Polydimethyl siloxane

Polydimethylsiloxane (PDMS) belongs to a group of polymeric organosilicon substances that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, and is particularly known for its strange rheological (or flow) properties. PDMS is optically clear, and, in general, is considered to be inert, non-toxic and non-flammable. It is often 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.

Chemistry

The chemical formula for PDMS is CH3[Si(CH3)2O]nSi(CH3)3, where n is the number of repeating monomer [SiO(CH3)2] units. Commercial synthesis can begin from dimethyldichlorosilane and water by the following net reaction:

n Si(CH3)2Cl2 + n H2O ? [Si(CH3)2O]n + 2n HCl

During polymerization, this reaction builds up potentially dangerous 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 treating procedure 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 perfect circumstances, each and every 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 control 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 various viscosities, ranging from a thin pourable liquid (when n is very low), to a dense rubbery semi-solid (when n is very high). PDMS molecules have rather 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 turn out to be loosely entangled when molecular weight is high, which results in PDMS’ uncommonly high level of viscoelasticity.

Mechanical qualities

PDMS is viscoelastic, meaning that at long flow times (or high temperatures), it behaves like a viscous liquid, similar to honey. However, at short flow times (or low temperatures), it behaves like an elastic solid, similar to rubber. In other words, if some PDMS is left on a surface overnight (long flow time), it will flow to cover the surface and mold to any surface flaws. Even so, if the same PDMS is rolled into a sphere and thrown onto the same surface (short flow time), it will bounce like a rubber ball.

Although the viscoelastic attributes of PDMS can be naturally observed using the simple experiment explained above, they can be more accurately measured using dynamic mechanical analysis. This involves using a specialized instrument to determine the material’s flow properties 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 (tan d « 0.001).

Chemical compatibility

After polymerization and cross-linking, solid PDMS samples will present an external hydrophobic surface. This surface will appear metallic and shiny, although the substrate is clear. 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 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 regularly required for 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 specific time hydrophobic recovery of the surface is unavoidable independently of the surrounding medium whether it is vacuum, air, or water.

Solid PDMS samples (whether surface oxidized or not) will not permit aqueous solvents to infiltrate and swell the material. Hence PDMS structures can be used in combination with water and alcohol solvents without material deformation.
Nevertheless the majority of organic solvents will diffuse into the material and make it swell,[3] making them incompatible with PDMS devices. Despite this, several 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 significant level. Solvents such as acetone, 1-propanol, and pyridine swell the material to a small level. Alcohols and polar solvents such as methanol, glycerol and water do not swell the material appreciably.

Applications

Many people are in a roundabout way familiar with PDMS because it is an important 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 element in silicone grease and other silicone based lubricants, as well as in defoaming agents, 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 implants, although this exercise has diminished considerably, due to safety issues.

Industrial uses

Dimethicone is also the active silicone fluid in automotive viscous limited slip differentials and couplings. This is usually a non-serviceable OEM element but can be replaced with mixed performance results due to variances in effectiveness caused by refill weights or non-standard pressurisations.

PDMS is commonly used as a stamp resin in the procedure of soft lithography, making it one of the most typical components used for flow delivery in microfluidics chips. The process of soft lithography consists of producing an elastic stamp, which allows 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 create devices that can be used in the areas of optic telecommunications or biomedical investigation. Nevertheless, this process still cannot be used for the industrial production of electronic components. In fact, the patterns are acquired by the process of stamping thanks to a shape (or stamp). This stamp is produced from the normal methods of photolithography or electron-beam technology. The resolution depends on the mask used and can reach 6 nm.

In Bio-MEMS, soft lithography is used thoroughly 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 taken off, even the smallest of details is left imprinted in the PDMS. With this particular PDMS block, hydrophilic surface modification is conducted using RF Plasma techniques. Once surface bonds are disrupted, usually a piece of glass slide is placed on the activated side of the PDMS (the side with imprints). Once the bonds relax to their normal state, the glass is completely sealed to the PDMS, hence creating a water-resistant channel. With these devices, investigators can utilize numerous surface chemistry techniques for various functions creating unique lab-on-a-chip devices for rapid parallel testing.

PDMS can be cross-linked into networks and is a commonly used system for studying the elasticity of polymer networks. PDMS can be directly designed by surface-charge lithography. PDMS is being used in the making of artificial Gecko adhesion dry adhesive materials, to date only in laboratory test quantities.

Medicine and cosmetics

Activated dimethicone, a mixture of polydimethylsiloxanes and silicon dioxide (sometimes called simethicone), is used in over-the-counter drug as an antifoaming agent and carminative.
PDMS is used variously in the cosmetic and consumer product business as well. For example, PDMS can be used in the treatment of head lice and dimethicone is used extensively in skin-moisturizing products where it is listed as an active component whose function is “skin protection.” 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.”

Other

This silicone can be found in several processed foods and fast food items such as McDonald’s Chicken McNuggets and Wendy’s French fries. PDMS is also used in analytical chemistry as a component of some types of SPME fibers. PDMS in a modified form is used as an herbicidal penetrant and is a critical component in water-repelling coatings, such as Rain-X