Chemistry Reference
In-Depth Information
End linking
Peroxide
thermolysis
High-energy
irradiation
M
c
Fi g u re 7.10:
Values of the ultimate strength shown as a function of the molecular weight between
cross links for three cross-linking methods.
This expectation is confirmed by an extensive work on tetrafunctionally
cross-linked PDMS networks. Some pertinent results are shown schemat-
ically as a function of the molecular weight between cross-links in
i g u re 7.10.
171
The largest values
f
*
r
were observed for the networks pre-
pared by selectively joining functional groups occurring as chain ends (or
as side groups) on the chains, which leads to low incidence of dangling
ends. As expected, the lowest values of the ultimate properties generally
occur for the networks cured by radiation (ultraviolet light, high-energy
electrons, and γ radiation). The peroxide-cured networks are intermediate
to these two extremes, with the ultimate properties presumably depend-
ing on whether or not the free radicals generated by the peroxide are suf-
ficiently reactive to cause some chain scission. Similar results were
obtained for the maximum extensibility. These observations are certainly
interesting, but they are somewhat deficient in that information on the
number of dangling ends is generally not available.
Quantitative information has been obtained using the specific chemi-
cal reactions used to form ideal elastomers but modified to prepare inten-
tionally nonideal networks containing known numbers and lengths of
dangling-chain irregularities (figure 7.11).
172
If more chain ends are pres-
ent than reactive groups on the end-linking molecules, then dangling
ends result and their number is directly determined by the stoichiometric
imbalance. The length of the dangling ends is of necessity the same as
those of the elastically effective chains, as shown in part
a
of the figure.
This constraint can be removed by separately preparing monofunctionally-
terminated chains of any desired lengths and attaching them as shown in
part
b
.
