Molecular damping: Mechanical response of self-assembled monomolecular layer to compression

We use a displacement-controlled dual double-cantilever-based surface-force-type apparatus to dynamically probe perfluorooctyl trichlorosilane monolayers self-assembled on aluminum and silicon substrates of 1 nm and 0.7 nm rms (root mean square) roughness, respectively, using a 1.12 mm diameter ruby sphere of 0.25 nm rms roughness. We record stiffness and damping constant as a function of compression load and deconvolute the elastic modulus using contact mechanical formulations. When mechanical intervention is limited to the terminal end of the molecule there is a strong viscous response and a low level of elastic response in consonance with the ability of the molecule to generate conformational defects freely. When the intervention penetrates into the molecular backbone the damping disappears dramatically and the molecule registers at a contact mean pressure of about 0.2 GPa a monotonic and steep rise in elastic resistance in response to further intervention by the probe. We offer a physical explanation of the process and describe this change as due to a phase transition from a liquidlike to a solidlike state as indicated by a large increase in relaxation time constant.