Session B34: Confinement, Dynamics, and Ion Interactions in Ion-Containing Polymers II

Ion Confinement in Self-Assembled Precisely Segmented Polyolefin Ionomers

Confining ions within self-assembled nanoscale structures is demonstrated in a series of single-ion conducting segmented polyolefins, nominally multiblock copolymers. We report a series of precisely-segmented polyethylene-like ionomers containing sulfonate groups (PES) with Li+, Na+, Cs+, or NBu4+ counterions synthesized from step-growth polymerization. At room temperature, the PES ionomers with long methylene units are semicrystalline with well-defined nanoscale ionic layers with spacings influenced by the spacer length and cation type. In situ X-ray scattering measurements reveal that the layered ionic aggregates in some of these polymers transform, upon melting the PE segments, into gyroid morphologies. The gyroid structure can further evolve into hexagonal symmetry as T increases. The ion transport behavior of these polymers is strongly dependent on the ionic aggregate morphologies. Specifically, a 3D interconnected gyroid morphology exhibits higher ionic conductivity than the isotropic layered or hexagonal morphologies. This innovative and versatile molecular design of ionomers leads to unprecedented percolated gyroidal ionic aggregate morphologies that provide a continuous pathway for improved ion transport.
  

Unraveling how nanoconfinement and phase-separation affect the transport properties of ionomer membranes

Transport of water and ions through ionic polymer membranes depends on multi-scale membrane structure. This talk will focus on the nano-scale structure-transport relations of ionomer membranes. We will present studies on the temperature dependence of diffusion as a function of water uptake in a perfluorosulfonic acid (PFSA) membrane. Using NMR diffusometry and MD simulations, we are able to probe the activation energy of water diffusion (E_a), which arises from molecular scale interactions and reflects the nano-scale environment of a water molecule. By comparing E_a in the PFSA membrane to E_a in a free liquid environment, we are able to identify two structural primary features of ionomer membranes, nanoconfinement and nano-scale phase separation, that affect membrane transport. Nanoconfinement alters local energetics of water molecules and can prompt formation of more ordered water structures. Nano-scale phase separation creates a local environment for water molecules that is closer to that of pure water, and thus gives rise to E_a values that approach that of pure water. These findings shed light on the fundamental aspects of structure-transport interplay in ionomer membranes.   

Polymeric Ionic Liquid-Ligand Gels Exhibiting Transient Gel Behavior and Multivalent Ion Conductivity

The conduction of metal ions through solid polymeric electrolytes relies on favorable but highly dynamic interactions between mobile metal cations and polymer-bound solvating functionalities. Metal-ligand interactions have shown promise as solvating functionalities in solid polymer electrolytes due to their ability to strongly solvate metal salts while simultaneously conducting mono and multivalent metal ions in the solid state. These interactions simultaneously act as reversible cross-links, enhancing the storage modulus of the material over short timescales. Controlled poly(methyl acrylate)s with imidazole chain ends are synthesized as model polymers to investigate metal-ligand bond lifetimes and design rules for solid polymer electrolytes. Materials with a controlled number of imidazole ligands per chain are used to generate dynamic star-like and network topologies which relax to a disordered melt through metal-ligand bond dissociation. The network-forming materials are further used to probe the kinetics of cation-ligand association when the identity and valency of the metal cation is varied. Scaling arguments based on oscillatory rheology and electrical impedance measurements are used to support a hopping-type mechanism of ion conduction in metal-ligand coordinating polymers.   

Hydroxide conducting block copolymers

Hydroxide ion conducting block copolymers have the potential to possess the multiple orthogonal properties required for anion exchange membranes to enable long-lasting alkaline fuel cell performance, and therefore can accelerate the advancement of the alkaline fuel cell, a low-cost alternative to the well-adopted commercial proton exchange membrane fuel cell. In this work, an overview of hydroxide ion transport (a property that is proportional to fuel cell performance) in block copolymers will be presented and the subsequent impact of block copolymer morphology on ion transport (conductivity), where the careful design of block copolymer chemistry and chain architecture can accelerate hydroxide ion transport and subsequently alkaline fuel cell performance.   

Simulation of Ion Transport through Percolated Aggregates in Precise Sulfophenylated Polyethylene Ionomers

Ionomers present an attractive potential alternative to standard electrolytes in Li ion batteries due to their capacity to function as single-ion conductors. When the spacing between adjacent pendant groups is precisely controlled, ionic groups within ionomers have been shown to self-assemble to form aggregates with well-ordered morphologies. The structure of these aggregates can have a significant impact upon ion conductivity. In order to gain greater understanding at the microscopic level, we perform fully atomistic molecular simulations to probe the assembled ionic aggregates of sulfophenylated polyethylene ionomers with a spacing of five carbon atoms between pendant groups (p5PhS), neutralized with a series of cations (Li, Na, and Cs). We find that the ionic aggregates form percolating clusters provided the polymers are neutralized with a sufficiently high fraction of cations. Li and Na aggregates take ribbon-like configurations, while Cs aggregates are more disordered. In each case the ions are able to slowly diffuse from sulfonate group to sulfonate group through the percolated aggregates, with larger ions exhibiting larger mean-squared displacements. Finally, structure factors computed from simulation agree reasonably well with experimental X-ray scattering data.   

Mechanisms of Ion Transport in Block Copolymeric Polymerized Ionic Liquids

We present the results of a multiscale simulation framework investigating the ion transport mechanisms in multicomponent polymerized ionic liquids. Three different classes of polymeric ionic liquid systems, viz., random copolymers, lamellae forming block copolymers and homopolymers are constructed at the coarse-grained scale, and their atomistic counterparts are derived by using a reverse mapping method. Using such a framework, we investigate the influence of morphology on ion transport properties of such polymerized ionic liquids. Our results for ion mobilities are in qualitative agreement with experimental observations. Further analysis of random copolymer and block copolymer systems reveal that the reduced ion mobilities in such systems arise from the influence of architecture and/or morphology on ion coordination and intramolecular hopping events.   

Composition fluctuation in weakly heterogeneous dielectric medium containing ions

Impacts of salt doping on morphological behavior of block polymers, though well-documented, remain poorly understood. Selective ion solvation has been identified as a driving force for ordering, and its effects studied using mean-field models. We present an analysis of the
composition fluctuation that couples, through the heterogeneous dielectric profile, to the ionic solvation in the weakly heterogeneous regime. The free energy is mapped onto the Brazovskii form. The scattering peak is found to depend on salt-doping, as a result of the shift of the
critical point and the pairwise Coulomb interaction. Phase diagrams are presented under different doping conditions, revealing the competition between selective solvation and fluctuation. The need of consistently parameterizing permittivity and dispersion interaction is discussed.   

Permeation and copermeation behavior of methanol and acetate in cation exchange membranes

The permeation of solutes, molecules and ions, through hydrated polymer membranes is of critical importance for many applications. We utilize in situ ATR FTIR spectroscopy to quantify the multicomponent transport of molecules and polyatomic ions through ion exchange membranes. We examine the individual and co-permeation of methanol and acetate across commercially available membranes, Nafion 117, and synthesized polyether-based membranes of varied and tunable ion content and charge type. Membrane permeability and selectivity calculated from single component permeation experiments are compared to those calculated for solutes in multicomponent permeation experiments. In Nafion 117 and model cation exchange membranes we find distinct differences in transport behavior of the acetate anion while the behavior of methanol remains unchanged. We also find no difference in transport behavior for these solutes in uncharged polyether membranes. We attribute this emergent transport behavior of the acetate anion to specific solute-solute-membrane interactions which likely emerge from the electrostatic screening of interactions between the acetate anion and membrane-bound anions.   

Percolated ionic aggregates in precise sulfophenylated polyethylene ionomers: Morphology and ion transport

We present a set of precise single-ion conducting ionomers that demonstrate decoupled transport of metal cations within self-assembled percolated aggregates in glassy polymer matrices. These precise ionomers consist of a polyethylene backbone with a sulfonated phenyl group pendant on every 5^th carbon, that is fully neutralized by a counterion X (Li⁺, Na⁺, or Cs⁺), p5PhSA-X. The morphologies of these ionomers are characterized with X-ray scattering, and the ion transport properties are characterized with electrical impedance spectroscopy. Both experiments are performed under vacuum, from room temperature up to 180°C. Atomistic molecular dynamics simulations elucidate that the structure of the aggregates in the ionomers is a percolated network. The characteristic length scales of these percolated aggregates as measured by X-ray scattering are ~2nm and independent of ion type. There is good agreement between simulations and experimental X-ray scattering data. The ionomers exhibit conductivity of 10^-7 to 10^-6 S/cm at 180°C and demonstrate Arrhenius behavior up to 180°C. This indicates that the ion transport is decoupled from the polymer backbone, which is consistent with a percolated aggregate within which ions travel.   

Investigation of monomer segment distributions, chain conformations, and lithium salt solvation in self-assembled, tapered block polymer electrolytes

Tapered block polymers (TBPs) contain modified monomer segment distributions (e.g., gradient or random copolymer regions) at the chemical junction between two homogeneous blocks. Nanostructured polystyrene-block-poly(oligo-oxyethylene methacrylate) (PS-b-POEM) TBP electrolytes have exhibited improved ionic conductivities, shear moduli, and processabilities in comparison to their conventional block polymer analogues. In this work, we studied the microscopic characteristics of TBPs that impart these enhanced properties. The monomer segment distributions of lithium salt-doped normal-, inverse-, and non-tapered PS-b-POEM TBPs were obtained via X-ray reflectivity, and these distributions also were successfully modeled through coarse-grained molecular dynamics simulations that included strong ion solvation effects. This combined experimental-computational approach allowed the segregation strengths, chain conformations, and ion solvation energies of the salt-doped TBPs to be quantified as a function of taper sequence and salt concentration. By understanding how these polymer assembly and ion solvation behaviors affect ion transport, we can guide the rational design of higher-performance polymer electrolytes.   

Model single ion conducting polymer networks for understanding the impact of ion content, crosslink density, and side chain length on Li transport

Single ion conducting polymer networks were designed containing tethered anions of bis(trifluoromethane sulfonamide) (TFSI), an acrylic backbone, and ethylene oxide (EO) side chains and crosslinkers to develop fundamental structure property relationships. The crosslinking density was varied from 1-50 % mol of the starting monomers, while the crosslinker was an EO diacrylate with 11, 22, or 33 atoms between acrylate groups. The Li to EO ratio was set by the density of fixed TFSI sites and spanned 1:20 to 1:100, while the length of EO side chain on non-ionic monomers was 11 or 22 atoms is achieved. By systematically manipulating parameters, a 3 order of magnitude difference in ionic conductivity and a 70 C shift in Tg was observed indicating the importance of design in such networks. Conductivities approaching 10^-5 S/cm are reported in dry, single ion conducting networks which is comparable to the state-of-the art. Adding plasticizer further increases the conductivity. The observed structure-conductivity trends provide insight into the design of single ion conductors for a broad range of energy applications.   

Salt-tethered nanoparticles in solvent: A potential high conductivity, high lithium transference number electrolyte system

Improving the lithium transference number in electrolytes, while maintaining high ionic conductivity, is crucial for reliable and high-performing lithium-ion battery technologies. In this work, using computer simulations, we model a relatively new class of electrolyte system, reported in a recent work [Chem. Mater. 2013, 25, 6, 834-839], wherein nanoparticles cofunctionalized with polymeric ligands and tethered lithium salts, are dispersed in a solvent host. We employ a sequential combination of Molecular Dynamics and kinetic Monte Carlo simulations, to study ion transport in this system. Our results are qualitatively consistent with the experimental findings. Specifically, we explain the interesting dependence of ionic conductivity on nanoparticle composition and anion chemistry. Further, we predict significant improvement in conductivity with high dielectric constant solvents. These results suggest that such electrolyte systems can potentially exhibit high conductivity, along with high lithium transference number.   

Cluster Cohesion Effects on Segmental Dynamics in Ionic Polymer Solutions: Molecular Dynamics Simulation Studies

A small number of ionizable groups tethered to a polymer backbone restricts their macroscopic dynamics. Here, using molecular dynamic (MD) simulations, the dynamics of sulfonated polystyrene (PSS) in toluene solutions are studied as the ionic domains are perturbed by small amounts of ethanol. The static and dynamic structure factors, S(q) and S(q,t) were calculated. S(q) exhibits a characteristic ionic domain signature that is affected by ethanol content. S(q,t), analyzed by KWW resolved two dynamic regions. At low ethanol concentrations, segmental dynamics increase, followed by constrained motion with increased alcohol concentration. Initially, the ionic domains swell, enabling segmental motion. Higher ethanol concentrations collapse the PS segments restricting the motion. These MD results are in excellent agreement with our neutron spin echo data.