Zhengfang Chen

New understanding in the influence of melt structure and β-nucleating agents on the polymorphic behavior of isotactic polypropylene

The β-nucleation behavior of isotactic polypropylene (iPP) is a fascinating and important issue in polymer physics; however, little about this phenomenon or its physical nature has been clearly understood. In the present study, by tuning the heating temperature (fusion temperature, Tf), the amount of ordered structures in iPP melt was controlled. In this way, the influence of five types of representative β-nucleating agents (β-NA) on the crystallization behavior of iPP with different melt structures (i.e. the amount of ordered structures) was comparatively studied by differential scanning calorimetry (DSC), polarized optical microscopy (PLOM), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and rheological measurement. A surprising synergetic effect was observed between β-NAs with α-/β-dual polymorphic selectivity (dual-selective β-NA) and the ordered structure of iPP, resulting in significant increases of the β-nucleation efficiency and the β-phase proportion of the sample. PLOM observation of the crystallization process confirmed that with the presence of ordered structures in iPP melt, a mass of dark, small crystal embryos derived from self-nuclei uniformly distributed in the melt and exhibited β-nucleation efficiency. This work provides the first evidence that for iPP nucleated with dual-selective β-NA, the ordered structures play a determining role in the β-nucleation of iPP. Under the influence of the dual-selective β-NA, the ordered structures exhibited β-nucleation efficiency and therefore encouraged β-nucleation. A possible mechanism was proposed.