Sol-gel reaction influence

Different characteristics of solvents seriously affect the sol-gel reaction in solution. This in turn influences the physico-mechanical properties of the resultant rubber-silica hybrid composites. Bandyopadhyay et al. [34,35] have carried out extensive research on stmcture-property correlation in sol-gel-derived rubber-sihca hybrid nanocomposites in different solvents with both chemically interactive (ENR) and noninteractive (ACM) mbber matrices. Figure 3.12 demonstrates the morphology of representative ACM-sihca and ENR-sihca hybrid composites prepared from various solvents. In all the instances, the concentration of TEOS (45 wt%), TEOS/H2O mole ratio (1 2), pH (1.5), and the gelling temperature (ambient condition) were kept unchanged. 

The influence of the polymeric substituent alpha to the silicon atom on the subsequent sol-gel reaction kinetics has not been addressed in detail. Many workers have described the effect of organic substituents on the hydrolysis and condensation kinetics of conventional sol-gel monomers. For example, Schmidt and co-workers have studied the effects of organic substituent on low 

Alkoxide-based systems are more complex because more parameters influence the sol-gel reactions (see below). This gives more possibUities to control the texture and properties of the obtained materials. 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

The host of modified monomers that have been included in sol-gel reactions, both in homopolymerization and copolymerization reactions, have been mentioned previously. In addition to the monomer composition, many other reaction parameters can be varied, including temperature, and the nature and amounts of catalysts, cosolvents, or reaction modifiers (such as surfactants and buffers). Each of the above variables, as well as the postprocessing steps, contribute to the ultimate outcome of the sol-gel reactions and can have a strong influence on the ultimate structure and properties of a material. 

There is a wide variety of both synthetic and natural crystalline fillers that are able, under specific conditions, to influence the properties of PP. In PP nanocomposites, particles are dispersed on the nano-scale. " The incorporation of one-, two- and three-dimensional nanoparticles, e.g. layered clays, nanotubes, nanofibres, metal-containing nanoparticles, carbon black, etc. is used to prepare nanocomposite fibres. However, the preparation of nanofilled fibres offers several possibilities, such as the creation of nanocomposite fibres by dispersing of nanoparticles into polymer solutions, the polymer melt blending of nanoparticles, in situ prepared nanoparticles within a polymeric substrate (e.g. PP/silica nanocomposites prepared in situ via sol-gel reaction), " the intercalative polymerization of the monomer. 

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