Polymers: Molecular Weight Distribution

Recall that polymers always possess a higher weight average molecular weight than number average molecular weight unless all polymer chains have the same molecular weight.

As a measure of molecular weight distribution in polymers, the polydispersity index, or PDI, is evaluated.

The polydispersity index is the ratio of weight average to number average molecular weight.

The minimum polydispersity index value for a polymer is one. This value indicates that all chains in the polymer have the same molecular weight, making it a monodisperse polymer.

This never occurs in synthetic polymers, so all synthetic polymers have a polydispersity index greater than one and are, therefore, known as polydisperse polymers.

A polydispersity index close to one indicates a narrow distribution of molecular weights among the various polymer chains.

In turn, higher polydispersity index values suggest a broad distribution of molecular weights.