A silicon-29 NMR study of the structural intermediates in low pH sol-gel reactions
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I. INTRODUCTION Sol-gel chemistry offers unique possibilities for designing and making inorganic and organic-inorganic polymeric materials which, among many diverse applications, can be coated as films or processed to give solid materials with a wide variety of uses (e.g., catalyst supports, fibers, or monoliths, to name only a few). The starting materials and chemical reactions involved often make it easy to introduce organic or inorganic groups into the growing polymer. This facet of the process holds the greatest promise for new materials but because of the innumerable possibilities, is also the most complex. The starting materials and reaction conditions determine what properties materials made via the sol-gel process will have. It has been pointed out that the structures of sol-gel polymers, at the gel point, influence final material properties.1"3 It is also believed that the early structural history plays a large part in determining the overall structure of the polymers at gelation.1'2'4 In the design of materials it is most convenient and efficient to make alterations at this early point in the material's history. Therefore, a sound understanding of this early history is most appropriate. One approach, already suggested,5 would be to use different monomer starting materials. Monomers of various structures (linear chains or ring compounds) were used and the authors watched the incorporation of these units into the polymer. Silicon-29 NMR can also be used to observe the formation of different molecular building blocks by simply varying the starting materials and reaction conditions. If a systematic strategy for formation of the desired molecular building blocks can be developed, this should be much easier and more economical than using starting materials that are difficult to prepare. The use of proton NMR6 and silicon-29 NMR7 has provided valuable information on the hydrolysis and polymerization reactions. In this report silicon-29 NMR is used to identify reacting chemical species in selected tetramethyl and tetraethyl orthosilicate (TMOS and TEOS) sol-gel systems. The concentrations of these oligomeric molecular building blocks are determined and early growth of the polymer followed. New silicon-29 NMR assignJ. Mater. Res., Vol. 4, No. 2, Mar/Apr 1989
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ments allow us to follow structural variations into very large polymers.
II. EXPERIMENTAL The silicon-29 NMR spectra were obtained at 99.36 MHz on a Bruker AM-500 spectrometer. Teflon tubes and a specially designed probe were used to eliminate background glass signals.8 Inverse-gated decoupling was used, the decoupler on during acquisition and off during the relaxation delay, to suppress any negative nuclear Overhauser effect (nOe). Silicon relaxation times are very long (up to 100 s and sometimes even longer), so chromium acetylacetonate [Cr(acac)3] was added at approximately 0.015 M to reduce the spin-lattice relaxation time. The effect of Cr(acac)3 on the sols was minimal. No change in the appearance
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