How Molecular Weight Averages are Calculated

Polymers are comprised of a mixture of molecules possessing different molecular weights. This distribution in molecules arises from the polymerization process used to synthesize the polymer. Depending on the polymerization process and conditions, polymers can have molecular weight distributions which are polydisperse or monodisperse. There are three main average molecular weights used to characterize a polymer:

a) Number-Average Molecular Weight:

b) Weight-Average Molecular Weight:

c) Z-Average Molecular Weight:

A widely used measure of the molecular weight distribution of a polymer sample is the polydispersity index (PDI). It is the ratio of Mw over Mn and for polymers having monodisperse distributions, the PDI tends towards 1. The scheme below demonstrates how Mn, Mw, and Mz are calculated based on a column calibration curve.

Graph and Equations for the Calculation of Molecular Weight Based on a Column Calibration Curve

Short Forms for Molecular Weight in Equations

The concentration, ci, of all fractionated polymer species comprised in polymer population is calculated from the RI detector using the equation below:

Formula to Calculate The Concentration of All Fractionated Polymer Species

Individual Components of the Polymer Concentration Formula

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