LOW-SPEED FLOTATION EQUILIBRIUM OF MACROMOLECULES IN THE ANALYTICAL ULTRACENTRIFUGE

The application of low-speed flotation equilibrium in the analytical ultracentrifuge for molecular weight analysis of a system of macromolecules in solvent of higher density is described. The method is illustrated by application to a poly[(isoprene)-b-(ethylene oxide)] block copolymer system in chloroform. Solute distributions recorded by using Rayleigh interference optics and the determination of weight-average molecular weights (both "whole-cell" and "point" averages) are specifically considered, although extension of the method to data recorded from other optical systems (namely, absorption and Schlieren) and for the determination of other averages (number, z, etc.) is indicated. The results are compared with a conventional low-speed sedimentation equilibrium of this polymer with water as solvent, where the polymer forms micelles. From comparison of the two results an estimate for the aggregation number is given for the micelles. © 1990 American Chemical Society.