Molecular weight and viscosity of polyethylene glycol

There is a certain relationship between the molecular weight of polyethylene glycol (PEG) and its viscosity, but this relationship is not as simple as nonlinear. Specifically, the viscosity of polyethylene glycol is influenced by various factors such as its molecular weight, molecular structure, and solution conditions, mainly reflected in the following aspects:

1. Low molecular weight polyethylene glycol: When the molecular weight of polyethylene glycol is small (usually less than 600), the intermolecular interactions, especially the hydrogen bonding effect between hydroxyl groups, are more significant, which can lead to higher viscosity. For example, PEG with a molecular weight of approximately 180 can achieve relatively high viscosity levels.

2. Medium molecular weight polyethylene glycol: When the molecular weight approaches 600, an interesting phenomenon is that due to the length of the molecular chain being neither too long nor too short, the hydrogen bonding effect between molecules and the fluidity of the molecular chain reach an equilibrium state. At this point, the viscosity of polyethylene glycol actually reaches a relatively low value, about 2.0.

3. High molecular weight polyethylene glycol: As the molecular weight further increases (over 600), although the distance between hydroxyl genes on the molecular chain increases and it becomes difficult to form effective hydrogen bonds, the increase in molecular chain length leads to enhanced interactions between molecular segments, hindering the movement of the molecular chain and causing viscosity to rise again with the increase in molecular weight.

In summary, the viscosity of polyethylene glycol first increases with the increase of molecular weight, and after reaching a specific range (around 600), the viscosity drops to its lowest point. Then, as the molecular weight continues to increase, the viscosity gradually increases again. In addition, this relationship is also influenced by external conditions such as temperature, as changes in temperature can affect the intermolecular interactions and the flexibility of molecular chains. In practical applications, viscosity method is often used to determine the relative molecular weight of polyethylene glycol, which relates its intrinsic viscosity and molecular weight based on specific empirical equations.