Fracture of polymers

I present a model study of brittle fracture of polymers. I show that the re lation nu=nu( F) between the crack-tip velocity nu and the driving force F exhibit discontinuous transitions and hysteresis. For short polymers, at th e onset of crack propagation the polymer chains separate by pulling out the molecular chains, while a discontinuous transition to a new state of rapid crack motion occurs at higher driving force, where the polymer chains brea k. In contrast to earlier atomistic studies of crack motion involving britt le materials such as silicon, for short polymers we find (at zero temperatu re) that the onset of crack motion is continuous, i.e., the crack-tip veloc ity nu increases continuously from zero when F is increased above the criti cal value F (c) necessary for starting crack motion. The difference is attr ibuted to inertia, which is less important during pull-out of the molecular chains, than in models where the crack motion involves breaking strong sho rt-ranged bonds. I also present a detailed study of the polymer chain pull- out process. (C) 1999 American Institute of Physics. [S0021-9606(99)50119-0 ].