Chemistry Reference
In-Depth Information
chains—that is, to an inadequacy in the Gaussian distribution function.
Specifically the Gaussian approximation does not assign a zero probability
to a configuration unless the end-to-end separation is infinite. Since the
increase in modulus had generally been observed only in networks that
can undergo strain-induced crystallization, however, the increase could
be due to the crystallites acting as additional cross links.
Reinforcement resulting from strain-induced crystallization is identified
by the fact that the higher the temperature, the lower the extent of crystal-
lization and the worse the ultimate properties. The effects of an increase in
swelling were found to parallel those for an increase in temperature, as ex-
pected, since diluent also suppresses network crystallization.
167,
168
On the
other hand, in those cases where the upturns are due to limited chain exten-
sibility, increased temperature has little effect on the upturns.
96
Also, in
these cases swelling can even make the upturns more pronounced because
of the already-imposed stretching of the chains from the dilational effects
of the swelling. Thus, the effects of temperature or swelling represent a way
to determine whether the upturns are due to strain-induced crystallization
or to a non-Gaussian contribution arising from limited-chain extensibility.
Attempts were made to observe upturns from non-Gaussian effects in
noncrystallizable networks using end-linked, noncrystallizable model
PDMS networks described section 5.3. These networks have high extensi-
bilities, presumably because of the very low incidence of dangling-chain
network irregularities. Extensibility is particularly high when networks
are prepared from a mixture of very short chains (around a few hundred g
mol
-1
) with long chains (around 18,000 g mol
-1
), giving a bimodal distribu-
tion of network chain lengths.
169
Apparently the short chains are impor-
tant because of their limited extensibilities, and the long chains because
of their ability to retard the rupture process. Such “bimodal” model net-
works are discussed further in section 7.3.2
Stress-strain measurements on bimodal PDMS networks exhibited up-
turns in modulus that were much less pronounced than those in crystal-
lizable polymer networks. The upturns are independent of temperature
and are not diminished by incorporation of solvent. These characteristics
are expected for the case of limited chain extensibility.
166,
170
7.2.5 Dangling-Chain Networks
Since dangling chains are imperfections in a network structure, their
presence should have a detrimental effect on ultimate properties such as
the tensile strength, as gauged by the nominal stress,
f
*, at rupture,
f
*
r
.
