Elastomeric Networks - The Polysiloxanes - page 172

M S / M L
Fi g u re 7. 24:
Maximum tensile strengths in tearing for bimodal elastomers as a function of the ratio of
the molecular weights of the short and long chains.
important to establish the molecular weight at which this decrease occurs
for a variety of deformations and a number of different elastomers.
Some Rheovibron viscoelasticity results have been reported for bi-
modal PDMS networks. 270 Measurements are first carried out on uni-
modal networks consisting of the chains used in combination in the
bimodal networks. One of the important observations was the depend-
ence of crystallinity on the network chain-length distribution.
Some measurements have been made on the permanent set of PDMS
networks in compressive cyclic deformation (figure 7.25). 271 here ap-
peared to be less permanent set or creep in the case of the bimodal elasto-
mers, consistent in a general way with some early results for polyurethane
elastomers. 272 Specifically, cyclic elongation measurements on unimodal
and bimodal networks indicated that the bimodal ones survived many
more cycles before fatigue failure. The number of cycles to failure is ap-
proximately an order of magnitude higher for a bimodal network, at the
same modulus (Mod10) at 10% deformation 96 Also viscoelastic effects in
bimodal networks are not simple averages of contributions from the short
and long chains. 270, 273
7.3.2.6 Results on Nonmechanical Properties
Birefringence can be used to characterize non-Gaussian behavior in PDMS
bimodal elastomers. 96, 274 - 278 A large decrease the stress-optical coefficient
(ratio of birefringence to stress) was observed over a relatively small range
in elongation, 78 presumably due to limited extensibility of the short chains.