Phase-separation induced extraordinary toughening of magnetic hydrogels

Phase separation markedly influences the physical properties of hydrogels. Here, we find that poly (N, N-dimethylacrylamide) (PDMA) hydrogels suffer from phase separation in aqueous sodium hydroxide solutions when the concentration is higher than 2 M. The polymer volume fraction and mechanical properties show an abrupt change around the transition point. We utilize this phase separation mechanism to synthesize tough magnetic PDMA hydrogels with the in-situ precipitation method. For comparison, we also prepared magnetic poly (2-acrylamido-2-methyl-propane sulfonic acid sodium) (PNaAMPS) magnetic hydrogels, where no phase separation occurs. The phase-separated magnetic PDMA hydrogels exhibit an extraordinarily high toughness of ∼1000 J m^-2; while non-phase-separated magnetic PNaAMPS hydrogels only show a toughness of ∼1 J m^-2, three orders of magnitude lower than that of PDMA hydrogels. This phase separation mechanism may become a new approach to prepare tough magnetic hydrogels and inspire more applications.