Covalently Attached Monolayers of Oligo(dimethylsiloxane)s on Silica:  A Siloxane Chemistry Approach for Surface Modification

This work describes the synthesis of oligo(dimethylsiloxane) [−Si(CH 3 ) 2 O−] n monolayers supported on silica. The fundamental properties of siloxane (Si−O−Si) backbones, for example, high flexibility and high thermal stability, set siloxane monolayers apart from self-assembled monolayers derived from long-chain alkylsilanes, alkylthiols, and so forth and provide attractive features for using “siloxane chemistry” for the modification of surfaces. Oligo(dimethylsiloxane) surfaces were prepared by reaction of low molecular weight α,ω-dichloro-dimethylsilane/oligo(dimethylsiloxane)s [Cl[Si(CH 3 ) 2 O] n −Si(CH 3 ) 2 Cl, n = 0, 1, 2, 3, 4] with mesoporous precipitated silica (pore diameter 15 nm). Reactions with silicas of different degrees of hydration (dried at 200 °C in a vacuum, “as received”, and saturated with water vapors) were studied under two conditions: (1) in the vapor phase and (2) in solution. Reactions of Cl[Si(CH 3 ) 2 O] n −Si(CH 3 ) 2 Cl with dry silicas yield surfaces with bonding densities of 2.35 ± 0.15 [Si(CH 3 ) 2 O] groups per nm 2 corresponding to a single layer of dimethylsiloxy groups on the surface. Fourier transform infrared (FTIR) spectroscopy indicates that the major products of these reactions are poly(dimethylsiloxane) loops covalently attached to silica by two Si S −O−Si bonds. Reactions of Cl[Si(CH 3 ) 2 O] n −Si(CH 3 ) 2 Cl with wet silicas yield oligomeric dimethylsiloxane surfaces with apparent bonding densities up to 11.4 [OSi(CH 3 ) 2 ] groups per nm 2 . The bonding density of dimethylsiloxane can be controlled with high precision by varying of the amount of water preadsorbed to the silica; this provides a conventional method for manipulating the surface bonding process. In situ FTIR studies of reaction kinetics indicate that reactions of Cl[Si(CH 3 ) 2 O] n −Si(CH 3 ) 2 Cl with wet silica proceed through hydrolysis and polycondensation yielding linear OH-terminated oligomeric dimethylsiloxane. These oligomers reacted with the silica surface through Si S −O−Si bonds yielding covalently attached oligomeric dimethylsiloxane surfaces. The excess of poly(dimethylsiloxane) is adsorbed on the surface and is removed after rinsing with solvents. The thermal stability of dimethylsiloxane surfaces was studied using thermogravimetric analysis. Monomeric dimethylsiloxane surfaces on silica show remarkable thermal and oxidative stability, and no mass loss was observed below 550−650 °C in air. Oligomeric dimethylsiloxane surfaces on silica show a maximum rate of mass loss at 400−450 °C, which is close to that of poly(dimethylsiloxane) (silicones). It is suggested that degradation of the oligomeric surfaces proceeds by depolymerization of dimethylsiloxane chains, while oxidative destruction is a more likely pathway for the degradation of a monomeric dimethylsiloxane surface.