Chemistry Reference
In-Depth Information
7.3.2 Bimodal Networks
7.3.2.1 Introduction
“Bimodal” elastomers prepared by end linking have very good ultimate
properties, which underlies recent interest in such materials.
71,
96,
139,
199-204
There have been several patents in the area.
205-207
In additional to this ex-
perimental work, there are now theoretical studies addressing the novel
properties of bimodal elastomers.
208-223
7.3.2.2 Materials and Synthetic
Techniques
Most bimodal networks have been prepared from PDMS, [-Si(CH
3
)
2
O-].
203,
204
PDMS is readily available with either hydroxyl or vinyl end groups and the
reactions in these groups are relatively free of complicating side reactions. The
end-linking reactions have generally involved hydroxyl-terminated
chains, which are readily obtained from the usual ring-opening polymer-
ization of the corresponding cyclic trimer or tetramer.
224
The ends of the
chains react with the alkoxy groups in a multifunctional organosilicate, as
described in chapter 3. In the application considered here, a mixture of
short and long hydroxyl-terminated polymers is end linked. The end link-
ing can take place in either the undiluted state or in solution.
225
Polyure-
thane elastomers have also been studied in this way.
226
In some cases, the
end linking was carried out in two steps.
227
An alternative approach in-
volves the addition reaction between vinyl groups at the ends of a polymer
chain and the active hydrogen atoms on silicon atoms in the [Si(CH
3
)HO-]
repeat units in an oligomeric poly(methyl hydrogen siloxane).
One can also introduce short chains by using a trifunctional end linker
with its fourth group able to associate with a similar group from another
trifunctional end linker.
228
Yet another alternative is to have potential
cross-linking sites that are closely spaced in one part of the chain back-
bone but widely spaced in another part.
143,
229
All these approaches can be
extended to higher modalities (trimodal, etc.).
There is evidence of large-scale supramolecular structures in end-
linked PDMS elastomers, particularly in the case of bimodal distribu-
tions.
230-233
Small-angle neutron scattering on bimodal networks of
poly(tetrahydrofuran) suggests segregation of short and long chains.
234
The distribution of network chain lengths in a bimodal elastomer can
be much different from the usual unimodal distribution obtained in less-
controlled methods of cross linking. Figure 7.15 shows a schematic
