TRIBOLOGICAL BEHAVIOR OF A POLYMER-GRAFTED ON SILANIZED SILICA PROBEDWITH A NANOTIP

The frictional forces between grafted layers on silica and a nanotip h ave been investigated as a function of the tip velocity. A comparative study has been performed between the friction behavior of the trietho xysilane molecules and polymer grafted on the silica. The polymer, a s ubstituted polyacetylene, has been grafted following a two-step proces s. The silica surface is first pretreated with the triethoxysilane mol ecules, then the polymer is grafted on the silane molecules acting as a coupling agent. This two-step process allows the polymer to be firml y fixed. The good reproducibility of the data is accompanied by a robu stness in the friction behavior. Both the silane molecules and the pol ymer grafted on the coupling agent show a linear increase of the force of friction with the logarithm of the sliding velocity. For the polym er, the force of friction is doubled that measured for the silane mole cules and the forces of friction are found to be linearly dependent of the effective applied load. These two results are also supported by t he measurement of the dynamic friction coefficient of the two grafted layers. The trends in these friction data have been found to be amenab le to an analysis based upon a simple stress-modified thermally-activa ted Eyring model. A good consistency of the evolution of the different parameters, shear strengths, and barrier heights, computed with the m odel 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 w e 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.