Chemistry Reference
In-Depth Information
are familiar with the now-commonplace constant-temperature bath con-
taining “silicone oil,” which operates at high temperatures for years with-
out any evidence for thermal degradation. Body implants made of
polysiloxanes show little evidence of degradation, hydrolytic or otherwise,
after decades of useful service, due to their resistance to hydrolysis and
oxidation. The inertness of the siloxanes should not be much of a surprise,
if one thinks of them as simple hydrocarbon modifications of the silicates
we commonly refer to as “glass.” In spite of this robustness, polysiloxanes
do
not
present severe environmental problems. For example, in the case of
a spill, or rupture of an electrical device such as a transformer, the poly-
mers released degrade completely and relatively rapidly under normal en-
vironmental conditions.
239-240
Examples of polysiloxane degradation studies include the role of surfac-
tants in suppressing aging of silica-PDMS gels,
242
the effects of pigments
on the stability of montmorillonite-PDMS composites,
243
and the use of
NMR and mass spectrometry to characterize degradation processes.
244
Degradation can occur in water, in air, and particularly in the soil
245
when polymers come into contact with one or more reactive species, such
as nitrate ion present in natural waterways. Nitrate is a source of atomic
oxygen and, from it, hydroxyl radicals, which initiate the degradation pro-
cess. Another reagent is ozone, split by UV (ultraviolet) light into oxygen
atoms, followed again by the production of hydroxyl radicals.
It is interesting to note that UV light itself has very little effect on the
siloxane structure. Only the very shortest wavelengths present in sun-
light have any influence and, in this case, generate methyl radicals from
the side groups. Polysiloxanes are generally resistant to all types of radia-
tion,
246 -250
particularly if they contain aromatic groups (e.g., the phenyl
groups in poly(methylphenylsiloxane)).
Even when methyl radicals are replaced by silanol units, the surface of
the material does not remain hydrophilic (water-wettable) very long.
Either the silanol groups condense with other silanol units to restore the
siloxane structure or unmodified chain segments migrate to the surface.
In any case, a “self-repair” mechanisms underlies the “recoverability” of
siloxane surfaces.
Clay minerals present in many soils have high interfacial areas with
strongly acidic groups on their surfaces. These materials can react with
siloxane chains and reorganize them into much smaller molecules. In fact,
water readily reacts with the Si-O bond in the presence of catalytic
amounts of either acids or bases. Some of these small molecules are vola-
tile enough to evaporate into the atmosphere. Others become capped with
silanol (-SiOH) groups that frequently makes them water-soluble, and
