Dimethyl silicone oil: Why does its viscosity vary so much? ​

In industrial production and daily life, dimethyl silicone oil (PDMS) is a common material, ranging from a smoothing additive in cosmetics to a mechanical lubricant. The viscosity of dimethyl silicone oil varies greatly, with some as low as 5 cSt and others as high as over 1,000,000 cSt. Where does this huge viscosity difference come from? This article will reveal the mystery of the viscosity of dimethyl silicone oil from the perspective of its chemical structure.

I. Degree of polymerization is the core factor determining PDMS viscosity

The molecular structure of PDMS is linear, with repeating units of  -Si(CH₃)₂-O- , its general formula can be expressed as:

(CH₃)₃SiO-[Si(CH₃)₂-O-]ₙ-Si(CH₃)₃

Where n is the degree of polymerization (the number of repeating units).

Higher degree of polymerization (larger n): The longer the molecular chain, the stronger the van der Waals forces (mainly dispersion forces) between molecules, and the higher the degree of entanglement between chains, leading to poorer fluidity and higher viscosity. High-polymerization silicone oil is suitable for damping oil, sealant, and other applications with high viscosity requirements.

Lower degree of polymerization (smaller n): The molecular chain is shorter, the intermolecular forces are weaker, there is less entanglement, the fluidity is better, and the viscosity is lower. This type of low-viscosity silicone oil is often used in release agents and light smoothing agents in cosmetics.

For example:

Low-viscosity PDMS (such as 100 mm²/s) usually has a lower degree of polymerization (n is approximately tens);

High-viscosity PDMS (such as 100,000 mm²/s) can have a degree of polymerization as high as hundreds or even thousands.

The relationship between viscosity and degree of polymerization is non-linear, specifically showing an exponential growth trend. This is because as the molecular chain length increases, the probability and intensity of inter-chain entanglement increase geometrically, causing the viscosity to increase rapidly after the degree of polymerization reaches a critical value.

II. Minor influence of end group type on viscosity

The end group structure at both ends of the molecular chain has a certain regulatory effect on the viscosity of dimethyl silicone oil. Common end group types of dimethyl silicone oil include hydroxyl (-OH) and methyl (-CH₃).

When the end group is hydroxyl, the hydroxyl groups of adjacent molecular chains can interact with each other through hydrogen bonds, enhancing intermolecular forces and slightly increasing the viscosity of the system.

When the end group is methyl, due to the chemical inertness of methyl, hydrogen bonds cannot be formed, and only van der Waals forces exist between molecules. Therefore, at the same degree of polymerization, the viscosity of methyl-capped silicone oil is lower than that of hydroxyl-capped silicone oil.

The effect of end groups on viscosity is significantly lower than that of the degree of polymerization. Only when the degree of polymerization is similar, the viscosity difference caused by the end group difference is more obvious. For example, in molecular chains with the same degree of polymerization, the viscosity of hydroxyl-capped silicone oil is usually 5%-10% higher than that of methyl-capped silicone oil, but this difference can be ignored for long-chain silicone oil.

III. Other minor factors

In addition to the degree of polymerization and end groups, the following factors have a fine-tuning effect on the viscosity of dimethyl silicone oil:

Temperature When the temperature increases, the kinetic energy of molecular thermal motion increases, the entanglement between molecular chains weakens, and the intermolecular forces decrease, leading to a decrease in viscosity; the opposite is true when the temperature decreases. Therefore, silicone oil products usually indicate "kinematic viscosity (40℃)" as a standard parameter to ensure the comparability of viscosity data from different samples.

Molecular weight distribution Under the condition of the same average degree of polymerization, the width of the molecular weight distribution has a significant impact on viscosity. When both short-chain and long-chain molecules are present in the system, short-chain molecules can fill the gaps between long-chain molecules, reducing inter-chain frictional resistance and leading to a decrease in overall viscosity; while systems with a narrower molecular weight distribution have higher viscosity stability.

Although these factors do not change the order of magnitude of viscosity, they need to be precisely controlled in industrial production to ensure the consistency of product performance.

The diversity of dimethyl silicone oil viscosity stems from the controllability of its chemical structure: by changing the degree of polymerization, the order of magnitude of viscosity can be controlled, and combined with end group modification and process optimization, fine adjustment of viscosity can be achieved, resulting in a series of products covering the range from low to ultra-high viscosity to meet the needs of different application scenarios.