Molecular motion in polyethylene

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Abstract
Molecular motion has been studied in a highly branched polyethylene, DYNK, and in a linear polyethylene, Marlex 50. The studies centered around the use of nuclear magnetic resonance spectroscopy, with additional information from measurements of x‐ray diffraction, dielectric relaxation, and the diffusion of small molecules into the polymers. The nuclear resonance studies extended over a temperature range of 80 to 410°K., and studies by the other techniques included some measurements at low and high temperatures. It is shown that crystallinity in the linear polyethylene persists to within only a few degrees of the “melting point” of the gross polymer, and that chain rotation within the crystallites is comparatively restricted until shortly below the temperature of fusion. On the other hand, chain rotation within the crystallites of the branched polyethylene is much freer, presumably due to the presence of lattice defects, such as might be caused by the incorporation of branch points into the crystallites. Crystallinity in the branched polyethylene disappears at much lower temperatures than in the linear polymer. There is considerable motion of chain segments within the amorphous regions of both kinds of polyethylene, although at a given temperature the linear polymer exhibits more constraint than does the branched polymer. It is evident that the amorphous portions of both polymers possess the attributes of a viscous liquid, even at temperatures well below the melting point of the gross material. Activation energies and frequency factors for the motions of chain segments in the amorphous portions have been computed and are compared with the data from earlier measurements of dielectric and mechanical relaxation.

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