Yes, the viscosity difference of PDMS (polydimethylsiloxane, i.e. dimethyl silicone oil) is mainly caused by the different degree of polymerization, and is also slightly affected by the molecular structure (such as the end group type). The following is a detailed explanation from the perspective of chemical structure:
Degree of polymerization is the core factor that determines the viscosity of PDMS
The molecular structure of PDMS is linear chain, and the repeating unit is -Si(CH₃)₂-O-. Its general formula can be expressed as:
(CH₃)₃SiO-[Si(CH₃)₂-O-]ₙ-Si(CH₃)₃
Wherein, n is the degree of polymerization (the number of repeating units).
The higher the degree of polymerization (the larger n is): the longer the molecular chain, the stronger the van der Waals force (mainly dispersion force) between molecules, and the higher the degree of entanglement between chains, resulting in poor fluidity and greater viscosity.
The lower the degree of polymerization (the smaller n is): the shorter the molecular chain, the weaker the intermolecular force, the less entanglement, the better the fluidity, and the lower the viscosity
For example:
Low viscosity PDMS (such as 100 mm²/s) usually has a low degree of polymerization (n is about tens);
High viscosity PDMS (such as 100000 mm²/s) can have a degree of polymerization as high as hundreds or even thousands.
Secondary effect of end group type on viscosity
The end groups (groups at both ends of the molecular chain) of PDMS are usually -Si(CH₃)₃ (trimethylsilyl), but some special models may be terminated with hydroxyl (-Si (CH₃)₂-OH) or other groups.
When the end groups are inert groups (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), weak hydrogen bonds may form between molecules, which will make the viscosity at the same degree of polymerization slightly higher than that of PDMS with inert end groups, but the impact is much smaller than the difference in degree of polymerization.
Other minor factors
In addition to the degree of polymerization and the end group, temperature also significantly affects the viscosity of PDMS (silicone oil has the characteristic of a small viscosity-temperature coefficient, but the viscosity will still decrease at high temperatures), but this is a change in physical state, not an inherent viscosity difference caused by chemical structure.
In summary, the degree of polymerization is the most core factor that determines the viscosity of PDMS. The difference in molecular chain length directly leads to the difference in viscosity of different types of PDMS, while the influence of the end group and temperature is relatively minor. In practical applications (such as floor polishes, lubricants, etc.), it is through the selection of PDMS with different degrees of polymerization that the required viscosity performance is matched.
