Chemistry Reference
In-Depth Information
bimodal networks exceeds that of the unimodal ones (figure 7.20).
256
hese re-
sults imply that bimodality facilitates strain-induced crystallization,
96
possibly
through increased orientation of the longer, more easily crystallizable chains,
into crystallization nuclei.
257
Similar conclusions have been reached in studies
of bimodal networks of elongated poly(tetrahydrofuran),
258
but bimodality ap-
parently had little effect in the undeformed state.
259
In practical terms, the foregoing results demonstrate that short chains
of limited extensibility may be bonded into a long-chain network to im-
prove its toughness. It is also possible to achieve the converse effect. Thus,
bonding a small number of relatively long elastomeric chains into a short-
chain PDMS thermoset greatly improves both its energy of rupture and
impact resistance (figure 7.21).
260
Approximately 95 mol % short chains
gave the maximum effect for the molecular weights involved. Lower con-
centrations give smaller improvements and higher concentrations will
gradually convert the composite to a more rubberlike material.
7.3.2.5 Results in Other Mechanical Deformations
There are numerous other deformations of interest, including compres-
sion, biaxial extension, shear and torsion.
124,
261
Equibiaxial extension was
obtained by inflating sheets of unimodal and bimodal networks of
PDMS.
225,
262
Upturns in the modulus occur at high biaxial extensions, as
expected. Pronounced maxima precede the upturns (figure 7.22), which is
yet to be explained by molecular theories.
Bimodal, lower temperature
Bimodal, higher
temperature
Unimodal, higher
temperature
Unimodal, lower temperature
M
c
Fi g u re 7. 20 :
Ultimate strength, as represented by the modulus at rupture, shown as a function of the
molecular weight between cross links for a unimodal and bimodal elastomer compared at
two temperatures. The improvement is larger at the lower temperature, presumably due
to enhanced strain-induced crystallization.
