Crystallization of alpha and gamma phases in isotactic polypropylene with low ethylene content: Isothermal crystallization and secondary crystallization

摘要

Crystallization and melting behaviors of isotactic polypropylene (iPP) with low ethylene content during an isothermal process and on heating just after it are investigated by means of in situ x-ray diffraction. At every isothermal temperature investigated here, the formations of alpha and gamma phases are confirmed. The ratio of gamma to alpha increases with an increase in isothermal temperature. On heating just after the isothermal process, the gamma starts melting at a lower temperature than the alpha. This may be due to the small crystallite size of the gamma crystallization during the isothermal process. On cooling after the completion of the isothermal crystallization, the secondary crystallization accelerates, and the gamma phase crystallizes predominantly. The fractionation is considered to occur in the primary crystallization during the isothermal process. The low-molecular-weight portion of the materials, which remained in amorphous regions of the spherulite during the isothermal process, crystallizes into the gamma phase in the secondary crystallization. Furthermore, the fractionation behavior seems to be enhanced at higher temperatures. [References: 27]