Molecular weight determination of macromolecules with a new simplified and coherent light scattering method

Abstract

The classical method allowing for the determination of the weight ( M 2) of macromolecules from static light scattering (LS) and refraction measurements in dilute solutions is reexamined. A new theoretical equation, deduced from more consistent derivations and already tested with success on small molecules, is recalled. New arguments and practical examples (forty-one pure liquids of small-molecule compounds) corroborating the new method are detailed. The influence of interference effects on the scattered intensities and their variations versus the analysis angle θ are discussed both for macromolecules and small molecules. In some studied examples, it appears that the corrected LS equation leads to different M 2 values (larger) or (and), more probably, to different interference effects (lower). The reasons why the classical method leads to realistic values, in spite of its theoretical inconsistencies, are explained. The new method is tested for several solutions of cellulose ethers and polystyrenes, from measurements either performed on a multi-angle laser LS apparatus or on the usual θ = 90° LS register. The proposed method is suitable for practical applications because it combines more coherent theoretical equations with more easy experiments and with reliable M 2 determinations. As a consequence, it seems that a revision of some classical LS data for large macromolecules must be envisaged.