On the existence and hydrolytic stability of titanosiloxane bonds in the system: Glycidoxypropyltrimethoxysilane-water-titaniumtetraethoxide

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.