Session X49: Charged and Ion-Containing Polymers

Role of Ion Size and Valency in Mechano-Electrical Energy Conversion of Flexo-Ionic Polymers

The increasing environmental concerns and global warming, attributed to conventional energy production, have motivated researchers to scout for clean energy harvesting (EH) strategy from pollutant-free and natural energy resources. Inspired by naturally occurring mechano-electroactive materials, flexo-ionic devices consisting of a flexible solid polymer electrolyte membrane (PEM), consisting of an ionic compound and a polymer matrix host, laminated between two compliant carbonaceous electrodes were developed. The laminated flexo-ionic PEMs can generate electrical potential and current via ion diffusion/polarization by subjecting the PEM assembly under mechanical deformation. By varying cation sizes and valencies of ionic compounds in the development of these PEMs, the effects of ion size and valency on flexoelectric properties (i.e. output voltage and current/polarization), ionic conductivity, viscoelastic and mechanical properties were investigated as a function of ion concentration and mode of mechanical stimuli involving intermittent square (low frequency) and dynamic oscillatory (high frequency) displacement. The synergic effect of these factors and their roles in mechano-electrical energy conversion will be discussed.   

Flexoelectricity in polymer electrolyte membranes: ‘Polarity switching’ in solid polymeric ion conductors

The present article describes a novel phenomenon of polarity flipping during mechano-electrical energy conversion subjected to bending deformation of flexible polymer electrolyte membranes (PEM), which were originally developed for polymeric ion conductors in solid Li-ion batteries. This bending induced electrical polarization is analogous to flexoelectricity found in insulators and dielectric materials. However, the basic principle of flexoelectricity in PEMs operates based on electrical energy generation driven by ion polarization/depolarization across the PEM subjected to a pressure (or stress) gradient during bending. The observed flexoelectric coefficient (~323 μC/m), i.e., a measure of the converted mechano-electrical energy, is the highest among all flexoelectric materials hitherto reported. Of particular importance is that the present work is the first to demonstrate the occurrence of polarity switching, i.e., reversal of the polarization direction with increasing succinonitrile (SCN) concentration, attributable to changing ionic size of solvated Li-SCN complexes.   

Coarse-grained simulation of lithium dendrite suppression by flowable polymer coating

Coating lithium anode with flowable polymer layer may effectively suppress lithium dendrite growth at high current density.^[1] Although the underlying mechanism is poorly explored, the sensitivity on dielectric permittivity and thickness of the polymer layer has been identified.^[2] A coarse-grained simulation of lithium deposition in presence of polymer coating, which explicitly incorporates the dielectric heterogeneity, is presented. It was found that the more effective coatings are more adaptable, have modest elasticity, and maintain integrity during deposition. Higher polymer dielectric permittivity and coating thickness inhibit the growth of dendrite, but inevitably sacrifice battery performance.
[1] Zheng et al., ACS Energy Lett., 2016, 1:1247.
[2] Lopez et al., J. Am. Chem. Soc., 2018, 140:11735.   

Ion Transport in Well-Aligned Block Copolymer Electrolytes

The independently tunable electrical and mechanical properties of block copolymer electrolytes (BCPEs) makes them attractive candidates for ion exchange membranes. However, these materials often show a lower conductivity even after accounting for the reduced volume fraction of the conducting phase. This reduction in conductivity has been ascribed to the tortuosity of the conducting pathways, poorly connected domains, grain boundaries, and low mobility near the BCP interface. It is often difficult to differentiate these nanoscale hinderances to transport in microns-thick membranes due to the inability to fully control the orientation of the BCP domains. Here, we present a new approach to this problem wherein we can fully align the conductive domains of a BCPE along the direction of the electric field produced by interdigitated electrodes (IDEs), thus enabling us to divorce morphological from molecular-level effects. We observe that a low mobility region near the block copolymer interface might explain the lower conductivity of BCPEs compared to an equivalent volume fraction of homopolymer electrolyte.   

Phase Behavior of Salt Doped A/B/A-B Ternary Polymer Blends

The phase prism of a pseudoternary polymer blend system containing low molar mass poly(ethylene oxide) (PEO) and polystyrene (PS) homopolymers, a PS−PEO block copolymer (SEO), and lithium bis(trifluoromethane)sulfonamide (LiTFSI) is constructed. The addition of salt increases the segregation strength, and thus causes both macroscopic and microscopic phase separation. The phase behavior of the ternary system is studied in detail, including isothermal slices at different temperatures and isopleths with different phi_PEO/phi_h ratios. Also, a bicontinuous microemulsion (BμE) channel is located, and the structure as a function of salt concentration and temperature is investigated. Moreover, the congruency condition near the BμE channel will be discussed. This work may serve as a benchmark for experimental design of polyelectrolyte systems with tunable ionic conductivity and high mechanical modulus and help understand the physics underlying the structure and dynamics of ion-containing A/B/A-B ternary blends.   

Detection of the Order-to-Disorder Transition in Block Copolymer Electrolytes Using Quadrupolar 7Li NMR Splitting

Mixtures of block copolymers and salts have been studied for use as safe electrolytes for batteries with energy-dense lithium metal anodes. Locating the order-to-disorder transition (ODT) in these systems is important, because microscopic morphology has a profound impact on bulk properties including ionic conductivity and mechanical rigidity. In this study, the ordered morphologies in a series of mixtures of polystyrene-b-poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are shown to be aligned due to exposure to the magnetic field in an NMR instrument, which is confirmed using small-angle X-ray scattering (SAXS). This alignment results in quadrupolar triplet peak splitting in ⁷Li NMR, which disappears above the T_ODT. The T_ODT identified using this novel NMR method is consistent with that determined using SAXS.   

Counterion condensation and ionic conductivity in microphase separated block copolymer electrolytes

Ion-conducting polymer electrolytes are central components to many types of electrochemical cells spanning batteries, fuel cells, and water purification units. Ionic conductivity is an important property of these materials as it controls the thermodynamic efficiency of electrochemical systems. Many studies correlate micro-structure attributes of polymer electrolytes to ionic conductivity. However, the impact of counterion condensation, a proxy for the extent and strength of ion-pairing, on ionic conductivity has received less attention and its impact on ionic conductivity is not clearly known. This talk highlights our effort to study counterion condensation in thin film ion-conducting block copolymer electrolytes with precisely defined microstructures. Ionic conductivity and 2D AFM force mapping, combined with all-atomistic molecular simulations, substantiate that microphase separated block copolymer electrolytes are less prone to counterion condensation resulting in lower resistances to ionic charge transport. These findings motivate future studies to probe counterion condensation in precisely defined block copolymer electrolyte microstructures as function of solvation and charge density.   

Compositionally Asymmetric Block Polyelectrolyte Morphologies

The delicate balance of (short ranged) enthalpic interactions and entropic factors direct the self-assembly in nonionic diblock copolymers (BCPs) which is exploited in a broad range of applications, e.g., lithography, energy storage, membrane separations, and optics. We demonstrate that such behavior is profoundly altered when one block carries a charged trisaminocyclopropenium (TAC) ion at each monomer. These charged-neutral copolymers (CNBPs) display strongly asymmetric morphology maps with the unique aspect that the minority component, the charged block, has a strong propensity to form the continuous matrix. Such observations, coupled with the unexpectedly low TAC dielectric constant (~2.5) lead us to postulate that the CNBP morphology is strongly modified by long-range electrostatics. This conjecture is verified by detailed geometric calculations and quantitatively captured by a simplistic scaling model derived from surfactant self-assembly principles. This fundamental insight into the role of strong electrostatics on CNBP self-assembly has attractive implications for ion transport in polymeric media while simultaneously improving their mechanical properties.   

Mechanisms of Ion Diffusion as a Function of Microstructure in Ionomer Melts

Understanding how the mechanisms and relative rates of ion transport depend on melt morphology and polymer architecture is a fundamental issue in ionomers. The low dielectric constant of the polymer backbone drives microscale aggregation of charged-groups and counterions, which can greatly impact ion conductivity. For some melts, aggregates can percolate and provide pathways for free ions to move independent of polymer dynamics; in other cases, isolated aggregates are favored and transport happens through infrequent collision-exchange events. We perform molecular dynamics simulations of coarse-grained ionomer melts that span these morphologies to understand charge mobility as a function of polymer architecture and background dielectric constant. For percolated networks, counterion diffusion depends weakly on these variables and results from discrete step motions along more static pathways of bound charged-groups. In contrast, charge mobility varies widely for systems that form clusters as the probability of ion exchanges is sensitive to the length scale and dielectric nature of the polymer-backbone regions between clusters. We explicitly measure the time scales of these distinct mechanisms and show that they directly underlie diffusion coefficients of the charged species.   

Structural and Dynamical Properties of Water and Salt ions in Confined Geometries of Block Copolymers

Understanding the transport properties of water and salt ions in confined geometries of block copolymers is crucial for water purification including others nanotechnology and biotechnology. Here, we investigate the local diffusivities along with supporting structural properties of water and salt (Na⁺ and Cl^-) ions in the different regions from the pore with varieties of sizes made by morphologies of liner triblock copolymers such as lamella, cylinder, and gyroid using dissipative particle dynamics (DPD) mesoscale simulations. Besides, we extend our study to investigate the water and salt permeability through the membranes. Our results suggest that diffusivities of water and salt ions in the different region of pores are different than in the bulk phases. Under the confined conditions, the mobility of water and salt ions are perturbed by the additional interaction forces that arise from the blocks that have preferable water selectivity, which ultimately reduces the local molecular diffusion. Further, the geometries of block copolymers with different sizes of pore also influence the structural and dynamical properties of water and salt ions.   

Phase Separation of Polymer Mixtures with Highly Concentrated Ions

The fluctuation of electrostatic potentials and the dielectric contrast between species often play a crucial role in the liquid-liquid phase separation of electrolytes, but the effect of correlation between the two on the phase behavior remains elusive. Accordingly, we develop a Ginzburg-Landau theory that simultaneously accounts for the two effects for liquid mixtures containing polymer and ionic liquid. Our theory suggests that both effects can be equally important to consider the trend of the phase boundary of the liquid-liquid phase separation involving highly concentrated ions. Moreover, our present results of ionic liquids compare favorably with those calculated by the Gaussian approximation (or the one-loop expansion) of self-consistent field theory. We also show that a comparison between theory and experiment suggests that the fluctuation effect may also be significantly suppressed in ionic liquids.<div class="grammarly-disable-indicator"> </div>   

Effects of Molecular Polarity and Polymerization on Ion Solvation in Polymer Melts

We study the solvation energy of ions in polymerized and non-polymerized dipolar solvents using molecular dynamics simulations. We use a coarse-grained Stockmayer fluid model to observe the effects on solvation energy for monovalent, divalent, and trivalent ions due to varying solvent dipole moments. We show that increasing polymer chain lengths leads to significantly more negative solvation energies of ions. This is because in polymerized solvents, chain connectivity leads to higher local packing fraction of solvent dipoles near the ion and hence a stronger dielectric response. Also, there is a substantial difference between simulation results and the predicted Born solvation energy using Onsager theory for dielectrics. We hypothesize that the compressibility of solvents and local dipolar structure are key to understanding the difference between simulated and predicted results.   

Peering into Phase-Separated Perfluorinated Sulfonic-Acid Ionomers with Energy-Tunable X-rays

Ion-containing polymers such as perfluorinated sulfonic-acid (PFSA) ionomers play a critical role as the ion-conducting electrolyte in various electrochemical energy devices such as polymer-electrolyte fuel cells, redox flow batteries, and solar fuel generators. The ion transport properties in PFSAs are controlled by the nanoscale phase-separation of hydrophilic and hydrophobic domains. Next-generation membranes leverage new chemistries that enable good conductivity without requiring high relative humidity. As with well-studied PFSAs like Nafion, understanding the structure-property relationships in these new materials is critical to optimizing performance. Here, synchrotron X-rays with energies tuned to the sulfonic-acid functional group provide enhanced contrast to reveal the phase separation in dry and wet PFSA membranes with various chemistries. These studies elucidate connections among molecular architecture, morphology and transport properties that can be used as design rules for ionomer membranes and highlight the capabilities of X-rays to understand structure and chemistry in ion-containing polymers in general.   

Macroion Complexation and Conformation of Neutral Polymer in Solution

Unusual electric charging and conformational behavior of neutral polymers, such as poly (ethylene oxide) (PEO), in polar solvents have been reported in the past. To further explore the charging mechanism, we compare the effect of simple small ions and multivalent macroions on the electrical potential and conformation of PEO in solutions of varied polarity. Measured electrical potential indicates that multivalent macroions can effectively bind with PEO to cause significant increase of the effective charges of a PEO chain, which is accompanied with the collapse of PEO chain in polar solvents as determined by single-molecule laser spectroscopy. In contrast, little change of PEO electrical potential and conformation is observed with lithium salt added solutions. More interestingly, as further increasing macroion concentration, unconventional liquid-liquid separating coacervate complexation is observed with macroion-added PEO solution, leading to a facile process of PEO-based ionomers.   

Diffusion of Charged Macromoelcules under Strong Electric Repulsion

The complicity of diffusive motion of individual charged macromolecules under strong electrostatic interaction has been puzzling for decades. As an effort to explore the mystery, diffusion of polyelectrolyte molecules is investigated at single molecular level. By dual-color fluorescence correlation spectroscopy, the diffusion of different sodium polystyrene sulfonate (NaPSS) molecules is discriminated and their mutual correlation demonstrates the strong coupling of the diffusion of different molecules, leading to the abnormal diffusion of individual NaPSS molecules. After the proper correction of the correlation function, the self-diffusion of NaPSS demonstrates a two-stage feature – a fast and slow diffusion modes. The results have exposed a new species in the multiple diffusion modes in polyelectrolyte solution.