Nuclear Magnetic Resonance Studies of Polyethylene

Nuclear magnetic resonance (NMR) studies of three polyethylenes have been made over the temperature range −190°C to 120°C. Two components of the NMR line are observed—a narrow one identified with the ``amorphous'' regions and a broad one identified with the ``crystalline'' regions. Differences between polyethylenes in the shape of the narrow component are observed and discussed. The variation of line width of the two components with temperature indicates two motional transitions occurring in the ``amorphous'' regions and one in the ``crystalline'' regions over the temperature range studied. Energies of activation inferred for one of the ``amorphous'' transitions agree well with those determined from melt viscosity studies implying a similarity of molecular processes controlling the two measurements and allowing an interpretation of the one observed by NMR to be made. The NMR data are used to interpret (in terms of molecular motions) the three dispersion regions observed by dynamic mechanical measurements, with the following results: in the vicinity of −100°C, the amorphous regions undergo a transition involving mainly the onset of rotation of linear segments of the polymer chains. Between −35°C and 0°C (depending on the sample), the motion of amorphous chain segments containing branch points leads to a transition. In the neighborhood of 60–100°C (depending on the sample), there are the beginnings of observable motion of the chain segments within crystallites. Using a new method of decomposing the NMR line into its two components (based on the experimental line shape at low temperatures), the relative intensities of the two components (hence, the relative amounts of fixed and moving nuclei) are determined. This ratio, previously held to be identical with the ``percentage crystallinity,'' is shown to be inconsistent with this view and to support the molecular interpretation presented.