Session A34: Confinement, Dynamics, and Ion Interactions in Ion-Containing Polymers I

Ion Transport Mechanisms in Ionomers

An understanding of dynamics in single-ion conducting polymers, such as ionomers with ionic groups covalently bonded to the polymer backbone, is needed to design these polymers for use as electrolytes. In melt ionomers, the ions tend to self-assemble into nanoscale ionic aggregates, and the morphology of these aggregates affects both the ion and chain dynamics. I will describe the ionic aggregate and ion dynamics in atomistic molecular dynamics (MD) simulations of precise polyethylene-based ionomers. In particular, the mechanisms for ion transport are the same as those found in previous coarse-grained MD simulations. In systems with isolated ionic aggregates, ions move through a process of aggregates merging, rearranging, and breaking up. In systems with percolated ionic aggregates, ions "shuttle" along the ionic aggregate. From new CG simulations, we find that the diffusion constant for ions in systems with percolated ionic aggregates is inversely proportional to an ion "stepping" time scale, which quantifies the cation-anion association lifetime.   

Confined polyelectrolyte solution driven by an external electric field

The transport properties of a dilute polyelectrolyte solution, confined inside a charged cylinder and driven by a constant external electric field, are studied using coarse-grained molecular dynamics simulations. The polyelectrolyte is negatively charged and modeled as linear bead-spring chains with explicit monovalent counterions and implicit solvent. We find that, when the confinement walls are negatively charged, the polyelectrolyte mobility is independent of the confinement charge density, whereas, when they are positively charged, the counterion mobility exhibits an intriguing non-monotonic dependence on the surface charge density. We study the dependence of the transport features on the diameter of the confinement, the presence of polarization effects, and on multivalent salts. We also study the mobility of the polyelectrolyte solution when the counterions of the charged surface are placed outside the confinement. In this case, the mobilities remain practically unaffected by the surface charge density, unless excluded volume effects become important.   

Ion Correlations and Transference Number in Model Polymer Electrolytes: Effects of Ion Size and Dielectric Strength

Salt-doped polymers have potential as safe electrolytes for batteries but suffer from low ion conductivity. Using bulky anions with delocalized charge may reduce ion agglomeration and increase conduction. However, size asymmetry between ions may increase preferential solvation of cations versus the larger anions, lowering the transference number t₊ (fraction of conductivity contributed by the cation). Here, we use coarse-grained molecular dynamics simulations, including a 1/r⁴ potential to capture size-dependent solvation effects, to relate polymer and ion chemistry to t₊ and overall conductivity. We calculate conductivity from ion mobilities in an external electric field, which improves accuracy versus the typical use of fluctuation dissipation relationships. We find that there is a discrepancy in t₊ estimated from ion diffusion constants and t₊ calculated from ion mobilities, especially at large ion size asymmetry or when ion-polymer interactions are strong. By understanding the impact of ion size, polarizability, and polymer dielectric strength on ion correlations, diffusion, and transference number, we aim to help guide design of future materials with improved conduction.   

Influence of Water Content and Morphology on Proton Transport in Biocompatible Conductive Polymer Membranes

Proton transport plays a critical role in many biological processes, including chemical signaling to cells within the human body. Devices that control the flow of protons to biological systems for medicinal and therapeutic purposes are known as bioprotonic devices, and they depend on conductive polymer membranes to facilitate proton transport. The development of bioprotonic devices demands proton-conducting polymers with suitable transport properties and biocompatibility. Herein, the results of theoretical investigations on the transport properties of candidate polymer membranes for bioprotonic devices are described. The effects of water content and morphology on proton transport in these candidate polymer systems are established. This provides a theoretical basis for the design optimization of bioprotonic devices that incorporate biocompatible proton-conducting polymer membranes.   

Effects of Homopolymer Additives on Conductivity of Salt-Doped Block Copolymers from Molecular Dynamics Simulations

As promising solid electrolyte materials for batteries, salt-doped block copolymers (BCP) simultaneously have ionic conductivity and mechanical robustness, due to the combination of two distinct polymer types. Recent experiments revealed that homopolymer additives, at high enough molecular weight (MW), can increase ionic conductivity in such systems. To understand this effect and guide further study, we employ coarse-grained molecular dynamics (MD) simulations. We analyze the microphase distribution of added homopolymers in salt-doped BCPs and relate this to the overall dynamics and ion conductivity. In particular, we find that longer homopolymers form an interlayer in the middle of the conducting microphase, while shorter chains are more fully integrated across the conducting microphase. This leads to a greater degree of overlap of the ion and homopolymer density profiles with increasing homopolymer MW, which may explain the increased conduction with MW. Ion concentration, segregation strength, and ion solvation energy also affect the ion distribution and can potentially be adjusted to enhance ionic conductivity.   

Can solvation free energy rationalize the phase behavior of ion-doped copolymers?

Previous experimental studies have established the pronounced effects of salt-doping on the morphological behavior of copolymers. The ionic solvation free energy has been recognized as an important factor for this doping effects, which has been encapsulated into an effective χ parameter that scales linearly with salt concentration. However, no general agreement between experimental and theoretial phase diagrams has been reached. By employing a recently developed ‘free’ ion model for salt-doped copolymers and cultivating the rich experimental data in literature, we show that, for two different molecular weights and over a range of compositions, the morphological varications can be captured by the solvation free energy alone, if the solvation radius is treated as an adjustable parameters.   

Densely Grafted Polyelectrolyte Brushes Trigger “Water-in-Salt” like Scenarios and Ultraconfinement Effect

Polyelectrolyte (PE) brushes find use in a plethora of applications like current rectification, ion sensing, biosensing, nanoscale energy conversion etc. We carry out all-atom Molecular Dynamics (MD) simulations of fully ionized PE brushes for various degrees of polymerization and grafting densities. Our brush height results show an excellent match with the existing scaling laws of the non-linear osmotic brush regime. We observe a most remarkable ultraconfinement effect created by the brushes, quantified by orders of magnitude reduction in the mean squared displacement (MSD) of counterions and water molecules. Moreover, we observe that within the PE brushes, the counterion – PE brush complex supersedes the water molecules by both mass and volume above a critical grafting density. This gives rise to a hitherto unknown “water-in-salt” like behavior inside the brushes, with the counterions acting as the cations and the PE brush repeating units acting as the anions. Our calculations reveal a significant lowering of the dielectric constant of water within the brushes due to strong electrostatic binding with the negatively charged PE’s.   

Ion Transport in Pendant and Backbone Polymerized Ionic Liquids

Polymerized ionic liquids (PILs) are single ion conductors, in which one of the ionic species is incorporated in the polymer chain while the other is nominally free to be transported. PILs are attractive as electrolytes in battery applications and other areas where liquid electrolytes are undesirable. In PILs, the ionic species can either be directly incorporated into the polymeric backbone (backbone PILs (B-PILs)) or placed as pendant groups to the chain (pendant PILs (P-PILs)). Here, we examined the ion transport, morphology, and dynamics of imidazolium-based pendant and backbone PILs with TFSI, CPFSI, and NfO counter-anions. We found that P-PILs yielded higher ionic conductivity when scaled to Tg, but B-PILs exhibited higher ionic conductivity on an absolute temperature scale, likely because of differences in the Tgs of the two systems. We also found that ion transport for B-PILs was coupled to the segmental dynamics below Tg, where the decoupling of ionic conductivity from segmental relaxation was observed for P-PILs. This study gleans insight on relating conductivity of equivalent backbone and pendant PIL structures to morphology, leading to a deeper understanding on the fundamental relationship between conductivity and morphology in PILs.   

Tailoring Ion Transport Properties of Block Copolymer Electrolytes with End-functionalized Homopolymer Addition

Block copolymer electrolytes based on poly(ethylene oxide) (PEO) have been regarded as promising candidates for solid-state lithium batteries, attributed to the good lithium salt-solvating ability of PEO chains and their high ionic conductivity. However, the inherent crystallinity of PEO is tied to a drastic reduction in ionic conductivity at room temperature, limiting its uses in practically viable batteries. In this study, we report a new means of controlling PEO crystallinity of block copolymers by blending end-group functionalized PEO homopolymers. It has been found that the ionic conductivity and morphology of the resultant blends are related to the type and number of terminal moieties in embedded PEO homopolymers. This result suggests that chain folding of PEO is largely modulated by the end-group-driven intermolecular interactions in PEO phases. It has further shown that ion transport properties of blend electrolytes can be improved by this blend approach.   

Mechanisms of Ion Transport in Polymeric Ionic Liquids

Polymeric ionic liquids (PILs) are an emerging class of materials which combines the attractive properties of ionic liquids with the sequence complexity and mechanical characteristics of macromolecules. While significant advances have occurred in the context of synthesis and characterization of such materials, comparatively less understanding exists on the mechanisms underlying ion transport in such materials. In this talk, I discuss some recent developments in the context of PILs trelating to the issue of ion transport in such materials.We focus on the mechanisms of ion transport in such materials, the influence of counterions, and whether such materials do live upto the promise of hgh transference numbers.
  

Quantifying Intrinsic Interfacial Transport Properties in Block Copolymer Electrolytes

Nanostructure-forming block copolymer electrolytes are of great interest related to their application towards a variety of electrochemical devices. However, questions remain about the nature of ion transport through these nanostructured materials. Specifically, decoupling extrinsic structural effects of tortuosity and grain boundaries from intrinsic phenomena occurring near the block copolymer domain interface has been challenging. This is due to the difficulty in precisely controlling or quantifying the film structure. Here, we present a new platform to probe defect-free single grains of any block copolymer electrolyte. We specifically focus on a model system of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with LiTFSI to quantitatively demonstrate that interfacial mixing is the predominant factor in reducing ionic mobility near the interface. Using SCFT calculations we directly correlate the interfacial width to reduced ionic conductivity as a function of lithium salt concentration. These findings are supported by atomistic simulations, which give further insight into the exact mechanisms by which this mixing layer restricts ion motion.
  

Multifunctional polymer electrolyte networks for energy harvesting and storage

A novel polymer electrolyte membrane (PEM) was developed based on poly(ethylene imine) (PEI)-co-polyethylene glycol diglycidyl ether (PEGDGE) network, containing lithium tri(fluoromethane sulphonyl) imide (LiTFSI) salt and succinonitrile (SCN) plasticizer. The polymer electrolyte multifunctional-networks have numerous advantages by virtue of the ion-dipole complexation facilitating facile ion conduction, multi-hydrogen bonding for self-healing to prevent electrode cracking, and covalently bonded networks to afford mechanical support. The tensile strength, modulus, and elongation-to-break of the PEI-co-PEGDGE/SCN/LiTFSI PEM films are all high, suggestive of highly flexible and stretchable nature. A wide electrochemical stability window of -0.5 ~ 5.1 V was achieved. In the symmetric Li/PEM/Li cell tests, dendrite formation of lithium crystal was discerned after cycling for 720 h, which motivates us to investigate the mechanism of Li⁺ dendritic growth through the solid PEM network and develop strategy for preventing such dendritic growth during charge/discharge cycling. The same polymer network showed mechanoelectrical response to bending, exhibiting a flexoelectric coefficient of ~190 μC/m due to polarization/depolarization of oppositely charge dipoles, and dissociated ions.   

Low-Voltage Reversible Electro-Adhesion of Ionoelastomer Junctions

An ionoelastomer is a soft and liquid-free ion conducting network formed by polymerization of an ionic liquid monomer and crosslinker into an elastomer network, such that one ion species is anchored by the network while the other species is mobile. An ‘ionic double layer’ (IDL) is formed at the interface between two oppositely charged ionoelastomers, analogous to the depletion (or space charge) layer formed at a p/n junction of electronic semiconductors. Here, we investigate how the voltage drop across the IDL can be modulated to reversibly control the adhesion between two ionoelastomers. The large electric field developed across the IDL allows for strong adhesion at potentials as low as ~ 1 V, while conventional dielectric electro-adhesives typically require much higher operating voltages (> 1 kV). These ionoelastomer electro-adhesives are also more efficient with regard to force capacity per electrostatic energy, and robust to defects or damage. Our findings provide new fundamental insight into low voltage electro-adhesion and broaden its possible applications.