Structure of Soluble Silicates
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DALE W. SCHAEFER and KEITH D. KEEFER, Sandia National Laboratories, Albuquerque,
NM 87185
ABSTRACT Small angle x-ray scattering (SAXS) is the technique of choice for the determination of structure on the 10-1000A scale. We have used this technique to study the growth and topology of the macromolecules which precede gelation in several chemical systems used in sol-gel glass technology. The results show that branched polymers, as opposed to colloids, are formed. The alcoholic silica system is akin to organic systems where gelation occurs through growth and Data are reported from crosslinking of chain molecules. both the Porod and Guinier regions of the SAKXScurve and these data are interpreted in terms of geometrical structures The repredicted by various disorderly growth processes. sults indicate that the degree of crosslinking can be conThe degree of crosslinking trolled by catalytic conditions. may, in turn, control phase separation and processability to a dense glass.
INTRODUCTION Although there has been substantial research in the last decade on solis known about the structure of solution gel derived materials, little precursors. This paucity of knowledge extends to structures at all levels; In this work we use small angle molecular, macromolecular, and microscopic. x-ray scattering (SAXS) to probe the structure of the macromolecular precursors to the gel phase in systems of interest in glass technology. The System This work centers on the growth of macromolecules resulting from the hydrolysis and condensation of silicon tetraethoxide (Si(OC 2 H5 ) 4 , TEOS): Si(OR)
4
+ H20
2Si(OR) 3 0H
+
Si(OR) 3 0H + ROH
(1) (2)
+ (RO) 3 SiOSi(OR) 3 + H20
Reaction In these reactions R is a proton or an alkyl or silicate group. Details of the compositions are given (1) is either acid or base catalyzed. The in Table 1. Table 2 gives some physical properties of the systems. solvent, n-propanol, is chosen to facilitate study of the reaction kinetics [1]. Further details on the chemical and by gas-liquid chromatography (GLC) The three compositions physical conditions are published elsewhere [2]. studied differ in catalyst (acid vs. base) and in the H2 0/TEOS ratio used for the hydrolysis. In order to eliminate the effects of phase separation and precipitation, a two-step hydrolysis process was used to prepare all the solutions. The first step consisted of mixing at 60'C tetraethylorthosilicate (TEOS), alcohol, water, and acid (HCl) in a molar ratio 1:3:1: This initial water addition equals one fourth the stoichiometric 0.0007. After 1.5 amount required to fully hydrolyze the TEOS to monosilicic acid. h, additional water plus base or acid were added at 40'C for the second hydrolysis step.
Mat.
Res. Soc. Symp.Proc. Vol.
32 (1984) Published by Elsevier Science Publishing Co.,
Inc.
2
TABLE I. Sample
n-prop
A2 A3 B2
32.8 18.4 39.2
TABLE II.
Compositions Investigated (mol%) H20
TEOS
HCI
55.7 75.5 47.9
10.9 6.1 12.9
0.632 0.005 0.010
[2].
NH40H
H2 0/TEOS
0.016
5.1 12.4 3.7
Summary of Physical Properties [
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