  Thermal welding of polymer-polymer interfaces is important for integrating
polymeric elements into devices. When two different polymers are joined, the
strength of the weld depends critically on the degree of immiscibility. We
perform large-scale molecular dynamics simulations of the structure-strength
relation at immiscible polymer interfaces. Our simulations show that
immiscibility arrests interdiffusion and limits the equilibrium interfacial
width. Even for weakly immiscible films, the narrow interface is unable to
transfer stress upon deformation as effectively as the bulk material, and chain
pullout at the interface becomes the dominant failure mechanism. This greatly
reduces the interfacial strength. The weak response of immiscible interfaces is
shown to arise from an insufficient density of entanglements across the
interface. We demonstrate that there is a threshold interfacial width below
which no significant entanglements can form between opposite sides to
strengthen the interface.
