Flaw Sensitivity of Tough and Self-Healing Hydrogels with Hierarchical Structure

Unintentional flaws in materials dramatically reduce their strength and lifespan once they exceed a critical size. Compared to brittle materials such as glass and ceramics, viscoelastic hydrogels, as soft materials, exhibit a much greater length of flaw sensitivity length. Understanding the molecular mechanisms that govern flaw sensitivity in viscoelastic materials is crucial for optimizing their applications and designing tough materials. Herein, we use viscoelastic polyampholyte hydrogels as a model system to investigate how the hierarchical structure, including the transient network, primary network, and bicontinuous phase-separated structure, affects the flaw sensitivity length (c _c). Our findings reveal that c _c is strongly influenced by the primary network but only weakly dependent on the transient network. Furthermore, we estimate the prefactor (1/k) in the relationship between c _c and the fractocohesive length (Γ/W*), given by c _c = (1/k)Γ/W*, which is derived from elastic fracture mechanics, for both viscoelastic and relatively elastic soft materials. Additionally, we identify a universal correlation between c _c and the crack blunting indicatorthe ratio of fracture stress to Young's modulus (σ _f/E)which is applicable to both tough elastic and viscoelastic materials. This study offers insights into the mechanisms governing flaw sensitivity in viscoelastic materials.