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.

由非共价交联的三维聚合物网络和大量水组成的物理水凝胶由于其优异的机械性能（如高韧性，自修复，自恢复和形状记忆）而备受关注。但是，尚未很好地研究物理水凝胶的疲劳行为，尤其是非共价键在疲劳过程中的作用。在这里，我们研究两种基于氢键（H键）的物理水凝胶的疲劳损伤和疲劳断裂：氢键弱的聚（乙烯醇）/聚丙烯酸（PVA / PAA）凝胶（弱凝胶）和具有强H键合的PVA / PAA-LiCl凝胶（强力凝胶）。对于未切割样品的疲劳损伤，通过拉伸诱导的H键重排，弱凝胶和强凝胶都会在小变形中疲劳增强，但是会因链条滑动而在大变形中疲劳损坏。对于预切割样品的疲劳断裂，弱凝胶和强凝胶均未显示可测量的疲劳阈值，但强凝胶可缓解裂纹扩展。这些结果表明，固有的弱非共价键不能有效地提高物理水凝胶的抗疲劳性。这项工作为开发新型抗疲劳水凝胶提供了深刻的见解。