DSC and morphological studies on the crystallization behavior of β-nucleated isotactic polypropylene composites filled with Kevlar fibers

The crystallization behavior of beta-nucleated isotactic polypropylene (PP) composites filled with Kevlar fibers (KFs), as well as that of non-nucleated PP/KF composites for comparison, was investigated using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The morphological observations revealed that the KF addition could induce thick alpha-transcrystalline layer around their surfaces in PP/KF composites, while no obvious transcrystalline layer could be detected in beta-nucleated PP/KF composites. Detailed DSC investigations suggested that for the PP/KF composites, the dominant modification was alpha-form, and the crystallization process of matrix was promoted by KF addition, as illustrated by faster isothermal crystallization rate, shorter induction time, and higher crystallization temperature. However, for beta-nucleated PP/KF composites, the main modification was beta-form, and their crystallization characteristics were independent of KF addition, indicating that the alpha-nucleating effect of KFs was absent in this system. The DSC results were confirmed by further rheological and wide angle X-ray diffraction (WAXD) studies. The mechanism of the formation of transcrystalline layer was also discussed.