An unusual transition from point-like to fibrillar crystals in injection-molded polyethylene articles induced by lightly cross-linking and melt penetration

Abstract

Recently, a melt penetrating process in which a first polymer melt is rapidly penetrated by a second polymer melt has been realized on our home-made multi-melt multi-injection molding (MMMIM) machine. Although great deformation can be provided by the rapid melt penetration process, it has been found that hardly any oriented crystalline structures can be kept and formed due to the quick chain relaxation at high temperatures. In the present work, lightly cross-linked structures were introduced to prolong the relaxation time of linear high density polyethylene (HDPE) molecular chains. The hierarchical structures of MMMIM samples were characterized by scanning electron microscopy (SEM), polarized light microscopy (PLM) and two-dimensional small angle X-ray scattering (2D-SAXS). It was found that the melt penetrating process promoted the formation of cylindritic crystalline structures in the subskin layer, whereas only isotropic spherulites were formed in the subskin layer of the corresponding conventional injection molding (CIM) sample. From linear to lightly cross-linked macromolecular chain structures, a transition from cylindritic structures composed of banded-spherulites along the flow direction towards shish–kebab-like structures was observed in the subskin layer of the MMMIM samples, and also the distances between two nuclei decreased as well as the orientation degree increased gradually in the transition layer. These results indicate that lightly cross-linked HDPE structures with longer relaxation times are beneficial to keep the point-like nuclei along the flow direction and are helpful for the transition to shish–kebab-like structures with thread-like nuclei. Modified models are proposed to interpret the mechanism of the formation of shish–kebab-like structures under the melt penetrating of samples with lightly cross-linked structures.