No abstract

Concepts of viscoelastic behavior of bulk polymer systems were used to describe maxima in the friction behavior of self-assembled monolayers ͑SAMs͒ measured with lateral force microscopy ͑LFM͒ as a function of sliding velocity and applied normal load in the presence of solvents ͑plasticizers͒. The objective of this study was to investigate whether decreases in the chain density of the SAMs caused maxima in the friction force to shift to higher sliding velocities; such a shift is indicative of shorter chain relaxation times in less dense, less ordered SAMs. Complete SAMs were formed from octadecyltrichlorosilane ͑OTS͒ and partial SAMs were formed from n-octadecylmethyldichlorosilane ͑2Cl͒ and n-octadecyldimethyldichlorosilane ͑1Cl͒, all on silicon/silicon dioxide substrates. With decreasing chain density, solvent partitioning into the monolayer should increase as OTSϽ2ClϽ1Cl. In butanol and pentanol, the maxima in the friction force shifted to higher sliding velocities with decreasing chain density, and the relaxation times calculated for the partial 2Cl films were an order of magnitude shorter than those for SAMs of OTS. For both SAMs of OTS and 2Cl films, maxima shifted to lower sliding velocities with increases in the applied normal load and with increases in the chain length n of the solvent. The higher compressibility of 2Cl films caused greater shifts in the maxima for similar increases in the applied normal load. The increase with n was consistent with both a mechanism of solvent partitioning controlled by the free volume distribution in the SAM and a mechanism of insertion. The relaxation times of the alkyl chains were related to a molecular model of energy dissipation involving the adsorption and desorption of the chain ends to and from the surface of the probe tip.