Steric Exclusion Chromatography (including the Chromatography of Polymers in an Aqueous Solution)

This chapter describes the different difficulties encountered when studying a new polymer by GPC or SEC. This technique is known as liquid chromatography in which a soluble polymer is eluted through a porous gel filling a column. The different molecular weights are separated following their hydrodynamic volumes compared with the pore diameters. It appeared in the sixties firstly in an aqueous medium. The main factors playing a role in the elution through the porous support are examined. Especially, the SEC behaviour of water-soluble polymers is discussed introducing the behaviour in aqueous medium where H bonds and hydrophobic interactions are important. Examples of dextrans and neutral oligosaccharides, rich in -OH groups are discussed showing that weak adsorption increases the elution volumes when eluted in water. Other important interactions concern the electrostatic interactions causing exclusion from the gels and changes in the polyelectrolyte conformation. Elution with monovalent electrolytes (NaNO3 or NaOAc) around 0.1M is recommended. SEC of charged oligosaccharides, hyaluronan, pectins and chitosan are briefly described. Fortunately, new equipment appeared progressively and especially in 1983 the multiangle laser light scattering (MALLS) was introduced, which is probably the most useful detector to associate with the differential refractometer. In that case, Mw is obtained independently of the elution volume as soon as there are no aggregates and good solubility of the polymer tested in the solvent selected. To conclude, it is necessary to insist on the quality of the polymeric solution avoiding the presence of aggregates which may be identified by dynamic light scattering (DLS). In their presence, even after filtration on 0.2 µm pore membrane, the SLS overestimates the Mw.

Keywords: Aqueous medium, Chromatography, GPC, Molar mass, Persistence length, Polyelectrolyte, SEC.