Chemistry Reference
In-Depth Information
consist of
chains
of sulfur atoms.
293
If such polysulfidic cross links can
themselves act as elastomeric network chains, then a bimodal network is
produced, albeit inadvertently (figure 7.26). Calculations are in good
agreement with experiment.
294
A similar situation may occur in the case of networks end linked using
the addition reaction involving vinyl chain ends and hydrogen atoms in an
oligomeric poly(methyl hydrogen siloxane).
96,
154,
295-298
In the case of in-
complete reactions, the segments between the reacted silicon atoms on the
oligomer may be long enough to act as elastically effective chains in a bi-
modal structure. Finally, a bimodal chain-length distribution has also
been proposed to explain some unusual properties of polysiloxane net-
works that were post cured,
299
elastomers prepared from two cross-linking
systems,
300
and elastomers reclaimed by ultrasonic devulcanization.
301,
302
7.3.2.8 Other Materials in Which Bimodality Might Be Advantageous
There are other cases where a bimodal distribution of chain length or some
other physical property can be advantageous, possibly through the idea of
a “delegation of responsibilities.”
96
In thermosets there seems to be an improvement in mechanical prop-
erties when the polymer being cured has a bimodal distribution of molec-
ular weight.
303
In this case, the improvements may be due to different
morphologies and degrees of inhomogeneity
304,
305
resulting from the fact
that the long chains in a bimodal distribution could have considerably
lower solubility than the short chains. Also, it is well known that the flow
Figure 7.26:
Sketch showing the difference between a monosulfidic (left portion) and a polysulfidic
cross link (right portion). In the latter case, the chains of sulfur atoms may act as addi-
tional, elastically effective chains in what is essentially a bimodal network.
