Although many studies have analyzed the various properties of glassy polymer thin films, the failure mode and nonlinear mechanical response have only recently received attention. Due to competing effects between reductions in the entanglement density and increased mobility near the free surfaces, it is not clear whether one should anticipate embrittlement or improved ductility. In this study, we constructed polymer thin films with a bidisperse molecular weight distribution by incorporating short chains as diluents into highly entangled polymer systems at different compositions and film thicknesses. Due to the dominance of the high-mobility region near the free surfaces, films approximately 10 monomer diameters thick are less ductile than thicker films composed of either a bidisperse blend or a homopolymer, despite having a similar entanglement density. The dependence of mechanical properties on the thickness is weak when the film is larger than 20 sigma, especially for highly entangled systems. With diluent polymers added, the films become less ductile, and the mechanical properties of the polymer thin films are affected most when the added diluents are unentangled polymers, as expected. Further, we observe a competing effect between the free surface and entanglement network, where the free surface interaction dominates the mechanical behavior for the thinnest films tested while the entanglement network plays a dominant role in thicker films.
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