Harder under stress
Hydrogels are generally weak materials because they contain only a small fraction of highly water-swollen polymeric material. Strength can be increased by adding additional crosslink or interpenetration networks with sacrificial bonds to improve toughness; however, these properties deteriorate upon stretching and may be slow to recover after relaxation. Liu et al. developed polyethylene glycol hydrogels crosslinked with moderate fractions of polymers that form sliding rings. These allow the chain to orient in parallel when stretched, which leads to rapid and reversible stress-induced crystallization and therefore much stronger hydrogels.
Science, aaz6694, this number p. 1078
Most resistant hydrogels are strengthened by introducing sacrificial structures that can dissipate input energy. However, because sacrificial damage cannot recover quickly, the toughness of these gels drops dramatically during consecutive cyclic charges. We propose a damage-free reinforcement strategy for hydrogels using strain-induced crystallization. For slip ring gels in which the polyethylene glycol chains are strongly oriented and mutually exposed under great strain, crystallinity forms and melts with elongation and shrinkage, resulting in both rapid recovery of almost 100%. extension energy and excellent toughness of 6.6 to 22 megajoules per square meter, which is an order of magnitude greater than the toughness of homogeneous covalently crosslinked polyethylene glycol gels.