Silicone rheology

Molecular weight and viscosity

A silicone’s molecular weight is a function of its degree of polymerization (DP) – the number of repeating Si-O-Si units in the polymer.

• The greater the degree of polymerization, the higher the molecular weight
• The higher the molecular weight, the longer the polymer
• The longer the polymer, the higher the viscosity
• The higher the viscosity, the more slowly the polymer will flow

Silicone fluids range in viscosity from 0.65 cs (thinner than water) to more than 20,000,000 cs (thicker than chewing gum).

Effect of temperature

The viscosity of organic materials, such as honey, is temperature-dependent. Think about what happens to honey if you put it in the freezer or in the microwave.

Question: What do you think would happen if you subjected a polydimethylsiloxane fluid to the same conditions? Answer: Not much! Silicones are extremely temperature-stable. They are also shear-stable, which is why polydimethylsiloxane fluids are used to calibrate viscometers!

Effect of shear forces

At low molecular weights, linear polydimethylsiloxane (PDMS) polymers behave essentially as Newtonian liquids (their viscosity is not influenced by shear rate). This is true to shear rates in excess of 1,000 s-1.

At molecular weights above 10,000, PDMS polymers become entangled and exhibit a viscoelastic response and shear-thinning behavior at modest shear rates. Learn more about this unique entanglement phenomenon.

Silicone polymers can also be engineered to be:

• Thixotropic (shear thinning) – Thixotropic silicones may appear to be solid at rest. However, they are easy to mix and spread. Once the mixing or spreading force is removed, they return to their original state. This is a benefit in hand creams, cosmetics, auto and furniture polishes, paints, and two-part moldmaking rubbers.
• Dilatant (shear resistant) – Dilatant silicones appear flexible, but harden when subjected to shear. The ability of dilatant silicones to become rigid upon impact has enabled the development of "intelligent" impact-resistant textiles. Read Silicones: Revolutionizing protective sports apparel. 

Silly Putty® is an example of a dilatant silicone. Silly Putty® is a registered trademark of Binney & Smith, Inc.

Viscosity and performance

Viscosity has relatively little effect on the chemical properties of a silicone fluid, but it does affect flow behavior and solubility.

• High-viscosity silicones flow more slowly than low-viscosity silicones, but they do flow. And both are capable of forming very thin films.
• High-viscosity silicones are less soluble in some solvents than low-viscosity silicones.