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Slide-Ring Materials Using Supramolecular Polyrotaxane : Topological Network with Entropy of Rings

We have recently developed a novel type of polymer network called slide-ring materials by cross-linking polyrotaxane, the supramolecular architecture with topological characteristics. In the network, polymer chains are topologically interlocked by figure-of-eight cross-links. Hence, these cross-links can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys. Because the cross-linking junction can move in the polymer network, the structure and physical properties of the polymeric materials are drastically different from conventional cross-linked or noncross-linked materials. For instance, the slide-ring gel or elastomer shows quite small Young’s modulus, which is not proportional to the cross-linking density and much lower than those of chemical gels with the same density. This arises from the difference in the molecular mechanism of the entropic elasticity : While the conformational entropy is mainly responsible for the elasticity in usual chemical gels or rubbers, the mechanical properties of the slide-ring gel should be inherently governed by the alignment entropy of free cyclic molecules in polyrotaxane as well as the conformational entropy of backbone polymer. This means that the softness in the slide-ring gel is due to the novel entropic elasticity, which is also expected to yield sliding state and sliding transition. Consequently, the slide-ring materials have the dynamic coupling between two kinds of entropy : the conformational entropy of strings and alignment entropy of rings.
The concept of the slide-ring gel is not limited to cross-linked gels but also includes elastomer, cross-linked polymeric materials without solvent. Accordingly it can be applied to wide area such as soft contact lens, paints, rubbers, soft actuator and so on. As a typical example, the scratch-resist properties of the self-restoring slide-ring elastomer were adopted into the top coating on the mobile phone.

Fig.1. Schematic diagram of slide-ring materials based on supramolecular network.

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