A quantum mechanical study of the bonding of a silyl ester to hydrated alumina

The bonding of methyltrimethoxysilane (MTMS), a model for alkoxysilane primers and adhesion promoters, to a hydrated alumina cluster has been studied with density functional theory. The most probable site for formation of an aluminosiloxane bond is at a surface hydroxyl defect site, such as might exist at a step dislocation on a hydrated alumina surface rather than the more symmetric tuck-in site with pseudo-C3v symmetry on the (0 0 0 1) surface. The energetics of the step-growth polymerization reaction of MTMS has also been studied, and it is concluded that the oligomerization/polymerization reaction, which consumes water and releases methanol, is practically athermal. Thus, the polymerization reaction is primarily driven by entropic factors. At low coverage, chemisorbed alkoxysilanes will most likely be unevenly distributed as determined by the irregularity of the surface defects and as influenced by the relative humidity at the time of application. At high coverage, well-aged alkoxysilanes on metal or semi-metal oxide surfaces are probably best represented as polymerized (thermoset) resins that are chemically bonded to the substrate at most, if not all, accessible hydroxyl sites.