On the concentration polarisation in molten Li salts and borate-based Li ionic liquids.

Electrolytes that transport only Li ions play a crucial role in improving rapid charge and discharge properties in Li secondary batteries. Single Li-ion conduction can be achieved via liquid materials such as Li ionic liquids containing Li+ as the only cations because solvent-free fused Li salts do not polarise in electrochemical cells, owing to the absence of neutral solvents that allow polarisation in the salt concentration and the inevitably homogeneous density in the cells under anion-blocking conditions. However, we found that borate-based Li ionic liquids induce concentration polarisation in a Li/Li symmetric cell, which results in their transference (transport) numbers under anion-blocking conditions (tabcLi) being well below unity. The electrochemical polarisation of the borate-based Li ionic liquids was attributed to an equilibrium shift caused by exchangeable B-O coordination bonds in the anions to generate Li salts and borate-ester solvents at the electrode/electrolyte interface. By comparing borate-based Li ionic liquids containing different ligands, the B-O bond strength and extent of ligand exchange were found to be directly linked to the tabcLi values. This study confirms that the presence of dynamic exchangeable bonds causes electrochemical polarisation and provides a reference for the rational molecular design of Li ionic liquids aimed at achieving single-ion conducting liquid electrolytes.

Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers.

We demonstrate that tetra-arm poly(ethylene glycol) gels containing highly concentrated sulfolane-based electrolytes exhibit high Li+ transference numbers. The low polymer concentration and homogeneous polymer network in the gel electrolyte are useful in achieving both mechanical reliability and high Li+ transport ability.

Direct Observation of Photo-Induced Reversible Sol-Gel Transition in Block Copolymer Self-Assembly Containing an Azobenzene Ionic Liquid.

Using atomic force microscopy, the photo-induced reversible changes in a block copolymer self-assembly containing an azobenzene ionic liquid, which undergoes sol-gel transition is directly observed. This is the first report on the sol-gel transition of an ABA-type block copolymer consisting of upper critical solution temperature (UCST)-type A blocks in a photoresponsive ionic liquid mixture. The sol-gel transition is accompanied by an order-to-disorder structural change, which subsequently induces a change in the ionic conductivity. Surprisingly, the photo-induced ionic conductivity and rheological changes occurs rapidly (≈30 s) despite the dense (≈80 wt%) polymeric system. The rapid structural change is probably attributable to the fast diffusion of the ionic liquid.

Effect of network homogeneity on mechanical, thermal and electrochemical properties of solid polymer electrolytes prepared by homogeneous 4-arm poly(ethylene glycols).

Mechanically tough solid polymer electrolytes (SPEs) are required to meet the demand for flexible and stretchable electrochemical devices for diverse applications, especially for wearable devices. It is well known that the inhomogeneity of a polymer network greatly affects its mechanical properties, but the evaluation of its effect on electrolyte properties including mechanical properties has not been accomplished yet because of the coexistence of various inhomogeneities (e.g., dangling bonds, loops, chain entanglements, and inhomogeneous distribution of cross-linking points). Herein, we discuss the effect of distribution of cross-linking densities in SPEs on its electrolyte properties by employing a model polymer network composed of a homogeneous 4-arm poly(ethylene glycol) (tetra-PEG) network and Li[TFSA] ([TFSA]: bis(trifluoromethanesulfonyl)amide). Tetra-PEGs having different molecular weights (Mn = 5, 10, 20, and 40 kDa) are subjected to the Michael addition reaction to induce network inhomogeneity while the average cross-linking densities are matched. It was found that thermal and ion transport properties of tetra-PEG SPEs do not depend on network inhomogeneity but on the average network size, which indicates that these properties reflect the averaged thermal fluctuation of polymer chains in terms of spatial and temporal dimensions. On the other hand, the mechanical toughness was largely dependent on the network homogeneity, and fracture strain, energy, and Young's modulus decreased by introducing network inhomogeneity. Rheological measurements showed that a transient cross-linking between Li cations and oxygens of tetra-PEG as well as the homogeneous distribution of the chemical cross-linking points contribute to the excellent mechanical properties of SPEs.

Photo/thermoresponsive ABC triblock copolymer-based ion gels: photoinduced structural transitions.

A photo/thermoresponsive ABC triblock copolymer-based ion gel exhibiting photoinduced structural transitions accompanied by significant rheological changes is newly developed. The ABC triblock copolymer comprises an ionic liquid (IL)-phobic A block, an IL-philic B block, and a photo/thermoresponsive C block containing azobenzene moieties. The IL-phobic A block forms a rigid micellar core in an IL over a wide temperature range and the photo/thermoresponsive C block undergoes upper critical solution temperature (UCST)-type phase transition in ILs. In concentrated polymer solution, the ABC triblock copolymer can form a percolated micellar network at low temperatures through aggregation of A and C blocks as physical crosslinks, bridged by IL-philic B blocks. In contrast, the ion gel undergoes structural transition to jammed micelles at high temperatures due to the disassembly of the thermoresponsive C block, resulting in significant softening of the ion gel. Importantly, the temperature dependences of the viscoelastic properties of the ion gel differ drastically depending on photo-irradiation conditions as the photoinduced isomerization of azobenzene moieties in the C block modulates the affinity between the polymer chain and IL. Utilizing this feature, photoinduced softening/hardening of the ion gel is realized at constant temperature. This study provides a promising strategy to control the rheological properties of nonvolatile soft materials via contactless light irradiation that could be exploited in various applications such as photoresponsive soft actuators and photo-healable soft materials.

Controlled Sol-Gel Transitions of a Thermoresponsive Polymer in a Photoswitchable Azobenzene Ionic Liquid as a Molecular Trigger.

Producing ionic liquids (ILs) that function as molecular trigger for macroscopic change is a challenging issue. Photoisomerization of an azobenzene IL at the molecular level evokes a macroscopic response (light-controlled mechanical sol-gel transitions) for ABA triblock copolymer solutions. The A endblocks, poly(2-phenylethyl methacrylate), show a lower critical solution temperature in the IL mixture containing azobenzene, while the B midblock, poly(methyl methacrylate), is compatible with the mixture. In a concentrated polymer solution, different gelation temperatures were observed in it under dark and UV conditions. Light-controlled sol-gel transitions were achieved by a photoresponsive solubility change of the A endblocks upon photoisomerization of the azobenzene IL. Therefore, an azobenzene IL as a molecular switch can tune the self-assembly of a thermoresponsive polymer, leading to macroscopic light-controlled sol-gel transitions.

Thermosensitive Phase Separation Behavior of Poly(benzyl methacrylate)/Solvate Ionic Liquid Solutions.

We report a lower critical solution temperature (LCST) behavior of binary systems consisting of poly(benzyl methacrylate) (PBnMA) and solvate ionic liquids: equimolar mixtures of triglyme (G3) or tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)amide. We evaluated the critical temperatures (Tcs) using transmittance measurements. The stability of the glyme-Li+ complex ([Li(G3 or G4)]+) in the presence of PBnMA was confirmed using Raman spectroscopy, pulsed-field gradient spin-echo NMR (PGSE-NMR), and thermogravimetric analysis to demonstrate that the complex was not disrupted. The interaction between glyme-Li+ complex and PBnMA was investigated via 7Li NMR chemical shifts. Upfield shifts originating from the ring-current effect of the aromatic ring within PBnMA were observed with the addition of PBnMA, indicating localization of the glyme-Li+ complex above and below the benzyl group of PBnMA, which may be a reason for negative mixing entropy, a key requirement of the LCST.

Self-Assembly of Polyether Diblock Copolymers in Water and Ionic Liquids.

Here the thermoresponsive self-assembly of diblock copolymers comprising poly(ethyl glycidyl ether) (PEGE) and poly(ethylene oxide) (PEO) in water and ionic liquids (ILs) is investigated. PEGE undergoes lower critical solution temperature (LCST) phase separation in both water and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2 mim][NTf2 ]), while PEO is a compatible segment for these solvents. The diblock copolymers, PEGE-b-PEO, undergo thermosensitive unimer-micelle transitions at temperatures close to the LCST point (TLCST ) of the PEGE homopolymer in water but not in [C2 mim][NTf2 ], even at temperatures much higher than TLCST . The difference in the thermoresponsivity of these solutions is explored using differential scanning calorimetry results from rather small magnitudes of the thermodynamic parameters for the phase transition of the PEGE segment in [C2 mim][NTf2 ], compared with those in water. Due to such small magnitudes, TLCST of the PEGE segment for the block copolymers in the IL is greatly affected by the elongation of soluble PEO segments.

From Macromolecular to Small-Molecular Triggers: Facile Method toward Photoinduced LCST Phase Behavior of Thermoresponsive Polymers in Mixed Ionic Liquids Containing an Azobenzene Moiety.

Instead of the reported photoinduced lower critical solution temperature (LCST) phase transition behavior in ionic liquids (ILs) achieved by photofunctional polymers, this study reports the facile photoinduced LCST phase behavior of nonfunctionalized polymers (poly(benzyl methacrylate) (PBnMA) and poly(2-phenylethyl methacrylate) (PPhEtMA)) in mixed ILs (1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)amide; [C1 mim][NTf2 ] and a newly designed functionalized IL containing an azobenzene moiety (1-butyl-3-(4-phenylazobenzyl)imidazolium bis(trifluoromethanesulfonyl)amide; [Azo][NTf2 ])) as a small-molecular photo trigger. Interestingly, the length of the alkyl spacer between the ester and aryl groups, which is the only structural difference between the two polymers, leads to two different photoresponsive LCST phase transition behaviors. On the basis of spectroscopic studies, the different phase transition behaviors of PBnMA and PPhEtMA may attribute to the different cooperative interactions between the polymers and [C1 mim][NTf2 ].

Thermoreversible nanogel shuttle between ionic liquid and aqueous phases.

We describe a nanogel that can reversibly shuttle between a hydrophobic ionic liquid (IL) phase and an aqueous phase in response to temperature changes. A thermosensitive diblock copolymer, consisting of poly(ethylene oxide) (PEO) as the first segment and a random copolymer of N-isopropylacrylamide (NIPAm) and N-acryloyloxysuccinimide (NAS) as the second segment, was prepared as a nanogel precursor using anionic ring-opening polymerization of EO followed by reversible addition-fragmentation chain-transfer (RAFT) polymerization of NIPAm and NAS. After the micellization of the diblock copolymer in an aqueous solution upon heating to temperatures higher than the lower critical solution temperature (LCST) of the second segment, a coupling reaction of the NAS group of the P(NIPAm-r-NAS) core with ethylenediamine gave a nanogel with a well-solvated PEO corona. The nanogel exhibited contrasting thermosensitivities in the aqueous and IL phases. Dynamic light scattering measurements revealed that the nanogel exhibited LCST phase behavior (low-temperature-swollen/high-temperature-shrunken) in the aqueous phase and the opposite upper critical solution temperature (UCST) phase behavior (high-temperature-swollen/low-temperature-shrunken) in hydrophobic ILs. The nanogel favored the aqueous phase at low temperatures and the IL phase at high temperatures because of the solubility changes in the PEO corona. Upon increasing the temperature, the nanogel underwent a swollen-to-shrunken phase change in the aqueous phase, a transfer from the aqueous phase to the IL phase, and a shrunken-to-swollen phase change in the IL phase. These processes were thermally reversible, which made the round-trip shuttling of the nanogel between the aqueous and IL phases possible.

Solvation Structure of Poly(benzyl methacrylate) in a Solvate Ionic Liquid: Preferential Solvation of Li-Glyme Complex Cation.

We report the solvation structure of a lower critical solution temperature (LCST)-type thermoresponsive polymer in a solvate ionic liquid (SIL, i.e., an ionic liquid comprising solvate ions) to elucidate the predominant interaction for the dissolution of the thermoresponsive polymer in SIL at low temperatures. The solvation structure of poly(benzyl methacrylate) (PBnMA) and a model compound of its monomer in a typical glyme-based SIL, [Li(G4)][TFSA] (G4: tetraglyme; TFSA: bis(trifluoromethanesulfonyl)amide), have been investigated using high-energy X-ray total scattering and all-atom molecular dynamics simulations. In the model compound/SIL system, the intermolecular components extracted from the total G( r)s revealed that the ester moiety of BnMA is preferentially solvated by Li cations through a cation-dipole interaction, which induces slight desolvation of the G4 molecules, and the aromatic ring of BnMA is secondarily solvated by the [Li(G4)] cation complex through a cation-π interaction with maintaining the complex structure. In contrast, TFSA anions are attracted only by the [Li(G4)] cation. These interactions result in the formation of a solvation layer of SILs around the aromatic ring, which plays a key role in the negative entropy and enthalpy of mixing. Meanwhile, in the polymer solution, the coordination number of the Li cation around the ester moiety significantly decreased. This could be ascribed to the steric effect of the bulky side chains, preventing the approach of the [Li(G4)] cation complex to the ester moiety located near the main chain. These solvation structures lead to small absolute values of negative entropy and enthalpy of mixing, which together are key factors to understand the LCST-type phase behavior in the IL system.

Viscoelastic change of block copolymer ion gels in a photo-switchable azobenzene ionic liquid triggered by light.

A photo-switchable ionic liquid solvent bearing an azobenzene moiety induced a viscoelastic change of block copolymer ion gels by light. ABA triblock copolymers having poly(phenethyl methacrylate) and poly(benzyl methacrylate) as A blocks exhibited opposite photoinduced rheological responses although they had only a tiny structural difference in the alkyl chain length.

LCST-type liquid-liquid phase separation behaviour of poly(ethylene oxide) derivatives in an ionic liquid.

We present a new series of polymer-ionic liquid solutions exhibiting LCST-type liquid-liquid phase separation behaviour, and reveal their phase behaviour and intermolecular interactions based on phase diagrams and NMR analysis.

Photohealable ion gels based on the reversible dimerisation of anthracene.

We report a photohealable ion gel based on the photodimerisation of anthracene as a dynamic covalent bond. A tetra-arm poly(ethylene glycol) terminally functionalised with anthracene was synthesised and combined with an ionic liquid to form an ion gel. The photodimerisation reaction was utilised to realise photohealing of the ion gels.

Photocurable ABA triblock copolymer-based ion gels utilizing photodimerization of coumarin.

Herein, we develop a photocurable ABA triblock copolymer-based ion gel, which can be converted from a thermally processable, physically crosslinked ion gel to a thermally and mechanically stable, chemically crosslinked ion gel via photoinduced dimerization. The A block consists of a random copolymer of N-isopropylacrylamide and a coumarin-containing acrylate monomer, while the B block consists of an ionic liquid-philic poly(ethylene oxide). Due to the upper critical solution temperature-type phase behavior of the A block, the ABA triblock copolymer undergoes gel-to-sol transitions in a hydrophobic ionic liquid as the temperature is increased. Furthermore, under ultraviolet (UV) light irradiation, the physical crosslinks formed by association of the A blocks in the gel at low temperatures become chemically crosslinked as a result of photodimerization of the coumarin moieties in the A block; this results in conversion from a thermo-reversible, physically crosslinked ion gel to a thermo-irreversible, chemically crosslinked ion gel. The rheological changes of the ion gel upon UV irradiation have been investigated in detail. In addition, photopatterning of the ion gel has been realized by exploiting the photocurable behavior of the ABA triblock copolymer in the ionic liquid.

Effect of core-shell micelle formation on the redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide).

Redox properties of phenothiazine-labeled poly(ethyl glycidy ether)-block-poly(ethylene oxide) (PT-EGE(n)-b-EO(m)) are reversibly changed by core-shell micelle formation. In the temperature range higher than the critical micellization temperature (cmt), the anodic potential of PT group positively shifts and concomitantly its anodic current decrease, or levels off compared to those of the reference polymer PT-EO(m) without the thermo-responsive EGE(n) segment. The former alteration is caused by incorporation of hydrophobic PT groups into a core of the micelle and the latter by the decrease in the diffusion coefficient of PT groups due to formation of the core-shell micelles. The cmt value and the temperature-dependent alteration in the redox properties strongly depend on the polymer structure, especially the length of thermo-responsive EGE(n) segment. The electrochemically determined hydrodynamic radii of the polymer aggregates seem to be overestimated, compared to the values reported for the aggregates of other thermo-responsive polymers with similar molecular weights, implying the presence of electrochemically inactive PT groups in the copolymers having longer thermo-responsive segments.

Printable polymer actuators from ionic liquid, soluble polyimide, and ubiquitous carbon materials.

We present here printable high-performance polymer actuators comprising ionic liquid (IL), soluble polyimide, and ubiquitous carbon materials. Polymer electrolytes with high ionic conductivity and reliable mechanical strength are required for high-performance polymer actuators. The developed polymer electrolytes comprised a soluble sulfonated polyimide (SPI) and IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim][NTf2]), and they exhibited acceptable ionic conductivity up to 1 × 10(-3) S cm(-1) and favorable mechanical properties (elastic modulus >1 × 10(7) Pa). Polymer actuators based on SPI/[C2mim][NTf2] electrolytes were prepared using inexpensive activated carbon (AC) together with highly electron-conducting carbon such as acetylene black (AB), vapor grown carbon fiber (VGCF), and Ketjen black (KB). The resulting polymer actuators have a trilaminar electric double-layer capacitor structure, consisting of a polymer electrolyte layer sandwiched between carbon electrode layers. Displacement, response speed, and durability of the actuators depended on the combination of carbons. Especially the actuators with mixed AC/KB carbon electrodes exhibited relatively large displacement and high-speed response, and they kept 80% of the initial displacement even after more than 5000 cycles. The generated force of the actuators correlated with the elastic modulus of SPI/[C2mim][NTf2] electrolytes. The displacement of the actuators was proportional to the accumulated electric charge in the electrodes, regardless of carbon materials, and agreed well with the previously proposed displacement model.

Driving mechanisms of ionic polymer actuators having electric double layer capacitor structures.

Two solid polymer electrolytes, composed of a polyether-segmented polyurethaneurea (PEUU) and either a lithium salt (lithium bis(trifluoromethanesulfonyl)amide: Li[NTf2]) or a nonvolatile ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide: [C2mim][NTf2]), were prepared in order to utilize them as ionic polymer actuators. These salts were preferentially dissolved in the polyether phases. The ionic transport mechanism of the polyethers was discussed in terms of the diffusion coefficients and ionic transference numbers of the incorporated ions, which were estimated by means of pulsed-field gradient spin-echo (PGSE) NMR. There was a distinct difference in the ionic transport properties of each polymer electrolyte owing to the difference in the magnitude of interactions between the cations and the polyether. The anionic diffusion coefficient was much faster than that of the cation in the polyether/Li[NTf2] electrolyte, whereas the cation diffused faster than the anion in the polyether/[C2mim][NTf2] electrolyte. Ionic polymer actuators, which have a solid-state electric-double-layer-capacitor (EDLC) structure, were prepared using these polymer electrolyte membranes and ubiquitous carbon materials such as activated carbon and acetylene black. On the basis of the difference in the motional direction of each actuator against applied voltages, a simple model of the actuation mechanisms was proposed by taking the difference in ionic transport properties into consideration. This model discriminated the behavior of the actuators in terms of the products of transference numbers and ionic volumes. The experimentally observed behavior of the actuators was successfully explained by this model.

Lower critical solution temperature phase behavior of linear polymers in imidazolium-based ionic liquids: effects of structural modifications.

The solubility and phase behavior of linear polymethacrylate polymers, primarily poly(phenylalkyl methacrylate)s, in imidazolium-based ionic liquids (ILs) were systematically studied by changing the structure of each component. Solutions of polymethacrylates in 1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([C(n)mim] [NTf2]) showed lower critical solution temperature (LCST) phase behavior, and the phase separation temperature (T(c)) could be varied by selecting an appropriate combination of a polymer and an IL. An increase in alkyl chain length between the phenyl and ester groups in the polymer side chain decreased the T(c); alternatively, substitution of the imidazolium cation with a longer alkyl chain increased the T(c). When the same anion was used, the miscibility of the polymer/IL system was mainly determined by the alkyl chain length. T(c) could also be varied by mixing two ILs in an appropriate ratio. In addition, the kinetics of the reversible phase transition phenomena exhibited by these polymers were examined. Redissolution kinetics were largely controlled by the magnitude of the difference between T(c) and the glass transition temperature (T(g)) of the polymer (T(c) - T(g)), in addition to the mutual affinity between the polymer and the IL.

Photoisomerization-induced tunable LCST phase separation of azobenzene-containing polymers in an ionic liquid.

4-phenylazophenyl methacrylate (AzoMA) and benzyl methacrylate (BnMA) were copolymerized to produce multistimuli-responsive polymers (P(AzoMA-r-BnMA)s) in a hydrophobic ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide ([C2mim][NTf2]), as the solvent. P(AzoMA-r-BnMA)s with a maximum of ca. 4 mol % AzoMA were soluble in [C2mim][NTf2] at low temperatures, and they underwent lower critical solution temperature (LCST) phase separation with an increase in temperature. Under UV and visible light irradiation, P(AzoMA-r-BnMA)s underwent reversible photochromism of trans-to-cis and cis-to-trans isomerization, respectively. The LCST temperature differences between trans- and cis-form polymers in the IL were as large as 22 degrees C. Reversible photoinduced phase separation of the polymers was achieved at a certain temperature; at this temperature, the cis-form polymers were soluble in the IL, but the trans-form polymers were not.