Mechanistic Origins of Yielding in Hybrid Double-Network Hydrogels

Hybrid double-network hydrogels comprise transiently and permanently cross-linked polymer networks and exhibit enhanced toughness, arising from a local yielding transition. Here, we examine the precise nature of this yielding transition by constructing a series of hydrogel designs from alginate and polyacrylamide (PAAm) networks, systematically controlling their cross-linking chemistry. Using large amplitude oscillatory shear (LAOS) rheology (LAOS), we show the presence of a hitherto unobserved two-step yielding process. Analysis of individual oscillatory cycles and the use of chaotropic/kosmotropic reagents shows that the first step of yielding is determined by the hydrogen bonding between the two polymer networks. These interactions also influence the second step of yielding, which we show is governed by the ionic interactions within the alginate network. This work demonstrates that interactions between are as crucial as interactions within the polymer networks and thereby provides insights into how the yielding in soft composite materials can be identified, adjusted, and controlled.