading to a decrease in T_g, and a decrease in the interatomic spacing as observed using WAXS. For the composition range we studied, addition of LiCl results in an increase of the molecular mobility through reduction of intra- and inter-chain crosslinking.   

Competition of folding-induced assembly and liquid-liquid phase separation produces diverse morphologies of peptide coacervates

Competition between short-range hydrogen-bonding driven intramolecular assembly coupled with long-range colloidal phase instability dictated by electrostatics can result in protein misfolding that has been linked to many neurodegenerative diseases. Specifically, during charge condensation of peptide mixtures such as polylysine and polyglutamic acid of the same chirality, a competition between complex coacervation and beta sheet assembly gives rise to a diversity of structures in solution spanning solid precipitates to network-like bicontinuous structures to liquid-like coacervate droplets. Our investigation into this system indicate that while precipitates dominate at higher peptide concentrations and coacervates dominate at lower peptide concentrations, this competition is highly dependent on the processing of the mixture (including mixing path and thermal history). This complex phase behavior suggests a competition between hydrogen bonding driven beta sheet assembly and electrostatics driven coacervate formation; the new results and its implications for protein folding and assembly will be discussed in this talk.   

Complex Micelle Morphology of DNA Block Copolymer and Ionic Liquid

DNA block copolymers are hybrid polymers that contain both DNA and synthetic polymer linked into a blocky chain structure. They also represent a special class of charged-neutral block copolymers that can potentially interact with cationic molecules to form polyelectrolyte complexes. In this study, we prepare DNA-poly(N-isopropylacrylamide) (DNA-PNIPAM) block copolymer and investigate its complexation with ionic liquid (IL). The block copolymer is synthesized via the coupling reaction of N-hydroxylsuccinimide ester functionalized PNIPAM and amine functionalized DNA. The DNA-PNIPAM block copolymer is then combined with 1-butyl-3-methylimidazolium hexafluorophosphate at different charge ratios to form polyelectrolyte complexes. We find that stable nano-sized complex micelles are formed over a range of charge ratios. The micelles prepared by following salt annealing at room temperature exhibit string-like morphology. Further treatment of the micelles at higher temperatures leads to the breakup of the string morphology into spheres and rods. Finally, we prepare complex micelles by conducting salt annealing at different temperatures to illustrate the interplay of DNA and IL charge interaction and PNIPAM thermoresponsive behavior on the micelle morphology.