Uniform silica particles, formation mechanism

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

The parameters of the pore structure, such as surface area, pore volume, and mean pore diameter, can generally be used for a formal description of the porous systems, irrespective of their chemical composition and their origin, and for a more detailed study of the pore formation mechanism, the geometric aspects of pore structure are important. This picture, however, oversimplifies the situation because it provides a pore uniformity that is far from reality. Thorough attempts have been made to achieve the mathematical description of porous matter. Researchers discussed the cause of porosity in various materials and concluded that there are two main types of material based on pore structure that can be classified as corpuscular and spongy systems. In the case of the silica matrices obtained with TEOS and other precursors, the porous structure seems to be of the corpuscular type, in which the pores consist of the interstices between discrete particles of the solid material. In such a system, the pore structure depends on the pores mutual arrangements, and the dimensions of the pores are controlled by the size of the interparticle volumes (1). 

Such a mechanism must have been involved in the formation of 200 nm spheres in a solution of pure silica sol prepared by hydrolyzing SiCU and removing HCl by electrodialysis, as reported by Radezewski and Richter (128b). The purified clear sol contained about 0.5% SiOj and the pH was 6.8. Similarly, uniform porous spherical silica particles up to 1 micron in diameter are formed by the aggregation of primary particles less than 5 nm in size formed by the hydrolysis of ethyl silicate in a water-alcohol-ammonia system as developed by Stdber and Funk (128c). 

Studies on randomness of filler distribution in polymethylacrylate nanocomposite are interesting. In this experiment, siUca particles were formed both before and after matrix polymerization. The results indicated that the concentration of silica was a controlling factor in the stress-strain relationship rather than the uniformity of particle distribution. Also, there was no anisotropy of mechanical properties regardless of the sequence of filler formation. This outcome cannot be expected to be duplicated in all other systems. For example, when nickel coated fibers were used in an EMI shielding application." When compounded with polycarbonate resin, fibers had a much worse performance than when a diy blend was prepared first and then incorporated into the polymer (Figure 7.1). In this case, pre-blending protected the fiber from breakage. 

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