Optimizing the fracture toughness of a dual cross-linked hydrogel via molecular dynamics simulation

In this work, a coarse-grained model is adopted to explore the fracture toughness of a dual cross-linked hydrogel which consists of a physically cross-linked network and a chemically cross-linked network. By calculating the fracture energy, the optimized fracture toughness of the hydrogel appears at the intermediate content of the chemical network. To understand it, the structure change of both the chemical network and the physical network is first characterized during the tensile process. For the chemical network, the fraction and rate of broken bonds gradually improve with increasing content of the chemical network while the strain range where the bond breakage occurs is reduced. For the physical network, the number of clusters and the interaction energy first increase and then decrease with increasing strain. This reflects the breakage and reformation of the physical network, which dissipates more energy and improves the fracture energy. Furthermore, by stress decomposition, the stress is mainly borne by the physical network at small strain and the chemical network at large strain, which proves their synergistic effect in enhancing the hydrogel. Then, the number of voids is calculated as a function of strain. It is found that the voids initiate in the weak region at small strain while in the position of the bond breakage at large strain. Moreover, the number of voids decreases with increasing content of the chemical network at small strain. Finally, the effect of the strength of the chemical network or the physical network on the fracture toughness is discussed. The optimized fracture toughness of hydrogel appears at the intermediate strength.