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Definition: The sum of the atomic weights of all atoms in a molecule. In most nonpolymeric materials the molecular weight is a fixed constant value. In high polymers, the molecular weights of individual molecules vary widely so that they must be expressed as- averages.

(1) The number-average molecular weight, Mn.

where

ni = number of molecules with molecular weight Mi
Wi = weight fraction of material having molecular weight Mi;Σ = ni = total number of molecules

(2) Weight Average Mw

(3) Z-average molecular weight, Mz

Methods for determining molecular weights include measurements of osmotic pressure, light scattering, sedimentation, equilibrium, dilute solution viscosity, freezing points, vapor pressure; and analyses of ultracentrifugation and spectroscopy. Also known as MOLECULAR WEIGHT DISTRIBUTION.
Definition Copyright ©1989 CRC Press LLC. All rights reserved.

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