Competitive Growth of α- and β-Crystals in β-Nucleated Isotactic Polypropylene under Shear Flow

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It has been well established that, although both shear flow and β-nucleating agent could separately induce β-crystals in isotactic polypropylene (iPP) in an efficient manner, their combination in fact depressed the content of β-crystals when compared with quiescently crystallized β-nucleated iPP. In the current study, in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were performed to investigate this behavior. The WAXD data obtained were quantitatively analyzed to determine the independent contributions of applied shear flow and added β-nucleating agent in terms of nucleation stage and subsequent α- and β-crystal growth stage. In the nucleation stage, the addition of β-nucleating agent increased the amount of β-nuclei, while the application of shear flow and the interactions between shear and β-nucleating agent enhanced the amount of α-nuclei (the amounts of α- and β-nuclei were in the same order of magnitude). As a result, in the initial crystallization, α- and β-crystals grew competitively, causing simultaneously increments of α- and β-crystals. However, in the growth stage, the growth rate of β-crystals was faster than that of α-crystals where the epitaxial growth of β-crystals on α-crystals also occurred (due to more favorable isothermal crystallization temperature for β-crystal growth). Consequently, the content of β-crystals became dominant in the limited growth space; however, it was still less than that formed from the quiescent isothermal crystallization of β-nucleated iPP. As the shear rate increased, more shear-induced α-nuclei were formed, further decreasing the amount of β-crystals. Nevertheless, when shear and β-nucleating agent coexisted, β-crystals emerged earlier than α-crystals. The SAXS results indicated that the combination of shear and β-nucleating agent changed the stacking manner of molecular chains, so that the long period of sheared, β-nucleated iPP was comparable to that of quiescently crystallized iPP.