The fractionation of polyethylene

Four of the new high‐density polyethylenes and one conventional high‐pressure, low‐density polyethylene have been fractionated by a fractional solution technique. The number‐average molecular weights, infrared absorption spectra, and densities of the fractions were measured by standard techniques, modified where necessary to accommodate the low solubility of the new high‐density polyethylenes. The results show: (1) The solubility of polyethylene is determined primarily by a shape parameter—number of methyl groups per hundred carbon atoms—rather than molecular weight. (2) Crystallinity is not a reliable index of the solubility of polythene unless considered in relation to molecular weight. (3) The unusual physical properties of the new high‐density polyethylenes arise from their having an essentially unbranched molecular structure rather than high molecular weights.