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Supramolecular slide-ring (SR) elastomers without solvent were fabricated by cross-linking of polymer-grafted polyrotaxane in which α -cyclodextrins (CDs) with poly- e -caprolactone (PCL) side chains are threaded on a polyethylene glycol (PEG) axial linear chain. The cross-links in the SR elastomers are capable of sliding along the backbone in response to applied stress. Mechanical properties of the SR elastomers with different cross-linking densities were compared with those of fixed cross-link (FC) elastomers prepared from PCL-grafted hydroxypropyl cellulose (HPC) to understand the influence of the slidable cross-links. The SR elastomers are found to undergo less strain-hardening at large strains and demonstrate higher strains at break than the conventional FC elastomers with the same Young's moduli.

The crack propagation behavior of pre-cut slide-ring (SR) gels with movable cross-links is investigated and compared to that of fixed cross-link (FC) gels under uniaxial stretching at a constant speed. Two fracture stages with 20-fold difference in crack velocity are observed for both SR and FC gels. At the beginning, the crack propagates slowly, and the crack propagation resistance is in good accordance with the evaluation based on tearing modulus Tr, the initial slope of J-integral – crack tip opening displacement (CTOD) curve. Higher Tr and slower crack velocity are observed in SR gels compared to FC gels with similar modulus. Moreover, Tr decreases and crack velocity increases with increasing modulus (cross-linking density) for FC gels, whilst both parameters are modulus-independent for SR gels. This means that the slow mode crack propagation resistance of SR gels is dominated by the sliding movement of cross-links instead of the cross-linking density. However, as the strain energy input exceeds a critical value, crack accelerates abruptly and overtakes the sliding dynamics of the cross-links, thus the fracture behavior of SR gels resembles that of FC gels. Our results have revealed the strong correlation between the cross-link mobility and crack propagation resistance of SR gels.

Polyrotaxane (PR), in which cyclic molecules are threaded into a linear polymer chain, has generated great interest because the sliding and rotation of the cyclic molecules on the axial polymer chain lead to unique functional nanomaterials with novel dynamical properties. A typical example of the functional materials is a polyrotaxane network, called slide-ring (SR) material, prepared by cross-linking the cyclic molecules on different PRs. The cross-links composed of two cyclic molecules in a shape of figure-of-eight slide along the polymer chains and the sliding motion gives rise to remarkable physical properties of the SR materials. In order to understand the unique features of the functional materials including SR materials and develop novel applications of PR, it is necessary to reveal the physical properties of PR, especially the sliding motion of the cyclic molecules in PR. In this article, we review the static structure and molecular dynamics of PR based primarily on our recent studies. Furthermore, the difference between SR materials and usual chemical gels in deformation behavior is also described. The findings summarized in this review indicate the significance of the sliding motion of cyclic molecules characterizing PR and SR materials.