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| Names | |
|---|---|
| IUPAC name poly(dimethylsiloxane) | |
Other names
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| Identifiers | |
3D model (JSmol) |
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| ChemSpider |
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| ECHA InfoCard | 100.126.442 |
| E number | E900(glazing agents, ...) |
| UNII | |
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| Properties | |
| CH3[Si(CH3)2O]nSi(CH3)3 | |
| Density | 0.965 g/cm3 |
| Melting point | N/A,vitrifies |
| Boiling point | N/A,vitrifies |
| Pharmacology | |
| P03AX05 (WHO) | |
| Hazards | |
| NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Polydimethylsiloxane (PDMS), also known asdimethylpolysiloxane ordimethicone, is asiliconepolymer with a wide variety of uses, fromcosmetics to industriallubrication andpassive daytime radiative cooling.[1][2][3]
PDMS is particularly known for its unusualrheological (or flow) properties. It isoptically clear and, in general,inert,non-toxic, andnon-flammable. It is one of several types ofsilicone oil (polymerizedsiloxane). The applications of PDMS range fromcontact lenses andmedical devices toelastomers; it is also present inshampoos (as it makes hairshiny andslippery), food (antifoaming agent),caulk,lubricants andheat-resistanttiles.
Thechemical formula of PDMS isCH3[Si(CH3)2O]nSi(CH3)3, wheren is the number of repeatingmonomer[Si(CH3)2O] units.[4] Industrial synthesis can begin fromdimethyldichlorosilane and water by the following net reaction:
The polymerization reaction evolveshydrochloric acid. For medical and domestic applications, a process was developed in which thechlorine atoms in thesilane precursor were replaced withacetate groups. In this case, the polymerization producesacetic acid, which is less chemically aggressive than HCl. As a side-effect, the curing process is also much slower in this case. The acetate is used in consumer applications, such as siliconecaulk andadhesives.
Hydrolysis ofSi(CH3)2Cl2 generates a polymer that is terminated withsilanol groups (−Si(CH3)2OH). These reactive centers are typically "capped" by reaction withtrimethylsilyl chloride:
Silane precursors with more acid-forming groups and fewer methyl groups, such asmethyltrichlorosilane, can be used to introducebranches orcross-links in the polymer chain. Under ideal conditions, each molecule of such a compound becomes a branch point. This can be used to produce hardsilicone 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.
Well-defined PDMS with a low polydispersity index and high homogeneity is produced by controlled anionic ring-opening polymerization ofhexamethylcyclotrisiloxane. Using this methodology it is possible to synthesize linear block copolymers, heteroarm star-shaped block copolymers and many other macromolecular architectures.
The polymer is manufactured in multipleviscosities, from a thin pourable liquid (whenn is very low), to a thick rubbery semi-solid (whenn is very high). PDMSmolecules have quite flexible polymer backbones (or chains) due to their siloxane linkages, which are analogous to theether linkages used to impart rubberiness topolyurethanes. Such flexible chains become loosely entangled whenmolecular weight is high, which results in PDMS' unusually high level ofviscoelasticity.
PDMS isviscoelastic, meaning that at long flow times (or high temperatures), it acts like aviscous liquid, similar to honey. However, at short flow times (or low temperatures), it acts like anelasticsolid, similar to rubber. Viscoelasticity is a form of nonlinear elasticity that is common amongst noncrystalline polymers.[5] The loading and unloading of a stress-strain curve for PDMS do not coincide; rather, the amount of stress will vary based on the degree of strain, and the general rule is that increasing strain will result in greater stiffness. When the load itself is removed, the strain is slowly recovered (rather than instantaneously). This time-dependent elastic deformation results from the long-chains of the polymer. But the process that is described above is only relevant when cross-linking is present; when it is not, the polymer PDMS cannot shift back to the original state even when the load is removed, resulting in a permanent deformation. However, permanent deformation is rarely seen in PDMS, since it is almost always cured with a cross-linking agent.
If some PDMS is left on a surface overnight (long flow time), it will flow to cover the surface and mold to any surface imperfections. However, if the same PDMS is poured into a spherical mold and allowed to cure (short flow time), it will bounce like a rubber ball.[4] The mechanical properties of PDMS enable this polymer to conform to a diverse variety of surfaces. Since these properties are affected by a variety of factors, this unique polymer is relatively easy to tune.[6] This enables PDMS to become a good substrate that can easily be integrated into a variety of microfluidic and microelectromechanical systems.[7][8] Specifically, the determination of mechanical properties can be decided before PDMS is cured; the uncured version allows the user to capitalize on myriad opportunities for achieving a desirable elastomer. Generally, the cross-linked cured version of PDMS resembles rubber in a solidified form. It is widely known to be easily stretched, bent, compressed in all directions.[9] Depending on the application and field, the user is able to tune the properties based on what is demanded.
Overall PDMS has a low elastic modulus which enables it to be easily deformed and results in the behavior of a rubber.[10][11][12] Viscoelastic properties of PDMS can be more precisely measured usingdynamic mechanical analysis. This method requires determination of 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.
Theshear modulus of PDMS varies with preparation conditions, and consequently dramatically varies in the range of 100 kPa to 3 MPa. Theloss tangent is very low(tan δ ≪ 0.001).[12]
PDMS ishydrophobic.[8]Plasmaoxidation can be used to alter the surface chemistry, addingsilanol (SiOH) groups to the surface. Atmospheric air plasma and argon plasma will work for this application. This treatment renders the PDMS surfacehydrophilic, allowing water to wet it. The oxidized surface can be further functionalized by reaction with trichlorosilanes. After a certain amount of time, recovery of the surface's hydrophobicity is inevitable, regardless of whether the surrounding medium is vacuum, air, or water; the oxidized surface is stable in air for about 30 minutes.[13] Alternatively, for applications where long-term hydrophilicity is a requirement, techniques such as hydrophilic polymer grafting, surface nanostructuring, and dynamic surface modification with embedded surfactants can be of use.[14]
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 mostorganic solvents willdiffuse into the material and cause it to swell.[8] Despite this, some organic solvents lead to sufficiently small swelling that they can be used with PDMS, for instance within the channels of PDMSmicrofluidic devices. The swelling ratio is roughly inversely related to thesolubility parameter of the solvent.Diisopropylamine swells PDMS to the greatest extent; solvents such aschloroform,ether, andTHF swell the material to a large extent. Solvents such asacetone,1-propanol, andpyridine swell the material to a small extent. Alcohols and polar solvents such asmethanol,glycerol and water do not swell the material appreciably.[15]
PDMS derivatives are commonsurfactants and are a component ofdefoamers.[16] PDMS, in a modified form, is used as anherbicidepenetrant[17] and is a critical ingredient in water-repelling coatings, such asRain-X.[18]
Dimethicone is used in the active silicone fluid in automotive viscous limited slip differentials and couplings.
PDMS is a common surface material used inpassive daytime radiative cooling as a broadband emitter that is high insolar reflectivity andheat emissivity. Many tested surfaces use PDMS because of its potentialscalability as a low-cost polymer.[19][20][21] As a daytime radiative cooling surface, PDMS has also been tested to improvesolar cell efficiency.[22]
PDMS is commonly used as a stamp resin in the procedure ofsoft lithography, making it one of the most common materials used for flow delivery inmicrofluidics chips.[23] 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 that can be used in the areas of optic telecommunications or biomedical research. The stamp is produced from the normal techniques ofphotolithography orelectron-beam lithography. The resolution depends on the mask used and can reach 6 nm.[24]
The popularity of PDMS in microfluidics area is due to its excellent mechanical properties. Moreover, compared to other materials, it possesses superior optical properties, allowing for minimal background and autofluorescence during fluorescent imaging.[25]
Inbiomedical (or biological) microelectromechanical systems (bio-MEMS), soft lithography is used extensively for microfluidics in both organic and inorganic contexts. Silicon wafers are used to design channels, and PDMS is then poured over these wafers and left to harden. When removed, even the smallest of details is left imprinted in the PDMS. With this particular PDMS block, hydrophilic surface modification is conducted usingplasma etching techniques. Plasma treatment disrupts surface silicon-oxygen bonds, and a plasma-treated glass slide is usually placed on the activated side of the PDMS (the plasma-treated, now hydrophilic side with imprints). Once activation wears off and bonds begin to reform, silicon-oxygen bonds are formed between the surface atoms of the glass and the surface atoms of the PDMS, and the slide becomes permanently sealed to the PDMS, thus creating a waterproof channel. With these devices, researchers can utilize various surface chemistry techniques for different functions creating unique lab-on-a-chip devices for rapid parallel testing.[7]PDMS can becross-linked into networks and is a commonly used system for studying the elasticity of polymer networks.[citation needed] PDMS can be directly patterned by surface-charge lithography.[26]
PDMS is being used in the making of syntheticgecko adhesion dry adhesive materials, to date only in laboratory test quantities.[27]
Someflexible electronics researchers use PDMS because of its low cost, easy fabrication, flexibility, and optical transparency.[28] Yet, for fluorescence imaging at different wavelengths, PDMS shows least autofluorescence and is comparable to BoroFloat glass.[29]
In stereo lithography (SLA) 3D printing, light is projected onto photocuring resin to selectively cure it. Some types of SLA printer are cured from the bottom of the tank of resin and therefore require the growing model to be peeled away from the base in order for each printed layer to be supplied with a fresh film of uncured resin. A PDMS layer at the bottom of the tank assists this process by absorbing oxygen : the presence of oxygen adjacent to the resin prevents it adhering to the PDMS, and the optically clear PDMS permits the projected image to pass through to the resin undistorted.
Activated dimethicone, a mixture of polydimethylsiloxanes andsilicon dioxide (sometimes calledsimethicone), is often used inover-the-counter drugs as anantifoaming agent andcarminative.[30][31] PDMS also works as a moisturizer that is lighter and more breathable than typical oils.
Siliconebreast implants are made out of a PDMS elastomer shell, to which fumedamorphous silica is added, encasing PDMS gel orsaline solution.[32]
PDMS is used variously in the cosmetic and consumer product industry as well. For example, dimethicone is used widely in skin-moisturizing lotions where it is listed as an active ingredient whose purpose is "skin protection." Some cosmetic formulations use dimethicone and related siloxane polymers in concentrations of use up to 15%. TheCosmetic Ingredient Review's (CIR) Expert Panel, has concluded that dimethicone and related polymers are "safe as used in cosmetic formulations."[33]
PDMS compounds such as amodimethicone, are effective conditioners when formulated to consist of small particles and be soluble in water or alcohol/act as surfactants[34][35] (especially for damaged hair[36]), and are even more conditioning to the hair than common dimethicone and/or dimethicone copolyols.[37]
A proposed use of PDMS is contact lens cleaning. Its physical properties of low elastic modulus and hydrophobicity have been used to clean micro and nano pollutants from contact lens surfaces more effectively than multipurpose solution and finger rubbing; the researchers involved call the technique PoPPR (polymer on polymer pollution removal) and note that it is highly effective at removing nanoplastic that has adhered to lenses.[38] The use of PDMS in the manufacture of contact lenses was patented (later abandoned).[39]
PDMS is effective for treatinglice in humans. This is thought to be due not to suffocation (or poisoning), but to its blocking water excretion, which causes insects to die from physiological stress either through prolonged immobilisation or disruption of internal organs such as the gut.[40]
Dimethicone is the active ingredient in an anti-flea preparation sprayed on a cat, found to be equally effective to a widely used more toxicpyriproxifen/permethrin spray. The parasite becomes trapped and immobilised in the substance, inhibiting adult flea emergence for over three weeks.[41]
PDMS is added to many cooking oils (as an anti-foaming agent) to prevent oil splatter during the cooking process. As a result of this, PDMS can be found in trace quantities in many fast food items such asMcDonald'sChicken McNuggets, french fries, hash browns, milkshakes and smoothies[42] and Wendy's french fries.[43]
Under European food additive regulations, it is listed asE900.
PDMS is widely used as acondom lubricant.[44][45]
Many people are indirectly familiar with PDMS because it is an important component inSilly Putty, to which PDMS imparts its characteristic viscoelastic properties.[46] Another toy PDMS is used in isKinetic Sand. The rubbery, vinegary-smelling silicone caulks, adhesives, and aquarium sealants are also well-known. PDMS is also used as a component insilicone grease and other silicone basedlubricants, as well as indefoaming agents,mold release agents, damping fluids,heat transfer fluids, polishes,cosmetics, hair conditioners, shininglatex, and other applications.
It can be used as asorbent for the analysis of headspace (dissolved gas analysis) of food.[47]
According toUllmann's Encyclopedia of Industrial Chemistry, no "marked harmful effects on organisms in the environment" have been noted for siloxanes. PDMS is nonbiodegradable, but is absorbed in waste water treatment facilities. Its degradation is catalyzed by variousclays.[48]
Amodimethicone is recognized for its extremely robust conditioning and for its ability to form clear products when used in high-surfactant shampoos. Amodimethicone is a useful ingredient in conditioners, gels, mousses, and permanents, but its use in shampoos has proved troublesome due to interactions between the cationic and the anionic surfactants, which can result in compatibility problems. However, the amodimethicone emulsion can be made compatible in high-surfactant-level shampoos
Amodimethicone is typically an emulsion-polymerized polymer; however, utilizing linear processing technology amodimethicone fluids may be prepared as neat fluids, and then emulsified by a mechanical process as desired. The most widely utilized amodimethicone emulsions contain as the surfactant pair either (1) tallowtrimonium chloride (and) nonoxy- nol-10, or (2) cetrimonium chloride (and) trideceth-10 or -12. These "uncapped" amino- functional silicone compounds may be characterized by a linear or branched structure. In either case, amodimethicone polymers will undergo a condensation cure reaction during drying to form a somewhat durable elastomeric film on the hair, providing wet- and dry- combing benefits, lowering triboelectric charging effects, and increasing softness of the dry hair. They are excellent conditioning agents, often found in conditioners, mousses, setting lotions, and less frequently in 2-in-1 shampoos
Amodimethicone is the most widely used amino-modified silicone. It has an aminopropyl group attached to the methyl group of Dimethicone. Amodimethicone of various degrees of amino modification are available as well as those that have POP, POE, or an alkyl group attached. Amino-modified silicones are cationic and affinitive to hair keratin. They are particularly highly affinitive to damaged hair, which is anionic due to the presence ofcysteic acid
...and amodimethicone, which is an amino-substituted silicone and silicone quats, which contain permanently quaternized ammonium groups. In general, amodimethicones and silicone quats condition better than dimethicones, which condition better than dimethicone copolyols
