Chemistry Reference
In-Depth Information
=
3
=
4
0
M
c
Figure 7.6:
Dependence of Mooney-Rivlin ratio, 2
C
2/2
C
1, on the molecular weight between cross
links. The factor 2
C
2
measures the departure from affineness as the elongation increases,
and 2
C
1
approximates the high-deformation modulus. The ratio decreases with decrease
in network chain molecular weight, and with increase in junction functionality, as pre-
dicted by theory.
98
H
H
H
6
+
HH
H
Fi g u re 7.7:
End linking by an addition reaction between vinyl groups at the ends of a polymer chain
and the active hydrogen atoms on silicon atoms in an oligomeric poly(methyl hydrogen
siloxane). The case shown gives a junction functionality,
φ
, of six.
7.2.3 Effects of Entanglements
Model networks may also be used to test of molecular predictions for the
modulus of a network of known degree of cross linking. Some experiments
on model networks
10,
96,
153,
154
have given values of the elastic modulus in
good agreement with theory. Others
155-160
have given values significantly
larger than predicted.
96
The increase in modulus has been attributed to
contributions from “permanent” chain entanglements of the type shown
in figure 7.9. The issue, however, has not been completely resolved.
Aspects of greatest importance appear to be (i) investigating the effects of
cross linking in solution, (ii) studying the effects of swelling on networks
cross linked in the bulk (dry) state, (iii) building on the demonstration by
