Controlled shear-induced molecular orientation and crystallization in polypropylene/talc microcomposites – Effects of the talc nature

Nowadays, innovation in the materials field, particularly for polymer materials undoubtedly requires the control of their structure at the nanoscale. Moreover, in the polymer processing tools, the material generally undergoes high strains. In our case, the goal is to determine the interactions between the filler and the matrix on the crystallization processes, especially for a high aspect ratio nucleating filler embedded in a semi-crystalline matrix submitted to a controlled strain field. In this study, PP/talc blends were developed by compounding in the melt state using a mini-extruder. Several talcs, different in morphology and particles size, have been studied to determine the influence of their dispersion and of their aspect ratio on the matrix crystallization kinetics. Particularly, one is natural talc and the other is synthetic. The dispersion and orientation of the talc lamellae were characterized by scanning electron microscopy (SEM) and Wide Angle X-Ray Scattering (WAXS). Moreover, the crystallization of the microcomposites polypropylene matrix was thoroughly analyzed in terms of kinetics and crystalline orientation. Experiments were performed using both a rheometer and a shearing hot stage mounted on a polarized light microscope or in a synchrotron X-ray beam. They revealed the influence of the nucleating effect depending on the filler, as well as of the shear conditions on both crystallization kinetics and crystalline orientation. Particularly, it was shown that there are two possible crystalline orientations with mother-daughter lamellar structure: one orientation is promoted by the nucleating effect of the aligned talc platelets and the other is directly due to the shear.