A Comparison between the Densification Kinetics of Colloidal and Polymeric Silica Gels
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A COMPARISON BETWEEN THE DENSIFICATION KINETICS OF COLLOIDAL AND POLYMERIC SILICA GELS C. J. BRINKER,* W. D. DROTNING,* AND G. W. SCHERER,** *Sandia National Laboratories, P. 0. Box 5800, Albuquerque, New Mexico 87185; **Corning Glass Works, Corning, New York ABSTRACT Silica gels were prepared by three methods in which the original silicate species varied from extended linear or randomly branched polymers to more highly crosslinked clusters to colloidal particles of anhydrous silica. During isothermal sintering experiments, the viscosities of the two polymer gels increased significantly (up to 3 orders of magnitude) while the isothermal viscosity of the colloidal gel was constant. Viscosity increases were explained by crosslinking and structural relaxation of the polymeric gels.
INTRODUCTION Most researchers (e.g. [1]) who have attempted to describe the gel - glass conversion for alkoxide-derived gels have utilized structural models based on Iler's representations of colloidal silica gels formed in aqueous solutions [2], i.e. the gels are considered to be composed of fully polymerized (anhydrous) particles (Fig. la and b). Gel densification has been considered to be essentially a sintering process [1], and differences in densification behavior have been attributed to differences in texture, e.g. the characteristic particle size, and/or differences in oxide composition, e.g. the densification temperature generally increases with T of a the corresponding melted composition. If these views are correct, it should be possible to quantitatively describe gel densification by application of a viscous sintering model. The purpose of the present investigation, therefore, is to compare the densification behavior of "polymer" gels (derived from metal alkoxides) to colloidal gels (composed of anhydrous, oxide particles) whose densification has previously been described by a viscous sintering model [3]. Differences in densification behavior between polymer and colloidal gels will be explained in terms of gel structure as determined by this and previous investigations of the sol - gel - glass conversion [4,5,6]. These differences will show that it is seldom appropriate to strictly employ Iler's models of aqueous silicate gels to describe the structures of metal-alkoxide-derived gels prepared from alcoholic solutions. GEL SYNTHESIS AND STRUCTURE Colloidal silica was prepared by flame oxidation of SiCl as described in reference 7. This technique results in fully dense, spherical SiO particles 10-100 nm in diameter which may be single or clustered. Gels were prepared by dispersing the particles in a non-polar solvent, e.g. chloroform, using long chain alcohols as a steric barrier to gelation. The addition of a strong base such as an amine deprotonates the surface silanols generating charges that cause gelation. The gel microstructure obtained, after drying for 72 h at room temperatur5 , is shown in FIG. lc. This xerogel has a density, P , of 0.54 g cm- (relative density, p0 /P = 0.25 where Ps= the density o• the melted glass), a su
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