Impact of Topological Defects on Fracture and Fatigue of Polymer Networks

Polymer networks are pervasive in biological organisms and engineering materials. Topological defects such as cyclic loops and dangling chains are ubiquitous in polymer networks. While fracture is a dominant mechanism for mechanical failures of polymer networks, existing models for fracture of polymer networks neglect the presence of topological defects. Here, we report a defect-network fracture model that accounts for the impact of various types of topological defects on fracture of polymer networks. We show that the fracture energy of polymer networks should account for the energy from multiple layers of polymer chains adjacent to the crack. We further show that the presence of topological defects tends to toughen a polymer network by increasing the effective chain length, yet to weaken the polymer network by introducing inactive polymer chains. Such competing effects can either increase or decrease the overall intrinsic fracture energy of the polymer network, depending on the types and densities of topological defects. This model provides theoretical explanations for the experimental data on the intrinsic fracture energy of polymer networks with various types and densities of topological defects. We further synthesize end-linked star-shaped polymer networks with controlled dangling-chain defects, experimentally investigating their impacts on fracture and fatigue of polymer networks through strain-induced crystallization. This work not only sheds light on the critical role of topological defects in toughening mechanism adopted in load-bearing biological tissues but also facilitate healthcare and sustainability challenges associated with the robustness and longevity of soft materials.