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)2Cl with n varying from 4 to 30] onto silica surfaces was developed by varying the surface preparation and the solvent used for the deposition process. A vapor phase deposition method was considered as the reference. The grafting method was evaluated by studying the wettability and the graft densities of the resulting monolayers. The chain conformation of the monolayers was determined by comparing the thicknesses measured by SE ellipsometry and AFM, and the orientation of the chains was evaluated by FT-IR in an attenuated total reflection (ATR) mode. By comparing the solvent and vapor phase deposition methods, it was demonstrated that the deposition process had a large influence on the structure of the grafted 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 solvent and by carefully carried out cleaning of the surface before deposition. 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 heterogeneous and do not display a preferential orientation, whereas long alkyl chains - C30 - lie on the silica surface without any organization.