Chemistry Reference
In-Depth Information
the PDMS is monitored as the chains become constrained by the growing
silica-like material. For testing purposes, this composite was intentionally
made to be inhomogeneous, with much larger amounts of silica on the
surface. The dark rim at the edge of the sample indicates a reduced mobil-
ity of the network chains due to the presence of the silica. The evolution of
the dark rim reveals the movement of the reaction front into the sample.
This technique is nondestructive, but if the sample can be sacrificed, then
slices can be further studied in a gradient column with regard to density,
by electron microscopy 100 or by x-ray or neutron scattering.
Sol-gel methods are quite general 36, 42 in that a variety of precursor
materials can be hydrolyzed, photolyzed, or thermolyzed to give reinforc-
ing, ceramic-type particles. Titanates, for example, can be hydrolyzed to
titania, aluminates hydrolyzed to alumina, and metal carbonyls photo-
lyzed or thermolyzed to metals or metal oxides. Magnetic particles would
be particularly interesting if they can be manipulated with an external
magnetic field during the curing process (section 9.2.5). It is also possible
to polymerize conducting polymers such as polyaniline within polysilox-
ane matrices. 101, 102 The method can be used in a variety of polymers (or-
ganic as well as inorganic, nonelastomeric as well as elastomeric). Even
nonpolar, purely hydrocarbon polymers can be reinforced, provided the
sol-gel precursor has sufficient miscibility in the matrix.
A variety of catalysts work well in hydrolyses reactions, including acids,
bases, and salts. 103 Basic catalysts give precipitated phases that are gener-
ally well-defined particles, whereas the acidic catalysts give more poorly de-
fined, diffuse particles. 4, 104 The solvent can be of considerable importance. 105
In some cases, particles are not formed at all, and bicontinuous (interpene-
trating) phases result. 37, 106, 107 Another approach uses poly(allylamine hy-
drochloride) in a buffered solution to obtain what was called “bioinspired
silica.” 108
These sol-gel polymer reactions can be carried out in three ways. 36, 42
In the first, the polymer is cross linked and swelled with the organome-
tallic reagent, which is then hydrolyzed in situ. In the second, hydroxyl-
terminated chains are blended with enough of the organometallic
compound (TEOS) to both end link the chains and generate silica by the
hydrolysis reaction. Thus, curing and filling take place simultaneously,
in a one-step procedure. In the third technique, TEOS is blended into a
polymer that has end groups (e.g., vinyl units) that are unreactive under
hydrolysis conditions. The silica is formed in the usual manner (Equa-
tion 9.1), and the mixture is dried. The resultant slurry of polymer and
silica is stable and can be cross linked at a later time using any of the
standard techniques, including peroxide cures, vinyl-silane coupling, or
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