Everything about Polydimethylsiloxane totally explained
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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. 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 (
polymerized
siloxane).
Chemistry
The
chemical formula for PDMS is
(H
3C)
3SiO[Si(CH
3)
2O]
nSi(CH
3)
3, where n is the number of repeating
monomer [SiO(CH
3)
2] units. Industrial synthesis can begin from dimethyl
chlorosilane and
water by the following net reaction:
» n [Si(CH
3)
2Cl
2] + n [H
2O] → [Si(CH
3)
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.
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'll 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'll 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's 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 (
).
Chemical compatibility
After
polymerization and
cross-linking, solid PDMS samples will present an external
hydrophobic surface. 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 frequently required for, for example 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's vacuum, air or water.
Solid PDMS samples (whether surface oxidized or not) won't 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 solvents will
diffuse into the material and cause it to swell,
Applications
Many people are indirectly familiar with PDMS because it's 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
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 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's 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.
PDMS can be
cross-linked 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"
(External Link
)
PDMS is also used in
analytical chemistry as a component of some types of
SPME fibers.
PDMS has also been rumored to be used in Leprino Food's "Pizza cheese" product, which is used by Pizza Hut on its pizzas
(External Link). Leprino Foods has replied that it isn't currently using the PDMS in the manufacturing of "Pizza Cheese", although it was never stated if they'd done so at one point in time.
(External Link)Further Information
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