Yielding of double-network hydrogels with systematically controlled Tetra-PEG first networks

Mechanical yielding of double-network (DN) hydrogels is a distinctive feature out of the classical polymer networks, which links to the toughening of the DN gels. Previous studies have focused on the effect of swelling on yield point; however, yield strain and yield stress could not be decoupled from each other which restricted the solid understanding of the micromechanical model of the yielding. In this study, we investigated the yield point of various DN gels where the first network parameters (preparation concentration, strand arm length and network connectivity) have been systematically varied using the well-established Tetra-PEG networks. This experimental approach clarified the universal relations of (1) yield elongation ratio to the finite extensibility of the first network and (2) yield stress to the number density of the first network strands regardless of the strand length, network connectivity and orientation. This research will lead to unveiling the true physical criterion of the yielding of double-network materials.