Sedimentation Equilibrium of Highly Nonideal Solutions of Polydisperse Polymers

Abstract

A new method of analysis is proposed for determination of the average molecular weight of a polymer sample, of its polydispersity, and of the thermodynamic characteristics of its solution. The method involves only a single sedimentation equilibrium experiment in the ultracentrifuge and is applicable even to highly nonideal solutions. It is based on the assumptions that (1) the distribution function of the polymer sample is reasonably well described by the unimodal function of the Schulz-Zimm type and that (2) the thermodynamic behavior of the solution obeys the same formula which was found useful for monodisperse samples. The method yielded plausible results both for highly nonideal solutions of polystyrene samples with a narrow distribution of molecular weights and for unfractionated samples of polyacrylamide with a broader distribution.Among the methods used for studying polymer solutions, the method of sedimentation equilibrium is potentially the most precise. Using interference optics, sapphire cell windows, and an automated plate reader, we can, under favorable circumstances, measure the profile (one hundred and more experimental points) of concentration and concentration gradient within the cell with a precision of 1-2%. Why is then this method so rarely used for the