Silicon backbone oxidation

Silicone rubber offers a set of unique properties to the market, which cannot be obtained by other elastomers. The Si-0 backbone provides excellent thermal stability and, with no unsaturation in the backbone, outstanding ozone and oxidative stability. The very low glass transition temperature, combined with the absence of low-temperature crystallization, puts silicones among the materials of choice for low-temperature performance. The fluoro-substituted versions provide solvent, fuel, and oil resistance along with the above-mentioned stability advantages inherent with the silicone backbone. 

Reversion and ring formation have been partially overcome through placement of chain-stiffening units in the silicone-backbone. Thus linear D2-m-carborane-siloxanes (11—14) with one to three trifluoropropyl moieties per repeating unit) exhibit better thermal and oxidative stability than silicones and fluorosilicones (J ). Initial degradation occurs in air about 300 to 350 C, almost 100 above that typically experienced for siloxanes and fluorosilicones. The carborane-siloxanes exhibit T s from -50 to 0 C ( 1 ). The carborane moiety also acts to inhibit formation of six-membered rings because of its size. 

The capped allyl polyalkylene oxide can also be based upon propylene oxide or a mixed ethylene oxide-propylene oxide copolymer. The resulting structure is a comb polymer, with pendant capped hydrophilic groups. In aqueous solution, the hydrophilic groups may form a sheath around the hydrophobic silicone backbone to minimize its contact with the water. 

The different structures affect how the molecules can pack at an interface. With pendant types in aqueous media, the silicone backbone aligns itself with the interface leaving the polyalkylene oxide groups projecting into the water. Linear types are considered to form a very flattened W alignment where the central silicone portion of the molecule aligns with the interface and the terminal groups are in the aqueous phase. The amounts to be added vary between 0.01% and 0.5% on the total formulation. 

Silicon carbide is also made as very strong fibres by the sequence of reactions shown in Equations 10.25 and 10.26, in which an infusible polymer containing the silicon backbone shown in Structure 10.6 is made first. This is converted by heat in an argon atmosphere into an isomer containing the silicon-carbon backbone shown in Structure 10.7, which can be melted and spun into fibres. The fibres are heated in air at 300 °C to partly oxidize them and make them infusible, and are then heated in nitrogen to 1 300 °C to convert them into SiC fibres ... 

The range of high-temperature rubbers is very small and limited to the silicones, already considered in this chapter, and certain fluororubbers. With both classes it is possible to produce polymers with lower interchain attraction and high backbone flexibility and at the same time produce polymers in which all the bonds have high dissociation energies and good resistance to oxidation. 

Poly(hydrosilane)s are stable compounds and can be manipulated in the air only for a short period since they are oxygen sensitive. In order to study the oxidation products, a xylene solution of poly(phenylhydrosilane)(Mw = 2340, Mw/Mn = 1.72) was refluxed (140 °C) for 12 h in a system exposed to the air [15]. Only minor changes were observed by GPC analysis whereas FTIR showed characteristic absorptions due to siloxane-type structures on the polymer backbone. A detailed NMR analysis, based on H NMR, Si INEPT and H- Si HMQC spectroscopies, indicated that the oxidized material contains the units 7-10 shown in Scheme 8.2. In particular, units 7,8 and 9+10 were present in relative percentages of 27%, 54% and 19%, respectively, which mean that more than 70% of the catenated silicons were altered. It has also been reported that silyl hydroperoxides and peroxides are not found as products in the autoxidation of poly(phenylhy-drosilane) [16]. 

The greater stability of sterically hindered siloxanes indicates that oxidation occurs at the silicon atom. Stability toward oxidative cleavage is dependent on both the nature of the organic groups and the backbone structure. 

Polysilanes are chemically inert to air and water at ordinary temperature, but their reactivity increases in solvent. In a solvent such as tetrahydrofuran, degradation of the Si—Si backbone by strong bases is quite rapid. Strong oxidizing agents like /77-chloroperbenzoic acid insert oxygen atoms between the silicons to produce Si—O—Si linkages in the backbone (120). 

Fluorosilicones can be compounded by the addition of mineral fillers and pigments. Fillers for such compounds are most commonly silicas (silicon dioxide), because they are compatible with the elastomeric silicon-oxygen backbone and thermally very stable. They range in surface areas from 0.54 to 400 m2/g and average particle size from 100 to 6 nm. Because of these properties, they offer a great deal of flexibility in reinforcement. Thus, cured compounds can have Durometer A hardness from 40 to 80. Other fillers commonly used in fluorosilicones are calcium carbonate, titanium dioxide, and zinc oxide. 

The alternatives to metal-based semiconductors are organic semiconductors, but they struggle to reach efficiencies of 5%, although there are hints in the scientific literature of polymers that are comparable to silicon in their semiconducting properties. Even so, polymers rely on double bonds along the backbone of the polymer to provide the structure along which electrons and holes can move, but double bonds are always susceptible to oxidation by the oxygen of the atmosphere so they have to be protected. 

---