Polydimethylsiloxane

The chemical formula for PDMS is CH3[Si(CH3)2O]//n//Si(CH3)3, where //n// is the number of repeating monomer [SiO(CH3)2] units. Industrial synthesis can begin from dimethylchlorosilane and water by the following net reaction: //n// Si(CH3)2Cl2 + //n// H2O → [Si(CH3)2O]//n// + 2//n// HCl During polymerization, this reaction evolves potentially hazardous hydrogen chloride gas. For medical uses, a process was developed in which the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side-effect, the curing process is also much slower in this case. This is the chemistry used in consumer applications, such as silicone caulk and adhesives. Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. Under ideal conditions, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. In a similar manner, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain. The polymer is manufactured in multiple viscosities, ranging from a thin pourable liquid (when //n// is very low), to a thick rubbery semi-solid (when //n// is very high). PDMS molecules have quite flexible polymer backbones (or chains) due to their siloxane linkages, which are analogous to the ether linkages used to impart rubberiness to polyurethanes. Such flexible chains become loosely entangled when molecular weight is high, which results in PDMS' unusually high level of viscoelasticity.
 * Polydimethylsiloxane** (PDMS) belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones.[1home] PDMS is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological (or flow) properties. PDMS is optically clear, and, in general, is considered to be inert, non-toxic and non-flammable. It is occasionally called **dimethicone** and is one of several types of silicone oil (polymerized siloxane). Its applications range from contact lenses and medical devices to elastomers; it is present, also, in shampoos (as dimethicone makes hair shiny and slippery), food (antifoaming agent), caulking, lubricating oils, and heat-resistant tiles.