Ex Situ and Modified Silicas

Separately prepared silica has long been used to reinforce polysiloxanes by blending them into the polymer prior to its cross linking. The technology of this ex situ method is well developed, including the characterization of the resulting composites. - Mesoporous silica 

One goal is to modify the surfaces of silica particles to improve bonding with PDMS, for example, with vinylethoxysilane or by si-lanization. Similarly, tetraphenyl-modified fumed silica has been used to increase PDMS radiation resistance. Such materials can be difficult to characterize quantitatively. For example, in some cases fumed silica particles in PDMS formed secondary domain structures that made it difficult to characterize nanoparticle formation by tapping-mode atomic force microscopy. Ultra small angle x-ray and neutron scatterings are useful for characterization of such complex morphologies. 

The layered silicate most often used to improve the properties of PDMS is montmorillonite. 204-225 improvements have also been reported upon the introduction of graphite, graphite oxide,mica, fluoromica, hectorite, ° fiuorohectorite, laponite, wollastonite, perovskite, ° sepiolite, and titanium-niobium oxide. Kaolin and exfoliated layered double hydroxides have also been used in PDMS nanocomposites. In some cases, diphenylsiloxane-clay hybrids have been used to reinforce polyethylene.  

Segmental motions of poly(methylphenylsiloxane) chains confined in intercalated nanocomposites have been studied by quasielastic neutron scattering and, surprisingly, were found to be faster than in the bulk polymer.  

Computations have been carried out to better understand the structures and properties of these types of nanocomposites. One subject of particular interest is the observed enhanced gas transport performance of PDMS nanocomposite membranes containing layered silicates, which is not expected based on studies of magnetically alligned particles disuc-ssed in the next section. Clays have also been used to improve the properties of silsesquioxane polymers.  

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