The effect of temperature and annealing on the phase composition, molecular mobility and the thickness of domains in high-density polyethylene

Molecular mobility, the thickness of domains and the amount of rigid, semi-rigid, and soft fractions of high-density polyethylene (HDPE) were characterized as a function of temperature and annealing time using time- and frequency-domains proton solid-state NMR. These experiments established the temperature range for which the largest differences are observed in molecular mobility in crystalline phase, semi-rigid crystal-amorphous interface, and soft fraction of the amorphous phase allowing accurate determination of the phase composition and the thickness of these domains. The domain thickness, which was determined by NMR, is in good agreement to those measured by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) on the same sample. Changes in molecular mobility in the amorphous phase upon increasing temperature and annealing are discussed. It is shown that annealing is accompanied by structural reorganizations in the amorphous layer adjacent to the lamella surface causing a continuous shift of the interface towards the inner part of the amorphous regions and thus reducing the thickness of the amorphous layer. A recently introduced method was used for measuring the thickness of domains by a spin-diffusion NMR experiment with a double-quantum dipolar filter. The temperature dependence of the spin diffusivities is reported for the three phases of HDPE. For the first time results of spin-diffusion experiment performed by time-domain low-field NMR and frequency-domain high-field NMR are compared.