Silicon-oxygen bond polymers

Silicones are probably best known for their application as sealants and as release materials for pressure sensitive adhesives [107]. The silicone polymer combines an inorganic backbone made from silicon-oxygen bonds with organic substitution on the silicon atom. This repeating unit, shown below is called a siloxane. 

Silicate minerals are classified by the degree to which silicon-oxygen bonded networks (polymers) form (Fig. 4.4). The degree of polymerization is measured by the number of non-bridging oxygens (i.e. those bonded to just one Si44). 

The enhanced thermal stability of the silicones has been postulated by Andrianov and Sokolov (26) to be due to (1) the high energy of the SiOSi bond, (2) the stability of organic radicals to oxidation at the silicon atom, (3) stable SiOSi bond formation within the polymer after partial degradation, and (4) the formation of a layer of silicon-oxygen bonds on the surface of the polymer during thermal oxidation. 

Silicon s bond to oxygen is two-and-a-half times the strength of a silicon-silicon bond so strong that in nature, silicon is exclusively bound to oxygen. We could conceive of a biochemistry based on silicon-oxygen polymers of the form —Si—O—Si—O—Si—O— artificial polymers of this type (silicones) are common. Unfortunately, the silicon-oxygen bond is too strong. It is hard to break—as one would want to in order to work with biomolecules. 

The silicone-oxygen bond has one of the highest torsional mobilities in any polymer backbone, e.g., Tg of polyfmethyl siloxane) is -130 °C. Polydimethylsiloxane (-[Si(CH3)20] -) is more stable to thermooxidative attack than is the aliphatic C-O bond but is sensitive to certain reagents such as acids and bases. 

This later led to the development of the most popular silicone polymers, the polysiloxanes, which, at high molecular weight, exhibit rubbery properties. Such elastomers are generally obtained from a cyclosiloxane monomer by a ring-opening reaction, as shown in Equation (A). Because of the unusual ease of rotation around the silicone-oxygen bond, the polymer chain is much more flexible than carbon-based polymers, so that a semi-liquid material is obtained, even at molecular weights of several hundred thousand. Fine... 

To a great extent, the chemistry of silicon is the chemistry of the silicon-oxygen bond. Just as carbon forms unending C—C chains, the —Si—O— grouping repeats itself endlessly in a wide variety of silicates, the most important minerals on the planet, and in silicones, synthetic polymers that have many appUcations ... 

Polydimethylsiloxane is the base polymer for silicone formulations employed in outdoor high voltage systems, not only in the case of coatings but also for composite insulators [3, 9-11]. It is a widely applied polymer within the silicone family [42], with remarkable and sometimes unique properties, which enable applications not only in outdoor installations but also in many technical fields [42-47]. The polymer chain is composed of silicone oxygen bonds, accompanied by two side methyl groups connected to the siHcone atom, as shown in Figure 1.4, where the monomer unit is illustrated. 

These properties are strongly correlated to the monomer structure and especially to the presence of the silicone oxygen bond, the weak intermolecular forces that appear within the material and the flexibility of the polymer chain [43]. 

Consequently, the presence of the silicone oxygen bond in the material monomer results in the development of a strong structure when compared to other polymers within the same application field, where the polymer chain is composed of carbon atoms instead of silicone. Nevertheless, it must be noted that the polar nature of the silicone oxygen bond results in a material that is vulnerable to the action of hydrolysis [44], which, however, appears to be notable for pH values less than 2.5 or greater than 11 [48]. 

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