Chemistry Reference
In-Depth Information
prior to incorporating the long ones.
207,
241,
242
The short-chain component
can even be prepared separately, ground up, and then blended into an-
other elastomeric network.
243
The resulting network can be spatially as
well as compositionally heterogeneous, in that many of the short chains
will be segregated into densely cross-linked domains that are only lightly
cross linked to other such domains.
241
These networks could serve as
models for inhomogeneously cross-linked elastomers, such as those cured
by thermolysis of a partially miscible peroxide.
Another approach is based condensation and addition end-linking re-
actions carried out simultaneously and independently, which can give rise
to the interpenetrating network structure.
96
The distribution of network
chains can be made bimodal, even though the short chains and long chains
communicate only through their entanglements. Finally, it is also possible
to reinforce any type of bimodal network with filler particles, thereby fur-
ther improving its mechanical properties.
96
Measurements of the mechanical and optical properties of bimodal net-
works as a function of temperature and degree of swelling were used to
test further the conclusion cited in section 7.3.2.4 that crystallization was
not the origin of the improved properties.
96,
203,
204,
244,246
For example,
stress-strain measurements on such bimodal PDMS networks exhibited
upturns in modulus that were much less pronounced than those in crystal-
lizable polymer networks such as natural rubber or
cis
-1,4-polybutadiene.
The top portion of figure 7.18 illustrates upturns in the case of
A
B
C
D
0
α
-1
Fi g u re 7.18:
Schematic stress-strain isotherms in elongation for a unimodal elastomer in the Mooney-
Rivlin representation of modulus against reciprocal elongation. The isotherms are repre-
sented as the dependence of the reduced stress ([
f
*] =
f
*/(α - α
-2
) on reciprocal elongation.
(
f
*=
f/A
*,
f
= elastic force,
A
* = undeformed area, α = elongation). The top three are for a
crystallizable network: curve
A
for a relatively low temperature,
B
for an increased tem-
perature, and
C
for the introduction of a swelling diluent. Isotherm
D
is for an unswollen
unimodal network that is inherently noncrystallizable.
