1.The degree of polymerization is the core factor determining the viscosity of PDMS

The molecular structure of PDMS is linear chain, with the repeating unit being -Si(CH₃)₂-O-, and its general formula can be expressed as:
(CH₃)₃SiO-[Si(CH₃)₂-O-]ₙ-Si(CH₃)₃
Here, n represents the degree of aggregation (the number of repeating units).

The higher the degree of polymerization (the larger the n) : the longer the molecular chain, the stronger the van der Waals forces (mainly dispersion forces) between molecules, and the higher the entanglement degree between chains, resulting in poorer fluidity and greater viscosity.

The lower the degree of polymerization (the smaller the n), the shorter the molecular chain, the weaker the intermolecular forces, the less entanglement, the better the fluidity, and the lower the viscosity.

For example:
The degree of polymerization of low-viscosity PDMS (such as 100 mm²/s) is usually relatively low (n is approximately several tens).
The degree of polymerization of high-viscosity PDMS (such as 100,000 mm²/s) can be as high as hundreds or even thousands.

2. Secondary Influence of End Group Types on Viscosity

The terminal groups (the groups at both ends of the molecular chain) of PDMS are usually -Si(CH₃)₃ (trimethylsiloxy), but some special models may use hydroxyl (-Si(CH₃) ₂-OH) or other groups for terminal capping.

When the end group is an inert group (such as trimethylsilyl), there is no additional interaction between the molecular chains, and the viscosity is only dominated by the degree of polymerization.

If the end groups are active groups (such as hydroxyl groups), weak hydrogen bonds may form between molecules, which will cause the viscosity at the same degree of polymerization to be slightly higher than that of inert end group PDMS, but the impact is much smaller than the difference in degree of polymerization.

3. Other secondary factors

Apart from the degree of polymerization and end groups, temperature also significantly affects the viscosity of PDMS (silicone oil has the characteristic of a small viscosity-temperature coefficient, but its viscosity still decreases at high temperatures), but this is a physical state change rather than an inherent viscosity difference caused by chemical structure.

In conclusion, the degree of polymerization is the most crucial factor determining the viscosity of PDMS. The difference in molecular chain length directly leads to the viscosity variation among different types of PDMS, while the influence of end groups and temperature is relatively secondary. In practical applications (such as floor polishes, lubricants, etc.), it is precisely by choosing PDMS with different degrees of polymerization that the required viscosity performance is matched.