Role of Tethered Ion Placement on Polymerized Ionic Liquid Structure and Conductivity: Pendant versus Backbone Charge Placement

Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes

Transport of ions through solid polymeric electrolytes (SPEs) involves a complicated interplay of ion solvation, ion-ion interactions, ion-polymer interactions, and free volume. Nonetheless, prevailing viewpoints on the subject promote a significantly simplified picture, likening ion transport in a polymer to that in an unstructured fluid at low solute concentrations. Although this idealized liquid transport model has been successful in guiding the design of homogeneous electrolytes, structured electrolytes provide a promising alternate route to achieve high ionic conductivity and selectivity. In this perspective, we begin by describing the physical origins of the idealized liquid transport mechanism and then proceed to examine known cases of decoupling between the matrix dynamics and ionic transport in SPEs. Specifically we discuss conditions for "decoupled" mobility that include a highly polar electrolyte environment, a percolated path of free volume elements (either through structured or unstructured channels), high ion concentrations, and labile ion-electrolyte interactions. Finally, we proceed to reflect on the potential of these mechanisms to promote multivalent ion conductivity and the need for research into the interfacial properties of solid polymer electrolytes as well as their performance at elevated potentials.

Cross-Linking and Evaluation of the Thermo-Mechanical Behavior of Epoxy Based Poly(ionic Liquid) Thermosets

Poly(ionic liquids) (PILs) and ionenes are polymers containing ionic groups in their repeating units. The unique properties of these polymers render them as interesting candidates for a variety of applications, such as gas separation membranes and polyelectrolytes. Due to the vast number of possible structures, numerous synthesis protocols to produce monomers with different functional groups for task-specific PILs are reported in literature. A difunctional epoxy-IL resin was synthesized and cured with multifunctional amine and anhydride hardeners and the thermal and thermomechanical properties of the networks were assessed via differential scanning calorimetry and dynamic mechanical analysis. By the selection of suitable hardeners, the glass transition onset temperature (Tg,onset) of the resulting networks was varied between 18 °C and 99 °C. Copolymerization of epoxy-IL with diglycidyl ether of bisphenol A (DGEBA) led to a further increase of the Tg,onset. The results demonstrate the potential of epoxy chemistry for tailorable PIL networks, where the hardener takes the place of the ligands without requiring an additional synthesis step and can be chosen from a broad range of commercially available compounds.

Coarse-grained simulations of ionic liquid materials: from monomeric ionic liquids to ionic liquid crystals and polymeric ionic liquids

Ionic liquid (IL) materials are promising electrolytes with striking physicochemical properties for energy and environmental applications. Heterogeneous structures and transport quantities of monomeric and polymeric ILs are intrinsically intercorrelated and span multiple spatiotemporal scales, which is more feasible for coarse-grained (CG) simulations than atomistic modelling. Herein we constructed a novel CG model for ethyl-imidazolium tetrafluoroborate ILs with varied cation alkyl chains ranging from C₂ to C₂₀, and the interaction parameters were validated against representative static and dynamic properties that were obtained from atomistic reference simulations and experimental characterizations at relevant thermodynamic states. This CG model was extended to study thermotropic phase behaviors of monomeric ILs and to explore ion association structures and ion transport quantities in polymeric ILs with different architectures. A systematic analysis of structural and dynamical quantities identifies an evolution of liquid morphology from homogeneous to nanosegregated structures and then a smectic mesomorphism via a gradual lengthening of cation alkyl chains, and thereafter a distinct structural transition characterized by a monotonic decrease in orientational and translational order parameters in a sequential heating cascade. Backbone and pendant polymeric ILs exhibit evident anion association structures with cation monomers and polymer chains, and striking intra- and interchain coordinations between cation monomers owing to an intrinsic polymer architecture effect. Such a peculiar ion pairing association leads to a progressive increase in anion intrachain hopping probabilities, and a concomitant decrease in anion interchain hopping events with a gradual lengthening of polymeric ILs. The anion diffusivities in polymeric ILs are intrinsically correlated with ion pairing association lifetimes and ion structural relaxation times via a universal power law correlation D ∼ τ^-1, irrespective of polymer architectures.

Aqueous Formulation of Concentrated Semiconductive Fluid Using Polyelectrolyte Coacervation

Conjugated polyelectrolytes (CPEs), which combine π-conjugated backbones with ionic side chains, are intrinsically soluble in polar solvents and have demonstrated tunability with respect to solution processability and optoelectronic performance. However, this class of polymers often suffers from limited solubility in water. Here, we demonstrate how polyelectrolyte coacervation can be utilized for aqueous processing of conjugated polymers at extremely high polymer loading. Sampling various mixing conditions, we identify compositions that enable the formation of complex coacervates of an alkoxysulfonate-substituted PEDOT (PEDOT-S) with poly(3-methyl-1-propylimidazolylacrylamide) (PA-MPI). The resulting coacervate is a viscous fluid containing 50% w/v polymer and can be readily blade-coated into films of 4 ± 0.5 μm thick. Subsequent acid doping of the film increased the electrical conductivity of the coacervate to twice that of a doped film of neat PEDOT-S. This higher conductivity of the doped coacervate film suggests an enhancement in charge carrier transport along PEDOT-S backbone, in agreement with spectroscopic data, which shows an enhancement in the conjugation length of PEDOT-S upon coacervation. This study illustrates the utilization of electrostatic interactions in aqueous processing of conjugated polymers, which will be useful in large-scale industrial processing of semiconductive materials using limited solvent and with added enhancements to optoelectronic properties.

Influence of Charge Fraction on the Phase Behavior of Symmetric Single-Ion Conducting Diblock Copolymers

A series of symmetric poly[(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) propyl sodium sulfonate methacrylate)]-block-polystyrene (PsOEGMA-PS) diblock copolymers were synthesized as a model system to probe the effect of charge fraction on the phase behavior of charged-neutral single-ion conducting diblock copolymers. Small-angle X-ray scattering (SAXS) experiments showed that increasing the charge fraction does not alter the ordered phase morphology (lamellar) but increases the order-disorder transition temperature (T_ODT) significantly. Additionally, the effective Flory-Huggins interaction parameter (χ_eff) was found to increase linearly with the charge fraction, similar to the case of conventional salt-doped diblock copolymers. This indicates that the effect of counterion solvation, attributed to the significant mismatch between the dielectric constant of each block, provides the dominant effect in tuning the phase behavior of this charged diblock copolymer. We therefore infer that electrostatic cohesion (local charge ordering induced by Coulombic interactions), which is predicted to suppress microphase separation and lead to asymmetric phase diagrams, only plays a minor role in this model system.

Quantitative Evidence of Mobile Ion Hopping in Polymerized Ionic Liquids

Atomistic molecular dynamics simulations were performed, and an extensive set of analyses were undertaken to understand the ion transport mechanism in the polymerized ionic liquid poly(C₂VIm)Tf₂N. The ion hopping events were investigated at different time scales. Ion hopping was examined by monitoring the instantaneous cation-anion association and dissociation. Ion diffusion was subsequently evaluated with correlation functions and the calculation of relaxation times at different time scales. Dynamical heterogeneity in the mobility of the ions was observed with only a small portion of the anions classified as fast mobile ions. The mobile ions were characterized as the ones traveling farther than a certain distance during a characteristic period, which was much longer than the time scale of the instant ion pair dissociation. Effective hopping of the mobile ions contributed to the diffusivity which was dominated by interchain hopping and generally facilitated with five associating cations from two different polymer chains. Mobile anions had relatively fewer associating cations from more associating chains than immobile anions. The stringlike cooperative motion was observed in the mobile anions. The string length was determined to decrease with increasing temperature. These findings provided an in-depth understanding of the ion transport in polymerized ionic liquids and important information for the rational design of novel materials.

Facile fabrication of robust gel poly(ionic liquid) electrolytes via base treatment at room temperature

This article reports a facile fabrication of robust gel poly(ionic liquid) electrolytes via base treatment.

Critical Role of Ion Exchange Conditions on the Properties of Network Ionic Polymers

Ionic polymers are important in a wide range of applications and can exhibit widely different properties depending on the ionic species. In the case of single ion conducting polymers, where one charge is attached to the backbone or as a side group, ion exchange is performed to control the mobile species. While the conditions are often specified, the final ion content is not always quantified, and there are no clear criteria for what concentration of salt is needed in the exchange. A series of ammonium network ionic polymers with different precise carbon spacers (C4-C7) between ionic junctions were synthesized as model systems to understand how the ion exchange conditions impact the resultant polymer properties. The initial networks with free bromide anions were exchanged with 1.5, 3, or 10 equiv of lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt in solution. For networks with seven carbons between cross-links, increasing the LiTFSI concentration led to an increase in ion exchange efficiency from 83.6 to 97.6 mol %. At the highest conversion, the C7 network showed a 4 °C decrease in glass transition temperature (T_g), a 50 °C increase in degradation temperature, 12-fold lower water uptake from air, and a greater than 10-fold increase in conductivity at 90 °C. These results illustrate that properties such as T_g are less sensitive to residual ion impurities, whereas the conductivity is highly dependent on the final exchange conversion.

Nonmonotonic Influence of Size of Quaternary Ammonium Countercations on Micromorphology, Polarization, and Electroresponse of Anionic Poly(ionic liquid)s

The size influence of quaternary ammonium countercations in poly[4-styrenesulfonyl(trifluoromethylsulfonyl)imide][tetraalkylammonium] (P[STFSI][Nnnnn], n = 1, 2, and 3) poly(ionic liquid)s on dielectric polarization and the stimuli-responsive electrorheological effect is investigated by dielectric spectroscopy and rheology, and the microstructure-level understanding behind the influence is analyzed by Raman and X-ray scattering spectra. The size influence of quaternary ammonium cations is found to be nonmonotonic. The largest electrorheological effect accompanied by best polarization properties is demonstrated in P[STFSI][N2222]. Raman spectra and activation energy measurements demonstrate that the nonmonotonic influence originates from the fact that, compared to small N1111⁺ and large N3333⁺, intermediate N2222⁺ as countercations can contribute a higher mobile ion number and lower activation energy barrier of ion dissociation and motion. But the experimental values of activation energy are not consistent with theoretically calculated values by considering the ion pair electrostatic potential and elastic force contribution of the matrix. By X-ray scattering and diffraction characterizations, it is clarified that the nonmonotonic influence and the inconsistency of activation energy originate from the size influence of Nnnnn⁺ on the micromorphology of P[STFSI][Nnnnn]. Compared to the semicrystalline structure of P[STFSI][N1111] and the ionic aggregation structure of P[STFSI][N3333], the relatively uniform amorphous structure of P[STFSI][N2222] may be responsible for its lower activation energy barrier of ion motion. This study further provides insights into the design and preparation of future poly(ionic liquid)-based electrorheological materials by considering not only molecular structure but also micromorphology.

Role of Multivalent Interactions in Dynamic-Template-Directed Assembly of Conjugated Polymers

Dynamic-template-directed assembly is a promising method to enhance molecular ordering and electronic properties of solution-coated polymer semiconductor thin films over a large area. In this work, we establish that multicomponent dynamic templates of complementary chemistries can promote polymer crystallization through cooperative multivalent interactions. We investigate this phenomenon using a combination of templating substrates including a fluoropolymer, a hydrogen-bonded liquid, and an ionic liquid (IL). Template-dependent multiscale morphology is studied by a comprehensive set of characterization techniques to understand how introducing diverse chemical moieties modulates polymer assembly. Our results clearly confirm synergistic effects between components of complementary chemistries constituting the dynamic template. The relative degree of crystallinity is improved by 50-150% for films deposited on multicomponent dynamic templates compared to their neat constituents. In addition, macroscopic alignment is increased significantly (2-5 times) compared to single-component templates. As a result, highly anisotropic charge transport is observed with apparent hole mobilities up to 3.6 cm² V^-1 s^-1. In contrast, such a synergistic effect is not observed when using a multicomponent dynamic template of comparable chemistries (i.e., IL and polymerized IL). We elucidate the origin of this synergistic effect by using attenuated total reflectance Fourier transform infrared spectroscopy and isothermal titration calorimetry. When the dynamic template comprises two or more components interacting with complementary binding sites on the conjugated polymer (CP) (esp. backbone vs side chain), the template-polymer interactions is significantly enhanced compared to the sum of single component contributions. These results provide valuable insights into surface-directed CP crystallization during large-area solution coating. Template dynamics is rarely studied and represents a new opportunity for guiding assembly of soft functional matter.

Ion transport in backbone-embedded polymerized ionic liquids

We use atomistic computer simulations to examine ion-transport phenomena for backbone polymerized cationic liquids with bistrifluoromethylesulfonylimide (TFSI^-) counterions. We consider a system in which the polymerized cation moiety is the imidazolium ring and study the structural characteristics and ion mobilities for cases in which the cations are separated by four, six, and eight methylene units on the backbone. A pendant polymerized ionic liquid, 1-butyl-3-vinylimidazolium, is compared to the backbone series across ion coordination and hopping features. The anion diffusivity in backbone polymerized cationic liquids is found to decrease with increasing spacer length, which is shown to result from a decrease in intramolecular and intermolecular hopping frequencies due to an increasing distance separating imidazolium moieties. In comparison with pendant polymerized ionic liquids, we observe that the participation rates of intermolecular hopping events in the backbone polymers far exceed that of the pendant, and the intrapolymeric ionic coordination profile shows the TFSI^- of the pendant polymer with a high propensity for coordination by multiple imidazolium, compared with one monomer from a given polymer for the backbone series. Despite these differences, backbone polymerized ionic liquids are seen to possess correlated diffusivity and ion-association relaxation times, in a manner similar to the results observed in past studies for pendant variants.

Effect of Network Architecture and Linker Polarity on Ion Aggregation and Conductivity in Precise Polymerized Ionic Liquids

Four polymerized ionic liquids (PILs) were systematically designed to study the effect of polymer architecture and linker polarity on ion aggregation and transport. Specifically, linear and network PILs with the same ammonium cations (Am) and bis(trifluoromethane)sulfonimide (TFSI) anions were prepared by step-growth polymerization, and polarity was tuned by incorporating two precise linkers, either polar tetra(ethylene oxide) (4EO) linker or nonpolar undecyl (C11) linker. The glass transition temperature (T_g) substantially increased with the nonpolar C11 linker or upon cross-linking to form a network. The low wave-vector (q) ion aggregation peak from wide-angle X-ray scattering (WAXS) was not observable in the linear 4EO PIL, while it was most pronounced in the network C11 PIL. The network C11 PIL exhibited the strongest decoupling, where the ionic conductivity at T_g is greater than 1 order of magnitude higher than the other PILs. This systematic comparison suggests that network structure and nonpolar linkers can promote both ion aggregation and ionic conductivity close to T_g.

Healable, Highly Conductive, Flexible, and Nonflammable Supramolecular Ionogel Electrolytes for Lithium-Ion Batteries

High-performance solid-state electrolytes with healability to repair mechanical damages are important for the fabrication of Li-ion batteries (LIBs) with enhanced safety and prolonged service life. In this study, we present the fabrication of healable, highly conductive, flexible, and nonflammable ionogel electrolytes for use in LIBs by loading ionic liquids and Li salts within a hydrogen-bonded supramolecular poly(ionic liquid) copolymer network. The ionogel electrolytes exhibit ionic conductivities as high as 10^-3 S/cm, which is comparable to the conventional liquid electrolytes. The Li/LiFePO₄ battery assembled with the ionogel membrane exhibits excellent cycling performance and delivers a steady high discharge capacity of 147.5 mA h g^-1 and Coulombic efficiency of 99.7% after 120 cycles at the charge/discharge rate of 0.2 C. Importantly, the ionogel membranes can heal damages outside or inside a battery because of the reversible nature of the supramolecular interactions between the components. The damaged ionogel membranes after being healed can effectively restore the original performance of the LIBs.

Effect of Polymerized Ionic Liquid Structure and Morphology on Shockwave Energy Dissipation

The ability of nanosegregated polymerized ionic liquids (PILs) to dissipate shockwave energy is investigated for a series of imidazolium-based PILs with varying alkyl spacer length. The PILs are designed to have similar glass transition temperatures but different structures. X-ray scattering analysis reveals that each of the amorphous PILs exhibit distinct nanoscale structural heterogeneity, depending on the length of the chain spacer. We find that a higher structural heterogeneity, determined from the intensity of the intercluster scattering peak, in the PILs with longer alkyl spacers results in greater shockwave energy dissipation. In addition, we observe the crystalline phase is less effective at dissipating shockwave energy than the amorphous phase due to the close packed morphology and slow kinetics.

Impact of building block structure on ion transport in cyclopropenium-based polymerized ionic liquids

Cation geometry, size, and polarity all contribute to conductivity in PS-TAC PILs, with highest conductivity from the isopropyl cation geometry.

Covalently Crosslinked 1,2,3-Triazolium-Containing Polyester Networks: Thermal, Mechanical, and Conductive Properties

Azide-alkyne "click" cyclization was used to prepare a series of polymerizable acetoacetate monomers containing a 1,2,3-trizolium ionic liquid group. The monomers were subsequently polymerized using base-catalyzed Michael addition chemistry, producing a series of covalently crosslinked 1,2,3-triazolium poly(ionic liquid) (TPIL) networks. Structure-activity relationships were conducted to gauge how synthetic variables, such as counteranion ([Br], [NO3], [BF4], [OTf], and [NTf2]), and crosslink density (acrylate/acetoacetate ratio) effected thermal, mechanical, and conductive properties. TPIL networks were found to exhibit ionic conductivities in the range of 10-6-10-9 S/cm (30 °C, 30% relative humidity), as determined from dielectric relaxation spectroscopy, despite their highly crosslinked nature. Temperature-dependent conductivities demonstrate a dependence on polymer glass transition, with free-ion concentrations impacted by various ions' Lewis acidity/basicity and ion mobilities impacted by freely mobile anion size.

Syntheses, characterizations and functions of cationic polyethers with imidazolium-based ionic liquid moieties

Cationic polyethers with ionic liquid groups are characterized with deliquescence, ionic conductivity and miscibility in ionic liquid.

Polymerized Ionic Liquids: Correlation of Ionic Conductivity with Nanoscale Morphology and Counterion Volume

The impact of the chemical structure on ion transport, nanoscale morphology, and dynamics in polymerized imidazolium-based ionic liquids is investigated by broadband dielectric spectroscopy and X-ray scattering, complemented with atomistic molecular dynamics simulations. Anion volume is found to correlate strongly with T_g-independent ionic conductivities spanning more than 3 orders of magnitude. In addition, a systematic increase in alkyl side chain length results in about one decade decrease in T_g-independent ionic conductivity correlating with an increase in the characteristic backbone-to-backbone distances found from scattering and simulations. The quantitative comparison between ion sizes, morphology, and ionic conductivity underscores the need for polymerized ionic liquids with small counterions and short alkyl side chain length in order to obtain polymer electrolytes with higher ionic conductivity.

Metal-Containing Poly(ionic liquid) Membranes for Antibacterial Applications

Imidazolium-type metal-containing ionic liquid (IL) monomers and their corresponding poly(ionic liquid) (PIL) membranes coordinated with CuCl₂ (PILM-Cu), FeCl₃ (PILM-Fe), or ZnCl₂ (PILM-Zn) were synthesized. The effect of metal ions on the antimicrobial activities against both Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) was investigated. Compared with pristine PILM-Br membrane, PILM-Cu, PILM-Fe, and PILM-Zn membranes exhibit enhanced antibacterial activities due to the attributes of both imidazolium cations and metal-containing anions. Furthermore, all of the metal-containing PIL membranes present low hemolysis toward human red blood cell and high long-term antibacterial stability, even after immersion in water for 90 days, demonstrating clinical feasibility in topical applications.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Enhanced Ionic Conductivity of a 1,2,3-Triazolium-Based Poly(siloxane ionic liquid) Homopolymer

A 1,2,3-triazolium-based poly(siloxane ionic liquid) (PSIL) is synthesized by UV-triggered thiol-ene ligation between a poly[(mercaptopropyl)methylsiloxane] and a tailor-made vinyl-functionalized triethylene glycol-based 1,2,3-triazolium ionic liquid. The quantitative nature of the thiol-ene coupling is demonstrated by ¹H and ¹³C NMR, whereas properties of this new PSIL are discussed based on solubility, size exclusion chromatography, differential scanning calorimetry, thermogravimetric analysis, and broadband dielectric spectroscopy measurements. Besides exhibiting low glass transition temperature (T_g = -62 °C) and high thermal stability (T_d10 = 284 °C), this new class of poly(1,2,3-triazolium) demonstrates the highest value of bulk anhydrous ionic conductivity reported to date for PILs (σ_DC = 7 × 10^-5 S cm^-1 at 30 °C).