Kohzo Ito

Recent advances in the preparation of cyclodextrin-based polyrotaxanes and their applications to soft materials

The present review article deals with recent novel studies on the preparation and application of polyrotaxanes comprised of cyclodextrins (CDs) and various linear polymers, especially poly(ethylene glycol) (PEG). First, a brief introduction of the historical background of the pioneering work on the preparation of an inclusion complex and polyrotaxane is provided. Subsequently, the authors have focused on the recently developed solvent systems for the polyrotaxane. These new solvents are interesting from two fundamental viewpoints: (1) from the perspective of the clarification of the hydrogen-bonding-based dissolution mechanism of polyrotaxanes; and (2) from the practical viewpoint of the preparation of modified polyrotaxanes or slide-ring gels containing ionic liquids. A wide variety of polyrotaxane derivatives, whose cyclodextrin moiety was modified to carry various functional groups, and their intriguing characteristics are introduced in this article. Finally, many instances of the application of the PEG-CD polyrotaxane to soft materials, such as gels, molecular tubes and multivalent ligand systems, are summarized.

Extremely stretchable thermosensitive hydrogels by introducing slide-ring polyrotaxane cross-linkers and ionic groups into the polymer network

Stimuli-sensitive hydrogels changing their volumes and shapes in response to various stimulations have potential applications in multiple fields. However, these hydrogels have not yet been commercialized due to some problems that need to be overcome. One of the most significant problems is that conventional stimuli-sensitive hydrogels are usually brittle. Here we prepare extremely stretchable thermosensitive hydrogels with good toughness by using polyrotaxane derivatives composed of α-cyclodextrin and polyethylene glycol as cross-linkers and introducing ionic groups into the polymer network. The ionic groups help the polyrotaxane cross-linkers to become well extended in the polymer network. The resulting hydrogels are surprisingly stretchable and tough because the cross-linked α-cyclodextrin molecules can move along the polyethylene glycol chains. In addition, the polyrotaxane cross-linkers can be used with a variety of vinyl monomers; the mechanical properties of the wide variety of polymer gels can be improved by using these cross-linkers.

Atomic force microscopy observation of insulated molecular wire formed by conducting polymer and molecular nanotube

Inclusion complex formation between a conducting polymer, polyaniline (PANI) with emeraldine base, and a molecular nanotube synthesized from α-cyclodextrin (α-CD) has been studied by atomic force microscopy. We observed a rodlike inclusion complex of PANI and the molecular nanotube on mica substrate at room temperature. The height of this structure is nearly equal to the outside diameter of α-CD and almost uniform along the whole length of the structure, which indicates that a conducting wire of PANI is fully covered by molecular nanotubes as insulator. Accordingly, this inclusion complex can be regarded as insulated molecular wire.

Temperature dependence of inclusion-dissociation behavior between molecular nanotubes and linear polymers

We have investigated the inclusion-dissociation behavior between molecular nanotubes and linear polymers in solutions by measuring the induced circular dichroism of the mixture of the molecular nanotubes, which are composed of α-cyclodextrins linked by three cross-linking bridges, and poly(ethylene glycol) modified with azobenzene. It was found that the inclusion complex between the nanotubes and the linear polymers was formed at room temperature, and that the polymers were dissociated from the nanotubes with increasing temperature, as expected theoretically.

Conductivity measurements of individual poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) nanowires on nanoelectrodes using manipulation with an atomic force microscope

We have prepared conducting polymer nanowires of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) with diameters under 10 nm by a molecular combing method, and have measured the conductivity of the individual PEDOT nanowires on platinum nanoelectrodes using manipulation with an atomic force microscope (AFM). The temperature dependence of the conductance was explained well by a quasi-one-dimensional variable range hopping model. The conductivity of two single nanowires was determined to be 0.6 and 0.09S∕cm, which is of the same order as that of PEDOT/PSS films. After all the nanowires crossed over the nanoelectrodes were cut off with AFM manipulation, the current was drastically decreased down to the background level. These results directly indicate that the conductivity was derived from the PEDOT nanowires on the nanoelectrodes.

Pressure‐Responsive Polymer Membranes of Slide‐Ring Gels with Movable Cross‐Links

Polymer membranes comprising slide-ring gels with movable cross-links exhibit a nonlinear pressure-dependence in the fluidic flow rate. The proportional constant between the flow rate and pressure significantly changes at a critical pressure. The slide-ring gels are promising polymer membrane materials, which would allow for the on-off control of fluid permeability using an imposed pressure.

Mechanics of slide-ring gels: novel entropic elasticity of a topological network formed by ring and string

Slide-ring (SR) gels are polymer networks with movable cross-links that are prepared by cross-linking polyrotaxane (PR) in which many cyclic molecules are threaded into a linear polymer chain. The elastic modulus E of SR gels shows a unique dependence on cross-linking density: at a high cross-linking density, E decreases with increasing cross-linking density. This tendency is not in agreement with conventional rubber elasticity theory. In order to explain this abnormal dependence, we propose a novel molecular theory for SR gels, which considers the alignment entropy of cyclic molecules on polymer networks. The alignment yields new entropic elasticity of the slidable network in SR gels.

Insulation effect of an inclusion complex formed by polyaniline and ?-cyclodextrin in solution

The insulation effect of an inclusion complex formed by polyaniline with emeraldine base as a conducting polymer and β-cyclodextrin as insulators of cyclic form has been studied using the oxidation by iodine in solution of N-methyl-2-pyrrolidone. When polyaniline is oxidized by iodine in the solution, the solution color changes from blue to violet. We observed little change in color at 273 K or a considerable delay of the spectral change at 288 K in the solution of the inclusion complex and iodine in contrast to that in solution of polyaniline and iodine without β-cyclodextrin. This suggests that the conjugated conducting polymer in the inclusion complex is almost completely insulated by the threaded cyclodextrin molecules from the chemical oxidation by iodine and that the oxidation at 288 K proceeds after the inclusion complex dissociates into polyaniline and cyclodextrin in the solution. Copyright

Sliding mode of cyclodextrin in polyrotaxane and slide-ring gel

The sliding modes of cyclic molecules in polyrotaxane and of cross-linking junctions in topological gel or slide-ring gel were investigated by quasi-elastic light scattering. We found that a polyrotaxane sparsely including α-cyclodextrin showed the sliding mode in solution other than the self- and cooperative-diffusion modes whereas the sliding mode was not observed in a polyrotaxane densely including α-cyclodextrin. After gelation of the sparse polyrotaxane, the self-diffusion mode of the polyrotaxane disappeared but the sliding mode was still observed. This indicated that the figure-of-eight cross-links in the slide-ring gel slid in the polymer network, passing through the polymer chains.

Size and number density of precrystalline aggregates in lysozyme crystallization process

Using dynamic light scattering, we investigated supersaturated aqueous solutions of hen egg white lysozyme. We could observe the formation of aggregates only in solutions, from which crystals grew within a few days. The aggregates were grouped into smaller “units” and larger “clusters.” The units consisted of a few molecules, whereas the clusters grew from about 100 nm to 1 μm. At the beginning of aggregation, the number density of the units decreased, while that of the clusters increased. At this stage, unit-cluster aggregation proceeded. At the next stage, the number density of the units became constant, while that of the clusters began to decrease, which means that the units stopped aggregating and cluster-cluster aggregation started. The aggregation mechanism for the clusters fit well with the diffusion limited cluster aggregation model, but this model alone could not explain that the aggregates separated into two groups, corresponding to units and clusters, and that the units stopped aggregating duri...