Recyclable, Healable, and Tough Ionogels Insensitive to Crack Propagation

Abstract

In practical applications, long-term load cycles tend to cause fatigue damage to the ionogels and induce cracks, reduce their stability and accuracy, greatly decrease their service life, and increase operating costs. At present, no effective strategy has been developed to fundamentally solve the problem of crack propagation sensitivity of ionogels. For example, double network (DN) gels (such as poly(1-acrylamido-2-methylpropane sulfonic acid) (PAMPS)/ polyacrylamide (PAAM) hydrogel [18] ) prevent crack propagation through the fracture of primary and sacrificial networks. However, covalent crosslinking network is usually irreversible, and fatigue resistance is limited under long-term cyclic loading. [9,[18][19][20][21][22] Moreover, introducing reversible dynamic bonds (such as ionic and hydrogen bonds) into polymer networks can effectively enhance the selfhealing ability during fatigue damage or crack propagation. [13,[23][24][25][26][27] However, due to the lack of an additional energy dissipation mechanism, irreversible fatigue damage can be caused by crack propagation during long-term cyclic loading. Furthermore, reversible bonds cannot dissipate the stress concentration at the pre-cut crack tip and are unable to prevent crack propagation. [21] Microgels have been used as DN structure to enhance the mechanical properties of hydrogels. [21,28,29] The matrix polymers are chemically crosslinked rather than linear polymer segments, which greatly limits the deformability of the hydrogels. In addition, the chemically crosslinked single network outside the microgels leads to the catastrophic expansion of the crack at the notch of the hydrogels. Therefore, the crack propagation insensitivity and fatigue resistance were sacrificed. Addition of nanocomposites improved the toughness and strength of the hydrogels through the interaction between surface functional groups and external polymer matrix (hydrophobic association by amphiphilic triblock copolymer, strong hydrogen bond, and coordination bond). [19,22,[30][31][32][33] The hydrogels need residence time to recover the damaged mechanical properties under cyclic load due to the unstable mechanical properties of reversible bonds.Recyclability and mechanical stability of polymer networks are contradictory properties. The covalently crosslinked rigid network ensures the stability of the polymer framework at the expense of the regeneration and recycling capacity. The noncovalent bond in the reversible dynamic bond system is a weak Most gels and elastomers introduce sacrificial bonds in the covalent network to dissipate energy. However, long-term cyclic loading caused irreversible fatigue damage and crack propagation cannot be prevented. Furthermore, because of the irreversible covalent crosslinked networks, it is a huge challenge to implement reversible mechanical interlocking and reorganize the polymer segments to realize the recycling and reuse of ionogels. Here, covalent crosslinking of host materials is replaced with entanglement. The entangled microdomains are used as...

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