Takeshi Ueki

Heterogeneous Slow Dynamics of Imidazolium-Based Ionic Liquids Studied by Neutron Spin Echo

We have investigated structure and relaxation phenomena for ionic liquids 1-octyl-3-methylimidazolium hexafluorophosphate (C8mimPF6) and bis(trifluoromethylsulfonyl)imide (C8mimTFSI) by means of neutron diffraction and neutron spin echo (NSE) techniques. The diffraction patterns show two distinct peaks appeared at scattering vectors Q of 0.3 and 1.0 Å(-1). The former originates from the nanoscale structure characteristic to ionic liquids and the latter due to the interionic correlations. Interestingly, the intensity of the low-Q peak drastically grows upon cooling and keeps growing even below the glass transition temperature. The NSE measurements have been performed at these two Q positions, to explore the time evolution of each correlation. The relaxation related to the ionic correlation (ionic diffusion) is of Arrhenius-type and exhibits nonexponential behavior. The activation energy (Ea) of the ionic diffusion, which is linked to viscosity, depends on the type of anion; the larger is the anion size, the smaller Ea becomes for most of anions. On the other hand, two kinds of relaxation processes, slower and faster ones, are found at the low-Q peak position. The most significant finding is that the fraction of the slower relaxation increases and that of the faster one decreases upon cooling. Combining the NSE data with the diffraction data, we conclude that there exist two parts in ILs: one with the ordered nanostructure exhibiting the slow relaxation, and the other with disordered structure showing faster relaxation. The structure and dynamics of ILs are heterogeneous in nature, and the fraction of each part changes with temperature.

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.

High-performance ion gel with tetra-PEG network

In this paper, we show a free-standing highly ion-conducting ionic liquid (IL)-polymer electrolyte, Tetra-PEG ion gel, prepared by incorporating imidazolium-based ILs into very much lower concentration (3–6 wt%) of tetra-arm poly(ethylene glycol), Tetra-PEG. The ionic conductivities of the free-standing Tetra-PEG ion gels were nearly equal to those of pure ILs, indicating a realization of liquid-like conductivity in a solid-state material. The Tetra-PEG ion gels showed advanced mechanical properties demonstrated by the results of compression and stretching tests.

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.

Thermosensitive, Soft Glassy and Structural Colored Colloidal Array in Ionic Liquid: Colloidal Glass to Gel Transition

A novel soft material comprising thermosensitive poly(benzyl methacrylate)-grafted silica nanoparticles (PBnMA-g-NPs) and the ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfonyl)amide ([C(2)mim][NTf(2)]), was fabricated. The thermosensitive properties were studied over a wide range of particle concentrations and temperatures. PBnMA-g-NPs in the IL underwent the lower critical solution temperature (LCST) phase transition at lower temperatures with a broader transition temperature range as compared to the free PBnMA solution. Highly concentrated suspensions formed soft glassy colloidal arrays (SGCAs) exhibiting a soft-solid behavior and angle-independent structural color. For the first time, we report a discrete change in the angle-independent structural color of SGCAs with temperature because of a temperature-induced colloidal glass-to-gel transition. The interparticle interaction changed from repulsive to attractive at the LCST temperature, and it was characterized by a V-shaped rheological response and a direct electron microscope observation of the colloidal suspension in the IL. With unique rheological and optical properties as well as properties derived from the IL itself, the thermosensitive SGCAs may be of interest as a new material for a wide range of applications such as electrochemical devices and color displays.

Self‐Beating Artificial Cells: Design of Cross‐Linked Polymersomes Showing Self‐Oscillating Motion

Biomimetic cross-linked polymersomes that exhibit a self-beating motion without any on-off switching are developed. The polymersomes are made from a well-defined synthetic thermoresponsive diblock copolymer, and the thermoresponsive segment includes ruthenium catalysts for the oscillatory chemical reaction and vinylidene groups to cross-link the polymersomes. Autonomous volume and shape oscillations of the cross-linked polymersomes are realized following redox changes of the catalysts.

Self-oscillating micelles

Rhythmic oscillation of the scattering intensity and hydrodynamic radii of a block copolymer solution driven by the Belousov-Zhabotinsky (BZ) reaction was demonstrated without any on-off switching of external stimuli. This is the first report on a synthetic block copolymer that realizes the novel concept of self-assembly assisted by a dissipative structure.

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.

Thermodynamic study on phase transitions of poly(benzyl methacrylate) in ionic liquid solvents

The lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate) (PBnMA) in room-temperature ionic liquids (ILs) was studied by considering the effect of the chemical structure of ILs on the phase separation temperature (Tc). It was found that the LCST behavior of PBnMA was observed in ILs containing [NTf2] anions and PF6 anions. Tc changed significantly with a small change in the chemical structures of the cations. High-sensitivity differential scanning calorimetry (DSC) was successfully performed for studying the LCST phase separation of PBnMA in two different imidazolium-based ILs. Endothermic peaks corresponding to the phase separation of PBnMA from the ILs were clearly observed at ca. 100 °C in the DSC thermograms. It was experimentally verified for the first time that the negative enthalpy and entropy change of mixing of PBnMA in ILs caused the LCST phase separations. The absolute values of the thermodynamic parameters for the phase transition of PBnMA in ILs obtained in this study were much lower than those reported in previous studies for aqueous polymer solutions that exhibit LCST phase behavior, such as poly(N-isopropylarylamide) and poly(vinyl methyl ether). Small changes in the thermodynamic parameters resulted in a large change in the phase separation temperature even by small changes in the chemical structure of the ILs and polymers. The microscopic desolvation process detected from the DSC measurements was inferred to have occurred before the macroscopic phase separation detected from turbidity measurements. The dependence of the endothermic peak temperatures on the DSC scan rate was observed even at slow scan rates. These results indicate that the phase separation of PBnMA from ILs is characterized by extremely slow kinetics.

Self‐Oscillating Vesicles: Spontaneous Cyclic Structural Changes of Synthetic Diblock Copolymers

A large variety of synthetic vesicles has been created for potential engineering applications and as model systems which mimic living organisms. In most cases, the structure is designed to be thermodynamically stable. However, mimicking dynamic behaviors of living vesicles still remains undeveloped. Herein, we present a synthetic vesicle which shows autonomous disintegration-reconstruction cycles without any external stimuli and which is similar to those in living organisms, such as in the nuclear envelope and synaptic vesicles. The vesicle is composed of a diblock copolymer which has a hydrophilic and a thermosensitive segment. The thermosensitive segment includes a redox moiety that acts as a catalyst for an oscillatory chemical reaction and also controls the aggregation temperature of vesicles. Furthermore, autonomous fusion of vesicles is also observed during the cycles.