SILICON AND PHOSPHORUS ALKOXIDE MIXTURE - SOL-GEL STUDY BY SPECTROSCOPIC TECHNICS

Sol-gel synthesis of mixtures of tetraethoxysilane and a phosphorus alkoxide [P(OEt)(3) or (OEt)(2)P-O-P(OEt)(2) or PO(OEt)(3)] have been studied by H-1, (CSi)-C-13-Si-29 and P-31 liquid and solid state NMR, infrared and raman spectroscopies. This study shows different behaviors towards hydrolysis for these three different phosphates and phosphites. P(OEt)(3) almost instantly reacts with water to form an intermediate species HPO(OEt)(2), which slowly evolves first to HPO(OH)(OEt), then to HPO(OH)(2) a few days later. For (OEt)(2)P-O-P(OEt)(2), the P-O-P bond is broken when water is added, then the same intermediates are formed faster. PO(OEt)(3) is hydrolyzed much slower than the other alkyl phosphates. After ten months, triethoxyphosphate is quantitatively present in the sol with little PO(OH)(OEt)(2) species. All these hydrolyzed species are well characterized. Only the system which contains the tetraethoxysilane and the triethoxyphosphite P(OEt)(3) forms a few P-O-P and P-O-Si bonds in the gel. Hydrolysis of tetraethoxysilane is much faster than that of phosphorus alkoxides and the conventional Q(2), Q(3) and Q(4) condensed silicon species form the gel three dimensional network.