Study on the exfoliation mechanism of graphene nanoplatelets in the polypropylene/graphene nanoplatelets composites under the elongational flow generated by convergent-divergent channels.

Exfoliation of graphene nanoplatelets (GNP) in polymer melts is one of the key challenges when applying melt mixing for preparing polymer-based composite. And the GNP can be exfoliated better within the polymer matrix under the elongational flow field than under the shear flow field. Here, we design three extrusion dies with different convergent-divergent channels to explore the exfoliation mechanism of GNP in polymer melt under elongational flow. The flow field in the convergent-divergent channels is numerically simulated, and the results demonstrate that elongational flow plays a dominant role in the convergent-divergent channel when compared to shear flow. The exfoliation states of GNP in different channels are assessed by the thickness and interlayer space of GNP using analysis of scanning electron micrographs, transmission electron micrographs, and X-ray diagrams. And a significant enhancement (increased by 103%) in the in-plane thermal conductivity of PP (polypropylene)/GNP composites extruded by the designed convergent-divergent channel further demonstrates the GNP are exfoliated efficiently under the elongational flow. Furthermore, an exfoliation model is used to estimate the required stress and explore the exfoliation mechanism of GNP. And the exfoliation mechanism of GNP is associated with the applied stress and the angle between the applied stress and the normal direction of GNP. This work could provide experimental and theoretical guidance for the preparation of high-performance composite filled with layered nanosheets. [ABSTRACT FROM AUTHOR]