Chemistry Reference
In-Depth Information
Table 7.1.
PDMS NETWORKS COMPARED AT APPROXIMATELY CONSTANT
MODULUS
v
2S
a
t
eq
(hr)
[
f
[f*](equil)/ [f*](init)
2
C
2
(N mm
-2
)
1.00
0.70
0.95
0.062
0.75
0.48
0.98
0.057
0.62
0.10
0.99
0.059
0.55
0.30
0.99
0.062
0.48
0.02
1.00
0.067
0.40
0.03
1.00
0.039
0.30
0.00
1.00
0.031
a
Volume fraction of polymer in the solution being irradiated during the cross-linking reaction, which is
generally somewhat larger than the volume fraction,
v
2c
, actually incorporated in the gel.
Reprinted with permission from J. E. Mark et al.,
Physical Properties of Polymers
. Copyright 1984 Amer-
ican Chemical Society.
84,
97
fraction of polymer present during the cross linking. ([
f
*] is defined in sec-
tion 7.1.3). Figure 7.2 qualitatively explains these observations. When a
network is cross linked in solution and the solvent then removed, the
chains collapse in such a way that there is reduced overlap in their configu-
rational domains, which reduces chain-junction entangling embodied in
the various constraint theories.
Solution cross-linked elastomers also exhibit stress-strain isotherms
in elongation that are closer in form to those expected from the simplest
molecular theories of rubberlike elasticity.
87
Specifically, there are large
decreases in the Mooney-Rivlin 2
C
2
correction constant described in
Figure 7.2:
Typical configurations of four chains emanating from a tetrafunctional cross link in a
polymer network. In the left sketch, the network was prepared in the undiluted state, and
in the right sketch, it was prepared in solution and then dried. When a network is cross
linked in solution and the solvent then removed, the chains collapse in such a way that
there is reduced overlap in their configurational domains.
