Polydimethylsiloxane
1151127
222623453
2008-06-30T09:10:23Z
Peterlewis
813120
/* Applications */
{{Chembox new
| Name = Polydimethylsiloxane
| ImageFile = pdms.png
| ImageName = PDMS
| ImageFile1 = Silicone-3D-vdW.png
| ImageName1 = PDMS
| IUPACName = poly(dimethylsiloxane)
| OtherNames = PDMS<br />dimethicone<br />E900
| Section1 = {{Chembox Identifiers
| CASNo = 63148-62-9
}}
| Section2 = {{Chembox Properties
| Formula = (C<sub>2</sub>H<sub>6</sub>OSi)<sub>''n''</sub>
| Density = 965 kg m<sup>−3</sup>
| BoilingPt = below about 200 °C
| MeltingPt = [[n/a|N/A]] ([[vitrification|vitrifies]])
}}
}}
'''Polydimethylsiloxane''' (PDMS) is the most widely used [[silicon]]-based [[organic compound|organic]] [[polymer]], and is particularly known for its unusual [[rheology|rheological]] (or flow) properties. Its applications range from contact
lenses and medical devices to elastomers, in shampoos (as dimethicone makes hair shiny and slippery), caulking, lubricating oils and heat resistant tiles.
PDMS is optically clear, and is generally considered to be inert, non-toxic and non-flammable. It has been assigned [[CAS number]] 63148-62-9, and is occasionally called '''dimethicone'''. It is one of several types of [[silicone oil]] ([[polymer]]ized [[siloxane]]).
==Chemistry==
The [[chemical formula]] for PDMS is
(H<sub>3</sub>C)<sub>3</sub>SiO[Si(CH<sub>3</sub>)<sub>2</sub>O]<sub>n</sub>Si(CH<sub>3</sub>)<sub>3</sub>, where n is the number of repeating [[monomer]] [SiO(CH<sub>3</sub>)<sub>2</sub>] units. Industrial synthesis can begin from dimethyl[[chlorosilane]] and [[water]] by the following net reaction:
:n [Si(CH<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] + n [H<sub>2</sub>O] → [Si(CH<sub>3</sub>)<sub>2</sub>O]<sub>n</sub> + 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 [[adhesive]]s.
Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce [[Branching (chemistry)|branches]] or [[cross-link]]s in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard [[silicone resin]]s. 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 [[viscosity|viscosities]], ranging from a thin pourable liquid (when n is very low), to a thick rubbery semi-solid (when n is very high). PDMS [[molecule]]s have quite flexible polymer backbones (or chains) due to their siloxane linkages, which are analogous to the [[ether]] linkages used to impart rubberiness to [[polyurethane]]s. Such flexible chains become loosely entangled when [[molecular weight]] is high, which results in PDMS having an unusually high level of [[viscoelasticity]].
==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|thumb|Hardened PDMS cylinder on the glass of a [[stereomicroscope]] light.]]
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>).<ref name="pdms_mechanical">Lotters, J. C.; Olthuis, W.; Veltink, P. H.; Bergveld, P. "[http://dx.doi.org/10.1088/0960-1317/7/3/017 The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications]" J Micromech Microeng 1997, 7, (3), 145-147.</ref>
==Chemical compatibility==
After [[polymerization]] and [[cross-linking]], solid PDMS samples will present an external [[hydrophobic]] surface.<ref name="pdms_review">McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M. "[http://www3.interscience.wiley.com/cgi-bin/abstract/68503440/ABSTRACT Fabrication of microfluidic systems in poly(dimethylsiloxane)]" Electrophoresis 2000, 21, (1), 27-40.</ref> This surface chemistry makes it difficult for polar solvents (such as water) to wet the PDMS surface, and may lead to adsorption of hydrophobic contaminants. [[Plasma (physics)|Plasma]] [[oxidation]] can be used to alter the surface chemistry, adding [[silanol]] (SiOH) groups to the surface. This treatment renders the PDMS surface [[hydrophilic]], allowing water to wet (this is frequently required for, e.g. water-based [[microfluidics]]). The oxidized surface resists adsorption of hydrophobic and negatively charged species. The oxidized surface can be further functionalized by reaction with trichlorosilanes. Oxidized surfaces are stable for ~30 minutes in air, after a certain time hydrophobic recovery of the surface is inevitable independently of the surrounding medium whether it is vacuum, air or water.<ref name="hydrophobic_recovery">H. Hillborg, J.F. Ankner, U.W. Gedde, G.D. Smith, H.K. Yasuda and K. Wikstrom, "[http://dx.doi.org/10.1016/S0032-3861(00)00039-2 Crosslinked polydimethylsiloxane exposed to oxygen plasma studied by neutron reflectometry and other surface specific techniques]" Polymer 2000, 41, (18), 6851–6863.</ref>
Solid PDMS samples (whether surface oxidized or not) will not allow aqueous solvents to infiltrate and swell the material. Thus PDMS structures can be used in combination with water and alcohol solvents without material deformation. However most [[Organic compound| organic]] solvents will [[Diffusion|diffuse]] into the material and cause it to swell,<ref name="pdms_review" /> making them incompatible with PDMS devices. Despite this, some organic solvents lead to sufficiently small swelling that they can be used with PDMS, for instance within the channels of PDMS microfluidic devices. The swelling ratio is roughly inversely related to the [[solubility parameter]] of the solvent. [[Diisopropylamine]] swells PDMS to the greatest extent, solvents such as [[chloroform]], [[ether]], and [[THF]] swell the material to a large extent. Solvents such as [[acetone]], [[1-propanol]], and [[pyridine]] swell the material to a small extent. Alcohols and polar solvents such as [[methanol]], [[glycerol]] and water do not swell the material appreciably.<ref name="solvent_review">Lee, J. N.; Park, C.; Whitesides, G. M. "[http://dx.doi.org/10.1021/ac0346712 Solvent Compatibility of Poly(dimethylsiloxane)-Based Microfluidic Devices]" Anal. Chem. 2003, 75, (23), 6544-6554.</ref>
==Applications==
Many people are indirectly familiar with PDMS because it is an important (4%) 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 [[lubricant]]s, as well as in defoaming agents, [[Release agent|mold release agents]], damping fluids, [[heat transfer]] fluids, polishes, [[cosmetics]], hair conditioners and other applications. PDMS has also been used as a filler fluid in [[breast implant]]s, 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. The process of soft lithography consists of creating an elastic stamp which enables the transfer of patterns of only a few nanometers in size onto glass, silicon or polymer surfaces. With this type of technique it is possible to produce devices which can be used in the areas of optic telecommunications or biomedical research. However, this process still can not be used for the industrial production of electronic components. In fact, the patterns are obtained by the process of stamping thanks to a shape (or stamp). This stamp is produced from the normal techniques of photolithography or electron-beam technology. The resolution depends on the mask used and can reach 6 nm<ref>{{cite book |last = Waldner |first = Jean-Baptiste |authorlink = Jean-Baptiste Waldner |title = Nanocomputers and Swarm Intelligence |publisher = [[ISTE]] [[John Wiley & Sons]] |place = London |date = 2008 | pages = P92-93 |isbn = 1847040020}}</ref>.
PDMS can be [[cross-link]]ed into networks and is a commonly used system for studying the elasticity of polymer networks.
PDMS can be used in the treatment of [[head lice]].
Dimethicone is also used widely in skin moisturizing lotions, listed as an active ingredient whose purpose is "skin protectant." Some cosmetic formulations use dimethicone and related siloxane polymers in concentrations of use up to 15%. The Cosmetic Ingredient Review's (CIR) Expert Panel, has concluded that dimethicone and related polymers are "safe as used in cosmetic formulations" [http://www.pgbeautyscience.com/en_UK/glossary/dimethicone_en.html]
PDMS is used as an environmentally-friendly [[dry cleaning]] solution under the licensed name [[GreenEarth Cleaning]].
PDMS is also used in [[analytical chemistry]] as a component of some types of [[Solid phase microextraction|SPME]] fibers.
==See also==
*[[Dimethylsiloxane|Siloxanes]] and other [[organosilicon]] compounds
*[[Polymethylhydrosiloxane]] (PMHS)
==References==
<references />
{{E number infobox 900-909}}
[[Category:Silicones]]
[[Category:Food additives]]
[[Category:Cosmetic chemicals]]
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