The initiation and growth mechanism of cavitation effect during stretching of high-density polyethylene

Abstract

Cavitation is a critical phenomenon to determine the mechanical performance and failure behavior of semicrystalline polymers. However, the initiation and growth mechanism of cavitation is still under debate. In this work, the cavitation behavior of high-density polyethylene (HDPE) was investigated systematically from the perspectives of crystal thickness and free-volume pores in amorphous phase, employing the in situ wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and minute-scale positron annihilation lifetime spectroscopy (PALS) analysis. It is found that cavities are originated from the coalescence of pre-existing free-volume pores, not by the expansion of individual free-volume pore. For the chloroform modified sample, solvent fills the amorphous phase and reduces the fraction of free volume by 47.2%, which prevents the coalescence of free-volume pores and thus the initiation of cavitation is suppressed completely. On the other hand, annealing improves the lamellae thickness of PE and can undergo higher negative pressure generated by the surface tension of growing cavities, resulting in the most pronounced cavitation effect. These findings provide new insights into the cavitation mechanism of semicrystalline polymers.