Chromatographic fractionation of polyethylene

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Abstract
A new method has been developed for the fractionation of polyethylene, providing more reliable data on molecular weight distribution in a fraction of the time required by previous methods. The fractionation is carried out on a column of glass beads, using a combination of solvent and temperature gradients by a method similar to that used by Baker and Williams for polystyrene. The lowest temperature in the column is above the melting point of the polyethylene, so that fractionation takes place by partition of the polymer molecules between two liquid phases in transient equilibrium. Because of the combined solvent and temperature gradients, the polymer is transferred from the stationary to the moving phase many times in its passage down the column; consequently, the fractions are considerably narrower than those obtained from a single‐step fractionation. A simple apparatus which will fractionate up to 5 g. of polyethylene in a 24‐hr. period is described. The effect of such experimental variables as column load, solvent flow rate, and the type of solvent gradient was determined. Experimental data obtained on several commercial polyethylenes are given.