Micro- and nano-structure in polypropylene/clay nanocomposites

Polypropylene (PP)/clay nanocomposites prepared by melt blending using different clays and coupling agents based on maleic anhydride-grafted PP (MA-PP) were studied. Clay dispersion using field emission gun scanning electron microscope (FEG-SEM) and transmission electron microscopy (TEM), and PP matrix morphology were characterized. Clay dispersion was improved in the presence of MA-PP, as shown by the higher particles surface density (number of particles/mm²) at all micro-, sub-micro- and nano-levels. The PP spherulite diameter was affected by both the presence of MA-PP and clay dispersion. Clay intercalation, characterized by both complementary X-ray diffraction (XRD) and TEM, was greatly influenced by the characteristics of MA-PP. The use of low molecular weight (M_w) MA-PP led to a good and uniform intercalation but with no further possibility to exfoliation. The use of higher M_w MA-PP led to a heterogeneous intercalation with signs of exfoliation. The crystallization behavior of nanocomposites was studied by differential scanning calorimetry (DSC). When fine clay dispersion was achieved with MA-PP, clay-nucleating effect was limited and lower crystallization temperature and rates were observed. It was also shown by wide angle X-ray diffraction (WAXD) that clay induced some orientation of α-phase PP crystallites.