The increase in the interfacial fracture energy (G(c)) with increasing interfacial width (a(i)) goes through a transition at a critical value of ai that is unique to each polymer-polymer system. This transition point does not scale with the bulk entanglement spacing (d(t)) for different systems, implying that the role of chain friction in reinforcing these interfaces is more important than previously thought. A theoretical model has been developed to calculate G(c) as a function of the interfacial stress transfer due to individual polymer chains. When including the effects of chain friction only, the model reproduces the nonuniversal behavior of G(c) with respect to a(i)/d(t) but yields poor fits for a(i)/d(t) > 1. The effects of entanglements are then added by calculating the fraction of entangled chains as a function of a(i)d(t). This contribution, although not material specific, matches the qualitative behavior of G(c) for large values of a(i)/d(t), When both contributions are included in the model, excellent fits are obtained for all data sets.