Chemistry Reference
In-Depth Information
FIGURE16.10
Positively charged dendrimers, and neutral dendrons, used for the elaboration
of functionalized silica.
Phosphorus dendrimers are usable to create new silica materials controlled at the
nanometric scale. The concomitant use of the positively charged dendrimers
37-G
n
or
38-G
n
(up to generation 8), cationic surfactants (cetyltrimethylammonium bromide
(CTAB)) and sodium silicate in water allowed the synthesis of mesostructured
nanoporous silica including dendrimers. We have shown that relatively important
amounts of dendrimers (up to 26% in weight) can be incorporated into hexagonal
silica phases during the structuring process. In these conditions, the inclusion of
dendrimers does not modify the honeycomb structure characteristic of the MCM-41
phase, as well as its narrow pore size distribution of about 25 A
and its specific surface
(Figure 16.10, left). These hybrid nanocomposites possess original properties, in
particular the unprecedented possibility to selectively remove the surfactant while
keeping the dendrimer inside the material, while noncovalently linked. Furthermore,
all the dendrimers, and particularly their end groups are fully accessible [47].
In another approach to functionalized silica, we have covalently grafted dendrons
39-G
3
functionalized by Si(OEt)
3
at their core. The cohydrolysis and polyconden-
sation of these macromolecules with a defined and varying number of equivalents of
tetraethoxysilane (TEOS) was carried out via sol-gel protocol, giving rise to dendron-
silica xerogels. The texture (porosity) of materials was determined byBET (Brunauer,
Emmett, and Teller) measurements. A narrow pore size distribution was obtained in
several cases (Figure 16.10, right). As the type of terminal groups of the dendrons can
be easily varied, this process allowed the synthesis of numerous types of functio-
nalized silica; we have in particular used as terminal groups of these dendrons
phosphines, dyes, nitriles, and amines [48].
Besides silica, less classical periodic hybrid organic-inorganic materials with
hierarchical structures and complex forms were obtained using as nanobuilding
