Clay Nanosheet-Based Nanocomposite Supramolecular Hydrogel Enabling Rapid, Reversible Phase TransitionOnly withVisible Light.

Abstract

High mechanical properties and rapid sol/gel phase transition are mutually exclusivein the hydrogels reported to date, most likely because the 3D crosslinked networks of mechanicallyrobust hydrogels comprise bundled thick fibers that are not rapidly dissociable or formable.Herein, we report a visible light-responsive hydrogelthatshowed a rapid, reversible sol/gel phase transition despite its relatively high mechanical properties (storage modulus ~1000Pa).To construct its 3D crosslinked network, we used a design strategy analogous to that employed for our highly water-rich yet mechanically robust nanocomposite supramolecular hydrogel("aqua material").In this case, multiple poly(ethylene glycol) chains carryingortho-tetramethoxyazobenzenetermini (AzoPEG) were noncovalently crosslinked by clay nanosheets (CNSs) with surface-immobilizedβ-cyclodextrin units using their seven guanidinium ion(Gu+) pendants (GuCD)via a multivalent salt-bridge.Whenexposed to visible light at 625 and 450 nm, the azobenzene termini isomerized fromtrans-to-cisandcis-to-trans, respectively, and were detached from and attached to the surface-immobilizedGuCD units.The advantage of thisCNS-based nanocomposite supramolecular system is its simple 3D network structure, which formsand breaksrapidly without slow chain entangling and disentangling processes.