The role of crystallinity in the crystallographic texture evolution of polyethylenes during tensile deformation

The crystallographic texture evolution of rapidly crystallized high-density polyethylene (HDPE) and ethylene-1-octene copolymer prepared with a Ziegler–Natta (LLDPE) catalyst, was studied during tensile deformation using wide-angle X-ray diffraction WAXD. The popLA software suite, a methodology based on spherical harmonics for texture analysis, was utilized to produce recalculated pole figures and orientation distribution function plots from the raw data. An important aspect of this work has been the in situ measurement of texture during tensile deformation and the subsequent measurement of texture in the relaxed samples. The difference in molecular structure of the polyethylenes had a strong effect in the initial texture as well as in the rate of texture evolution during deformation. Texture evolves slowly in the HDPE while a fast drastic change in texture is observed in LLDPE after yield. At higher strains the texture of LLDPE was basically unchanged revealing not only a strong (100)[001] ‘c-axis’ texture component, but also other weaker texture components such as (010)[001], (1×0)[010], (011)[100] and (201)[1̄02]. Furthermore, while relaxation after unloading mitigated or eliminated two of the preferred texture components in HDPE strengthening the (001) component aligned along the extension axis, it was observed that the texture of LLDPE did not undergo any significant changes after relaxation. At large strains (ε>1.0), microfibers formed in both HDPE and LLDPE. A lamellar substructure that differs between the HDPE and LLDPE is observed by AFM images of drawn materials. The role of possible slip mechanisms and stress relaxation in the evolving crystallographic texture of these polyethylenes is discussed.