Chemistry Reference
In-Depth Information
a crystallizable elastomer. Upturns decrease or disappear upon either an
increase in temperature or addition of a diluent, as shown by two of the
additional isotherms. The upturns due to crystallization are absent in the
case of an elastomer that is inherently noncrystallizable, such as a ste-
reoirregular polyacrylate.
In related experiments, temperature was found to have little effect on
the Mooney-Rivlin isotherms for bimodal networks of (noncrystallizing)
PDMS, as would be expected if limited chain extensibility causes the
upturn (lower two isotherms in figure 7.19). Also, stress-temperature
(“thermoelastic”) and birefringence-temperature measurements showed
no discontinuities or discernible changes of slope. Rather strikingly, swell-
ing can even make the upturns in modulus
more pronounced
(upper iso-
therm in figure 7.19).
166,
170,
247
Apparently, the enhanced upturns are due
to the chains being stretched in the solvent dilation process, prior to fur-
ther stretching in the elongation experiments. In contrast, the upturns in
crystallizable polymer networks
disappear
upon sufficient swelling.
A final experiment of relevance concerns the spatially heterogeneous
PDMS networks in which the short chains are clustered. If the upturns in
modulus were due to some type of intermolecular organization such as
crystallization, then the behavior would presumably have been affected
by this change in spatial heterogeneity. Instead, there was no discernible
effect on the measured elastomeric properties. Also spectroscopy shows
that bond-angle deformation was not significant in highly elongated
PDMS elastomers.
248
The foregoing findings argue against the presence of any crystallization
or other type of intermolecular ordering. The upturns thus do seem to be
C
B
A
0
α
-1
Fi g u re 7.19:
Schematic Mooney-Rivlin isotherms for a noncrystallizable bimodal network: curve
A
for
a relatively low temperature,
B
for an increased temperature, and
C
for the introduction
of a swelling diluent.