Chemistry Reference
In-Depth Information
PDMS SOFT CONTACT LENS
Eye
O
2
Hydrophilic
coating
Figure 5.7:
An example of the use of grafting to change only the surface properties of a polymeric
material.
303
Reproduced by permission of the American Chemical Society.
The contact lens application just cited illustrates the use of one of the
most striking properties of PDMS, its superb transparency.
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220
PDMS
can retain this transparency even when filled (reinforced) with rather
large amounts of silica. The transparencies
219,
221
and indices of refrac-
tion
222
of polysiloxane have been of considerable interest in general.
5.5 DIELECTRIC CONSTANTS AND DIPOLE MOMENTS
Some of the earliest relevant work in this area involved experimental
studies of the dielectric relaxation times, polarizations, and dipole mo-
ments of the small molecules hexamethyldisiloxane, hexaethyldisiloxane,
hexamethylcyclotrisiloxane, and octamethylcyclotetrasiloxane.
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Research on the corresponding polymers has focused almost exclu-
sively on PDMS, but there has been some work on poly(diethylsiloxane).
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In the case of the PDMS, most studies have involved the linear chains,
31,
225-235
some the cyclics,
233-235
and at least one cross-linked elastomer.
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Improvements in dielectric properties continue to be important goals in
the case of the polysiloxanes.
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5.6 STABILITY, SAFETY ASPECTS, AND ENVIRONMENTAL IMPACTS
Siloxane polymers possess a number of properties that seem almost con-
tradictory. One example is the tremendous stability and durability shown
by polysiloxanes in a wide variety of applications.
239-241
One reason for
this stability is the fact that the chain is already in a high oxidation state,
and reduction takes place only at very high temperatures. Most scientists
