The Effect of Hydrolysis Conditions on the Structure and Growth of Silicate Polymers
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THE EFFECT OF HYDROLYSIS CONDITIONS ON THE STRUCTURE AND GROWTH OF SILICATE POLYMERS K. D. Keefer, Sandia National Laboratories,
Albuquerque, New Mexico
87185
ABSTRACT Small angle scattering experiments have demonstrated that the structure of the silicate species produced by the hydrolysis of silicon alkoxides in non-aqueous solvents ranges from extended, weakly cross-linked polymers to highly condensed, colloidal particles. In contrast, inorganic, aqueous silicate solutions yield primarily colloidal particles because the silicate species have a number of different silanol sites available and the preferred condensation reaction is that of weakly condensed species with highly cross-linked branch sites, such as those on an amorphous silica surface. It is proposed that in the alkoxide systems, however, the hydrolysis reaction may control the number and type of silanol sites available for condensation. In acid catalyzed reactions, the rate of hydrolysis of a silicate tetrahedron tends to decrease as alkoxide groups are removed. This favors the production of silanol sites on the end of chains, thus generating linear polymers. In base catalyzed reactions, it is argued that each subsequent hydrolysis of a tetrahedron should proceed more rapidly than the previous one, producing numerous branch points which are the preferred sites for condensation. INTRODUCTION
The hydrolysis of silicon tetraethoxide (Si(0C2• H)A) and subsequent condensation of the resulting silanols (ESioH groups) may be described by the following set of reactions: ESiOEt + H20 O* Si0H + Et0H (1) 2ESiOH
-
,
SiOSiE
+ H20
(2)
The gels which result from these reactions differ in several ways, depending on the water content of the system and whether acid or base was used to catalyze the hydrolysis. First, measurements of the small angle scattering[lJ from these systems show that in acidic solutions or at low water concentration, the product of these reactions is weakly crosslinked and polymeric in nature. In alkaline solutions or at high water concentrations, silica forms more highly crosslinked polymers, even fully dense colloidal particles[2]. Second, analyses of the solutions show that in acidic solutions, hydrolysis tends to go to stochiometric completion, but in basic solutions it does not[9], the tetraethoxide monomer being particularly resistant[3]. Third, despite the degree of hydrolysis in solution, dried gels prepared from acidic solutions often have more alkoxy groups per unit surface area than gels prepared in basic solutions[4] indicating that reesterification (the reverse of hydrolysis) is more important in acidic systems. These differences can be accounted for by some simple chemical arguments steming from the mechanisms of the reactions. This reaction occurs via one of two different reaction mechanisms, depending on the type of catalyst and each mechanism favors a different degree of branching. In Mat.
Res.
Soc..Symp. Proc.
Vol. 32
(1984) a Elsevier Science Publishing Co..
Inc.
16
the initial stage of the reaction, even a smal
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