Persistence of regions with high segment density in polyethylene melts

Proton nuclear magnetic resonance (NMR) spin‐spin relaxation measurements were made on three commercial‐grade polyethylenes in the melt state, free of solvent. All samples exhibit a three‐component relaxation behavior, with components being assigned to amorphous low‐molecular weight material (non‐network fraction), amorphous entangled network fraction, and an ordered or high‐segmental‐density fraction, in order of decreasing relaxation times. Sample thermal history is shown to have a considerable effect on the overall relaxation behavior, and therefore on the relative amounts of each of the three components in the melt. An adequate thermal treatment of samples produces an equilibrium melt with invariant composition of the three fractions. The effects of thermal history on the relative amount of high‐segment‐density regions in the melt parallels its effect on the fraction of crystalline material in the solid polymer. These results are evidence for the persistence of ordered regions in polyethylene at temperatures well above the crystalline melting point of the polymer. We further comment on the nature of the two slower relaxing components and present examples of how the components manifest themselves in other polymer characterization techniques.