Chemistry Reference
In-Depth Information
Aliphatic or aromatic peroxide curing agents can also be used, by reac-
tions with vinyl side chains or even saturated alkyl groups.
136-137
Specific
peroxides are chosen on the basis of their decomposition temperatures,
and the reaction products they leave behind after the curing process is
complete. Some peroxides used are bis(2,4-dichlorobenzoyl)peroxide,
benzoyl peroxide, dicumyl peroxide, and di-
t
-butyl peroxide.
117-120, 138
PDMS has also been cured using UV (ultraviolet) radiation,
139 -142
gamma or electron beams,
143-150
and laser irradiation.
151
Thermal cures are
also available.
20, 152-155
Also relevant here are physically cross-linked fluo-
rosilicone elastomers obtained by self-assembly and template polycon-
densation of tailored building blocks.
156
Evaluation of the results of any of these curing processes is important,
including rheological characterization.
157
Control of the material during
cross linking is quite important since movement of the polymer during
the curing process can affect the mechanical properties of the resulting
elastomer.
158
Some modeling of the vulcanization process has been car-
ried out. Additional work could be very useful for optimizing the proper-
ties of the resulting elastomers.
159
Using cyclic polystyrene and cyclic PDMS can give PDMS networks
with movable cross-link sites
160
and some chemistries can yield elasto-
mers in which the cross linking is reversible.
161
Also, including the proper
functional groups on the chains can make PDMS elastomers self-healing
after rupture.
162-163
In an extension of these ideas, the cross linking of en-
capsulated PDMS resins has been used for the self healing of high-
temperature cured epoxies.
164
There has been some interest in cross-linking in solution, since the net-
work chains in the subsequently dried elastomer are “super compressed.”
These materials can have some unusual properties, including unusually
high extensibility.
165-167
Mechanical property measurements are the most common way to char-
acterize cross linking.
115,
168
Different curing methods can give different
mechanical properties.
169
New characterization methods are being devel-
oped, including fluorescence,170
170
small-angle neutron scattering,
171
and
multiple quantum
1
H NMR.
The compounding process can be very complicated.
104, 107
The amount of
some ingredients used is fixed by the stoichiometry of the reaction in
which they participate. The end-functionalized polymer and the end-
linking agent are in this category. The relative amounts of other ingredi-
ents are often chosen by experience or by trial and error. After all the
amounts have been selected, they are mixed (“compounded”) in conven-
tional equipment (e.g., a Banbury mixer). Although the resulting mixture