Solid-state 13C NMR investigation of the structure and dynamics of highly drawn polyethylene—detection of the oriented non-crystalline component

Abstract

The structure and dynamics of highly drawn polyethylene samples were studied by solid-state 13C NMR spectroscopy. The analyses of the 13C spin–lattice relaxation time (T1C) and the 13C spin–spin relaxation time (T2C) have revealed that at least three components with different T1C and T2C values, which correspond to the crystalline, less mobile non-crystalline, and rubbery amorphous components, exist for these materials, as in the case of isothermally crystallized samples. However, another component with a mass fraction of 0.13–0.18 exists which has a 13C chemical shift very close to that of the orthorhombic crystalline phase but has an extremely small T1C. Since this component is believed to have the all-trans conformation, it is termed fast all-trans. The chemical shift anisotropy (CSA) spectra for various samples that have small T1C values have been recorded and resolved into those of the non-crystalline and fast all-trans components. As expected, the CSA spectra of the less mobile non-crystalline and rubbery amorphous components that have the smallest T1C values display only a slight asymmetry. In contrast, the CSA spectrum of the fast all-trans component displays higher asymmetry. However, the spectrum is still much narrower than that of the normal orthorhombic crystalline phase, indicating a high degree of motional averaging. It is proposed that this component should be a highly oriented non-crystalline component, which may exist as taut tie-molecules traversing the non-crystalline region. To account for the narrow CSA, this component must undergo rapid fluctuation with large amplitudes at the torsional potential minimum in each C–C bond and possibly an additional random jump or diffusional rotation around the chain axis. Additional measurements obtained by aligning the draw axis of the sample parallel or perpendicular to the static magnetic field indicate that the fast all-trans component is oriented along the drawing direction and subjected to rapid motion around the chain axis.