Xiangfang Peng

Transcrystallization in nanofiber bundle/isotactic polypropylene composites: effect of matrix molecular weight

Polyamide 66 (PA 66) nanofiber bundles were first electrospun and then introduced into isotactic polypropylene (iPP) melts to prepare nanofiber bundle/iPP composites. To reveal the influences of matrix molecular weight (Mn) on the transcrystalline layer, three kinds of iPP with different Mn were adopted. Polarized optical microscope was employed to investigate the transcrystallinity. In the presence of PA 66 nanofiber bundle, the heterogeneous nucleation distinctly happened in iPP melts. Moreover, the higher the iPP Mn, the denser the nuclei. Both a decrease in matrix Mn and an increase in isothermal crystallization temperature led to an increase in the induction time. The maximum temperature at which the transcrystalline layer can be optically observed increased with the increase of Mn. The growth rate of transcrystallinity decreased with the increasing Mn and crystallization temperature. Moreover, selective melting of the transcrystalline layers confirmed that it was merely composed of α form crystal for all composites.

Suppression of β-crystal in iPP/PET Fiber Composites

The β-crystal formed in PP/PET fiber composites was investigated. The results indicate that PET fibers (PF) can preferably lead to α-crystal formation on their surface. Besides, α-crystals occur earlier than those in the bulk. The β-crystal, might be induced by temperature gradient, only formed away from the PF in composites with lower content of PF. The higher the content of PF is, more possible the PF network is constructed. The transcrystallinity induced by PF will rapidly occupy the region between the adjacent PFs. Consequently, owing to the spatial confinement, β-form is suppressed in the composites with higher content of PF.

Effect of Stretching on β-Phase Content of Isotactic Polypropylene Melt Containing β-Nucleating Agent

Pure isotactic polypropylene (iPP) and that containing 0.2 wt% of a β-nucleating agent (β-NA) were extruded through a slit die. Simultaneously, the extruded melt was stretched at the die exit with different stretching rates (SR). The change of β-phase content with different SR was investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results indicate that, for pure iPP, the content of β-phase first increases with increasing SR till it reaches a maximum and then it gradually decreases. However, for the case of β-nucleated iPP, it decreases monotonously with increasing SR. The spatial confinement is considered as the best explanation for the suppression of β-phase in the nucleated iPP melt upon stretching.

Skin Thickness and β-Crystals Development in Injection-Molded iPP Along the Flow Direction

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one. The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex.

Competing effect of shear and β-nucleating agent on the crystallization of injection-molded iPP

Abstract In this study, crystallization at different depth in the injection-molded iPP, filled with β-nucleating agent (β-NA), was investigated. According to WAXD result, one observes that the fraction of β-crystals unevenly distributes along the depth of the injection-molded parts. For iPP with lower content of β- NA, larger amount of β-crystals near the surface is shear induced; meanwhile, for the parts with higher content of β-NA, larger amount of β crystals away from the surface is β-NA induced. From the results, it could be suggested that the traditional role of nucleation agent, i.e., heterogeneous nucleation, under complex thermo-mechanical conditions during injection molding, might be suppressed.