Strain-induced crystallization and phase separation used for fabricating a tough and stiff slide-ring solid polymer electrolyte.

The demand for mechanically robust polymer-based electrolytes is increasing for applications to wearable devices. Young's modulus and breaking energy are essential parameters for describing the mechanical reliability of electrolytes. The former plays a vital role in suppressing the short circuit during charge-discharge, while the latter indicates crack propagation resistance. However, polymer electrolytes with high Young's moduli are generally brittle. In this study, a tough slide-ring solid polymer electrolyte (SR-SPE) breaking through this trade-off between stiffness and toughness is designed on the basis of strain-induced crystallization (SIC) and phase separation. SIC makes the material highly tough (breaking energy, 80 to 100 megajoules per cubic meter). Phase separation in the polymer enhanced stiffness (Young's modulus, 10 to 70 megapascals). The combined effect of phase separation and SIC made SR-SPE tough and stiff, while these mechanisms do not impair ionic conductivity. This SIC strategy could be combined with other toughening mechanisms to design tough polymer gel materials.

Bioinspired Oxidation-Resistant Catechol-like Sliding Ring Polyrotaxane Hydrogels.

Adaptable hydrogels have been used in the biomedical field to address several pathologies, especially those regarding tissue defects. Here, we describe unprecedented catechol-like functionalized polyrotaxane (PR) polymers able to form hydrogels. PR were functionalized with the incorporation of hydroxypyridinone (HOPO) moieties into the polymer backbone, with a degree of substitution from 4 to 22%, depending on the PR type. The hydrogels form through the functionalized supramolecular systems when in contact with a Fe(III) solution. Despite the hydrogel formation being at physiological pH (7.4), the HOPO derivatives are extremely resistant to oxidation, unlike common catechols; consequently, they prevent the formation of quinones, which can lead to irreversible bounds within the matrix. The resulting hydrogels demonstrated properties lead to unique hydrogels with improved mechanical behavior obtained by metallic coordination crosslinking, due to the synergies of the sliding-ring PR and the non-covalent (reversible) catechol analogues. Following this strategy, we successfully developed innovative, cytocompatible, oxidative-resistant, and reversible crosslinked hydrogels, with the potential of being used as structural self-materials for a variety of applications, including in the biomedical field.

Cross-Linking-Filler Composite Materials of Functionalized Hexagonal Boron Nitride and Polyrotaxane Elastomer.

This study demonstrates cross-linking-filler composites in which covalent bonds between the fillers and polymer chains act as the main cross-linking points for the development of flexible and thermally conductive materials. Cross-linking-filler composites are fabricated using functionalized hexagonal boron nitride (hBN) fillers and polyrotaxane, called slide-ring polymers. The hBN particles modified with a carbon layer were produced by plasma processing in hydroquinone aqueous solution and functionalized with isocyanate groups. As the functionalized hBN provides cross-linking points for polyrotaxane, the cross-linking-filler composites can reduce cross-linking agents among polyrotaxane and exhibit considerable flexibility. Young's moduli of the cross-linking-filler composites are much lower than those of previously reported polyrotaxane composites while retaining their toughness. These values are relatively close to those of unfilled polyrotaxane elastomers, despite containing hBN fillers with a content of 50 wt %. Thus, the cross-linking-filler composites exhibit a combination of flexibility and thermal conductivity, which few hBN/elastomer composites have achieved.

Stabilization and Movable Ligand-Modification by Folate-Appended Polyrotaxanes for Systemic Delivery of siRNA Polyplex.

To achieve a systemic targeted delivery of siRNA using polymeric carriers, there is a dilemma between ligand modification and stabilization of the polyplex. Namely, ligand modification often leads to destabilization of the polyplex in the blood circulation. In fact, we previously developed cyclodextrin (CD)/polyamidoamine dendrimer conjugates (CDE) as siRNA carriers, and the interaction of CDE/siRNA was decreased by the conjugation with folate-polyethylene glycol, leading to the destabilization. To overcome this dilemma, in this study, folate-appended polyrotaxanes (Fol-PRX) were developed. Fol-PRX stabilized CDE/siRNA polyplex by intermolecularly connecting CDE molecules through a host-guest interaction between adamantane at the terminals of Fol-PRX and ß-CD in the polyplex. Moreover, the intermolecular connection of the polyplex with Fol-PRX provided movable folate moieties on the surface. As a result, Fol-PRXs enhanced the in vivo antitumor activity of the polyplex after intravenous administration, suggesting their utility as the dual-functional materials for systemic delivery of siRNA polyplexes.

A neuromechanical model for Drosophila larval crawling based on physical measurements.

BACKGROUND: Animal locomotion requires dynamic interactions between neural circuits, the body (typically muscles), and surrounding environments. While the neural circuitry of movement has been intensively studied, how these outputs are integrated with body mechanics (neuromechanics) is less clear, in part due to the lack of understanding of the biomechanical properties of animal bodies. Here, we propose an integrated neuromechanical model of movement based on physical measurements by taking Drosophila larvae as a model of soft-bodied animals. RESULTS: We first characterized the kinematics of forward crawling in Drosophila larvae at a segmental and whole-body level. We then characterized the biomechanical parameters of fly larvae, namely the contraction forces generated by neural activity, and passive elastic and viscosity of the larval body using a stress-relaxation test. We established a mathematical neuromechanical model based on the physical measurements described above, obtaining seven kinematic values characterizing crawling locomotion. By optimizing the parameters in the neural circuit, our neuromechanical model succeeded in quantitatively reproducing the kinematics of larval locomotion that were obtained experimentally. This model could reproduce the observation of optogenetic studies reported previously. The model predicted that peristaltic locomotion could be exhibited in a low-friction condition. Analysis of floating larvae provided results consistent with this prediction. Furthermore, the model predicted a significant contribution of intersegmental connections in the central nervous system, which contrasts with a previous study. This hypothesis allowed us to make a testable prediction for the variability in intersegmental connection in sister species of the genus Drosophila. CONCLUSIONS: We generated a neurochemical model based on physical measurement to provide a new foundation to study locomotion in soft-bodied animals and soft robot engineering.

High-yield one-pot synthesis of polyrotaxanes with tunable well-defined threading ratios over a wide range.

In this work, we report a high-yield one-pot synthesis of polyrotaxane (PR), composed of (2-hydroxypropyl)-α-cyclodextrin (hpCD) and polyethylene glycol (PEG), with well-defined hpCD threading ratios controllable across a wide range from 0.64% to 10%. In hpCD/PEG aqueous solutions, hpCDs are well dispersed and threaded spontaneously into hpCDs to form a pseudo-PR (pPR) structure. The homogeneous dispersion of hpCDs results in a well-defined threading ratio of hpCDs on PEG, which is suggested by the fact that the dispersity of the molecular weight distribution of PR is almost the same as that of pure PEG. The well-defined hpCD threading ratio of the PRs can be controlled over a wide range by tuning the hpCD concentration in the pPR solutions.

Fracture Behavior of Polyrotaxane-Toughened Poly(Methyl Methacrylate).

The fracture behavior of polyrotaxane (PR)-modified poly(methyl methacrylate) (PMMA) was investigated. PR is a supramolecule with rings threaded onto a linear backbone chain, which is capped by bulky end groups to prevent the rings from de-threading. The ring structure is α-cyclodextrin (CD), and it can be functionalized to enhance its affinity with the hosting polymer matrix. Adding only 1 wt % of PR containing methacrylate functional groups (mPR) at the terminal of some of the polycaprolactone-grafted chains on CD promotes massive crazing, resulting in a significant improvement in fracture toughness while maintaining the modulus and transparency of the PMMA matrix. Dynamic mechanical analysis and atomic force microscopy studies reveal that mPR strongly interact with PMMA, leading to higher molecular mobility and enhanced molecular cooperativity during deformation. This molecular cooperativity may be responsible for the formation of massive crazing in a PMMA matrix, which leads to greatly improved fracture toughness.

Polymer Brush Formation Assisted by the Hierarchical Self-Assembly of Topological Supramolecules.

The development of methods for the polymer brush layer formation on material surfaces to improve the surface properties has been researched for decades. Here, we report a novel approach for the formation of a polymer brush layer on materials and the alteration of the surface properties using a pseudo-polyrotaxane nanosheet (PPRNS). In the PPRNS, ß-cyclodextrin (CD) selectively covered the central poly(propylene oxide)29 segment of the carboxyl-terminated poly(ethylene oxide)75-b-poly(propylene oxide)29-b-poly(ethylene oxide)75 (COOH-EO75PO29EO75) triblock copolymer to form columnar crystals. The EO chains of COOH-EO75PO29EO75 then adopt polymer brush conformations and exhibit an oil-repellent property on the material surfaces. Based on the flexibility derived from the nanosheet structure, the PPRNS showed high adhesion to the Blu-ray disk substrate (1D bending), polystyrene spherical beads (2D bending), and random rough surface of pork skin. The PPRNS is expected to become a new method for obtaining polymer brush layers and improving the surface properties irrespective of the material type.

Hydrogen-Bonded Structure of Water in the Loop of Anchored Polyrotaxane Chain Controlled by Anchoring Density.

Hydrogen-bonded network of water surrounding polymers is expected to be one of the most relevant factors affecting biocompatibility, while the specific hydrogen-bonded structure of water responsible for biocompatibility is still under debate. Here we study the hydrogen-bonded structure of water in a loop-shaped poly(ethylene glycol) chain in a polyrotaxane using synchrotron soft X-ray emission spectroscopy. By changing the density of anchoring molecules, hydrogen-bonded structure of water confined in the poly(ethylene glycol) loop was identified. The XES profile of the confined water indicates the absence of the low energy lone-pair peak, probably because the limited space of the polymer loop entropically inhibits the formation of tetrahedrally coordinated water. The volume of the confined water can be changed by the anchoring density, which implies the ability to control the biocompatibility of loop-shaped polymers.

Direct enhancement of intercomponent interactions in polyrotaxane and its pronounced effects on glass state properties.

Strong interactions between the host cyclodextrin and the threading guest polymer were introduced by selective modifications to the polymer of a polybutadine-based polyrotaxane. The changes in the intercomponent interactions influenced the mobility of the threading polymer that was confined in the glassy host framework, resulting in different mechanical properties.

Softness, Elasticity, and Toughness of Polymer Networks with Slide-Ring Cross-Links.

Slide-ring (SR) gels cross-linked by ring molecules are characterized by softness (low Young's modulus), elasticity (low hysteresis loss), and toughness (large fracture energy). In this article, the mechanical and fracture properties of SR gels are reviewed to clarify the physical understanding of the relationship between the molecular-level sliding dynamics of the slide-ring cross-links and macroscopic properties of SR gels. The low Young's modulus and large fracture energy of SR gels are expressed by simple equations as a function of the degree of sliding movement. The dynamic fracture behaviors of SR gels gives us the time scale of the sliding dynamics of the cross-links, which is at the micro-sec scale. The fast sliding motion of the cross-links leads to the elasticity of the SR gels. The SR concept can be applied to solvent-free elastomers and composite materials.

Tough hydrogels with rapid self-reinforcement.

Most tough hydrogels are reinforced by introducing sacrificial structures that can dissipate input energy. However, because the sacrificial damage cannot rapidly recover, the toughness of these gels drops substantially during consecutive cyclic loadings. We propose a damageless reinforcement strategy for hydrogels using strain-induced crystallization. For slide-ring gels in which polyethylene glycol chains are highly oriented and mutually exposed under large deformation, crystallinity forms and melts with elongation and retraction, resulting both in almost 100% rapid recovery of extension energy and excellent toughness of 6.6 to 22 megajoules per cubic meter, which is one order of magnitude larger than the toughness of covalently cross-linked homogeneous gels of polyethylene glycol.

Molecular Recognition of Fluorescent Probe Molecules with a Pseudopolyrotaxane Nanosheet.

Pseudopolyrotaxane nanosheets (PPRNS) are ultrathin two-dimensional (2D) materials fabricated via supramolecular self-assembly of ß-cyclodextrin (ß-CD) and poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers. In this study, the molecular loading of various fluorescent probe molecules onto PPRNS was systematically investigated. 1H NMR study for R6G absorption to PPRNS indicated that the small hydrophobic groups, such as the methyl group, of R6G were absorbed by PPRNS. Consistently, the fluorescent probes without methyl groups were not absorbed. These results indicate that PPRNS has a molecular recognition absorption property based on the host-guest interaction of the functional groups on probe molecules and molecular-sized spaces of PPRNS surfaces, which may be vacant ß-CDs and voids between ß-CD columns. The absorbed amount of the molecular probes onto PPRNS was investigated by UV-vis spectra, and the absorption behavior could be described well by the Langmuir absorption isotherm. This is consistent with the suggested model that the probes are absorbed onto the PPRNS surfaces. This study demonstrates that PPRNSs can be applied as adsorbents for toxic compounds, drug delivery systems, and 2D sensors.

Precise control of cyclodextrin-based pseudo-polyrotaxane lamellar structure via axis polymer composition.

Self-assembly of cyclodextrin (CD) with guest polymers has attracted much attention owing to its biocompatibility and accessibility. In this study, we investigate the composition effect of poly(ethylene oxide)m-b-poly(propylene oxide)n-b-poly(ethylene oxide)m (EOmPOnEOm) triblock copolymers on lamellar or plate structures formed by complexation with ß-CD. EO5PO29EO5, EO14PO29EO14, and EO75PO29EO75 show periodic lamellar morphology consisting of single-crystalline pseudo-polyrotaxane (PPR) nanosheets with a thickness equal to the central PO length. This is because ß-CDs selectively cover the PO component and cause the microphase separation between ß-CD and EO layers. The thickness of the EO layers increases linearly with increasing number of EO units, which suggests that the EO chains are constrained into virtual cylinders with the diameter of the ß-CD. This means that we can precisely control the thickness of both the crystal (ß-CD and PO) and the amorphous (EO) layers in the lamellar structure. In contrast, EO2PO29EO2 forms a thin plate structure, where not only PO but also EO chains are covered with ß-CD. Furthermore, the length of the central PO component is necessary to form the lamellar structure with the phase separation between the ß-CD and EO layers. These findings provide a more fundamental understanding to enhance the variety and applicability of CD-based self-assembled materials.

Slide-Ring Material/Highly Dispersed Graphene Oxide Composite with Mechanical Strength and Tunable Electrical Conduction as a Stretchable-Base Substrate.

The development of stretchable elastomer composites with considerable mechanical strength and electrical conductivity is desired for future applications in communication tools, healthcare, and robotics. Herein, we have developed a novel stretchable elastomer composite by employing a slide-ring (SR) material as a matrix for restoration and graphene oxide (GO) as a precursor for a conductive filler. Highly dispersed GO in an organic solvent, prepared via a new method developed by the authors, allowed the uniform dispersion of GO into the matrix by simply mixing the solvent and SR. The resultant SR/GO composite exhibited considerably high mechanical toughness and cyclic durability. These properties were approximately maintained after pulse laser irradiation to add electrical conductivity on the composite by photoreducing of the dispersed GO, and its electrical conductivity was higher than that of the SR/graphene, carbon nanotubes, or graphite composites. The potential of the SR/GO composite as a stretchable base substrate for wearable devices was demonstrated by producing a prototype humidity sensor, a human motion monitoring sensor, and an electrical heater based on the composite with conductive circuits drawn using pulse laser patterning.

Anisotropic Amorphous X-ray Diffraction Attributed to the Orientation of Cyclodextrin.

The beauty of cyclic molecules is reflected in their host-guest complexation reactions, as well as their unique X-ray diffraction patterns. Cyclodextrins, the longest known host molecules with rigid ring structures, show anisotropic X-ray diffraction characteristic of their single-molecule structure, rather than their intermolecular relationships. Amorphous derivatives of α-cyclodextrin exhibit broad and strong halo diffractions in the solid, melted, and dilute solution states. The diffraction angle corresponds to the intramolecular distance between neighboring glycosidic oxygen atoms located at the vertices of a regular hexagonal array. Because the hexagon is parallel to the aperture plane of the rigid cyclic molecule, the diffraction appears only in the direction parallel to this plane. The anisotropy was confirmed by stretching an amorphous thermoplastic polymer threaded through the inclusion cavities of a sequence of cyclodextrins. The resultant unique anisotropic X-ray diffraction suggests the possible use of rigid cyclic molecules as molecular orientation probes.

Adhesion Force Analysis of Dynamic Polymer Brushes.

Spontaneous surface segregation of amphiphilic diblock copolymers at the water interface from the elastomeric portion was utilized for the fabrication of hydrophilic brushes, named as "dynamic polymer brush". Observation of the dynamic polymer brushes appears only when immersed in water and demands advanced experimental techniques for embedded interfaces such as neutron reflectivity. Measurement of the hydrophobic interaction at the polymer/water interface is not only an alternative method to monitor the brush but also reveals its unique surface properties. We carried out adhesion force measurements using atomic force microscopy with a hydrophobic probe for measuring the hydrophobic interactions of dynamic polymer brushes in water. Dynamic polymer brushes showed reduced hydrophobic interaction, which becomes more significant at higher graft density. Moreover, a unique transitional response to the applied pressure was observed for the dynamic polymer brush: the adhesion force was almost zero at low applied pressure and increased by further increasing the applied pressure. This phenomenon may indicate reallocation or retraction of the block copolymer chains from the contact area by the applied pressure, which are the unique characteristics of nonbound dynamic polymer brush chains. We also conducted adhesion force imaging and proved that dynamic polymer brushes form uniform layers without any defects, irrespective of brush density, which suggests that the interaction between the dynamic polymer brush chains is that of repulsion.

Interfacial Energy Measurement on the Reconstructive Polymer Surface: Dynamic Polymer Brush by Segregation of Amphiphilic Block Copolymers.

Herein, the interfacial energy of a reconstructive polymer surface formed by segregation is analyzed by measuring the change in the size of elastomer thin films floating on water. When a system in which amphiphilic diblock copolymers are mixed with the hydrophobic elastomer is in contact with water, surface reconstruction is triggered by the segregation of copolymers with a gain in the hydration energy of the hydrophilic blocks. The hydrophilic brush layer spontaneously formed at the elastomer-water interface is named the dynamic polymer brush. Although it is anticipated that the interfacial energy will significantly decrease in the dynamic polymer brush system, a direct measurement of the interfacial energy of the reconstructive interface is a challenge. We propose a novel method to measure the interfacial energy of a reconstructive polymer surface by measuring the deformation of elastomer thin films floating on water and apply it to the dynamic polymer brush system. The interfacial energy of the dynamic polymer brush formed by the segregation of amphiphilic diblock copolymers with longer hydrophilic chains drastically decreased to zero due to the high hydration energy of hydrophilic chains. Based on the neutron reflectometry results, the graft density and thickness of the dynamic polymer brush system floating on water were found to be lower than those of the system fixed onto solid substrates. This indicates that the floating system can respond to an external environment with a high degree of freedom (graft density, brush thickness, and interface area).

Molecular Dynamics Simulation and Theoretical Model of Elasticity in Slide-Ring Gels.

In this study, molecular dynamics (MD) simulations were carried out on the uniaxial deformation of slide-ring (SR) networks with slidable cross-links to understand the relationship between the sliding of the cross-linking points and the Young's moduli of SR gels, which are lower than those of covalently cross-linked gels with the same cross-linking densities. The slidability of the cross-links in SR gels was characterized by the rate of change of the segment number between the cross-links, Nslide, estimated by the MD simulation. We have successfully constructed a molecular model for the elasticity of SR gels and proposed a simple equation for the Young's moduli of SR gels as a function of Nslide. The theoretical model was compared with the MD simulation results and experimental data.

Synthesis of Poly(Methyl Methacrylate)-Based Polyrotaxane via Reversible Addition-Fragmentation Chain Transfer Polymerization.

A polyrotaxane (PR) with poly(methyl methacrylate) (PMMA) as the main chain polymer was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Because of the special mechanism of RAFT, the suprastructure of a PMMA-based PR is established by synthesizing inclusion complexes of methyl methacrylate and gamma-cyclodextrin (γCD) into the middle of the poly-N-(3-dimethylamino) propyl methacrylamide segments. The presence of threaded γCD was determined via diffusion ordered spectroscopy from the alignment of the mobility of γCD and the main chain polymer. A PMMA-based PR with 2-20% CD coverage and a molecular weight of 7K-60K g/mol of PMMA-based PR was synthesized with a targeted molecular structure by mediating the RAFT polymerization. The PMMA-based PR prepared in this study is expected to be suitable for wide applications of tough materials with good heat resistance. Moreover, the investigation of this synthetical approach opened possibilities for more variety of PR with controllable properties.