  Entanglement of polymer chains is ubiquitous in elastomers, gels, and
biological tissues. While the effects of chain entanglement on elasticity and
viscoelasticity of polymer networks have been intensively studied, it remains
elusive how chain entanglement affects fracture and fatigue of polymer
networks. In this paper, using polyacrylamide hydrogels as a model material, we
systematically compare fracture toughness and fatigue threshold of polymer
networks with various levels of chain entanglement. We find that the fracture
toughness and fatigue threshold of an unentangled polymer network are almost
the same, although the unentangled polymer network still contains non-ideal
features including topological defects (i.e., dangling chains and cyclic loops)
and structural heterogeneity (i.e., non-uniform chain lengths and non-uniform
functionalities). In contrast, the fracture toughness of an entangled polymer
network can be over ten times (up to 16 times) higher than its fatigue
threshold, indicating substantial toughness enhancement due to chain
entanglement. Different from the conventional toughness enhancement due to bulk
dissipation of polymer networks, the toughness enhancement by chain
entanglement requires low stress-stretch hysteresis (<10%) of the bulk
entangled polymer networks. We attribute the toughness enhancement in entangled
polymer networks to a new dissipation mechanism, near-crack dissipation, which
is possibly induced by pull-out of chains and delocalized damage of chains
around the crack tip.
