Chemistry Reference
In-Depth Information
It is possible to determine the amount of silicon present by pyrolysis to
silica, followed by atomic absorption methods.
Both mass spectrometry and gas chromatography are used to identify
and determine the amounts of volatile siloxane-type materials.
1,
174
he
more important nonvolatile materials, including polymers, can be charac-
terized by liquid chromatography and by gel permeation chromatography
(GPC). When used analytically, these techniques give molecular weight
distributions; used preoperatively, they yield narrow molecular weight
distribution fractions that are suitable for determining structure-
property relationships. Average molecular weights can be determined by a
variety of techniques, including dilute solution viscometry, osmometry,
ultracentrifugation, and light scattering intensity measurements. Other
techniques for determining molecular weight distributions include frac-
tional precipitation and gradient elution. Extractions with supercritical
fluids seem particularly promising in this regard.
177
Thermal properties are measured and evaluated by common thermal
analysis instruments. For identification of transition temperatures, heat
of fusion, differential thermal analysis (DTA), and differential scanning
calorimetry (DSC) are available. Thermal stability is measured by thermo-
gravimetric analysis (TGA), although this technique can give overly opti-
mistic results unless used with great care.
Rheological measurements are of central importance in the processing
of siloxane polymers. Typical studies include determination of the de-
pendence of the bulk viscosity on the average molecular weight, molecular
weight distribution, and rate of shear. Characterization of the effects of
branched chains or reinforcing fillers present is also of great importance.
2
Most siloxane polymers are excellent insulators so electrical properties
are important for many applications. Such properties include resistivity,
dielectric constant, dielectric losses, dielectric strength (resistance to
electrical breakdown), and power factors.
2
The use of siloxane polymers in applications such as separation mem-
branes, drug release systems
178 -179
and blood oxygenators requires exten-
sive permeability studies. These applications also involve measurements
of diffusivity and solubility.
2
For some specialized applications, optical properties can be of crucial
importance. Two examples are contact lenses and interlayers for glass
windshields. Here, transparency is of primary importance, but index of
refraction (
n
) is important for matching values of
n
for polymers and fill-
ers.
2, 180-181
Applications in the biomedical area require extensive testing of biocom-
patibility,
182
including acute, dietary and implant testing, and monitoring
