Detecting “Perfect Resolution” Local Polydispersity in Size Exclusion Chromatography

Abstract

The objective of this work is to obtain a simple method for detecting local polydispersity. Local polydispersity is the presence of a variety of different types of molecules at the same retention volume in SEC. One source of local polydispersity is axial dispersion. However, the topic of this paper is the detection of local polydispersity which is independent of axial dispersion effects This “perfect resolution” local polydispersity can occur because SEC separates on the basis of molecular size in solution and thus for complex polymer molecules, such as copolymers or branched polymers, a variety of combinations of molecular weight and composition can produce the same molecular size. In conventional SEC interpretation, it is assumed that with high resolution columns local polydispersity is absent. Highly misleading analyses can result if this assumption is invalid. Two very simple methods were developed in this work. The first method enabled polystyrene-poly(dimethyl siloxane) blends to be examined for local polydispersity by regenerating the chromatogram after sample preparation using an adsorption cartridge and comparing it to the original concentration chromatogram. The second method involves the use of a dual detector SEC to examine the change in apparent local intrinsic viscosity caused by sample preparation utilizing precipitation. Two-dimensional solubility parameters were used for solvent/nonsolvent selection. The method was demonstrated on a polystyrene-poly(dimethyl siloxane) blend.