Cross-Condensation Reactions in an Organically Modified Silica Sol-Gel
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S. PRABAKAR*, R. A. ASSINK*, N. K. RAMAN and C. J. BRINKER*,, *UNM/NSF Center for Micro-Engineered Ceramics, University of New Mexico, Albuquerque, NM 87106 National Laboratories, Albuquerque, NM 87185.
**Sandia
ABSTRACT High resolution 29Si NMR has been used to study the extent of cross condensation taking place in a hybrid organic/inorganic sol-gel system. Tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) sol-gels were chosen for this purpose. The sols were prepared by acid catalyzed hydrolysis of TEOS and MTEOS with a H2 0/Si ratio of 0.3. 29Si NMR shows signals due to both self-condensation and cross-condensation between TEOS and MTEOS. Resonance assignments were made by comparing the positions and intensities of peaks in the spectra of single and multicomponent systems. It was found that, within experimental error, the self- and cross-condensation rates are equal and that extensive molecular level mixing takes place during the early stages of the reaction. INTRODUCTION The chemical synthesis of glasses and ceramics provides a convenient method for producing high purity, homogenous materials [1,2]. A general strategy for the design of improved materials has been the synthesis of multicomponent systems. These efforts may take the form of organic/organic copolymers, organic/inorganic hybrids and multicomponent inorganic materials. The effectiveness of the strategy often depends on the degree of mixing of the components. Mixing can be achieved on an intimate level when covalent bonds are formed between components. While a variety of techniques have been employed to study the chemical structure of silica sol-gels [1-3], corresponding studies of hybrid organic /inorganic silica solgels are scarce [4]. When TEOS is reacted with MTEOS, three kinds of condensed species are possible, two due to self-condensation ((i) and (ii)) and one due to cross-condensation (iii): T+T ---> TI Q+ Q ---> QQ T+ Q ---> TQ
(i) (ii) (iii)
where T represents the silicon in MTEOS and Q represents the silicon in TEOS. Fyfe et al. [4] studied an amorphous solid material prepared in this manner by 29Si CP/MAS NMR. Since the resonances of the solid material were broad, it was difficult to distinguish between species due to self- and cross-condensation. A two-dimensional 1H/ 29Si NMR experiment, however, was able to detect the presence of cross-condensed species. In this study we investigate a hybrid sol-gel during the early stages of the reaction using liquid state high resolution 29 Si NMR. Because of the increased resolution, we are able to directly observe resonances attributable to species formed by cross-condensation reactions. This work was supported by the U.S. Department of Energy under Contract DE-AC0494AL8500. 979 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL Sols in the system x MTEOS : (100-x) TEOS (where x = 0, 50, 100, all in mole percent) were prepared by the acid catalyzed hydrolysis of the respective alkoxides in ethanol with an H 2 0/Si ratio (R) of 0.3. Chromium acetylacetona
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