Self-Healing Supramolecular Hydrogel: Polyrotaxane Cross-Linked By Host-Guest Interactions

Abstract

Self-healing materials, which can extend the service life of objects, have attracted much attention in efforts to create a sustainable society. One of the most important approaches to obtain self-healing materials is non-covalently bonded systems. Unlike covalently bonds, non-covalent interactions such as hydrogen bonds, π-π stacking interactions, metal-ligand coordination bonds, and host-guest interactions [1] are reversible. It has been recently reported that materials recover their original mechanical property when they are damaged. This self-healing property occurs by introducing a non-covalent interaction site into the end or side chain of a polymer. In this regard, the healing time is affected by the mobility of the polymer chain because self-healing occurs by the formation of non-covalent bonds between the responsible sites via non-covalent interactions in the polymer. To address the expected practical demands and requirements, the development of a self-healing material that recovers in a short time is in high demand. For this purpose, we employ polyrotaxanes (PRxs), which are a class of supramolecular threaded macromolecules in which cyclic molecules are threaded onto linear polymers.[2] The advantage of PRxs is that threaded cyclic compounds are freely mobile along the linear polymer because they are non-covalently assembled between the linear polymer and many cyclic molecules. Thus, compared to functional groups conjugated to a conventional polymer side chain, the functional group conjugated with the cyclic molecules in PRx can effectively bind to the target. Herein we report a self-healing material based on PRx through host-guest interactions. This material was prepared by radical copolymerization of acryl amide (AAm) and the host monomer in the presence of PRx introduced with guest molecules. The resulting material exhibits a self-healing property, and its healing speed is faster than a self-healing material without PRx. [1] (a) T. Kakuta, Y. Takashima, M. Nakahata, M. Otsubo, H. Yamaguchi, A. Harada, Adv. Mater. 2013, 25, 2849-2853. (b) M. Nakahata, Y. Takashima, A. Harada, Macromol. Rapid. Commun. 2016, 37, 86-92. [2] (a) Y. Kobayashi, Y. Nakamitsu, Y. Zheng, Y. Takashima, H. Yamaguchi, A. Harada, Chem. Commun. accepted (b) A. Harada, J. Li, M. Kamachi, Nature 1992, 356, 325-327. Figure 1. (a) Chemical structures of Ad(x)DMEDAPRx–βCD gel, Ad(x)–βCD gel, and AAm(x) gel. (b) Schematic illustration of the self-healing feature of Ad(x)DMEDAPRx–βCD gel. Here, x represents the mol% of the cross-linking unit. Figure 1