Preparation, Analysis, and Degradation - The Polysiloxanes - page 19

toxicological effects such as carcinogenicity, mutagenicity, teratogenicity,
and bacterial or fungal colonization. 2
In the case of siloxane elastomers, the testing of mechanical properties
is of particular importance. Elongation or tensile measurements are used
almost to the exclusion of other types of mechanical tests, probably be-
cause of their simplicity. In this way, structural information is obtained
about the networks, such as their degrees of cross linking. 168 Measure-
ments of the ultimate strength (modulus at rupture), and the maximum
extensibility (elongation at rupture) are also important, as is recovery
after compression. 183 Relating such properties to the chemical nature of
the siloxane polymer, to the curing conditions, and to the nature and
amount of any reinforcing filler is a task of paramount importance in the
area of elastomeric applications. 168
Many applications of siloxane materials involve such a complicated
array of properties that the ultimate evaluation has to involve a “use” test.
Many surface applications, such as release coatings, are in this category.
In this approach, the prospective material is tested directly under stand-
ard conditions chosen to mimic those under which the material would ac-
tually be employed. 1-2, 174 The advantage of such a test is its direct
connection with the desired application. The major disadvantage results
from the fact that the underlying reasons why a material fails are not un-
covered in a global test. One purpose of such analysis is determination of
the extent to which a polymer has degraded under a specific set of
conditions.
2.7 DEGRADATION
Polysiloxanes are known to degrade in acids and in bases, 33 in the presence
of clay-like materials, 184 and even in the presence of minerals such as alu-
minum oxide. 185
Degradation of PDMS at elevated temperatures, 186-191 and at various
levels of humidity 192 is relevant to many applications. Decomposition
products have also been reported after siloxane pyrolysis. 193-194 Simula-
tions have been carried out on the thermal decomposition of PDMS using
a reactive force field,195 195 or density functional theory. 196 Other studies have
focused on the effects of photons, 197 high-energy protons 198 or 4 He ion
beams and flammability.199-202 199-202 Ablation and deposition of PDMS with
x-rays have also been reported. 203 Degradation of biodegradable polysilox-
anes is of particular importance, 204 as is their behavior in biomedical ap-
plications in general. 205