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The influence of two kinds of polymers (polystyrene (PS), acrylonitrile−styrene copolymer (SAN)) on the crystallization behavior of isotactic polypropylene (iPP) has been investigated by means of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The current experimental results indicated that these two kinds of polymers with low concentration, as special β-nucleating agents, can induce the β-iPP polymorph during quiescent melt crystallization. The nucleating activity of SAN or PS significantly depends on its concentration, molecular structure, and thermal history of processing. The content of β-crystal form increases with the increasing crystallization temperature or nucleating agent (SAN or PS) percentage, reaches a maximum value, and then decreases as the temperature or nucleating agent percentage further increases. Under the same crystallization condition, SAN is more effective than PS on inducing a higher level of β-crystal form in iPP. Besides, variable temperature WAXD and POM experiments have been used to investigate the polymorphism and the crystalline phase transformation of iPP. It was proved that the β-crystal form of iPP is a thermodynamically metastable structure and will gradually transform to a new kind of α‘-crystal form with the rise in crystallization temperature. Because of different structural characteristics, the polymeric nucleating agents exhibit nucleation and crystallization mechanism which differ from the traditional low molecular weight β-nucleating agents of iPP.

This article reports crystallization behaviors of isotactic polypropylene (iPP) with an aryl amide derivative (TMB‐5) as β‐form nucleating agent. The effects of nucleating agent concentration, thermal history and assemble morphology of nucleating agent on the crystallization behaviors of iPP were studied by differential scanning calorimetry, X‐ray diffraction, and polarized optical microscopy. The results indicated that the TMB‐5 concentration should surpass a threshold value to get products rich in β‐iPP. The diverse morphologies of TMB‐5 are determined by nucleating agent concentration and crystallization condition. At higher concentrations, the recrystallized TMB‐5 aggregates into needle‐like structure, which induces mixed polymorphic phases on the lateral surface and large amount of β modification around the tip. High β nucleation efficiency was obtained at the lowest studied crystallization temperature, which is desirable for real molding process. TMB‐5 prefers to recrystallize from the melt at higher concentration and lower crystallization temperature. The difference in solubility, pertinent to concentration and crystallization temperature, determined the distinct crystallization behaviors of iPP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1725–1733, 2008