Physical and chemical properties of silicone
A unique semi-organic structure
The siloxane backbone efficiently uses one of the lowest surface energy
organic groups – methyl (CH3) – to its highest level of
Polar inorganic backbone
Long and strong Si-O bond; open Si-O-Si angle
Low barrier to rotation
Low rotation energy
Nonpolar organic substituents (methyl)
Shorter Si-C bond
No steric hindrance to the methyl group
Ease of reorientation
Weak intermolecular forces
Opportunity to substitute other functional groups
A cost-effective molecular architecture
Long bond length combined with a wide bond angle and a low barrier to
rotation give polydimethylsiloxane (PDMS) outstanding flexibility, internal
mobility, and large free volume. This:
Enables functional groups to align efficiently to the most compatible
Reduces competition among functional groups.
Lowers functionality requirements.
As a result, PDMS polymers are able to perform in applications where rigid
organic polymers would require higher concentrations of expensive functional
Strength and stability
The siloxane backbone's high bond energy of ~445 kJ/mol along with its
"inert" methyl (CH3) functional groups combine to make PDMS
a very chemically stable material that:
Resists temperature extremes, weathering, aging, oxidation, moisture, many
chemicals, and ultraviolet radiation.
Is generally non-irritating.
Wetting, water repellency, release – With a low surface tension of
20.4 mN/m, PDMS easily wets most surfaces. Plus, its methyl groups align in the
most favorable position to create water-repellent films and good release
Film forming – With a critical surface tension of wetting of 24 mN/m,
which is higher than its own surface tension, PDMS can flow over itself. This
enables it to greatly outperform hydrocarbon in forming extremely thin
(monomolecular) self-leveling films. Learn about other
differences between silicone and carbon-based chemistry.
Explore interface and surface applications for silicones.
Easy spreading and flow – PDMS is very shear stable. Because it
experiences very little internal friction, PDMS spreads and flows more easily
than same-viscosity hydrocarbon fluids in a wide variety of mechanical (shear)
All-temperature performance – PDMS remains liquid at low
temperatures, even at high molecular weights. Its extremely low glass
transition temperature (pour point), high thermal stability, and less
temperature-dependent viscosity enable it to perform across a broader
temperature range than most hydrocarbon fluids. Typical use temperatures for
PDMS can range from below -40 to above 150°C (below -40 to above 302°F).
Learn more about silicone rheology.
Other benefits of silicone’s open molecular structure
Breathability – Silicones are highly permeable to oxygen,
nitrogen, and water vapor (but not to water molecules). For example, silicone
elastomers form breathable barriers that prevent water from entering while
allowing internal moisture to escape. This makes silicone-coated raincoats more
comfortable to wear. It also prevents moisture from building up inside of
buildings sealed with silicone.
Plus, compared to other polymers, PDMS polymers are very permeable to the
diffusion of various substances, gases or active drugs, which makes them
valuable in healthcare applications.
High flexibility – Silicones have a very springy nature. You can
compress them, stretch them, bend them, smash them, and spread them (over and
over again), and they will simply bounce back with their properties and volume
intact. This makes them ideal for creating flexible molds and coatings, and for
sealing expansion joints in buildings and bridges.
The presence of groups other than methyl along the polymer chain enables
silicone properties to be modified. Learn how organic modification changes the way silicones behave and bridges
the gap between silicone and organic chemistries.
Did you know ... PDMS has the lowest recorded surface
shear viscosity, the highest permeability coefficient for N2 and
O2, and the lowest known glass transition temperature of any