Chemistry Reference
In-Depth Information
PDMS is relatively weak and stands to benefit most from reinforcement.
PDMS is easily absorbs the precursor materials generally used in the sol-
gel process. Nearly monodisperse silica microparticles can be obtained
using siloxane elastomer mixtures.
27
In some cases, the PDMS has been
part of a copolymer, with ureas,
28
imides,
29
amideimides,
30,
31
and diani-
lines.
20
In other approaches, the particle surface is modified,32
32
for exam-
ple, with a polysiloxane.
33
Siloxane/silica nanocomposites have also been
used as “culture-stone-protective materials.”
34
9.2 FILLERS IN ELASTOMERS
9.2.1 Approximately Spherical Particles
Sol-gel hydrolysis and condensation can be carried out within a poly-
meric matrix to generate
particles
of the ceramic material, typically with
an average diameter of a few hundred angstroms.
13-15,
35-37
Silica has been
of particular interest in this regard
38
because of its ecodesign, sustain-
ability, and recyclability.
39
The use of hard, polysilicate particles has been
explored in considerable detail.
40,
41
The polymer typically has end groups,
such as hydroxyls, that can participate in the hydrolysis-condensation
reactions.
42,
43
In some cases, amphiphilic PDMS networks have been
used.
44
Polysiloxanes have been particularly useful as templates and building
blocks for nanostructured materials.
45,
46
End groups improve bonding be-
tween the two rather disparate phases, but bonding agents may also be
introduced for this purpose.
47
Considerable reinforcement of elastomers,
including those prepared from PDMS, can be achieved. Some PDMS sam-
ples have even been electrospun into fibers and fiber mats.
48
In another
novel approach, an emulsion technique was used to prepare particles in
which the PDMS is entrapped in a silica-like matrix.
49
It is also possible to
use simultaneous vapor-phase polymerization to make hybrid organic/in-
organic thin films.50
50
Many of these nanocomposites have been characterized by nuclear mag-
netic resonance (NMR) spectroscopy,
51-54
single-molecule spectroscopy,
55
and small-angle scattering.
37,
56-58
Some applications require other measure-
ments, such as permeability
59
or ionic conductivity.
60
In the case of permea-
bility, the presence of the filler can substantially reduce flux by blocking
low-resistance pathways and by modification of the elastomer in the vicin-
ity of the filler (figure 9.1).
Other properties of PDMS-silica nanocomposites that have been re-
ported include the glass transition temperatures, segmental dynamics,
61-63
