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

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