Crystalline Structure of Injection Molded β-Isotactic Polypropylene: Analysis of the Oriented Shear Zone

Although both shear flow and beta-nucleating agent (beta-NA) could separately induce beta-crystal in isotactic polypropylene (iPP), their combination, particularly in the typical industrial processes, in fact has received comparatively little attention. In the current study, two-dimensional wide-angle X-ray diffraction and small-angle X-ray scattering (2D-WAXD/SAXS) measurements were performed to investigate the effect of beta-NA on the crystalline structure of the oriented shear zone in injection molded iPP. It is observed that, regardless of the beta-NA concentration, parent daughter structure of a-crystal can be formed in iPP. Furthermore, the fraction of daughter lamellae elevates with the increasing concentration of beta-NA. Interestingly, unexpected scattering patterns of (300) reflection for beta-crystal, which is similar with that for parent daughter lamellar branching of a-crystal, is exclusively found in iPP with higher concentration of beta-NA (1.0 wt %). The most possible explanation is that the addition of high content of beta-NA lowers the free energy barrier. Additionally, the same change tendency of long period, crystal lamellar thickness and lateral dimension, d-spacing and crystallite size is found, viz., they first increase and then decrease with the increasing beta-NA content. The results demonstrate that the concentrations of beta-NA have a significant effect on the crystal grain structure under the practical molding process.