Crystallization kinetics of melt-mixed 3D hierarchical graphene/polypropylene nanocomposites at processing-relevant cooling rates

Knowledge of the solidification behaviour of isotactic polypropylene (iPP) composites with reduced graphene oxide nanoplatelets is key to open the possibility to widespread use of these high performing nanocomposites. The crystallization behaviour of these systems, with filler content in the range of 0.2-3.5 wt% has been inves-tigated by differential scanning calorimetry (up to 100 degrees C/min) and ex-situ structural and morphological characterization of samples prepared at processing-relevant cooling conditions (up to 2000 degrees C/s). Compared to the self-nucleated neat iPP, the nucleation efficiency was estimated to vary from 44 to 93% when increasing the filler content. Such a high nucleating efficiency has not been reported yet for a nanocomposite with iPP matrix. This result is due to the very good dispersion of the filler in these melt-mixed graphene-based polypropylene systems. The nucleation ability of the graphene filler does not reach a saturation in the concentration range studied. The gap between laboratory and industrial cooling rate scale is here reduced thanks to the information achieved from the fast-cooling experiments. By varying the filler amount in the investigated range, a significant shift of the cooling rate window in which the transition from alpha-to mesophase dominated crystallization takes place was detected. Notably, with 3.5 wt% graphene platelets, alpha-phase crystals are predominantly present in the material even after quenching at 1000 degrees C/s, similar to highly nucleated commercial iPP grades.

Automated summary

Understanding the solidification behavior of iPP composites with reduced graphene oxide nanoplatelets is crucial for their widespread use. This study investigated the crystallization behavior of these composites and found that the nucleation efficiency of the graphene filler is high even at low concentrations, with a significant impact on the crystallization transition.