Effect of Surfactant Oligomerization Degree on Lubricant Properties of Mixed Surfactant-Diblock Copolymer Films

Abstract

In this study, we investigate the coadsorption and the behavior under shear of mixtures of an anionic–neutral hydrophilic–hydrophilic diblock copolymer and oligomeric cationic surfactants on mica surfaces. Three different ammonium quaternary oligomeric surfactants are studied: a monomer, a gemini, and a trimer. Using both an Atomic Force Microscope (AFM) and a Surface Force Apparatus modified for nanotribology studies (SFA-N), we compare the morphology, the interactions between two coated surfaces, and the frictional properties of the resulting film for the three surfactants investigated. While the cohesion of the trimeric and gemini self-assembled layers is markedly improved by the presence of the copolymer, a less significant effect is observed for the monomeric surfactant. Several shear-induced dynamic transitions between states of low- and high-friction forces are observed in all cases. The nature and the characteristic times of these transitions are strongly influenced by the oligomerization degree of the assembled surfactant.