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

The bonding of methyltrimethoxysilane (MTMS), a model for alkoxysilane prim ers 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 ex ist at a step dislocation on a hydrated alumina surface rather than the mor e symmetric tuck-in site with pseudo-C-3v symmetry on the (0001) surface. T he energetics of the step-growth polymerization reaction of MTMS has also b een studied, and it is concluded that the oligomerization/polymerization re action, 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 d istributed as determined by the irregularity of the surface defects and as influenced by the relative humidity at the time of application. At high cov erage, well-aged alkoxysilanes on metal or semi-metal oxide surfaces are pr obably best represented as polymerized (thermoset) resins that are chemical ly bonded to the substrate at most, if not all, accessible hydroxyl sites. (C) 2001 Elsevier Science B.V. All rights reserved.