Structural evolution of tensile deformed high-density polyethylene at elevated temperatures: Scanning synchrotron small- and wide-angle X-ray scattering studies

The structural evolution of an ice-quenched high-density polyethylene (HDPE) subjected to uniaxial tensile deformation at elevated temperatures was examined as a function of the imposed strains by means of combined synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) techniques. The data show that when stretching an isotropic sample with the spherulitic structure, intralamellar slipping of crystalline blocks was activated at small deformations, followed by a stress-induced fragmentation and recrystallization process yielding lamellar crystallites with their normal parallel to the stretching direction. Stretching of an isothermally crystallized HDPE sample at 120 °C exhibited changes of the SAXS diagram with strain similar to that observed for quenched HDPE elongated at room temperature, implying that the thermal stability of the crystal blocks composing the lamellae is only dependent on the crystallization temperature. The strain at a characteristic transition point associated with the first indication for the occurrence of a fibrillar structure remains essentially constant in spite of the large changes in drawing temperature and crystalline thickness. In addition, WAXS experiments were used to probe the texture changes accompanying the uniaxial elongation and yield the relationship between the orientational order parameters associated with the crystallites and the amorphous chain segments, and the imposed strain. The results support the existence of intralamellar slip processes from the very beginning of tensile deformation.