Chemistry Reference
In-Depth Information
Very little work has been done on elastomers in torsion (twisting a cy-
lindrical sample around its long axis). More results are presumably forth-
coming, particularly on bimodal networks and on networks containing
some of the unusual fillers introduced in situ by hydrolysis of organome-
tallic compounds such as silicates or titanates.
96,
264
In any case, the same
types of bimodal PDMS networks showed rather different behavior in tor-
sion.
265
Specifically, ambiguous upturns in modulus were observed at large
deformations. It has not yet been established whether the lack of upturn
is due to the inability to reach sufficiently large torsions or whether there
is some inherent difference in this type of deformation.
Tear tests have been carried out on bimodal PDMS elastomers
266-269
using the standard “trouser-leg” method. Tear energy is considerably in-
creased for a bimodal distribution, with documentation of the effects of
compositional and ratio of molecular weights of the short and long
chains. The increase in tear energy did not seem to depend on tear rate,
266
an important observation that suggests that viscoelastic effects are not
of paramount importance in explaining the observed improvements.
Figure 7.23 shows some of these results schematically.
A subsequent series of shear tests
267
established the dependence of the
tearing properties on the composition of the bimodal networks and the
lengths of the chains used to prepare them. The maxima the tearing
energy versus the amount of the short-chain component locates the com-
position giving the greatest increases in tear energy. Tensile strength de-
pends on the ratio,
M
S
/
M
L
, of molecular weights of the two components
(figure 7.24). The observed increase in strength with deceased molecular
weight of the short chains must eventually decrease reverse when the
chains become too short to have any elastic effectiveness at all. It is
Bimodal
Unimodal
Log Tearing Rate
Fi g u re 7. 23:
Tearing energies for a unimodal and bimodal elastomer as a function of tearing rate
