Grafting of alkylchlorosilanes onto silica from solution for adhesion enhancement

An efficient grafting process of monofunctional alkylchlorosilanes [general formula: CH3-(CH2)(n-1)-Si (CH3)(2)Cl with n varying from 4 to 30] onto si lica surfaces was developed by varying the surface preparation and the solv ent used for the deposition process. A vapor phase deposition method was co nsidered as the reference. The grafting method was evaluated by studying th e wettability acid the graft densities of the resulting monolayers. The cha in conformation of the monolayers was determined by comparing the thickness es measured by SE ellipsometry and AFM, and the orientation of the chains w as evaluated by FT-IR in an attenuated total reflection (ATR) mode. By comp aring the solvent and vapor phase deposition methods, it was demonstrated t hat the deposition process had a large influence on the structure of the gr afted monolayers. The same structure as that from a vapor phase method can be obtained from a solvent deposition process by a suitable choice of the s olvent and by carefully carried out cleaning of the surface before depositi on. The organization of the grafted layers was found to be dependent on the length of the alkyl chains; i.e. uniform and well-ordered monolayers were obtained for a 'critical' length of the alkyl chain - C18. In fact, as the alkyl chain becomes short, the resulting layers are hydrophobic and heterog eneous and do not display a preferential orientation, whereas long alkyl ch ains - C30 - lie on the silica surface without any organization.