Hydrolytic damage study of the silane coupling region in coated silica microfibres: pH and coating type effects

Aqueous solutions of three silanes to cover silica microfibres were used, being the 3-aminopropyltriethoxysilane (APTES) and the 3-aminopropylmethyldiethoxysilane (APDES) the reagents for preparing them at the proportion of (APTES/APDES): 1/0, 1/1 and 0/1. The 1-pyrene sulphonyl chloride (PSC) fluorescent moiety was chemically attached to the silanised substrate via the sulphonamide formation. The hydrolytic degradation phenomenon of the silane coupling layer was studied as a function of: (i) temperature, (ii) coating layer type and (iii) pH (4, 7 and 10). The hydrolytic damage in the coupling region of the silica microfibres composite materials occurs under an equilibrium process. It was obtained the activation energies (Ea) for the hydrolytic damage considering the rate to reach the equilibrium. The values of Ea depended on the type of coating and on the pH. As a consequence, the rate of hydrolytic damage could be related to the proportion of Sisilane–O–Sisilane, while the OH− groups were thought to catalyse the reaction.