Effect of reaction conditions on surface properties of TEOS-TBOT-PDMS hybrid materials

Hybrid materials based on tetraethoxysilane (TEOS), tetrabutyl orthotitanate (TBOT), and hydroxyl terminated polydimethyl siloxane (PDMS) have been prepared and characterized. The effects of HCl concentration and PDMS molecular weight (M_W) have been analysed. The physical and chemical characteristics have been discussed based on the results obtained from the characterization by Raman spectroscopy, nitrogen adsorption, mercury intrusion porosimetry and inverse gas chromatography (IGC). Specific surface areas (SSA) and pore volumes increase with HCl concentration and are independent on the PDMS Mw. SSA values as high as 300 m² g^-1 have been obtained for hybrid materials prepared with HCl:(TEOS + TBOT) concentration of 0.3 and PDMS Mw of 550 g mol^-1. These materials present the higher pore volumes and lower pore sizes. However, for HCl:(TEOS + TBOT) concentration of 0.5, only macroporous materials are obtained with SSA values as low as 2-4 m² g^-1, and pore sizes close to 10 μm. In general the presence of Ti on the SSA and porosities give a little decrease in such values. Surface energies are consistent with those of organic-inorganic materials. Dispersive surface energies are higher for the Si-Ti hybrid materials and, for all of them, they decrease with both HCl concentration and PDMS Mw. This result can be assigned to the presence of a higher concentration of PDMS (or CH ₃ groups) on the surface of the hybrid material as well as the presence of disordered anatase-like structures. On the other hand, the acid and base properties are dependent on both HCl concentration and PDMS Mw. It has been observed that the acid and base parameters of the Si-Ti hybrid materials are influenced by the disorder degree of the amorphous titanium oxide present in them. As the disorder decrease both acid and base parameters increase.