Determination of molecular weight of polymers by viscosity method.

Determination of molecular weight of polymers by viscosity method.

It is a simple method for determining the molecular weight of polymers. Addition of polymers in the solvent increases the viscosity of the solvent, due to introduction of inhomogeneities by the polymers.

If η₀ is the viscosity of the solvent and η is the viscosity of the solution at the same temperature, then the relative viscosity is given by

Relative viscosity.

Specific viscosity: 

It is defined as the relative increase in viscosity and is given as

Specific viscosity.

Reduced viscosity:

It is defined as the relative increase in viscosity per unit concentration (C) of polymer and is given as

Reduced viscosity.

Intrinsic viscosity:

The reduced viscosity is dependent on concentration (C). If a graph is plotted between  reduced viscosity vs concentration then the extraplotation value when C=0 is called intrinsic viscosity.

Intrinsic viscosity.

Graph of reduced viscosity vs concentration.

Thus the value of intrinsic viscosity can be known easily from the value of reduced viscosity.
Staudinger showed that the intrinsic viscosity of polymer is related to the molecular weight of the polymer by relation

Relation between molecular weight and viscosity.

Where K and a are constant.
If the value of K and a are known, then the molecular weight (M) of polymer can be determined by knowing the value of viscosity. Next