The effects of physisorbed polymer molecules on carbon nanotubes dispersed in a soft polymer matrix on the resulting mechanical strength of the nanocomposite are reported. From measurements of the large-strain deformation of the nanocomposites, the shear strength, τ, of the nanotube/matrix interface was determined as a function of the interfacial polymer chain length and the chain density, Σ. The results show that the value of τ (per chain) increases with increasing chain length. τ likewise increases with Σ but then levels off above a critical value. These results are explained by the molecular friction of the adsorbed polymer chains sliding along the rubbery polymer matrix. The results can be used to guide the interfacial design of polymer nanocomposites to obtain ultimate macroscopic mechanical control. In particular, the monomeric friction coefficient, ξ1, could be used to adjust the macroscopic properties of this type of nanocomposite.