Polydimethylsiloxane
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| Polydimethylsiloxane | |
|---|---|
| Image:Pdms.png Image:Silicone-3D-vdW.png | |
| Systematic name | Polydimethylsiloxane |
| Other names | PDMS, dimethicone, E900 |
| Chemical formula | (CH3)3SiO[SiO(CH3)2]nSi(CH3)3 |
| Density | 965 kg/m3 |
| Melting point | xx.x °C |
| Boiling point | xx.x °C |
| Glass transition temperature | -125 °C |
| CAS number | [63148-62-9] |
| Disclaimer and references | |
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.
Contents |
[edit] Chemistry
The chemical formula for PDMS is (H3C)[SiO(CH3)2]nSi(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 + 2n HCl
During polymerization, this reaction evolves potentially hazardous hydrogen chloride gas. For medical uses, a process was developed where 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. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, 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 having an unusually high level of viscoelasticity.
[edit] Mechanical properties
PDMS is viscoelastic, meaning that at long flow times (or high temperatures), it 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.
Image:PDMS-cylindertje onder stereomicroscoop.jpg
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 as a calibration fluid for this type of experiment.
The shear modulus of PDMS varies with preparation conditions, but is typically in the range of 100 kPa to 3 MPa. The loss tangent is very low (<math>\tan\delta\ll0.001</math>).
[edit] Applications
Many people are indirectly familiar with PDMS because it is the primary component in Silly Putty, to which PDMS imparts its characteristic viscoelastic properties. The rubbery, vinegary-smelling silicone caulks, adhesives, and aquarium sealants are also well-known. 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, hair conditioner 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. It continues to be used in knuckle replacement implants, with good results.
Activated dimethicone, a mixture of polydimethylsiloxanes and silicon dioxide (sometimes called simethicone), is used in over-the-counter drug as an anti-foaming agent and carminative.
As a food additive, it has the E number E900 and is used as an anti-foaming agent and an anti-caking agent.
PDMS is commonly used as a stamp resin in the procedure of soft lithography, making it one of the most common materials used for flow delivery in microfluidics chips.
PDMS can be cross-linked into networks and is a commonly used system for studying the elasticity of polymer networks.
[edit] See also
- Polymethylhydrosiloxane (PMHS)
[edit] External links
| E numbers | ||
|---|---|---|
| Colours (E100-199) • Preservatives (E200-299) • Antioxidants & Acidity regulators (E300-399) • Thickeners, stabilisers & emulsifiers (E400-499) • pH regulators & anti-caking agents (E500-599) • Flavour enhancers (E600-699) • Miscellaneous (E900-999) • Additional chemicals (E1100-1599) | ||
| | ||
| Waxes (E900-909) • Synthetic glazes (E910-919) • Improving agents (E920-929) • Packaging gases (E930-949) • Sweeteners (E950-969) • Foaming agents (E990-999) | ||
| | ||
| Dimethyl polysiloxane (E900) • Beeswax (E901) • Candelilla wax (E902) • Carnauba wax (E903) • Shellac (E904) • Paraffins (E905) • Mineral oil (E905a) • Vaseline (E905b) • Microcrystalline wax (E905c) • Gum benzoic (E906) • Crystalline wax (E907) • Rice bran wax (E908) | ||
