Study of molecular motion in drawn polyethylene by broad-line nuclear magnetic resonance

Measurements have been made of the anisotropy in the nuclear magnetic resonance second moment for the proton resonance in drawn low density polyethylene films over the temperature range –196 to +60° C.The experimental results are first compared with the theoretical predictions of a model in which it is assumed that the polyethylene chains undergo coherent classical rotation about the chain axis. This model is successful in predicting the general pattern of anisotropy at temperatures close to the melting point.To account for the observed reduction in second moment at intermediate temperatures further calculations are described, based on a model in which the polyethylene chains twist about the chain axis, the total twist being shared equally between all monomer units. This does not account satisfactorily for the changes in second moment with temperature.It is concluded that satisfactory general agreement between theory and experiment is reached if the measured anisotropy at intermediate temperatures derives from rigid lattice intramolecular interactions only, and that the intermolecular interactions are averaged to a small value. It is suggested that the inter-chain averaging occurs by a chain-sliding process.At higher temperatures further reduction in the second moment occurs due to coherent chain oscillation about the chain axis, which finally leads to complete rotation at premelting point temperatures.