Polymer fractionation

Different molecules present in a polymer sample possess different molecular weight. The process of separating the unfractionated sample into different fractions of average molecular weight is known as polymer fractionation. There exist four methods for polymer fractionation. They are fractional precipitation, partial dissolution, gradient elusion and gel permeation chromatographic technique. In fractional precipitation technique, small quantities of non-solvent are added to a polymer solution in a poor solvent. This forces high molecular weight species to come out of the solution. In this technique, small quantities of non-solvent are added to a large volume of the solution. The relation between solvent power and molecular weight is conveniently used in partial dissolution technique. Here the polymer sample to be fractionated is taken in the form of tiny powder and solvent mixtures are prepared with different solvent powers. Partial dissolution technique is again used in a slightly different way in gradient elution technique. Here polymer is precipitated on a glass wool packed at the top of a column filled with silica or alumina. Gel permeation chromatographic technique is a faster fractionation technique. In this technique, a single liquid acts both stationary as well as mobile phases.