Gelation Point In Borosilicate Sols From Rheological Experiments
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ABSTRACT Dynamic oscillatory experiments are used to monitor the gelation of the borosilicate systems prepared through the sol-gel process from metal alkoxides. The rheological experiments show that tan 8 = G"/G' is independent of frequency at the gel point in agreement with the results of others on organic gelling systems. The dynamic moduli at the gel point followed power-law behavior with respect to frequency. The power-law exponent is found to be -0.70. The apparent fractal dimension, dy, of the network cluster at the gel point is determined. The dF values for the samples ranged from 2.5 to 3.8 depending on the final structure of the evolved products at the gel point. The large values (dF > 3) exclude a simple geometric interpretation of the results. The effect of processing parameters, such as composition of reactants and temperature, on the resulting microstructures near the gel point is discussed. INTRODUCTION Understanding gelation rheology is an important link in the processing of sol-gel precursor solutions. The information regarding gelation point is useful in processing applications. For example, quality fibers are obtained as reported by several researchersI when they are subjected
to spinning techniquesjust before the gel point. Most rheological studies have been conducted in steady shear flowsl,', 3 ,4 . Precursor solutions initially exhibit Newtonian shear behavior. As they begin to interact due to chemical reactions taking place in the system or due to evaporation, some solutions become shear thinning. When fully reacted, the solutions become solid and can be characterized by a modulus. Dynamic oscillatory experiments provide a good way to monitor structure development during the gelation process. This has been effectively demonstrated for polydimethylsiloxanes 5' 6, polyurethanes7'°'9, and some sol-gel systems - . Small-amplitude measurements have the advantage over steady measurements in that they do not disrupt the structure as it develops. Chambon and Winter 5' 6 showed that a good indicator of the gel point for many systems is for the ratio of viscous to elastic moduli, G"/G'= tan 5, to become independent of measuring frequency. We have applied this technique to borosilicate gels prepared from metal alkoxides. CHEMICAL ASPECTS On hydrolyzing metal alkoxides with water and catalyst in the presence of the solvent, small clusters are produced which grow by condensation of polymers or by aggregation. The clusters grow until a sample-spanning cluster is formed at the gel point. The reaction is generally carried out in a diluent such as ethanol. The rate and extent of the reactions depend on a number of parameters such as the ratios of water, reactants and the solvent, the nature and con785 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
17 centration of catalyst, and the temperature of the reaction. The pattern of hydrolysis and subsequent polycondensation differ according to whether the catalyst is acidic or basic. In the presence of acid, the hydrolysis of the silicon alk
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