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Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and its copolymer hydrogels are typical polyelectrolyte gels with extremely high swelling capacity that are widely used in industry. It's common to consider these hydrogels as weak materials that are difficult to toughen. Reported here is a facile strategy to transform swollen and weak poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid) P(AAm-co-AMPS) hydrogels to tough ones by forming strong sulfonate-Zr4+ metal-coordination complexes. The resultant hydrogels with moderate water content possess high stiffness, strength, and fracture energy, which can be tuned over 3-4 orders of magnitude by controlling the composition and metal-to-ligand ratio. Owing to the dynamic nature of the coordination bonds, these hydrogels show rate- and temperature-dependent mechanical performances, as well as good self-recovery properties. This strategy is universal, as manifested by the drastically improved mechanical properties of hydrogels of various natural and synthetic sulfonate-containing polymers. The toughened hydrogels can be converted to the original swollen ones by breaking up the metal-coordination complexes in alkaline solutions. The reversible brittle-tough transition and concomitant dramatic volume change of polyelectrolyte hydrogels afford diverse applications, as demonstrated by the design of a tubular grasper with holding force a thousand times its own weight for objects with different geometries. It is envisioned that these hydrogels enable versatile applications in the biomedical and engineering fields.
Yu et al. (Thu,) studied this question.