Rubbers dimethylsilicone

The elastomers consist of very high molecular weight (-0.5 X 10 ) linear gums cross-linked after fabrication. In order to achieve such polymers it is necessary that very pure difunctional monomers be employed since the presence of monofunctional material will limit the molecular weight while trifunctional material will lead to cross-linking. Where dimethylsilicone rubbers are being prepared, the cyclic tetramer, octamethylcyclotetrasiloxane, which may be obtained free from mono- and trifunctional impurities, is often used. This tetramer occurs to the extent of about 25% during the hydrolysis of dichlorosilanes into polymers. 

To obtain high molecular weight polymers the tetramer is equilibrated with a trace of alkaline catalyst for several hours at 150-200°C. The product is a viscous gum with no elastic properties. The molecular weight is controlled by careful addition of monofunctional material. 

In recent years there has been some interest in the ring-opening polymerisation of cyclic trimers using a weak base such as lithium silanolate which gives high molecular weight products of narrow molecular weight distribution ftee of cyclic materials other than the unreacted trimer. 

For reasons that will be explained in the next section the simple poly-dimethylsiloxane rubbers are seldom used today. 

Dimethylsilicone rubbers show a high compression set which can be reduced to some extent by additives such as mercurous oxide and cadmium oxide. These materials are undesirable, however, because of their toxicity. Substantially reduced compression set values may be obtained by using a polymer containing 

---

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

Indications are that cross-linking of dimethylsilicone rubbers occurs by the sequence of reactions shown in Figure 29.9. 

In 1979, Baadenhuijsen and Seuren-Jacobs [2] were the first to report on a FI gas diffusion separation system with a semi-permeable dimethylsilicone rubber membrane, used for the determination of carbon dioxide in plasma. In the same year. Zagatto et al.[3] introduced an isothermal distillation FI system in which ammonia diffused from a flowing donor liquid film across an air-gap and absorbed by a flowing acceptor film on the opposite side of the gap. However, later developments on gas diffusion separations mainly followed the approach of Baadenhuijsen and Seuren-Jacobs, obviously due to its simpler design and higher versatility. The first theoretical study on an FI gas-diffusion separation system was attempted by van der Linden [4], who used a tank-in-series model for the mathematical evaluation of the separation process. 

The first application of FI gas-diffusion separation reported by Baadenhuijsen [II] was on the determination of carbon dioxide in plasma by spectrophotometry, resulting in a fast automated procedure. A dimethylsilicone rubber membrane was used for separating the carbon dioxide from the acidified sample. Fan et al.[12] described a similar procedure using a PTFE microporous membrane. The procedure is described in detail in Sec. 9.5.2. 

---