Chemistry Reference
In-Depth Information
NMR
187-189
and positron annihilation lifetime spectroscopy
188
have
been used to characterize nanocomposites of this type.
9.2.4 Layered Fillers
Exfoliated layered particles such as the clays, mica, graphite, or graphite
oxide give effective reinforcement of polymers at loading levels much
smaller than in the case of spherical particles such as carbon black and
silica.
190-193
It should be recognized, however, that reinforcement is typi-
cally well below that expected for idealized sheet-like fillers.57
57
Graphite-
oxide reinforced PDMS, for example, gives a 71% enhancement of young's
modulus at 3 wt % loading.
194
This value, however, is just 8% that expected
for flat sheets. The shortfall is attributed to crumpling of the sheets.
57
Other
properties can also be substantially improved, including processability,
resistance to solvents, and reduced permeability and flammability.190-192,
190-192,
195-202
The viscoelastic properties of graphite oxide suspensions in PDMS
have also been studied.
203
The layered silicate most often used to improve the properties of PDMS
is montmorillonite.
119,
204-225
Improvements have also been reported upon
the introduction of graphite,
226
graphite oxide,
194
mica,
227,
228
fluoromica,210
210
hectorite,
207
fluorohectorite,210
210
laponite,
210
wollastonite,
229
perovskite,
230
sepiolite,
231
and titanium-niobium oxide.
232,
233
Kaolin
234
and exfoliated
layered double hydroxides
235
have also been used in PDMS nanocompos-
ites. In some cases, diphenylsiloxane-clay hybrids have been used to rein-
force polyethylene.
236
Segmental motions of poly(methylphenylsiloxane) chains confined in in-
tercalated nanocomposites have been studied by quasielastic neutron scat-
tering and, surprisingly, were found to be
faster
than in the bulk polymer.
237
Computations have been carried out to better understand the struc-
tures and properties of these types of nanocomposites.
238-243
One subject
of particular interest is the observed
enhanced
gas transport performance
of PDMS nanocomposite membranes containing layered silicates,
244
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 proper-
ties of silsesquioxane polymers.
245,
246
9.2.5 Magnetic and Metallic Particles
Incorporating reinforcing particles that respond to a magnetic field is
important with regard to aligning particles to improve mechanical
