D. Hoebbel et al., On the existence and hydrolytic stability of titanosiloxane bonds in the system: Glycidoxypropyltrimethoxysilane-water-titaniumtetraethoxide, J SOL-GEL S, 13(1-3), 1998, pp. 37-43
Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniuma
lkoxide are used for optical applications and require a high homogeneity on
the molecular level. The presence of heterometal titanosiloxanes, their di
stribution and hydrolytic stability should influence the homogeneity of the
se materials. Si-29 and O-17 NMR spectroscopy has been used to investigate
sols with molar ratios Si:Ti = 1 and H2O:OR (H) = 0.5-2.0 and their gels af
ter heat treatment at 130 degrees C. The presence of Si-O-Ti bonds in sols
with a low water content (H < 0.2) and in the corresponding gels was identi
fied by the high-field shift of the Si-29 NMR signals of T-1 and T-2 units
of up to 2-3 ppm compared to corresponding signals of home-condensed Si-O-S
i bonds. The existence of Si-O-Ti bonds in the sols is supported by O-17 NM
R spectra which show a characteristic signal around 340 ppm. A cleavage of
the Si-O-Ti bonds occurs with increasing water/OR ratio in the sols. The cl
eavage of the heterometal bonds and the building up of home-condensed speci
es leads to a separation into areas with predominantly Ti-O-Ti and Si-O-Si
bonds resulting in a decreased molecular homogeneity of the materials.