Chemistry Reference
In-Depth Information
moduli are significantly larger than the predicted ones. Part of the differ-
ence, but probably not all, is due to problems in bringing such lightly
cross-linked networks to elastic equilibrium in the unswollen state. Fi-
nally, the moduli tended to zero as the junction density went to zero as it
should since PDMS in the liquid state above
T
g
.
The departure of the experimental results from the simple molecular
theory when the network chains are long indicates the presence of addi-
tional contributions to the modulus. The question is whether the depar-
tures result from trapped entanglements or simply from the presence of
other chains sharing the volume of a given network chain. One important
difference between the two types of constraints involves their depend-
ence on network swelling. The localized, permanent entanglements should
be independent of swelling, while the more diffuse interchain interactions
should decrease with increased swelling. The contributions from trapped
entanglements should therefore persist even in highly swollen networks,
and therefore contribute to the phantom modulus. For these reasons, it is
quite important to carry out measurements of the modulus versus the
degree of swelling, with the results at the highest degrees of swelling pre-
sumably being least complicated by nonequilibrium effects.
If one makes a network in the extremely dilute state so that there is no
chain interpenetration during its formation, the result will be a “phan-
tom-structure state” in which the chains can seemingly pass through one
another. Experimentally, network formation in the dilute state decreases
both the modulus and its dependence on elongation (the “
C
2
effect”).
96
It
was observed that the number of entanglements resulting from the dis-
perse interpenetration of chains in the cross-linked state were far more
numerous than the specific localized points along the chain. Definitive
experiments of this type would do much to resolve the nature and impor-
tance of chain entanglements in network structures at elastic
equilibrium.
7.2.4 Interpretation of Ultimate Properties
Ultimate properties of unfilled elastomers at high elongations reveal how
model networks can clarify elastomer properties. An upturn in modulus
(as in figure 7.18-A) is frequently exhibited by elastomers at very high
elongation.
121,
124,
126,
165
The upturn is very important since it corresponds
to a significant toughening of the elastomer; its molecular origin,
however, has been the source of some controversy.
166
he upturn has
been widely attributed to the “limited extensibility” of the network
