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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 cSt (thinner than water) to more than 20,000,000 cSt (thicker than chewing gum).

Effect of shear forces

At low molecular weights, linear poly(dimethylsiloxane), or 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 Da, PDMS polymers become entangled and exhibit a viscoelastic response and a decrease in viscosity (or shear-thinning behavior) at modest shear rates. Learn more about how molecular weight and shear rate impact the viscosity of PDMS polymers.

Silicone polymers can also be engineered to be:

• Thixotropic – Thixotropic silicone formulations 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 solid-like state. This is a benefit in hand creams, cosmetics, auto and furniture polishes, paints, sealants, and two-part moldmaking rubbers.
• Dilatant (shear thickening) – Dilatant silicone formulations appear flexible, but harden when subjected to shear deformation.

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 exhibit more resistance to flow 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.

Effect of temperature

Think about what happens to the viscosity of honey if you put it in the freezer or in the oven.

Question: What do you think would happen if you subjected a polydimethylsiloxane fluid to the same conditions? Answer: Not much! The viscosity of silicones changes significantly less with temperature than does the viscosity of organic materials.