Chemistry Reference
In-Depth Information
irradiation. It is also possible to generate the catalyst and other reac-
tants in situ, to give composites of unusually high transparency.
109
Interesting “aging” effects are frequently observed in these systems. If
the precipitated particles are left in contact with the hydrolysis catalyst
and water they appear to reorganize, so that their surfaces become better
defined and their sizes become more uniform.
35
The process seems analo-
gous to the “Ostwald ripening.”
110
The reinforcing ability of such in situ generated particles has been
amply demonstrated for a variety of deformations, including uniaxial ex-
tension (simple elongation), biaxial extension (compression), shear, and
torsion.
36,
42,
111,
112
In the case of uniaxial extension, the reduced stress [
f
*
]
frequently increases by more than an order of magnitude, with the iso-
therms generally showing the upturns at high elongation, which is the
signature of good reinforcement.
113,
114
The left portion of figure 9.4 shows
typical results, where α is the extension.
115
As is generally the case in filled
elastomers, there is irreversibility in the isotherms, which is thought to be
due to irrecoverable sliding of the chains over the surfaces of the filler
particles. The right portion of the figure documents the reinforcement ob-
served in biaxial extension. The maxima and minima exhibited by such
results will be a challenge to those seeking a better molecular understand-
ing of filler reinforcement.
Silica 22.5%
1.5
18.8%
1.0
9.4%
0.5
0%
0.0
0
1
2
3
4
5
α
-1
Figure 9.4:
Stress-strain isotherms for PDMS-silica in situ-reinforced elastomers in elongation
(region to the left of the vertical dashed line, with α
-1
< 1), and in biaxial extension (com-
pression, to the right, with α
-1
> 1).
115
The filled symbols represent the data obtained out
of sequence to test for reversibility. [
f
*] is the reduced stress, ([
f
*] =
f
*/(α - α
-2
) where
f
*=
f/A
*,
f
= elastic force,
A
* = undeformed area and α = elongation.
