Study of a skin-core type of crystallinity distribution within polyethylene specimen crystallized under high pressure

The regularity of ‘skin-core’ type of crystallinity distribution within the polyethylene cylindrical specimen crystallized under high pressure was investigated via modulated differential scanning calorimeter (MDSC), wide angle X-ray diffraction meter (WAXD), dynamic mechanical analyzer (DMA) and scanning electron microscope (SEM). The relationship between the gradient of high pressure stress field and the ‘skin-core’ structure of polyethylene was also established. The results indicate that there exists a distinguished difference of crystallinity, morphology and dynamical mechanical properties between skin layer and core region within the cylindrical specimen crystallized under 650 MPa pressure. The measurement data of the non-reversible heat flow of MDSC and WAXD for both skin layer and core region are in accordance with that marked difference. According to the law of stress decay of high pressure stress field and the high pressure crystallization theory of polymers, it was found that the uneven distribution of high pressure stress field, especially, the high pressure decay effect at the edge of plunger and cylindrical channel of the mold is proposed as the principal factor to the formation of the ‘skin-core’ type of crystallinity distribution within the polyethylene cylindrical specimen crystallized under high pressure.