Fatigue behaviors of physical hydrogels based on hydrogen bonds

Physical hydrogels, composed of non-covalently crosslinking three dimensional polymer networks and a large amount of water, have drawn much attention due to their excellent mechanical properties such as high toughness, self-healing, self-recovery, and shape-memory. But the fatigue behaviors of physical hydrogels, especially the role of non-covalent bonds in the fatigue process, have not been well studied. Here we study fatigue damage and fatigue fracture of two kinds of physical hydrogels based on hydrogen bonding (H-bonding): poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) gels with weak H-bonding (weak gels) and PVA/PAA-LiCl gels with strong H-bonding (strong gels). For the fatigue damage of uncut samples, both the weak and strong gels are fatigue strengthened in small deformation by stretch-induced H-bonding rearrangement, but are fatigue damaged in large deformation by chain sliding. For the fatigue fracture of precut samples, both the weak and strong gels show no measurable fatigue threshold, but the strong gel can mitigate crack growth. These results reveal the intrinsically weak non-covalent bonds cannot effectively increase the fatigue resistance of physical hydrogels. This work provides a deep perspective for developing novel anti-fatigue hydrogels.