Influence of Added Salt on Chain Conformations in Poly(ethylene oxide) Melts: SANS Analysis with Complications

Metal-Halogen Interactions Inducing Phase Separation for Self-Healing and Tough Ionogels with Tunable Thermoelectric Performance

Ionic liquid-based thermoelectric gels become a compelling candidate for thermoelectric power generation and sensing due to their giant thermopower, good thermal stability, high flexibility, and low-cost production. However, the materials reported to date suffer from canonical trade-offs between self-healing ability, stretchability, strength, and ionic conductivity. Herein, a self-healing and tough ionogel (PEO/LiTFSI/EmimCl) with tunable thermoelectric properties by tailoring metal-halogen bonding interactions, is developed. Different affinities between polymer matrix and salts are exploited to induce phase separation, resulting in simultaneous enhancement of ionic conductivity and mechanical strength. Molecular dynamics (MD) simulations and spectroscopic analyses show that Cl- ions impair the lithium-ether oxygen coordination, leading to changes in chain conformation. The migration difference between cations and anions is thus widened and a transition from n-type to p-type thermoelectric ionogels is realized. Furthermore, the dynamic interactions of metal-ligand coordination and hydrogen bonding yield autonomously self-healing capability, large stretchability (2000%), and environment-friendly recyclability. Benefiting from these fascinating properties, the multifunctional PEO-based ionogels are applied in sensors, supercapacitors, and thermoelectric power generation modules. The strategy of tuning solvation dominance to address the trade-offs in thermoelectric ionogels and optimize their macroscopic properties offers new possibilities for the design of advanced ionogels.

Threading Subunits for Polymers to Predict the Equilibrium Ensemble of Solid Polymer Electrolytes

We present a computational method for polymer growth called "threading subunits for polymers (TSP)" that can efficiently sample solid polymer electrolyte structures with extended conformations. The TSP method involves equilibrating subunit (e.g., monomer) conformations that form favorable solvation ion shells, followed by consecutively connecting the subunits and minimizing the structures. The TSP method can sample polymers with good solvent-like conformations and from near-equilibrium structures in which ions are well-dispersed, avoiding unusual ion clustering under ambient conditions. Using the TSP method, the equilibration time can be reduced significantly by effectively sampling the polymer conformations near equilibrium. We anticipate that the TSP method can be applied to simulate various polymer electrolytes.

Fluorinated amphiphilic Poly(β-Amino ester) nanoparticle for highly efficient and specific delivery of nucleic acids to the Lung capillary endothelium

Endothelial cell dysfunction occurs in a variety of acute and chronic pulmonary diseases including pulmonary hypertension, viral and bacterial pneumonia, bronchopulmonary dysplasia, and congenital lung diseases such as alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV). To correct endothelial dysfunction, there is a critical need for the development of nanoparticle systems that can deliver drugs and nucleic acids to endothelial cells with high efficiency and precision. While several nanoparticle delivery systems targeting endothelial cells have been recently developed, none of them are specific to lung endothelial cells without targeting other organs in the body. In the present study, we successfully solved this problem by developing non-toxic poly(β-amino) ester (PBAE) nanoparticles with specific structure design and fluorinated modification for high efficiency and specific delivery of nucleic acids to the pulmonary endothelial cells. After intravenous administration, the PBAE nanoparticles were capable of delivering non-integrating DNA plasmids to lung microvascular endothelial cells but not to other lung cell types. IVIS whole body imaging and flow cytometry demonstrated that DNA plasmid were functional in the lung endothelial cells but not in endothelial cells of other organs. Fluorination of PBAE was required for lung endothelial cell-specific targeting. Hematologic analysis and liver and kidney metabolic panels demonstrated the lack of toxicity in experimental mice. Thus, fluorinated PBAE nanoparticles can be an ideal vehicle for gene therapy targeting lung microvascular endothelium in pulmonary vascular disorders.

Polyether-based solid electrolytes with a homogeneous polymer network: effect of the salt concentration on the Li-ion coordination structure

We report a solid polymer electrolyte with an ideal polyether network that was synthesized by using tetra-functional poly(ethylene glycol) (TetraPEG) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt. The solid TetraPEG electrolyte had few network defects (<5%) and exhibited high mechanical toughness by enduring approximately 11-fold elongation at a 1 : 10 ratio of Li salt to O atoms of PEG (Li/O_PEG). We found that the mechanical properties strongly depend on the Li/O_PEG ratio, which mainly contributes to the density of crosslinking points in the electrolyte. Raman spectroscopy and high-energy X-ray total scattering were used with all-atom molecular dynamics simulations to visualize the structural effects of Li-ion coordination in the TetraPEG network. At lower salt contents (Li/O_PEG = 1 : 10), Li ions were found to preferentially coordinate with O_PEG atoms rather than the TFSA anions to form crown ether-like Li⁺-PEG complexes as ion pair-free species. With increasing salt content, the TFSA anions partially coordinated with Li ions through O atoms of TFSA (O_TFSA) to afford contact ion pairs surrounded by both O_PEG and O_TFSA atoms. Finally, the ion pairing enhanced mononuclear ion pairs as well as multinuclear ionic aggregates when more Li salt was added. This structural change in the Li-ion complexes was directly reflected by the ion-conducting properties of the electrolyte. The TetraPEG electrolyte composed of the ion pair-free Li⁺ species (Li/O_PEG = 1 : 10) exhibited higher ionic conductivity, and the conductivity gradually decreased with increasing salt content because of extensive ion pairing for both mononuclear contact ion pairs and multinuclear aggregates. Regarding the electrochemical properties, the optimum electrolyte composition to realize a reversible Li deposition/dissolution reaction for a negative electrode was found to be Li/O_PEG = 1 : 4.

Polycation radius of gyration in a polymeric ionic liquid (PIL): the PIL melt is not a theta solvent

The conformation of the polycation in the prototypical polymeric ionic liquid (PIL) poly(3-methyl-1-aminopropylimidazolylacrylamide) bis(trifluoromethylsulfonyl)imide (poly(3MAPIm)TFSI) was probed using small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) at 25 °C and 80 °C. Poly(3MAPIm)TFSI contains microvoids which lead to intense low q scattering that can be mitigated using mixtures of hydrogen- and deuterium-rich materials, allowing determination of the polycation conformation and radius of gyration (R_g). In the pure PIL, the polycation adopts a random coil conformation with R_g = 52 ± 0.5 Å. In contrast to conventional polymer melts, the pure PIL is not a theta solvent for the polycation. The TFSI^- anions, which comprise 48% v/v of the PIL, are strongly attracted to the polycation and act like small solvent molecules which leads to chain swelling analogous to an entangled, semi-dilute, or concentrated polymer solution in a good solvent.

Nanostructured Block Polymer Electrolytes: Tailoring Self-Assembly to Unlock the Potential in Lithium-Ion Batteries

ConspectusIon-containing solid block polymer (BP) electrolytes can self-assemble into microphase-separated domains to facilitate the independent optimization of ion conduction and mechanical stability; this assembly behavior has the potential to improve the functionality and safety of lithium-ion batteries over liquid electrolytes to meet future demands (e.g., large capacities and long lifetimes) in various applications. However, significant enhancements in the ionic conductivity and processability of BPs must be realized for BP-based electrolytes to become robust alternatives in commercial devices. Toward this end, the controlled modification of BP electrolytes' intra-domain (nanometer-scale) and multi-grain (micrometer-scale) structure is one viable approach; intra-domain ion transport and segmental compatibility (related to the effective Flory-Huggins parameter, χ_eff) can be increased by tuning the ion and monomer-segment distributions, and the morphology can be selected such that the multi-grain transport is less sensitive to grain size and orientation.To highlight the characteristics of intra-domain structure that promote efficient ion transport, this Account begins by describing the relationship between BP thermodynamics (namely, χ_eff and the statistical segment length, b, which is indicative of chain stiffness) and local ion concentration. These thermodynamic insights are vital because they inform the selection of synthesis and formulation variables, such as polymer and ion chemistry, polymer molecular weight and composition, and ion concentration, which boost electrolyte performance. In addition to its relationship with local ion transport, χ_eff is also an important factor with respect to electrolyte processability. For example, a reduced χ_eff can allow BP electrolytes to be processed at lower temperatures (i.e., lower energy input), with less solvent (i.e., reduced waste), and/or for shorter times (i.e., higher throughput) yet still form desired nanostructures. This Account also examines the impact of electrolyte preparation and processing on the ion transport across nanostructured grains because of grain size and orientation. As morphologies with a 3D-connected versus 2D-connected conducting phase show different sensitivities to conductivity losses that can occur because of the fabrication methods, it is necessary to account for electrolyte processing effects when probing ion transport.The intra-domain and micrometer-scale structure also can be tuned using either tapered BPs (macromolecules with modified monomer-segment composition profiles between two homogeneous blocks) or blends of BPs and homopolymers, independent of the BP molecular weight and composition, as detailed herein. The application of TBPs or BP/HP blends as ion-conducting materials leads to improved ion transport, reduced χ_eff, and greater availability of morphologies with 3D connectivity relative to traditional (non-tapered and unblended) BP electrolytes. This feature results from the fact that ion transport is related more closely to the monomer-segment distributions within a domain than the overall nanoscale morphology or average polymer/ion mobilities. Taken together, this Account describes how ion transport and processability are influenced by BP architecture and nanostructural features, and it provides avenues to tune nanoassemblies that can contribute to improved lithium-ion battery technologies to meet future demands.

Conformation of poly(ethylene glycol) in aqueous cholinium amino acid hybrid solvents

HYPOTHESIS

Hybrid solvents based on cholinium amino acid ionic liquids ([Ch][AA] ILs) mixed with water are environmentally benign solvents with low toxicity. [Ch][AA] ILs are used in biomass pretreatment processes to dissolve targeted (macro)molecules such as lignin from lingnocellulose. Understanding how [Ch][AA] ILs dissolve polymers is therefore of great interest for the rational design of ILs towards industrial application. Variation of the IL anion and the water concentration are hypothesised to change the solvent properties of [Ch][AA] hybrid solvents. Therefore, we probe the solvent quality of [Ch][AA] aqueous solutions with different anions (glycinate, prolinate and argininate) and water concentration for the simple model solute poly(ethylene glycol) (PEG).

EXPERIMENTS

Partial phase diagrams were produced to probe the salting-out effect of [Ch][AA] ILs towards PEG (M_w = 38 kDa). Small-angle neutron scattering experiments of deuterated PEG in hydrogenous [Ch][AA] aqueous solutions were performed to determine the polymer radius of gyration at infinite dilution (R_g,0) via Zimm-plots. Polymer concentration dependent apparent R_g values were obtained fitting an excluded volume polymer model onto the scattering data. Blends of hydrogenous and deuterated PEG under zero average contrast conditions were analysed to probe R_g at high polymer concentrations.

FINDINGS

Hydrogen bond capacity of the anion is key to the salting-out effect of [Ch][AA] ILs on PEG. R_g,0 depends on anion species and water concentration. At IL:water = 1:30 (mole:mole) and 37 °C, cholinium argininate and cholinium glycinate are close to theta solvents while cholinium prolinate and dilute cholinium argininate (IL:water = 1:100) are between theta and good solvents.

A simple simulation model for complex coacervates

When oppositely charged polyelectrolytes mix in an aqueous solution, associative phase separation gives rise to coacervates. Experiments reveal the phase diagram for such coacervates, and determine the impact of charge density, chain length and added salt. Simulations often use hybrid MC-MD methods to produce such phase diagrams, in support of experimental observations. We propose an idealized model and a simple simulation technique to investigate coacervate phase behavior. We model coacervate systems by charged bead-spring chains and counterions with short-range repulsions, of size equal to the Bjerrum length. We determine phase behavior by equilibrating a slab of concentrated coacervate with respect to swelling into a dilute phase of counterions. At salt concentrations below the critical point, the counterion concentration in the coacervate and dilute phases are nearly the same. At high salt concentrations, we find a one-phase region. Along the phase boundary, the total concentration of beads in the coacervate phase is nearly constant, corresponding to a "Bjerrum liquid''. This result can be extended to experimental phase diagrams by assigning appropriate volumes to monomers and salts.

Formation of a C15 Laves Phase with a Giant Unit Cell in Salt-Doped A/B/AB Ternary Polymer Blends

Salt-doped A/B/AB ternary polymer blends, wherein an AB copolymer acts as a surfactant to stabilize otherwise incompatible A and B homopolymers, display a wide range of nanostructured morphologies with significant tunability. Among these structures, a bicontinuous microemulsion (BμE) has been a notable target. Here, we report the surprising appearance of a robust C15 Laves phase, at compositions near where the BμE has recently been reported, in lithium bis(trifluoromethane) sulfonimide (LiTFSI)-doped low-molar-mass poly(ethylene oxide) (PEO)/polystyrene (PS)/symmetric PS-b-PEO block copolymer blends. The materials were analyzed by a combination of small-angle X-ray scattering (SAXS), ¹H NMR spectroscopy, and impedance spectroscopy. The C15 phase emerges at a high total homopolymer volume fraction ϕ_H = 0.8 with a salt composition r = 0.06 (Li⁺/[EO]) and persists as a coexisting phase across a large area of the isothermal phase diagram with high PS homopolymer compositions. Notably, the structure exhibits a huge unit cell size, a = 121 nm, with an unusually high micelle core volume fraction (f_core = 0.41) and an unusually low fraction of amphiphile (20%). This unit cell dimension is at least 50% larger than any previously reported C15 phase in soft matter, despite the low molar masses used, unlocking the possibility of copolymer-based photonic crystals without compromising processability. The nanostructured phase evolution from lamellar to hexagonal to C15 along the EO60 isopleth (ϕ_{PEO,homo-LiTFSI}/ϕ_H = 0.6) is rationalized as a consequence of asymmetry in the homopolymer solubility limit for each block, which leads to exclusion of PS homopolymer from the PS-b-PEO brush prior to exclusion of the PEO homopolymer, driving increased interfacial curvature and favoring the emergence of the C15 Laves phase.