Cavitation in Tensile Stretched HDPE Revealed by Synchrotron Ultrasmall-angle X-ray Scattering

A series of high-density polyethylene samples with different microstructures obtained via isothermal crystallization at various temperatures and then cooling down to room temperature were used to explore the structural evolution and cavitation behavior during stretching by means of the ultrasmall-angle X-ray scattering and differential scanning calorimetry techniques. The samples crystallized at temperature higher than 110 degrees C then naturally cooled to room temperature possess two populations of lamellar stacks with different crystalline thicknesses. The thicker lamellae with well-organized structure were formed in the isothermal crystallization process, and chains crystallized at lower temperatures during cooling down yielded thinner defective lamellar crystallites. The melting temperatures of the two populations of lamellar stacks were around 133 and 110 degrees C, respectively. When deformed at 30 degrees C, the cavitation was observed in all samples and accompanied with strainwhitening phenomenon. It turned out that the cavities were more intense for the samples with thicker lamellae generated during isothermal crystallization. During deformation, the cavities first take place with the normal parallel to the stretching direction around yield point, and then changes the normal perpendicular to the tensile direction with the increase of strain. The length of cavities was calculated and the values were around 900 - 1200 nm. On the other hand, the extent of cavitation becomes smaller with the increasing ratio of the thinner lamellae crystallized during cooling down. Moreover, the plastic deformation of the lamellae is facilitated and the degree of cavitation become weaker with the increase of the stretching temperature.