Unusual hierarchical distribution of β-crystals and improved mechanical properties of injection-molded bars of isotactic polypropylene

Abstract

In this study, the microstructures, hierarchical distribution of β-phase crystalline morphology and mechanical properties of neat isotactic polypropylene (iPP) prepared by conventional injection molding (CIM) and gas-assisted injection molding (GAIM) were intensively examined. The obtained samples were characterized via two-dimensional small-angle X-ray scattering (2D SAXS), polarizing light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (XRD) and tensile testing. It was found that the strong shear effect introduced during the gas penetration of the GAIM process greatly influences the morphology and the formation of β-crystals in the pure PP specimens. Shish-kebab and β-cylindrite morphologies were observed simultaneously in the sub-skin layer of the GAIM part for the first time, where the flow pattern was complex and the shear strength was believed to be the maximum. Furthermore, an iPP specimen with β-crystals existing in the entire cross section of the molded bar were obtained for the GAIM sample without adding β-nucleating agents or other components. However, the pure iPP sample with a low content of β-crystals were only found in the skin layer of the CIM part. In addition, due the coexistence of shish-kebab and β-cylindrite structures, the mechanical properties of the GAIM specimen were significantly improved when compared with the CIM sample. Based on these experimental observations, a schematic illustration was proposed to interpret the mechanism of the formation of the unusual hierarchical distribution of β-crystals during the GAIM process.