Load-induced frictional transition for well-defined cyclic self-Assembled monolayers

Self-Assembled monolayer (SAM) with amphiphilic linear molecules has been studied as a surface modifier. In general, the surface properties of a SAM are dependent on the chain end chemistry of the molecules. Thus, a SAM composed of alkane loops from cyclic alkanedisulfide on a gold substrate, in which both sulfurs arc bound to gold, may possess unique surface properties due to the chain folding as well as a lack of chain ends. We here focus on the frictional properties of the linear and cyclic SAMs. For the cyclic SAM, the load dependence of the frictional force became more striking beyond a threshold. Such a frictional transition was not observed for the corresponding linear SAM. The load-induced transition in the frictional response from the alkane loops could be related to the conformational change of the alkane loops, which collapsed along the direction normal to the SAM surface.