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

Abstract

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 selfrecovery 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-Fe 3+ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe 3+ gel as the second chemical−physical network. By introducing Fe 3+ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe 3+ DN gels can achieve extremely high mechanical properties (σ f of ∼8 MPa, E of ∼8.8 MPa, and W of ∼16.7 MJ/m 3 ), 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-Fe 3+ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e., hydrogen bonds in the agar network and Fe 3+ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe 3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe 3+ 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, selfrecovery, 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.

Recent Advances in Metal‐Containing Polymer Hydrogels

“…[34] Then, a new type of Agar/PAMAAc-Fe 3+ DN hydrogels was obtained by introducing Fe 3+ ions into the second network to form strong coordination interactions. These DN hydrogels possess high mechanical properties, fast self-recovery (50% toughness recovery after 1 min of resting), and good fatigue resistance.…”

Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

“…For hydrogels that show promise in dissipating mechanical energy, many researchers have pursued insufficient study of their material through very low‐cycle tensile testing conditions (10 to 1000 cycles) with no systematic varying of the stress or strain the material experiences to determine a material's inherent resistance to fatigue. [ 4,59,60 ] It is impossible to extract information on a hydrogel's fatigue resistance from single stress amplitude tests, stopped at arbitrary cycle numbers, though many studies have focused on this analysis. [40–42, 61–66 ] An overarching goal of this work is to understand the tensile fatigue properties of PVA with and without additives.…”

Ultrastretchable, Super Tough, and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

“…The elastic modulus of NC‐DN hydrogels decreased from 120 to 4 kPa at a strain of 500% and 2000%, respectively (Figure d). To quantify the internal damage of NC‐DN gels, a softness parameter was defined as (1 − E X / E 0 ) × 100%, where E X and E 0 were elastic moduli at different strain and initial modulus, respectively. The softness parameter of NC‐DN hydrogels rapidly increased to 62% at a strain of 1000% and slowly increased to 94% at a strain of 2000%.…”