Stratiform β crystals in ultrahigh molecular weight polyethylene and β-nucleating agent-nucleated isotactic polypropylene at micro-injection molding condition

The crystalline and oriented morphologies of isotactic polypropylene (iPP), ultrahigh molecular weight polyethylene (UHMWPE) and β-nucleating agent (β-NA) blends molded by micro-injection were investigated via polarized light microscopy, scanning electron microscopy, differential scanning calorimetry and wide-angle X-ray diffraction. The results showed that the addition of β-NA raised the onset crystallization temperature and the relative crystallinity of the β crystals of the micro-injection molding (MIM) specimens because of its strong heterogeneous nucleating effect in the iPP matrix. The introduction of UHMWPE, because of its “maintaining-orientation” effect, increased the thickness of the shish-kebab structure in the skin layer and induced the formation of perfect β cylindrulites that are epiphytic and symmetrical on the surface of long fibrous crystals. Furthermore, stratiform β crystals induced by the combined effects of strong shear flow field and addition of UHMWPE and β-NA were observed and investigated. Such a unique structure provides an effective way to tune the mechanics of MIM parts.