Straightforward preparation of a tough and stretchable ion gel.

Ion gels, polymer networks swollen by ionic liquids, are expected to be applied to wearable devices that are tolerant to repeated stretching. High strength and excellent stretchability was achieved due to the numerous physical cross-links with abundant polymer chain entanglements in addition to a small number of immobile chemical cross-links, even though the ion gel was prepared by a facile methodology.

Ionic Liquid Interface as a Cell Scaffold.

In sharp contrast to conventional solid/hydrogel platforms, water-immiscible liquids, such as perfluorocarbons and silicones, allow the adhesion of mammalian cells via protein nanolayers (PNLs) formed at the interface. However, fluorocarbons and silicones, which are typically used for liquid cell culture, possess only narrow ranges of physicochemical parameters and have not allowed for a wide variety of cell culturing environments. In this paper, it is proposed that water-immiscible ionic liquids (ILs) are a new family of liquid substrates with tunable physicochemical properties and high solvation capabilities. Tetraalkylphosphonium-based ILs are identified as non-cytotoxic ILs, whereon human mesenchymal stem cells are successfully cultured. By reducing the cation charge distribution, or ionicity, via alkyl chain elongation, the interface allows cell spreading with matured focal contacts. High-speed atomic force microscopy observations of the PNL formation process suggest that the cation charge distribution significantly altered the protein adsorption dynamics, which are associated with the degree of protein denaturation and the PNL mechanics. Moreover, by exploiting dissolution capability of ILs, an ion-gel cell scaffold is fabricated. This enables to further identify the significant contribution of bulk subphase mechanics to cellular mechanosensing in liquid-based culture scaffolds.

Extremely Tough, Stretchable Gel Electrolytes with Strong Interpolymer Hydrogen Bonding Prepared Using Concentrated Electrolytes to Stabilize Lithium-Metal Anodes.

Extremely tough and stretchable gel electrolytes, which can be prepared by leveraging the strong interpolymer hydrogen bonding in concentrated lithium (Li)-salt electrolytes, are reported. These electrolytes can be realized by optimizing the competitive hydrogen-bonding interactions between polymer chains, solvent molecules, Li cations, and counteranions. Free polar solvent molecules, which typically impede interpolymer hydrogen bonding, are scarce in concentrated electrolytes; this feature can be exploited to prepare hydrogen-bonded gel electrolytes with unprecedented toughness. In contrast, free solvent molecules are abundant in electrolytes with typical concentrations, yielding considerably weaker gel electrolytes. The tough gel electrolyte can be used an artificial protective layer for Li-metal anodes, as it considerably enhances the cycling stability of a Li symmetric cell through uniform Li deposition/dissolution. Additionally, employing the gel electrolyte as the protecting layer significantly improves the cycling performance of the Li||LiNi0.6 Co0.2 Mn0.2 O2 full cell.

Controlling mechanical properties of ultrahigh molecular weight ion gels by chemical structure of ionic liquids and monomers.

A new class of ion gels, termed ultrahigh molecular weight (UHMW) gels, formed by physical entanglement of ultrahigh molecular weight polymers in ionic liquids, are synthesised using facile one step radical polymerisation with significantly low initiator conditions, and exhibit superior mechanical characteristics such as stretchability, recyclability, and room temperature self-healing ability. In this study, UHMW gels are synthesised using various combinations of monomer and IL structures, and the effect of their chemical structures on the physicochemical properties of UHMW gels are thoroughly investigated. UHMW polymers are prepared in situ for all combinations of ILs and monomers used in this study, indicating the wide applicability of this fabrication strategy. The structure-property relationships between chemical structures and mechanical properties of UHMW gels are investigated in detail. Furthermore, the differences in self-healing efficiency of UHMW gels depending on the chemical structure is discussed in terms of individual polymer conformation and polymer-polymer interaction based on molecular dynamics simulations.

Highly stretchable and self-healable polymer gels from physical entanglements of ultrahigh-molecular weight polymers.

Highly stretchable and self-healing polymer gels formed solely by physical entanglements of ultrahigh-molecular weight (UHMW) polymers were fabricated through a facile one-step process. Radical polymerization of vinyl monomers in ionic liquids under very low initiator concentration conditions produced UHMW polymers of more than 106 g/mol with nearly 100% yield, resulting in the formation of physically entangled transparent polymer gels. The UHMW gels showed excellent properties, such as high stretchability, high ionic conductivity, and recyclability. Furthermore, the UHMW gel exhibited room temperature self-healing ability without any external stimuli. The tensile experiments and molecular dynamics simulations indicate that the nonequilibrium state of the fractured surfaces and microscopic interactions between the polymer chains and solvents play a vital role in the self-healing ability. This study provides a physical approach for fabricating stretchable and self-healing polymer gels based on UHMW polymers.

Lithium-ion coordination-induced conformational change of PEG chains in ionic-liquid-based electrolytes.

We report the structure of poly(ethylene glycol) (PEG) in a imidazolium-based ionic liquid (IL) electrolyte containing lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) salt, as determined using Raman spectroscopy, high-energy X-ray total scattering (HEXTS), and molecular dynamics (MD) simulations. The Raman spectral study indicated that the TFSA anions bound to Li ions are desolvated when PEG is added to the LiTFSA/IL solution to form stable Li+-PEG complexes. Via quantitative analysis of the obtained Raman spectra, the desolvation number of the TFSA [nd, per one oxygen atom of the ethylene glycol unit (Opeg)] was determined to be ∼0.4, irrespective of the shape (star or linear) and molecular weight of the polymer. On the basis of radial distribution functions obtained from the HEXTS experiments and MD simulations, we demonstrated that the Li+-PEG complexation induces a conformational change of the PEG chain from gauche/anti-conformers to a syn conformer. This Li+-coordination-induced conformation resulted in a decrease in the radius of gyration (Rg) of the PEG chain, implying a folding behavior of polymer chains through multiple OpegLi+Opeg interactions.

Defect-free network formation and swelling behavior in ionic liquid-based electrolytes of tetra-arm polymers synthesized using a Michael addition reaction.

The gelation mechanism of tetra-arm poly(ethylene glycol) (TetraPEG) prepolymers via a Michael addition reaction was investigated from the viewpoint of chemical reaction kinetics. The polymer network was formed by mixing two different TetraPEGs functionalized with maleimide and thiol terminal groups (TetraPEG-MA and TetraPEG-SH) in aqueous solutions, and the gelation rate was strongly dependent on the solution pH. We found that the gelation reaction can be a second-order reaction when the acid-base equilibrium of the terminal SH groups (-SH ⇆ -S- + H+) was taken into account, resulting in a quantitative estimation of the rate constant (kgel) in the current polymer solution system. Based on the kgel value, the network connectivity (p), which corresponds to efficiency at the linking point, was evaluated to be p > 95% at the end of the reaction; thus, the resulting TetraPEG hydrogels have a homogeneous polymer network without network defects. We used the TetraPEG network as a polymer matrix in a lithium-ion battery gel electrolyte: dried TetraPEG gels were swollen with ionic liquid-based electrolytes containing Li salts to prepare TetraPEG ion gel electrolytes. Swelling behaviors of the TetraPEG network were characterized from the swelling rate and the equilibrium swelling ratio, and we found that these swelling behaviors were significantly affected by the Li-ion component. We concluded that an intermolecular interaction between Li-ions and the polymer (Li-ion coordination with the O atoms within the PEG chains) plays a key role in the fundamental physical properties of the gel electrolyte.