There are two basic approaches to prepare the supramolecular polymeric materials through host?guest interactions: (1) from a mixture of host and guest polymers, and (2) from polymerization of host and guest monomers. To create high strength and short self-healing materials, we chose the method of (2).
Prior to radical copolymerization, hydrophobic guest monomers (Adamantane, Ad), which are insoluble in water, were dissolved in corresponding aqueous solutions of βCDs to form inclusion complexes. After polymerization, the homogeneous solutions yielded hydrogels (Figure 1).
Figure 1. Chemical structures of the self-healable supramolecular hydrogel (a) and photographs of these hydrogels.
The β CDAd gel(7, 6) sample immediately mends after being broken; the gel can be lifted against its own weight. The repaired β CDAd gel(7, 6) adheres strongly without a crack after crushing and dropping (Figure 2a). 1-Adamantane carboxylic acid sodium salt (AdCANa, Figure 2 b) or β CD, in aqueous solutions, were used as the competitive molecules for the β CDAd gel(7, 6). In the presence of competitive molecules on the cut surface, the gels do not adhere within 24 h. The competitive molecules inhibit the complexation between the CD and guest units on the cut surface, which function as crosslinkers to adhere the two cut gels. We successfully prepared self-healing CDguest gels crosslinked between polymer chains with inclusion complexes. The self-healing behavior exhibited by forming inclusion complexes of the free CD and guest units on the cut surfaces (Figure 2b).
(Kakuta, T.; Takashima, Y.; Nakahata, M.; Otsubo, M.; Yamaguchi, H.; Harada, A., Adv. Mater. 2013, 25, 2849.)
Figure 2. Self-healing behavior of the host-guest gel.
