Improvement of Mechanical Strength and Fatigue Resistance of Double Network Hydrogels by Ionic Coordination Interactions

Double network hydrogels (DN gels) are considered as one of the toughest soft materials. However, conventional chemically linked DN gels often lack high self-recovery and fatigue resistance properties due to permanent damage of covalent bonds upon deformation. Current strategies to improve self-recovery and fatigue resistance properties of tough DN gels mainly focus on the manipulation of the first network structure. In this work, we proposed a new design strategy to synthesize a new type of Agar/PAMAAc-Fe³⁺ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe³⁺ gel as the second chemical–physical network. By introducing Fe³⁺ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe³⁺ DN gels can achieve extremely high mechanical properties (σ_f of ∼8 MPa, E of ∼8.8 MPa, and W of ∼16.7 MJ/m³), fast self-recovery (∼50% toughness recovery after 1 min of resting), and good fatigue resistance compared to properties of cyclic loadings by simply controlling acrylic acid (AAc) content in the second network. The high toughness and fast recovery of Agar/PAMAAc-Fe³⁺ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e., hydrogen bonds in the agar network and Fe³⁺ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe³⁺ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe³⁺ coordination interactions in mechanical strength, self-recovery, and fatigue resistance of DN gels. Different mechanistic models were proposed to elucidate the mechanical behaviors of different agar-based DN gels. Our results offer a new design strategy to improve strength, self-recovery, and fatigue resistance of DN gels by controlling the structures and interactions in the second network. We hope that this work will provide an alterative view for the design of tough hydrogels with desirable properties.