Ionically crosslinked chitosan/poly(acrylic acid) hydrogels with high strength, toughness and antifreezing capability.

A dual physically crosslinking (DPC) strategy is used to construct hydrogels with ultrahigh strength. First, polyelectrolyte complex (PEC) hydrogels were prepared through in situ polymerization of acrylic acid monomers in chitosan solutions. Subsequently, cations and anions were introduced into the PEC hydrogels to form strong electrostatic interactions with the polymer chains. The mechanical properties of the DPC hydrogels strongly depended on the ionic concentration and the valence state of the loading ions. The tensile strength of DPC-Na 8-20-2.0, DPC-Mg 8-20-2.0, DPCAl 8-20-2.0 and DPC-Sul 8-20-0.8 reached to 2.36, 12.59, 65.1 and 2.80 MPa, respectively, which were significantly higher than that of PEC 8-20 (0.29 MPa). Moreover, DPC-Na, DPC-Mg and DPC-Sul still maintained a good flexibility. Specifically, hydrogels of DPC-Ca exhibited ionic conductivity and freeze tolerance, which could be cooled to -20 °C without freezing. The DPC strategy opens an avenue to fabricate hydrogels with outstanding mechanical properties.