A simple method of estimation of the polydispersity index of narrow molecular weight distributions by using quasielastic light scattering data

Abstract

A simple method of estimation of the polydispersity index of narrow molecular weight distributions using quasielastic light scattering data has been suggested. The method approximates the relaxation times distribution by using the Pearson V distribution and iterates its parameters by the least squares method to an experimental correlation function. For the usual unimodal molecular weight distributions the polydispersity index estimate is good (with an error up to 0.1) at a determined value up to 1.2, rough at a determined value up to 1.5, while above this value the method is virtually impracticable. If the type of the molecular weight distribution is known (Schulz-Zimm, lognormal, Pearson V), the polydispersity value thus determined can be corrected, which would extend the applicability limits of the method. it is shown that for an adequate determination of the polydispersity index the noise level ought to be about 10-4.