Effects of Structure on the Tribological Properties of Organic Self-Assembled Molecular Layers

Abstract

The tribological properties of self-assembled monolayers (SAMs) on Au were investigated with a focus on the effects of the structure and end groups of the molecules upon the frictional properties of the modified surfaces of the SAMs. Three kinds of organothiols were used: HS-(CH₂)₁₁-(O-CH₂-CH₂)₃-OCH₃: EG3OMe, CH₂-(CH₂)₁₅-SH: HDT, and OH-(CH₂)₁₆-SH: HDTOH. The SAM surfaces were characterized by contact angle measurements of pure water, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. The frictional properties were examined using a pin-on-plate tribometer. The friction coefficients of the SAM surfaces were similar to the bare Au surface in the high normal load region. In the low normal load region, HDT SAMs exhibited lower friction coefficients than HDTOH SAMs. The end groups affected the friction coefficients for the same chain structures. The friction coefficients of the EG3OMe SAMs were lower than those of HDT SAMs. The HDT SAMs were thought to have well-organized structures. On the other hand, the EG3OMe SAMs probably had more flexible structures than the HDT SAMs because of their ethylene glycol chains, indicating that the chain structures as well as the end groups affected the friction coefficients of the SAMs.