Physical and chemical properties of silicone (polydimethylsiloxane)

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 effectiveness.

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 interface.
• 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 groups.

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.

Surface activity

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 properties.

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.

Rheological advantages

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) environments.

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.

Adaptability

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.