Stiff, strong, and tear-resistant physical hydrogels with widely tunable toughness by post-treatments

Synthetic polymeric hydrogel is a potential substitute for soft biological tissues. However, the poor mechanical properties of traditional synthetic hydrogels limit their applications in biological fields. Herein, a series of tough physical hydrogels have been prepared by micellar polymerization, in the existence of sodium bromide (NaBr), using methacrylic acid (MA) and stearyl acrylate (SA) as monomers, cetyltrimethyl ammonium bromide (CTAB) as cationic surfactant. The hydrogels exhibit excellent mechanical properties: modulus, toughness, and tearing fracture energy up to 7.8 MPa, 34 MJ m−3, and 16,600 J m−2, respectively. Moreover, it was found that the toughness of the hydrogels can be modulated in a very wide range by different post-treatments, e.g., dried/reswelling, freezing/thawing, or heated treatments. After post-treatments, the elastic physical hydrogels even turn into brittle plastics: modulus and toughness vary by 3 and 4 orders of magnitude, respectively. The mechanism for this wide tunability is attributed to the change of electrostatic attraction, crystallization, and phase separation during post-treatments.