Polydispersity index (PDI)
PDI is a measure of the distribution of molecular mass in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular masses in a batch of polymers.
The PDI has a value equal to or greater than 1, but as the polymer chains approach uniform chain length, the PDI approaches unity (1). For some natural polymers PDI is almost taken as unity. The PDI from polymerization is often denoted as:
Mn is more sensitive to molecules of low molecular mass, while Mw is more sensitive to molecules of high molecular mass.