High strength of hybrid double-network hydrogels imparted by inter-network ionic bonds

Interaction between networks has been proven to be of importance for mechanical property enhancement of double-network (DN) hydrogels. In this work, inter-network ionically cross-linked hybrid DN hydrogels have been prepared using poly(sodium 2-acrylamide-2-methylpropanesulfonate) (PNaAMPS) as the first network, and a co-polymer of a cationic monomer and acrylamide (AM) as the second network. Ionic complexes were formed between the two oppositely charged networks after dialyzing the hydrogels in water. Near the ion-balance point, hybrid DN hydrogels with enhanced strengths were achieved compared to the unmodified PNaAMPS/polyacrylamide (PAM) DN hydrogel. Synchrotron radiation small-angle X-ray scattering (SAXS) analyses were performed on the DN hydrogels. It was found that large and compact cross-linked domains were formed in the hybrid DN hydrogels, especially near the ion-balance point, as demonstrated by the SAXS results. These large and compact cross-linked domains, which indicate enhanced interactions between networks containing ionic bonds besides physical entanglements, dissipate additional concentrated stress, resulting in high strength of the hybrid DN hydrogels.