Session F10: Ion Containing Polymers - The Role of Structure and Dynamics I

Lyotropic Liquid Crystal Mesophases as a Platform for Understanding Proton Transfer in Hydrated Polymer Membranes

Few molecular design rules exist for guiding the development of proton transporting, acidic nanoporous polymer membranes for fuel cell and electrochemical device applications. Lyotropic liquid crystal (LLC) mesophases, which arise from small molecule surfactant self-assembly, comprise a model materials platform for investigating the molecular details of water-mediated proton transfer in monodisperse nanochannels lined with specific chemical functionalities. We describe studies of the aqueous phase behavior and ion conductivities of a new class of non-fluorinated alkylsulfonic acid amphiphile LLCs, which exhibit remarkably high proton conductivities ( $\ge $ 150 mS/cm at 25 $^{\circ}$ C and 100 {\%}RH) that rival those of the best polymeric, perfluorosulfonic acid proton conductors. Thus, the acidity of the pore functionality is not the primary determinant of ionic conductivity. Instead, our studies demonstrate that proton conductivity depends on maximizing proton activity within the nanochannels, which sensitively depends on the pore diameter and pore morphology.   

Electrostatic Correlations and the Polyelectrolyte Self Energy

We address the effects of chain connectivity on electrostatic fluctuations in polyelectrolyte (PE) solutions using a field-theoretic, renormalized Gaussian fluctuation (RGF) theory. As in simple electrolyte solutions (Z.-G. Wang, Phys. Rev. E. {\bf 81}, 021501 (2010)), the RGF provides a unified theory for electrostatic fluctuations, accounting for both dielectric and charge correlation effects in terms of the self-energy. Unlike simple ions, the PE self energy depends intimately on the chain conformation, and our theory naturally provides a self-consistent determination of the response of intramolecular chain structure to PE and salt concentrations. The theory captures the expected scaling behavior of chain size from the dilute to semi-dilute regimes; by properly accounting for chain structure the theory provides improved estimates of the self energy in dilute solution and correctly predicts the eventual $N$-insensitivity of the critical temperature and concentration of salt-free solutions of flexible PE. We show that the self energy can be interpreted in terms of an infinite-dilution energy $\mu^\text{el}_{m,0}$ and a finite concentration correlation correction $\mu^\text{corr}$ which tends to cancel out the former with increasing concentration.   

Concentration Dependence of Ionic Correlation and Its Relation to Non-Ideality

For salt-doped polymer electrolytes, a number of spectroscopic studies report that anions are more likely to be in contact with cations as salt concentration increases, implying that the ionic conductivity exhibits greater non-ideality, namely, growing deviation from Nernst-Einstein relation, due to the increased number of spatially-close ion pairs. However, the validity of this expectation remains unclear since the spectroscopic (structural) and the conductivity (dynamical) measurement resolve different time-scales, obscuring the fundamental relationship between the ion paring and the non-ideal conductivity. In this work, we investigate the above question based on equilibrium and non-equilibrium coarse-grained simulations for model polymer electrolytes. By quantifying the structural and dynamical correlation of ions in a distinctive statistical-mechanical manner, and comparing them with the concentration dependence of the non-ideality, we aim to provide microscopic insights about how the natures of the two correlations differ, and how they affect the conductivity as a function of concentration. Their relative significance compared to the concentration dependence of the matrix viscosity is also discussed.   

Phase behavior of acid-bearing block copolymers containing ionic additives

Ion-containing block copolymers, where one of the block is conducting ion and the other block is mechanically robust, are of interest for polymer electrolyte membranes. They can self-assemble into nanoscale morphologies in which the small amount of charges is known to make an impact on the phase behavior. Herein, we investigate the phase behavior of acid-bearing block copolymers with the addition of ionic liquids (ILs). In order to understand the role of thermodynamic interactions among the ionic moieties in determining the phase behavior, the ratio of cation and anion in ILs was systematically varied. Overall, the addition of ILs resulted in the increase in segregation strength of block copolymers, attributed to the selective incorporation of ILs in ionic domains. Interestingly, it has been further revealed the segregation strength can be largely modulated by varying the ratio of cation and anion in ILs. The results were rationalized by the balance of favorable thermodynamic interactions in IL-containing ionic phases and repulsive electrostatic interactions among oppositely charged ionic moieties. The effective Flory-Huggins interaction parameters were determined by fitting the scattering data of disordered phases based on the random phase approximation.   

Molecular dynamics simulation of low dielectric constant polymer electrolytes

Recent experimental studies measured the ionic conductivities of a series of poly(glycidyl ether)s with varying neat dielectric constants ($\epsilon$), viscosities ($\eta$), and glass transition temperatures (T$_{g}$), as hosts for lithium bistrifluoromethanesulfonimide (LiTFSI) salt. In such a context, it was demonstrated that the ionic conductivity of these polymer electrolytes was a function of $\epsilon$ rather than T$_{g}$ or $\eta$, suggesting that there may exist regimes in which ionic conductivity is not limited by slow segmental dynamics but rather by low ionic dissociation. Motivated by such results, we used atomistic molecular dynamics to study the structure and transport characteristics of the same set of host polymers. We found that the coordination number of TFSI$^{-}$ about Li$^{+}$ in the first solvation shell and the total fraction of free ions increased as a function of $\epsilon$, implying the polymer hosts enhanced ion dissociation. In addition, we found that increasing the dielectric constant of the host polymer enhanced self-correlated ion transport, as evidenced by an increase in the diffusion coefficients of each ion species. Overall, we confirmed that limited ion dissociation in low-$\epsilon$ polymer electrolyte hosts hampers ionic conductivity.   

Probing the Electromechanical Response Mechanism of Ionic Block Copolymers

One of the rapidly developing frontiers in research is focused on the design of materials that have targeted functionalities and tunable responses to external stimuli. A chief obstacle to achieving this capability is the lack of a fundamental understanding of how chemical structure and morphology give rise to macromolecular properties. Our research explores how polymer structure, counterion species, and film morphology affects the electromechanical response of ionic block copolymers when exposed to external electric fields using neutron reflectometry. Currently, the literature lacks a thorough understanding of how these factors drive the electrostatic and mechanical stresses that result in material deformation. The sensitivity of neutron reflectometry to scattering length density as well as subnanometer changes in thickness make it an ideal tool for investigating polymer and counterion mobility under applied fields. To this end, we have designed a vacuum chamber capable of applying electric fields to thin films during reflectometry measurements to probe electromechanical response.   

New Polymer Structures for Anion Exchange Membranes

New cationic membranes for alkaline fuel cells and other ion transport applications continue to generate interest in the polymer materials community. A current barrier in the field is the lack of consensus on structure-property relationships of polymers with backbone-tethered cations. Our group has demonstrated a number of polymer structures based on commercially-available polymers and inexpensive modification reagents. We have focused on poly(phenylene oxide) as a backbone platform and employed ammonium cations that have good stability in light of their low cost and reasonable performance. In previous work, we employed alkyl chains to cause phase separation in random copolymers. This phase separation increased the conductivity and decreased the water uptake of the materials. We have since continued to investigate crosslinking and interpenetrating networks as further methods to optimize the conductive, mechanical, and swelling properties of anion exchange membranes. This talk will highlight our recent work on new polymer structures and demonstrate how stability and conductivity can be increased by iterating on quaternary ammonium poly(phenylene oxide)-based materials.   

Insights on Li-TFSI diffusion in polyethylene oxide for battery applications

Improving the energy density, safety and efficiency of lithium-ion (Li-ion) batteries is crucial for the future of energy storage and applications such as electric cars. A key step in the research of next-generation solid polymeric electrolyte materials is understanding the diffusion mechanism of Li-ion in polyethylene oxide (PEO) in order to guide the design of electrolytes materials with high Li-ion diffusion while, ideally, suppress counter-anion movement. In this work we use computer simulations to investigate this long-standing problem at a fundamental level. The system under study has Li-TFSI concentration and PEO chain length that are representative of practical application specifications; the interactions of the molecular model are described via the PCFF+ all-atom force-field. Validation of the model is performed by comparing trends against experiments for diffusivity and conductivity as a function of salt concentration. The analysis of Li-TFSI molecular dynamics trajectories reveals that 1. for high Li-TFSI concentration a significant fraction of Li-ion is coordinated by only TFSI and consistently move less than PEO-coordinated Li-ion, 2. PEO chain motion is key in enabling Li-ion movement.   

Role of Semi-Crystalline Morphology on Proton Conduction Pathways in a Precise Sulfonic Acid Polyethylene

Linear polyethylenes with precisely periodic functional groups (precise polyethylenes) have been shown to exhibit unusually well-defined semi-crystalline morphologies containing layers of functional groups. These layers may be useful for protonic, ionic or molecular transport. Recently, a precise polyethylene was synthesized with sulfonic acid groups bonded to every 21$^{\mathrm{st}}$ carbon. We have studied the proton dynamics in this polymer using broadband dielectric spectroscopy at various hydration levels and thermal histories to evaluate the utility of these self-assembled pathways for improving transport. To understand the effect of structure on proton dynamics, detailed characterization has been carried out including X-ray scattering and atomistic molecular dynamics simulations. Comparable proton conductivities are measured in the semi-crystalline and amorphous states, showing that the greatly reduced chain mobility in the semi-crystalline state does not lead to a commensurate reduction in conductivity. This suggests that the proton pathways within the crystallites play a role in enhancing transport.   

The effect of varying linker length on ion-transport properties in polymeric ionic liquids

We report results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-(n-alkyl)imidazolium ionic liquids with $PF_6^-$ counterions. Consistent with experimental observations, we observe that the mobility of the $PF_6^-$ ions increases with increasing n-alkyl linker length. Analysis of our results suggests that the motion of $PF_6^-$ ions is driven by intermolecular ion hopping between chains, which in turn is influenced by ion-pair coordination numbers and intermolecular ionic separation distances. With increasing linker length, we observe 1) the anions coordinating less closely with cations and 2) intermolecular hopping distances decreasing.   

Ion Transport via Structural Relaxations in Polymerized Ionic Liquids

We study the mechanisms underlying ion transport in poly(1-butyl-3-vinylimidazolium-hexafluorophosphate) polymer electrolytes. We consider polymer electrolytes of varying polymerized ionic liquid to ionic liquid (polyIL:IL) ratios and use atomistic molecular dynamics (MD) simulations to probe the dynamical and structural characteristics of the electrolyte. Our results reveal that anion diffusion along polymer backbone occurs primarily \textit{via }the formation and breaking of ion-pairs involving \textit{three }polymerized cationic monomers of \textit{two }different polymer chains. Moreover, we observe that the ionic diffusivities exhibit a direct correlation with the structural relaxation times of the ion-pairs and hydrogen bonds (H-bonds). These results provide new insights into the mechanisms underlying ion transport in polymerized ionic liquid electrolytes.   

Understanding glass formation and ion transport in polymeric ionic liquids using computer simulations

Polymeric ionic liquids (PILs) are very promising materials to enable more environmentally stable high density energy storage devices. Realization of PILs providing high environmental and mechanical stability while maximizing ion conductivity would be accelerated by an improved molecular level understanding of their structure and dynamics. Extensive evidence suggests that both mechanical properties and ion conductivity in anhydrous PILs are intimately related to the PIL's glass formation behavior. This represents a major challenge to the rational design of these materials, given that the basic nature of glass formation and its connection to molecular properties remains a substantial open question in polymer and condensed matter physics. Here we describe coarse-grained and atomistic molecular dynamics simulations probing the relationship between PIL architecture and interactions, glass formation behavior, and ion transport characteristics. These studies provide guidance towards the design of PILs with improved stability and ion conductivity for future energy applications.   

Ionic-Functionalized Polymeric Microporous Materials

Ionic-functionalized microporous materials are attractive for gas adsorption and separation applications. In this study, we investigate the effect of changing ions (Li$^{+}$, Na$^{+}$, K$^{+}$, Rb$^{+}$, and Mg$^{2+}$) on the porosity, carbon dioxide (CO$_{2}$) gas adsorption, and selectivity in ionic functionalized polymers of intrinsic microporosity (IonomIMs). Structure generation and gas adsorption are studied using molecular dynamics and Monte Carlo simulations respectively. The IonomIMs show an enhanced performance for CO$_{2}$ selectivity in CO$_{2}$$/$CH$_{4}$ and CO$_{2}$$/$N$_{2}$ gas mixtures at pressure swing adsorption and vacuum swing adsorption conditions. For 100\% ionic concentration, ions with the same charge show a decrease in the adsorption capacity with increasing cation size. Mg$^{2+}$ has the highest pure CO$_{2}$ adsorption and lowest mixed gas separation performance. The increasing concentration of ions decreases the porosity of the framework and increases the tunability of structural and adsorption properties. Hence, the concentration of ions, size, and charge play a vital role in determining the optimum adsorbent for a targeted industrial application.