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Polydimethylsiloxane (PDMS) is the most widely used silicon-based organic polymer, and is particularly known for its unusual rheological (or flow) properties. It is optically clear, and is generally considered to be inert, non-toxic and non-flammable. PDMS has been assigned CAS number 63148-62-9, and is occasionally called dimethicone. It is one of several types of silicone oil.
The chemical formula for PDMS is
(CH3)3SiO[SiO(CH3)2]nSi(CH3)3, where n is the number of repeating monomer [SiO(CH3)2] units. 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), which become loosely entangled when molecular weight is high. This results in PDMS having an unusually high level of viscoelasticity.
Viscoelasticity means that at long flow times (or high temperatures), PDMS acts like a viscous liquid, similar to honey. However at short flow times (or low temperatures) it acts like an elastic solid, similar to rubber. In other words, if you leave some PDMS on a surface overnight (long flow time), it will flow to cover the surface and mold to any surface imperfections. However if you roll the same PDMS into a sphere and throw it onto the same surface (short flow time), it will bounce like a rubber ball.
Although the viscoelastic properties of PDMS can be intuitively observed using the simple experiment described above, they can be more accurately measured using dynamic mechanical analysis. This involves using a specialized instrument to determine the material's flow characteristics over a wide range of temperatures, flow rates, and deformations. Because of PDMS's chemical stability, it is often used a calibration fluid for this type of experiment.
Many people are indirectly familiar with PDMS because it is the primary component in Silly Putty, to which PDMS imparts its characteristic viscoelastic properties. PDMS is also used as a component in silicone grease and other silicone based lubricants, as well as in defoaming agents, damping fluids, heat transfer fluids, cosmetics and other applications. PDMS has also been used as a filler fluid in breast implants, although this practice has decreased somewhat, due to safety concerns.
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