We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly­(ethylene glycol) (PEG) cross-linked by a [c2]­daisy chain, which is a double-threaded [2]­rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/vis spectroscopy results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene unit in the [c2]­daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest that αCD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that photoisomerization allows αCD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of αCD with cis-stilbene is quite low. Thus, the sliding motion of the αCD unit shrinks the [c2]­daisy chain, leading to the contraction of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier weight compared to the dry gel).

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We prepared acrylamide monomers with permethylated cyclodextrins (PM-CDAAmMe) or peracetylated cyclodextrins (PAc-CDAAmMe). PM-CDAAmMe and PAc-CDAAmMe are soluble in various hydrophobic liquid acrylate monomers, and they can form inclusion complexes with guest monomers such as adamantane or fluoroalkyl groups tethered to a vinyl residue. The bulk polymerization of the liquid acrylate monomers with the PM-CDAAmMe or PAc-CDAAmMe monomers and the guest monomers gave highly flexible and tough elastomers. Tensile tests on the obtained supramolecular elastomers showed fracture strains of over 800% and fracture energies that were 12 times larger than those of covalently cross-linked conventional elastomers, indicating that the host–guest cross-linking made the supramolecular elastomers quite tough. During the deformation process, the applied stress is dispersed into the supramolecular elastomers by dissociation and recombination of the reversible host–guest complex. Moreover, these host–guest complexes also allow the adhesion of fractured pieces of the supramolecular elastomers without adhesives. The mechanical strength of the fractured elastomer was restored to ∼99% of its initial strength within 4 h. The self-healing properties can be attributed to the reversible cross-linking by the host–guest interactions.

Highly flexible and tough elastomers were obtained from the bulk copolymerization of a peracetylated cyclodextrin (CD) monomer and small alkyl acrylate main chain monomers without a guest monomer. The main chains penetrated the cavity of the CD units, and the CD units on the polymer chain acted as movable cross-linking points in the obtained elastomer. In contrast, the copolymerization using a bulky main chain monomer with bulky side groups gave linear polymers. The CD units with the bulky main chain polymer cannot serve as movable cross-linking points. Introducing movable cross-linking into poly­(ethyl acrylate) resulted in a higher fracture energy comparable to that of conventional rubbers because of the stress-dispersion properties related to the sliding motion of the movable cross-linking points. The movable cross-linkers disperse applied external stresses more effectively than an elastomer with reversible cross-linking at a high Young’s modulus (150 MPa). Movable cross-linking can be introduced to enhance the fracture energy of polymeric materials.

Self-healing materials constructed from host–guest interactions between macrocyclic compounds as host molecules and specific guest molecules.