Fracture and fatigue of ideal polymer networks

Abstract

Soft materials have enabled diverse modern technologies, but their practical deployment is usually limited by their mechanical failures. Fracture and fatigue of polymer networks are two important causes of mechanical failures of soft materials. A soft material fails by fracture when a monotonic load reaches its fracture toughness or fails by fatigue when a cyclic load reaches its fatigue threshold. The fracture toughness is usually much higher than the fatigue threshold for randomly crosslinked elastomers and gels. While fracture and fatigue have been extensively studied in randomly crosslinked elastomers and gels, they have not been comparatively studied in polymer networks with well-controlled architectures and defects. This work systematically studies the fracture and fatigue of ideal polymer networks with controlled densities of dangling-chain defects. We show that the fracture toughness and fatigue threshold of an ideal polymer network almost without defects are the same. After introducing a low density of dangling-chain defects into the ideal polymer network, its fracture toughness and fatigue threshold still maintain approximately the same. The fracture toughness of the ideal polymer network is also independent of the loading rate. We further use the recently developed defect-network model to explain the fatigue threshold (i.e., intrinsic fracture energy) of ideal polymer networks with controlled densities of dangling-chain defects.