Chemical Properties of Silicone Rubber
A polymer is a chain of small repeating units with an end group. With silicone the polymer chains i.e. siloxanes are formed with Si-O bonds. The polysiloxane backbone is a chain consisting of silicone and oxygen. Silicone is part inorganic, part organic. It shares the same Si-O backbone with inorganic materials such as quartz or glass. Silicones also contain organic groups which are attached to the silicon. These organic groups on the polysiloxane can be methyl, vinyl or phenyl groups.
The various silicone polymers are named after the organic side groups attached to the silicon atom. Each silicon atom has four chemical bonds, which is why silicone rubber is often abbreviated with a Q for quaternary groups. The polymers present in rubber can vary in structure e.g. with different chain lengths, with or without branching or with high or low vinyl content. This in turn, affects the ultimate properties of the rubber.
In polydimethylsiloxanes, the organic groups are methyl groups. They have little importance because they have no double bonds and do not react easily with peroxide crosslinkers. They can however be used as plasticizers.
This compound is similar to polydimeythl siloxane but some of the methyl groups have been replaced with vinyl groups. The double bond is a reactive group which is needed for crosslinking.
If a small number of the methyl groups in a VMG are replaced with phenyl groups, the polymer chains have less tendency to pack with adjacent chains at low temperature. This chemical structure of phenyl methyl silicone helps them retain flexibility at temperature as low as -80°C.