Self-Assembled Monolayers with a Controlled Density of Hydroxyl Groups: A Relevant Model to Investigate the Adhesion Properties of Epoxy Adhesives

Numerous studies reported on the importance of the presence of hydroxyl groups on the substrate to obtain good adhesion properties of epoxy adhesives. However, the required surface density of this functional group to ensure a cohesive failure pattern remained unclear. Indeed, when researchers usually varied this functional group density, they also often impacted the topography of the surface, in particular its roughness. Therefore, self-assembled monolayers (SAMs) with a controlled density of hydroxyl groups but extremely low roughness were fabricated in this study using mixed bromine-terminated monolayers as intermediates. Complete conversion of bromides to hydroxyl groups was achieved using mercury (II) oxide-assisted solvolysis and a subsequent thorough washing procedure. The characterization of surface chemistry and the monolayer structure were investigated by water contact angle measurements, spectroscopic ellipsometry, atomic force microscopy, and high-resolution X-ray photoelectron spectroscopy (XPS). Then, these well-controlled mixed monolayers could be used as model adherends for adhesion tests of a commercial epoxy adhesive. Peel tests revealed a threshold value close to 3 OH/nm2 to change from an adhesive to a cohesive failure pattern of the epoxy adhesive joint on these model surfaces. The methodology developed here was particularly relevant to investigate the adhesion mechanism of adhesive formulations on materials coated with an oxide layer since solely surface chemistry varied from one substrate to the other.