Surface and interface viscoelastic behaviors of thin polymer films investigated by nanoindentation

Abstract

Nanoindentation experiments were performed on poly(methyl methacrylate) films deposited on Si(100) substrates to investigate the effects of film thickness, loading rate, and molecular weight on the surface and interface viscoelastic behaviors. The film stiffness increased with the indentation depth due to the increase of the contact area and the effect of the rigid substrate. The film thickness and loading rate exhibited strong effects on the film stiffness, while the molecular weight effect was secondary. For film thickness greater than the unperturbed radius of gyration, the stiffness variation across the film thickness demonstrated the existence of three regimes of different viscoelastic behaviors. The loading rate effect on the viscoelastic behavior of the indented polymer films is interpreted in terms of the confinement between the indenter and the substrate surfaces that restricted the molecular chain mobility in the vicinity of the film/substrate interface. The through-thickness variation of the film stiffness (modified to exclude the viscous component) is examined in the context of a previous indentation model. The nanoscale viscoelastic behavior of the thin polymer films observed in this study has important implications in polymer nanostructuring and nanopatterning.