Chemistry Reference
In-Depth Information
distribution for the important example in which there is simultaneously a
large
number
fraction of short chains and a large
weight
fraction of long
chains. The major difference is the significant numbers of both very short
chains and very long chains, which contrasts sharply with the
small
amounts of such chains in a typical unimodal distribution. The case
shown, where the short chains predominate numerically, is of interest
with regard to improvements in mechanical properties.
96
Studies have
documented the effects of chain lengths, composition, and cross-link
functionality.
235-239
7.3.2.3 Testing of the Weakest Link
he weakest-link theory
198
was tested by preparing end-linked networks
containing increasing amounts of short chains, on the order of
10-20 mol %.
71,
199
In striking disagreement with the suggested mode of
elastomer failure, these elastomers showed no significant decrease in ul-
timate properties with increasing number of short chains. Networks are
apparently much more resourceful than given credit for in the weakest-
link theory. Apparently, the strain is continually being reapportioned
during deformation, such that the more easily deformed long chains bear
most of the burden of the deformation (figure 7.16). Thus, the short
chains do not contribute significantly until just prior to rupture. The flaw
in the weakest-link theory is the assumption that all parts of the network
deform affinely, whereas the deformation is markedly nonaffine.71,
71,
96
M
Figure 7.15:
Network chain-length distributions in which
N
M
is the number of chains in an infinitesi-
mal interval around the specified value of the molecular weight
M
. For reference purposes,
a unimodal distribution is shown between the two parts of the bimodal distribution.
