Multiple Cross-Linking-Dominated Metal–Ligand Coordinated Hydrogels with Tunable Strength and Thermosensitivity

Development of stimuli-responsive hydrogels with improving mechanical strength is highly important for widening the practical applications of the smart soft hydrogel materials. In this paper, the metal–ligand coordinated hydrogels with tunable strength and thermosensitivity were fabricated by a facile two-step method including polymerization and post cross-linking. Compressive tests indicate that the covalent cross-linking degree, coordinated cross-linking degree and the type of coordinated metal ions largely impact the mechanical properties. Four types of the first transition metal ions including Zn2+, Cu2+, Co2+, and Ni2+ were used for coordination, which enhanced the mechanical properties of the hydrogels to different extents. In particular, the compressive strength of Zn2+ coordinated hydrogel is significantly improved compared with other ions coordinated hydrogels. The deswelling tests and differential scanning calorimetry (DSC) characterizations proved that the type of metal ions has great influence on the lower critical solution temperature (LCST) of the hydrogels. Herein, we attribute the different mechanical strength and phase transition behavior to the relaxation times as well as the divers spatial cross-linking density of the hydrogels, which may provide a avenue for the design of high-strength and fast-response hydrogels.