  Large-scale molecular simulations are performed to investigate tensile
failure of polymer interfaces as a function of welding time $t$. Changes in the
tensile stress, mode of failure and interfacial fracture energy $G_I$ are
correlated to changes in the interfacial entanglements as determined from
Primitive Path Analysis. Bulk polymers fail through craze formation, followed
by craze breakdown through chain scission. At small $t$ welded interfaces are
not strong enough to support craze formation and fail at small strains through
chain pullout at the interface. Once chains have formed an average of about one
entanglement across the interface, a stable craze is formed throughout the
sample. The failure stress of the craze rises with welding time and the mode of
craze breakdown changes from chain pullout to chain scission as the interface
approaches bulk strength. The interfacial fracture energy $G_I$ is calculated
by coupling the simulation results to a continuum fracture mechanics model. As
in experiment, $G_I$ increases as $t^{1/2}$ before saturating at the average
bulk fracture energy $G_b$. As in previous simulations of shear strength,
saturation coincides with the recovery of the bulk entanglement density. Before
saturation, $G_I$ is proportional to the areal density of interfacial
entanglements. Immiscibiltiy limits interdiffusion and thus suppresses
entanglements at the interface. Even small degrees of immisciblity reduce
interfacial entanglements enough that failure occurs by chain pullout and $G_I
\ll G_b$.
