Tribological behavior of a polymer grafted on silanized silica probed with a nanotip

The frictional forces between grafted layers on silica and a nanotip have been investigated as a function of the tip velocity. A comparative study has been performed between the friction behavior of the triethoxysilane molecules and polymer grafted on the silica. The polymer, a substituted polyacetylene, has been grafted following a two-step process. The silica surface is first pretreated with the triethoxysilane molecules, then the polymer is grafted on the silane molecules acting as a coupling agent. This two-step process allows the polymer to be firmly fixed. The good reproducibility of the data is accompanied by a robustness in the friction behavior. Both the silane molecules and the polymer grafted on the coupling agent show a linear increase of the force of friction with the logarithm of the sliding velocity. For the polymer, the force of friction is doubled that measured for the silane molecules and the forces of friction are found to be linearly dependent of the effective applied load. These two results are also supported by the measurement of the dynamic friction coefficient of the two grafted layers. The trends in these friction data have been found to be amenable to an analysis based upon a simple stress-modified thermally-activated Eyring model. A good consistency of the evolution of the different parameters, shear strengths, and barrier heights, computed with the model is obtained, From these results and their interpretation one gets a step forward for more quantitative information to be extracted with an atomic force microscope. Also, with the help of the Eyring model we provide a qualitative interpretation of what process is taking place to explain the increase of dissipation when the sliding experiment is performed on the grafted polymer.