Experiments were conducted to evaluate the effect of areal attachment density of monodisperse tethered chains on the toughness of an impenetrable interface. The interfacial toughness was found to be sensitive to the areal attachment density, with a reduction in attachment density enhancing the toughness. These findings are consistent with the notion that when the tethered chains are in the stretched conformation, the areal attachment density must be reduced to permit mixing with the matrix free chains. We conclude that the observed improvements in interfacial toughness were a result of increased mixing, i.e. molecular level interaction, between tethered and matrix chains.