Study On β-iPP Induced By PET Fiber And In Confined Melt

The work of this paper is to study theβ-modification formed in isotactic polypropylene (iPP), and we have employed various ways to test it. Our work mainly includes two impotant parts:Firstly, polyethylene terephthalate (PET) fibers containing different content ofβ-nucleating agent (β-NA) were melt-spun from a reconstructive melt flow index rheometer at 270℃. The interfacial features of PET fiber/isotactic polypropylene (iPP) composites were investigated by polarized optical microscope (POM), differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). For the pure PET fiber reinforced iPP, the interfacial region is mainly composed of a-modification, though it has been demonstrated that PET fiber shows weak nucleating ability towards iPP matrix. More interestingly, when the amount ofβ-NA introduced into the PET fiber exceeds a critical value, the transcrystalline layer will be notably dominated byβ-modification. This method to prepare P-transcrystallinity is remarkably different from those produced by stress or temperature control. The present results point out a facile and promising technique to prepare richβ-transcrystallinity under the condition without stress.The result shows, for 0-PET/iPP and 0.5-PET/iPP, the crystals in both iPP matrix and those at the interface are a-phase. Furthermore, nuclei in the two areas appear at the same time. For 0.5-PET/iPP, although it hasβ-NA in the PET fiber, there is rarelyβ-NA on the lateral surface of the fiber, so it is hard to induceβ-transcrystallinity. However, for 1-PET/iPP,1.5-PET/iPP and 2-PET/iPP, the nuclei surrounding the fiber occur earlier than those in the matrix, indicating thatβ-NA loaded in the PET fiber shows heterogeneous nucleation towards iPP matrix. Furthermore, if the content ofβ-NA exceeds a critical value (i.e., 1wt.%), the PET fiber loadingβ-NA shows nucleating duality. Meanwhile, the PET fiber loaded the content ofβ-NA above the critical value will show nucleating durability. Secondly, the crystallization process of isotactic polypropylene (iPP) film was investigated systematically via polarized optical microscope (POM) at isothermal condition. The iPP film was sandwiched between two microscope slides. During the course of crystallization at an appropriate constant temperature, iPP is isolated from air by the microscope slides. In the incipient stage of the crystallization process, the spherulites formed in the melt are mainly of a-form, and there exist obvious flow in the melt. With the growth of crystallization, some spherulites impinged and confined melt are encapsulated by the crystallization phase in the late stage of crystallization. Vacuum bubbles result from the negative pressure buildup due to density change during crystallization in the inclusion of melts occluded by impinging spherulites. We can find obvious orientation of polymer chain, not only in the surface layer of the melt surrounding the vacuum bubbles but also in the narrow area among the impinged spherulites. The polymer chains are subjected to extension flow during the development of vacuum bubbles, which leads to shear flow in confined melt, thus bring forth lead to the formation of cc-row nuclei in these areas. As has been observed during the shear-induced crystallization of iPP, the row-nuclei formed in situ can also induce the growth of theβ-phase in this case. We support the point that the formation ofβ-phase are caused by the flow of the melt and the negative pressure in the inclusion. Through the experimental observation we also find that such effect cannot be observed during isothermal crystallization of pure iPP film or without upper surface.