Extracting the Elastic and Surface Properties of Thin Polymer Films and Self-Assembled Monolayers from Interfacial Force Microscope Experiments
K. M. Liechti
Research Center for the Mechanics of Solids, Structures and Materials, Department of Aerospace Engineering and Engineering Mechanics, The University of Texas, Austin TX 78712
In this study, an Interfacial Force Microscope (IFM) was used to probe the mechanical and surface properties thin films of two silane coupling agents deposited on silicon. The first was (3-aminopropyl)triethoxysilane (g-APS) in films of 50 nm and 4 mm on silicon. The other was self-assembled (2nm) monolayers of octadecile trichlorosilane (OTS) on silicon. The films were probed in the IFM apparatus with a Tungsten tip that had a 120 nm radius.
The mechanical behavior and work of adhesion of the thin films and monolayers were extracted from measured force-displacement curves and contact mechanics theories and finite element simulations. It was found that substrate effects could only be ignored if indentation depths were less than 10% of the contact radius, rather than the commonly used film thickness. The reduced modulus of the g-APS was typical of polymers. The IFM force profiles for OTS on silicon suggested that the OTS was nonlinearly elastic. This was confirmed in molecular dynamics analyses of simple stress states. Surface energies extracted from finite element analyses that incorporated the nonlinear behavior along with cohesive zone models were consistent with expectations.