Chemistry Reference
In-Depth Information
7.1.2 Cross Linking Under Unusual Conditions
The earliest studies of networks formed in solution were undertaken to
investigate some subtle aspects of the elastic free energy expression—
whether or not an additional term in the logarithm of the volume was
required.
79-86
Other studies focused on the properties of networks in general. As can
be gathered from chapter 4, it is difficult to obtain information on the to-
pology of a network. Some studies have therefore taken an indirect ap-
proach. Networks were prepared in a way as to simplify their topologies,
and their properties were measured and interpreted in terms of reduced
degrees of network-chain entanglement.
71,
82,
87-94
The two techniques employed involved separating the chains prior to
cross linking by either dissolution
84
or stretching.
95
After cross linking,
the solvent is removed or the stretching force is relaxed, and the network
is studied (unswollen) with regard to its stress-strain properties, typically
in elongation.
85,
86
Figure 7.1 schematically shows the preparation of networks by cross
linking in solution followed by removal of the solvent.
96
Success in obtain-
ing elastomers with fewer entanglements is supported by the observation
that such networks come to elastic equilibrium much more rapidly than
elastomers cross linked in the dry state. Table 7.1 shows results on PDMS
networks cross linked in solution by means of γ radiation.
84,
97
Note the
continual decrease in the time required to reach elastic equilibrium,
t
eq
, and
in the extent of stress relaxation as measured by the ratio of equilibrium
to initial values of the reduced stress, [
f
*], upon decrease in the volume
XL
Solvent removal
Dry network with fewer
entanglements
Fi g u re 7.1:
Cross linking in solution to prepare networks of simpler topology. XL is the cross-linking
step done in solution.
