Chemistry Reference
In-Depth Information
figure 7.18. There have also been studies of the segmental orientation
90
and viscoelastic properties
88
of these networks.
In terms of the constrained-junction theory,
71,
96
the constraint param-
eter
κ
is reduced for solution cross-linked elastomers, indicating decreased
chain-crosslink entangling. Specifically, the theory gives a good account
of these results when
κ
was decreased though a reduction in the volume
fraction
v
2c
of polymer present during the cross linking.
89,
91-93
The values
of
κ
obtained are within the range obtained in other comparisons of theory
and experiment, as are the values of an additional, relatively unimportant
heterogeneity parameter ζ.
98
The values of
κ
generally decrease with de-
crease in the volume fraction
v
2c
, and with increase in degree of cross link-
ing as represented by the Mooney-Rivlin constant 2
C
1
. The dependence of
κ
on
v
2c
is significantly stronger than that suggested by theory however,
indicating a particularly strong effect of dilution on the degree of network
chain interpenetration. Simulations have been carried out to further clar-
ify the structure-property relationships of these elastomers.
99
Removal of the solvent has the additional effect of putting the chains
into a “supercontracted” state (figure 7.3).
96
Experiments on strain-in-
duced crystallization carried out on such solution cross-linked elastomers
indicate that the decreased entangling is less important than the super-
contraction of the chains. Crystallization required larger values of elonga-
tion compared to elastomers cross linked in the dry state.
100,
101
he most
recent work in this area has focused on the unusually high extensibilities
of such elastomers.
99,
102-107
Unusual properties are also obtained for networks prepared and stud-
ied in the opposite way, specifically cross linking in the dry state and then
swelling the network prior to the measurements of mechanical proper-
ties.
71
The approach to elastic equilibrium is more rapid and the stress-
strain isotherms in elongation are closer to the form predicted by the
Solvent
removal
Fi g u re 7. 3:
“Supercompression” of the network chains when a network formed in solution is dried.
