Simultaneous Enhancement of Stiffness and Toughness in Hybrid Double-Network Hydrogels via the First, Physically Linked Network

Combining both chemical and physical cross-links in a double-network hydrogel (DN gel) has emerged as a promising design strategy to obtain highly mechanically strong hydrogels. Unlike chemically cross-linked DN gels, little is known about the fracture process and toughening mechanisms of hybrid chemically physically linked DN gels. In this work, we engineered tough hybrid DN gels of agar/polyacrylamide (Agar/PAAm) by combining two types of cross-linked polymer networks: a physically linked, first agar network and a chemical-linked, second PAAm network. The resulting Agar/PAAm exhibited high stiffness of 313 kPa and high toughness of 1089 J/m(2). We then specifically examined the effect of the first agar network on the mechanical properties of hybrid Agar/PAAm gels. We found that by controlling agar concentrations above a critical value, the physically linked agar network can simultaneously enhance both stiffness and toughness of Agar/PAAm DN gels, as evidenced by a linear relationship of elastic modulus and tearing energies of the gels as the increase of agar concentration. This toughening behavior is different from that of chemically linked DN gels. Complement to chemically linked DN gels, this work provides a different view for the design of new stiff and tough hydrogels using hybrid physical and chemical networks.