Silicon polymers characteristics

The synthesis of silicone polymers from prepolymers will allow one to obtain not only polyorganosiloxanes, but also polyelementorganosiloxanes of a more regular structure (unlike the existing processes of hydrolytic cocondensation of various organochlorosilanes which form polymers of a static composition). Therefore, the polymers obtained in this way will have improved chemical and physicochemical properties, and materials based on them will have valuable performance characteristics. Moreover, a new technique for obtaining polymers based on block oligomers will help to... 

The introduction of functional groups (halogen, CN, COOH, NH2, etc.) into the organic part of the molecule is a way for a controlled modification of silicone polymers. These groups increase the polarity of the molecule and the intermolecular forces of interaction between the chains, which has a positive effect on the mechanical and adhesive characteristics of the polymers. 

Silicone polymers have one more remarkable characteristic. They combine well with various organic and inorganic substances, which makes it possible to increase their strength, elasticity and especially glueing and adhesive ability to glass, metals and other materials. 

At a hrst glance, a bonded polymers may be considered as being characteristically different from n bonded polymers. However, there are several systems which truly behave very much like Jt polymers. Characteristic examples include polysilanes, whose backbone is a <7 bonded silicon zig-zag chain with alkylic side groups (Rabolt et al., 1986). The similarity to the Ji-.systems results from the fact that there is an efficient overlap of adjacent orbitals leading to conjugated [Pg.381]

While the silicone polymer phase provides a high enough oxygen permeability, it also dominates the surface characteristics, i.e., the surface of silicone hydrogel is too hydrophobic to be used as a contact lens. The silicone phase also readily absorbs... 

The hydrophobicity of the surface prevents the wetting by tear and tends to expose dry surface of a contact lens. Therefore, rapid dehydration of the corneal tissues could occur, which could cause the damage of corneal epithelium. However, this explanation seems to be oversimplified in light of the adsorption of protein, which makes a hydrophobic surface wettable by tear fluid, as described in Chapter 26. Moreover, the highly hydrophobic surface characteristic of silicone rubber tends to encourage the deposition of protein and mucus of the tear on the surface of the lens. Lipids and lipid-soluble materials follow the same track and eventually penetrate into the bulk phase of the contact lens. Because of these undesirable factors, the use of silicone contact lenses of various chemical compositions and with surface treatments has not been successful but rather disastrous because of the interfacial characteristics of silicone contact lens on the cornea, which cannot be oflfset by these efforts. It indicates that more profound surface modification to cope with the problems rather than mere surface treatment is needed in capitalizing on the advantageous bulk properties of silicone polymers. 

Changes in the crystallization behavior of the thermoplastic phase of the IPN have been observed in DSC (differential scanning calorimetry) experiments (i3). In Figure 7, a series of DSC scans on silicone-nylon 6,6 composites of various silicone contents are presented. The characteristic melt peak of the nylon matrix is observed in all of the thermograms, but a second, slightly lower temperature melt peak becomes more pronounced as the concentration or the cross-link density of silicone polymer in the IPN is... 

Furthermore, the molecular weights of silicone polymers are higher than their viscosities indicate. This is a function of the low intermolecular forces characteristic of organosilicones. 

Both poljmiers respond to ppb levels of NO2 at a sensitivity comparable to that of Pb phthalocyanine. The polymers exhibit good heat-resisting characteristics and are stable even in operating in the 200-250°C temperature region. Moreover the silicon polymer exhibits a negligible response to water vapor. 

In many cases, a material may exhibit the characteristics of both a liquid and a solid, and neither of the hunting laws will adequately describe its behavior. The system is then said to be in a viscoelastic state. A particularly good illustration of a viscoelastic material is provided by a silicone polymer known as bouncing putty. If a sample is rolled into the shape of a sphere, it can be bounced like a rubber ball, i.e., the rapid apphcation and removal of a stress causes the material to behave like an elastic body. If, on the other hand, a stress is applied slowly over a longer period the material flows like a viscous liquid, and the spherical shape is soon lost if left to stand for some time. Pitch behaves in a similar, if less spectacular, manner. 

A lengthy discussion follows, covering how the polymers behave in use, the spreading characteristics, and the many beneficial properties silicone polymers offer when delivered to keratinous surfaces. The chapter concludes with a thorough discussion of the safety and environmental impact of silicone-based polymers. 

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