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 beta-nucleating agent (beta-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 beta-NA raised the onset crystallization temperature and the relative crystallinity of the beta 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 13 cylindrulites that are epiphytic and symmetrical on the surface of long fibrous crystals. Furthermore, stratiform beta crystals induced by the combined effects of strong shear flow field and addition of UHMWPE and beta-NA were observed and investigated. Such a unique structure provides an effective way to tune the mechanics of MIM parts. (C) 2015 Elsevier Ltd. All rights reserved.