Chemistry Reference
In-Depth Information
thus environmentally degradable. At later stages in the process, even the
hydrocarbon groups are affected. Although the organosiloxane structure
is completely unknown in nature, the introduction of these small mole-
cules into the biosphere is thought to be entirely harmless. Furthermore,
some of the degradation processes lead ultimately to silica, water, and
carbon dioxide or inorganic carbonates.
239, 240
The degradative reactions experienced by siloxane chains usually gen-
erate silanol groups, often at the chain ends, and can produce various cy-
clics.
251
Pairs of such silanol groups can condense with one another,
forming new siloxane linkages. This condensation reaction provides an
interesting “healing” mechanism for the siloxane backbone.
Resistance to degradation or cross linking by radiation is also impor-
tant.
252,
253
Resistance is one advantage that poly(methylphenylsiloxane)
has over poly(dimethylsiloxane); the phenyl groups are efficient energy
sinks that suppress the usual effects of radiation on a polymer.
There is interest in synthesizing polysiloxane in environmentally
friendly supercritical carbon dioxide
.254
Studies of the interactions be-
tween PDMS and supercritical carbon dioxide have been reported.
255
his
medium has also been used in the preparation of composites between
PDMS and polystyrene
256
and nanoclays.
257
Supercritical fluids have been
used to study various properties
258
including surface behavior and carry-
ing out depth analyses in PE/PDMS composites.
259
Recycling of polymers has become an important environmental issue.
Not surprisingly given its relatively high cost, such studies are underway
on silicone elastomers as part of a general effort addressed to various
types of elastomeric materials, including ones that are heavily filled with
silica.
260-263
5.7 THERMODYNAMICS
Typical thermodynamic properties of the polysiloxanes, such as heat ca-
pacities, solubility parameters, thermodynamic interactions parameters,
and so on, have been extensively tabulated.
1,
99,
264
Of considerable interest
have been their low-temperature properties, particularly stiffening ef-
fects.
22
Static and dynamic light scattering measurements have even been
carried out on PDMS in supercritical carbon dioxide.
265
In some cases, dual
techniques have been employed, for example, the joint use of differential
scanning calorimetry
266
and Raman spectroscopy.
267
Equation-of-state
parameters for PDMS itself have been reported,
268-270
as have excess vol-
umes and chemical potentials for PDMS in several solvents.
271
