Hierarchical crystalline structures induced by temperature profile in HDPE bars during melt penetration process

The hierarchical crystalline morphologies and orientation structures across the thickness direction in high-density polyethylene (HDPE) molded bars were investigated via a novel melt-penetrating processing method named multi-melt multi-injection molding (M³IM). The samples with various mold temperatures (20, 40 and 60 °C) were prepared, and the effects of the external temperature profile on the evolution of crystalline microstructures were studied. With scanning electron microscopy (SEM), the transition of crystalline morphology from ring-banded structure to oriented lamellae was observed with decreasing mold temperature, and the oriented lamellae were formed at the sub-skin layer of the samples at the lowest mold temperature, which was further testified by differential scanning calorimetry (DSC). With the decline of mold temperature, the degree of orientation, obtained from two-dimensional small angle X-ray scattering (2D-SAXS), was increased and long periods rose a little. Thus, decreasing mold temperature was beneficial to the formation of orientation structures because the relaxation of chains was weakened.