Spinnability of Silica Sols: the Role of Alkoxy Group Exchange
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ABSTRACT The exchange and spinnability of several systems with the general form Si(OR) 4 (where R = CH 3 , C2 H 5 ) in ethanol, propanol and butanol were investigated. We have followed the alkoxy group exchange, the hydrolysis and condensation reactions and the Q distributions of these systems using 29 Si and 170 NMR. The observed differences in spinnability of the various sol-gel systems are attributed to the effect of the bulkiness of the alkoxy group on its reactivity. INTRODUCTION The sol-gel process provides a low temperature route for preparing glass fibers. The nature of the polymeric species strongly influences spinnability. Previous studies have indicated that a spinnable sol has a high viscosity without premature gelation [1-3]. These conditions appear to be controlled by two parameters. First, the sols were found to have an extent of reaction on the order of 0.7 where the extent of reaction is defined as (Q 1 + 2Q 2 + 3Q 3 +4Q 4 )/4. Second, the sols were found to have a greater than expected number of Q 2 and Q 3 species where the Q indicates that the silicon is tetrafunctional and the superscript refers to the number of bridging oxygens surrounding the silicon. These conditions were measured for an acid-catalyzed tetraethoxysilane (TEOS)-H 2 0-C 2 H 5OH by 2 9Si NMR spectroscopy. The requirement for a preponderance of Q2 and Q3 species is supported by the observation that TMOS sols, which react statistically and hence contain a lower fraction of Q2 and Q3 species, do not exhibit spinnability while TEOS sols, which react non statistically, do exhibit spinnability [3-5]. In order to have practical utility, a sol must be spinnable for an extended period of time. In this study we explore methods by which a non statistical distribution of Q species can be introduced into the sol, an appropriate extent of reaction reached quickly and then the reaction slowed so that the duration of spinnability can be extended. We investigated several systems with the general form Si(OR) 4 where R = CH 3 or C2 H5 in ethanol, propanol and butanol. In an acid catalyzed sol, the alkoxides and alcohols undergo rapid exchange. If tetramethoxysilane (TMOS) is mixed with butanol, an equilibrium distribution of methoxy and butoxy groups is quickly reached. We expect that this distribution will be non statistical because of the different sizes of the alkoxides. When these species undergo hydrolysis and subsequent condensation, the number of bridging oxygens surrounding the silicon will also be non statistical. The methoxy groups are expected to react rapidly so that the spinnability stage is quickly reached, while the remaining butoxy groups are expected to react more slowly so that length of the spinnability stage is extended. This work was supported by the U.S. Department of Energy under Contract DE-AC0494AL8500.
433 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL The alkoxy group exchange reactions of TMOS in ethanol, propanol and butanol were carried out by mixing TMOS in the appropriate alco
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