Multi-energy dissipation mechanisms in supramolecular hydrogels with fast and slow relaxation modes.

Reversible cross-links by non-covalent bonds have been widely used to produce supramolecular hydrogels that are both tough and functional. While various supramolecular hydrogels with several kinds of reversible cross-links have been designed for many years, a universal design that would allow control of mechanical and functional properties remains unavailable. The physical properties of reversible cross-links are usually quantified by thermodynamics, dynamics, and bond energies. Herein, we investigated the relationship between the molecular mobility and mechanical toughness of supramolecular hydrogels consisting of two kinetically distinct reversible cross-links via host-guest interactions. The molecular mobility was quantified as the second-order average relaxation time (〈τ〉w) of the reversible cross-links. We discovered that hydrogels combining fast (〈τ〉w = 1.8 or 18 s) and slowly (〈τ〉w = 6.6 × 103 or 9.5 × 103 s) reversible cross-links showed increased toughness compared to hydrogels with only one type of cross-link because relaxation processes in the former occurred with wide timescales.

Redox-responsive supramolecular polymeric networks having double-threaded inclusion complexes.

Stimuli-responsive hydrogels have attracted attention as soft actuators that act similarly to muscles. In this work, hydrogel actuators controlled by host-guest interactions have been developed. The introduction of a 1:1 inclusion complex into a hydrogel is a popular design for achieving a change in cross-linking density. To realize faster and larger deformation properties, the introduction of a 1:2 inclusion complex is effective because the alteration in cross-linking density in a hydrogel with 1:2 complexes is larger than that in a hydrogel with 1:1 complexes. A redox-responsive hydrogel actuator cross-linked with 1:2 inclusion complexes is designed, where γ-cyclodextrin (γCD) and viologens modified with an alkyl chain derivative (VC11) were employed as the host and guest units, respectively. γCD includes two VC11 molecules in its cavity. The obtained γCD-VC11 hydrogel cross-linked with the 1:2 complex showed faster and larger deformation behaviour than the αCD-VC11 and the ßCD-VC11 hydrogels cross-linked with a 1:1 complex. The deformation ratio and response speed of the γCD-VC11 hydrogel, which forms a supramolecular cross-linking structure by stimuli, are 3 and 11 times larger, respectively, than those of our previous hydrogel consisting of a ßCD/ferrocene 1:1 inclusion complex.