Nuclear Magnetic Resonance Study of Molecular Motion in Polyethylene Crystal

An NMR (nuclear magnetic resonance) study is made on the α or crystalline dispersion of polyethylene which has been found viscoelastically between room temperature and its melting point. Narrowing of NMR absorption line, increase of the mobile fraction and anisotropy of the narrowing are observed in the temperature range of the α dispersion both on melt grown and solution grown specimens. This α narrowing is found to occur at higher temperatures ( i.e. the relaxation time is longer) for specimen with thicker crystalline lamella and to be so anisotropic that it disappears when all the molecular axes are aligned parallel to the main magnetic field H 0 . The experimental results indicate that the molecular motion underlying the crystalline dispersion is the large amplitude torsional oscillation of folded polyethylene chains.