Synthesis and Properties of Slide-Ring Gels
We have recently developed a novel type of gel, called the slide-ring gel or topological gel, which is different from physical and chemical gels, utilizing supramolecular architecture with topological characteristics. In this gel, polymer chains with bulky end groups exhibit neither covalent cross-links, as in chemical gels, nor attractive interactions, as in physical gels, but are topologically interlocked by “figure-of-eight” cross-links. These cross-links can move along the polymer chains freely to equalize the tension of the threaded polymer chains, in a manner similar to pulleys; this is called the pulley effect. The slide-ring gel consisting of movable cross-links and free uncross-linked cyclic molecules shows peculiar mechanical properties, different from conventional chemical gels, due to microscopic slidability at the cross-link. In this review, we describe our recent designs of materials and findings on the mechanical properties of the slide-ring gels attributed to the slidable cross-links that are built on a synthetic breakthrough that yields a new series of slide-ring materials composed of various polymer backbones. The finding of a sliding transition reveals the dynamics of sliding, the significance of the alignment entropy of uncross-linked cyclic compounds, and helps in determining the mechanical properties of slide-ring materials.