Marked Sensitivity of Ultimate Elongation to Loading Axiality in Polyrotaxane Gels with Largely Slidable Cross Links.

Polyrotaxane (PR) gels with low ring densities have figure-of-eight cross links that can slide along network strands. The slidable cross links have a unique ability to increase the network strand length between adjacent cross links in the loading direction via chain supply from the stress-free direction, thereby enhancing the ultimate elongation (λm) of the gels. We reveal that this enhancement of λm due to the slidable cross links is pronounced specifically in uniaxial stretching, while it is considerably modest in biaxial stretching. The sensitivity of λm to loading axiality becomes larger as the ring densities decrease. The corresponding difference in λm is markedly larger for the PR gels with low ring densities than that for the networks with fixed cross links. The exceptional sensitivity of λm to loading axiality unveils a previously unidentified aspect of the chain-supply mechanism based on slidable cross links.

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.

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.

Analysis of High-Molecular-Weight Polyrotaxanes by MALDI-TOF-MS Using 3-Aminoquinoline-Based Ionic Liquid Matrix.

Polyrotaxane (PR) is a necklace-like supramolecule composed of cyclic components, such as cyclodextrin (CD), and a threading polymer capped with bulky end groups. PR exhibits peculiar mechanical properties attributed to the intermolecular cross-links with CD. Various CD molecules threaded on a linear PEG chain are often modified with chemical groups to add specific physicochemical properties. In general, the stoichiometry between CD and the PEG chain is a significant parameter that defines the unique physical properties of CD-based polyrotaxane (CD-PR). To date, mass spectrometry (MS) has been applied to investigate the molecular distribution of CD-PR, modifications of CD, and the threaded ratio of CD. However, only molecular weights (MWs) up to several 10s of kDa can be subjected to such analysis, whereas the MW of CD-PR used as industrial materials is much greater. Herein, we applied two ionic liquid matrices composed of 3-aminoquinoline and a high mass detector to analyze PRs using MALDI-TOF-MS. High to very high MW PRs in the range of 90-700 kDa were successfully analyzed using this method. The threaded ratio of CD was estimated from a single MW of CD, PEG, and PR. The ratios obtained were consistent with that obtained using 1H NMR. Furthermore, a single-stranded form of PR in γ-cyclodextrin threaded PR (γCD-PR) was clearly distinguished from a double-stranded form, which is only possible in γCD -PR because of its large host cavity.

Prolonged Glass Transition due to Topological Constraints in Polyrotaxanes.

Topological constraints in polyrotaxanes significantly affected their glass transition dynamics. The effects of the constraints were systematically studied using a series of different coverage glass-forming polyrotaxanes consisting of a common polymer and threaded ring molecule of varying ratios. Although their ratios were similar and hence exhibited similar Tg values by differential thermal analysis, mechanical relaxation was considerably prolonged with increasing coverage. The relaxation became a two-step process: a faster step at a common temperature near the Tg and another which was prolonged by the coverage increase. Relaxation dynamics analysis revealed that segment motions, which are cooperative translations of different components, freeze at considerably higher temperatures than the Tg with increasing coverage. This suggests that although the rings are released from conventional interactions at the Tg, their cooperative translational motions are significantly constrained by the threading polymers with increasing coverage.

Drastic Change of Mechanical Properties of Polyrotaxane Bulk: ABA-BAB Sequence Change Depending on Ring Position.

Polyrotaxane (PR), consisting of many ring molecules and an axis polymer, is a typical supramolecular structure with unique topological characteristics. In this study, we demonstrated the drastic change of the macroscopic mechanical properties depending on the ring position of PR in bulk. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer was employed as an axis polymer to control the position of ß-cyclodextrin (ß-CD). To transfer the ß-CD positions, hydroxypropyl groups (HPPR) and hydrophobic trimethyl silyl groups (TMS-HPPR), which have hydrophilic and hydrophobic ß-CD, respectively, were synthesized. ß-CDs in HPPR were localized on a central PPO segment and formed crystal domains. The axis polymer of HPPR could not bridge ß-CD crystal domains, resulting in a melt state at high temperature. On the other hand, ß-CDs in TMS-HPPR were transferred to both PEO segments and formed crystal domains. The axis polymer in TMS-HPPR could bridge the ß-CD crystal domains, resulting in an elastic state even at high temperature. We succeeded in demonstrating the potential ability of PR: the macroscopic mechanical properties of PR can be changed from a melt state to an elastic one by manipulating the ring positions.

Direct Determination of Cross-Link Density and Its Correlation with the Elastic Modulus of a Gel with Slidable Cross-Links.

The universal relationship between the elastic modulus and the cross-link density of a conventional rubber/gel has been demonstrated experimentally to be inapplicable to gels with slidable cross-links. Herein, we describe the synthesis of slide-ring (SR) gel networks devoid of intramolecular cross-links by the cross-coupling of two differently functionalized polyrotaxanes. The cross-link density was determined from the characteristic UV absorption attributed to the asymmetric cross-linked moiety. The cross-link density was shown to correlate considerably more weakly with the Young's modulus than conventional gels and rubbers that follow a universal proportional dependence. In addition, even at a similar cross-link density, the modulus appeared to be lower due to a lower density of cyclic components along the threading chain, i.e., the "coverage", though the data were limited in the narrow cross-link density range. These results might suggest a considerably lower contribution from the conformational entropy of chains associated with sliding through the cross-links and the counteracting entropy attributed to ring arrangement, though effects of the different persistence length due to the coverage difference could affect the modulus.

Polyrotaxane Brushes Dynamically Formed at a Water/Elastomer Interface.

Dense polymer brushes with closely packed rotaxane structures were formed at the interface of water and a styrene-butadiene elastomer by spontaneous segregation of an amphiphilic polyrotaxane (PR), a mechanically interlocked polymer consisting of hydrophobic polybutadiene threading through multiple hydrophilic γ-cyclodextrin (γ-CD) derivatives. Segregation of PR at the water/elastomer interface was suggested by X-ray photoelectron spectroscopy. The polymer brush structure at the water interface was investigated using neutron reflectometry. Brush structures were found to depend on the number of CDs on the PRs; the PR with a small number of CDs formed a thinner and homogeneous brush, whereas the PR with a higher number of CDs formed a thicker and less-ordered brush. These PR-brushes showed protein repulsion, resulting from the surface-hydrated brush layer preventing direct contact of proteins.

Pronounced effects of the densities of threaded rings on the strain-dependent Poisson's ratio of polyrotaxane gels with movable cross-links.

The density of threaded ring molecules (fCD) in polyrotaxane (PR) chains has pronounced effects on the strain-induced swelling of PR gels where the cross-linked ring molecules are slidable along the network strands. The equilibrium Poisson's ratio (µ∞), which is a measure of the strain-induced volume change, for the PR gel increases with an increase in elongation (λ) at moderate λ but becomes a constant value () at sufficiently large λ. When the modulus exceeds a threshold value (Ec), the λ dependence of µ∞ disappears due to the loss of the slidability of the cross-links. The fraction fCD significantly influences the values of and Ec. When fCD is sufficiently small (<14%), (≈0.25) agrees with the values of µ∞ for the classical gels in good solvents. When fCD is high (>25%), varies over a wide range (0.22 < < 0.33) depending on fCD and the cross-link concentration in a complicated way. The modulus Ec at fCD = 25% is more than twice as high as that at fCD = 5% due to the finite contribution of the larger amount of uncross-linked ring molecules via combinatorial entropy in the axial polymers. The origin of the markedly small values of µ∞ (less than 0.1) at small λ is also considered on the basis of the magnitude of the accompanying force reduction caused by the slidable function of the cross-links.

Ductile Glass of Polyrotaxane Toughened by Stretch-Induced Intramolecular Phase Separation.

A new class of ductile glasses is created from a thermoplastic polyrotaxane. The hard glass, which has a Young's modulus of 1 GPa, shows crazing, necking, and strain hardening with a total elongation of 330%. Stress concentration is prevented through a unique stretch-induced intramolecular phase separation of the cyclic components and the exposed backbone. In situ synchrotron X-ray scattering studies indicate that the backbone polymer chains slip through the cyclic components in the regions where the stress is concentrated.

Unusual Fracture Behavior of Slide-Ring Gels with Movable Cross-Links.

In this study, the quasi-static fracture behavior of slide-ring gels (SR gels), in which movable cross-links can slide on polymer chains, is for the first time investigated and compared to that of conventional polymer gels with fixed cross-links (FC gels). For the usual FC gels, there is a trade-off relation between toughness (fracture energy [Formula: see text]) and stiffness (Young's modulus E): with increasing cross-linking density, the Young's modulus E increases, while fracture energy [Formula: see text] decreases. However, SR gels show an unusual fracture behavior that contradicts this trade-off relation. The fracture energy of SR gels is independent of the Young's modulus, in other words, the cross-linking density; moreover, it rises with increasing slidable range of movable cross-links on polymer chains. A new molecular model is proposed by attributing the unusual fracture properties of SR gels to the relative sliding movement between polymer chains and cross-links. Utilizing this concept, simultaneous fulfillment of high stiffness and high toughness in polymer gels can be realized.

One-Pot Synthesis and Characterization of Polyrotaxane-Silica Hybrid Aerogel.

A novel kind of polyrotaxane-silica hybrid aerogel is successfully prepared via one-pot sol-gel synthesis in this work. The polyrotaxane can chemically interpenetrate with Si particles homogeneously in nanoscale, so as to shorten the gelation time and construct a flexible and mechanically strong skeleton. The supramolecular effect ascribable to the sliding motion of cyclic components in polyrotaxane is introduced into the hybrid aerogel for the first time. Compared with the brittle pure silica aerogel, the obtained polyrotaxane-silica hybrid aerogels show very low density, low thermal conductivity, and more than two orders magnitude improvement in the compression strength without compromising transparency.

Volume Phase Transitions of Slide-Ring Gels.

The volume phase transition of slide-ring gels with freely-movable cross-linking junctions was investigated. Ionic chemical gels with fixed cross-linking junctions undergo volume phase transitions when they have higher than the critical degree of ionization. However, the experimentally-observed critical ionization value for slide-ring gels is much higher than theoretical values for chemical gels. This difference indicates that the volume phase transition is significantly suppressed in slide-ring gels. The mesoscale structure at various swollen or shrunken states was also investigated by small angle X-ray scattering. Changes in the scattering patterns with shrinking slide-ring gels suggest microphase separation due to the sliding of cyclic molecules threaded along the axis of the polymer chains, which may suppress the volume phase transition. In addition, slide-ring gels absorbed/desorbed greater than equilibrium volumes in the shrinking/swelling processes and showed slow dynamics; these observations are also related to their sliding properties.

Synthesis, structure, and mechanical properties of silica nanocomposite polyrotaxane gels.

A significantly soft and tough nanocomposite gel was realized by a novel network formed using cyclodextrin-based polyrotaxanes. Covalent bond formation between the cyclic components of polyrotaxanes and the surface of silica nanoparticles (15 nm diameter) resulted in an infinite network structure without direct bonds between the main chain polymer and the silica. Small-angle X-ray scattering revealed that the homogeneous distribution of silica nanoparticles in solution was maintained in the gel state. Such homogeneous nanocomposite gels were obtained with at least 30 wt % silica content, and the Young's modulus increased with silica content. Gelation did not occur without silica. This suggests that the silica nanoparticles behave as cross-linkers. Viscoelastic measurements of the nanocomposite gels showed no stress relaxation regardless of the silica content for <20% compression strain, indicating an infinite stable network without physical cross-links that have finite lifetime. On the other hand, the infinite network exhibited an abnormally low Young's modulus, ~1 kPa, which is not explainable by traditional rubber theory. In addition, the composite gels were tough enough to completely maintain the network structure under 80% compression strain. These toughness and softness properties are attributable to both the characteristic sliding of polymer chains through the immobilized cyclodextrins on the silica nanoparticle and the entropic contribution of the cyclic components to the elasticity of the gels.

A significant impact of host-guest stoichiometry on the extensibility of polyrotaxane gels.

Modification of the guest polymer ends efficiently and substantially decreases the density of the threaded rings. The minimized ring density drastically facilitates chain sliding through the cross-links in polyrotaxane gels. This molecular design is like regulating the "pressure" of the rings confined in the interlocked architecture, which counteracts the sliding.

Polyrotaxane Glass: Peculiar Mechanics Attributable to the Isolated Dynamics of Different Components.

The molecular dynamics of condensed polymers are directly linked to practically important mechanical properties, such as glass transition behavior and impact strength. We present a unique viscoelastic glass-forming polyrotaxane that is comprised of a main chain polymer and threaded cyclodextrins. The abnormally broad glass transition phase is attributed to the segment motion, which is unprecedentedly insusceptible to cooperative motion with neighboring chains until vitrification is completed. Even in the glass state, in which the cyclic components, occupying >80 wt % of the material, are vitrified, the main chain polymer retains high mobility within the glassy porous framework. The anomalous mobility of the polymer chains causes strong subrelaxation that changed the elastic modulus about 3-fold. These results demonstrate that the unique dynamics and mechanics are attributable to the loose correlation between different components, suggesting the possible creation of unprecedented mechanical properties based on the molecular design of mechanically interlocked polymers.

Synthesis of graft polyrotaxane by simultaneous capping of backbone and grafting from rings of pseudo-polyrotaxane.

Graft polyrotaxanes, with poly(ε-caprolactone) (PCL) graft chains on the ring components were synthesized by the simultaneous ring-opening polymerization of ε-caprolactone from both ends of the backbone polymer, an end-functionalized polyethylene glycol (PEG) and the formation of inclusion complexes with α-cyclodextrin (α-CD). PEG with multiple functional groups at each end was prepared by the condensation of PEG-amine and D-gluconic acid; the PEG derivative formed an inclusion complex with α-CD. The polymerization of multiple hydroxy groups at the backbone ends resulted in a star-shaped end group, which served as a bulky capping group to prevent dethreading. In contrast, PEG with only one hydroxy group at each end did not produce polyrotaxanes, indicating that single PCL chains were too thin to confine α-CDs to the complex. In addition, the grafting polymerization proceeded properly only when robust hydrogen bonds formed between α-CDs were dissociated using a basic catalyst. Since the dissociation also induced dethreading, kinetic control of the polymerization and dissociation were crucial for producing graft polyrotaxanes. Consequently, this three-step reaction yielded graft polyrotaxanes in a good yield, demonstrating a significant simplification of the synthesis of graft polyrotaxanes.

[Risk-benefit analysis of 18FDG PET cancer screening].

The benefits of (18)F-fluorodeoxyglucose ((18)FDG) positron emission tomography (PET) cancer screening are expected to include a large population of examinees and are intended for a healthy group. Therefore, we attempted to determine the benefit/risk ratio, estimated risk of radiation exposure, and benefit of cancer detection. We used software that embodied the method of the International Commission on Radiological Protection (ICRP) to calculate the average duration of life of radiation exposure. We calculated the lifesaving person years of benefit to be obtained by (18)FDG PET cancer screening detection. We also calculated the benefit/risk ratio using life-shortening and lifesaving person years. According to age, the benefit/risk ratio was more than 1 at 35-39 years old for males and 30-34 years old for females. (18)FDG PET cancer screening also is effective for examinees older than this. A risk-benefit analysis of (18)FDG-PET/computed tomography (CT) cancer screening will be necessary in the future.

Cooperativity and selectivity in chemomechanical polyethylenimine gels.

Gel particles obtained from polyethylenimine (PEI) cross-linked with 10% ethylene glycol diglycidyl ether show considerable selectivity in size changes if exposed to solutions of different effector compounds. Monocarboxylic acids induce contractions of, e.g., -9% for benzoate and -40% for 2-naphthoate (all values in one dimension such as length). With diacids, the contractions increase to -29% for 1,4-benzene and -68% for 2,6-naphthalene disulfonate, indicating the significant contributions of noncovalent ionic cross-linking and of interactions with aromatic residues. A striking cooperativity is observed if aromatic compounds such as naphthoic acid are used simultaneously with amino acids as effectors. The combination with, e.g., 2-naphthoic acid and phenylalanine induces, e.g., 69% contraction, whereas the single effector compounds induce only 40% and 8%, respectively. As expected with contraction of chemomechanical polymers, the kinetics of effector absorption is significantly slower than that of the size changes, which take, e.g., 5 min to reach one-half of the final contraction; with smaller gel particles, the rates are significantly increased. Gravimetric determinations show that release of solvation water is largely responsible for the observed volume contractions. Copper or zinc ions induce small contractions, also in experiments with PEI solutions, but show no evidence of cooperativity in combination with amino acids or peptides. MAS NMR spectra of the gels induced by aromatic effector compounds exhibited moderate upfield shifts of the polymer backbone signals, as a result of ring current effects.