Crack formation in end-linked polymer networks

Macroscopic fracture of polymer networks is linked to the molecular behavior of chains within the network, yet the fundamental molecular-level processes that lead to nucleation of a crack and its subsequent propagation remain elusive. Damage zones can be very widely spread in a network containing topological defects, suggesting that there is a finite volume of fracture zone around the crack tip associated with the initiation of the crack, in contrast to the simple assumption of a sharp crack plane. Recent studies have revealed that depercolation of the fracture zone is a necessary criterion for crack propagation. However, the size of this fracture zone is not yet accurately known. To better understand the initiation of fracture, coarse-grained simulations are performed on end-linked networks containing a blend of “weak” polymer chains in a matrix of unbreakable “strong” chains. Chain breaking is studied as a function of the extent of tensile deformation for networks with varying ratios of weak chains below and above the percolation threshold to reveal the extent of localization of chain scission within specific network clusters. As the network is stretched, a series of such chain breaking events lead to depercolation of the fracture zone, eventually leading to macroscopic failure of the material.