Polypropylene-graphite nanocomposites made by solid-state shear pulverization: Effects of significantly exfoliated, unmodified graphite content on physical, mechanical and electrical properties

Nanocomposites made from polypropylene and as-received graphite were prepared by solid-state shear pulverization (SSSP) as a function of graphite loading (0.3-8.4 wt%). X-ray diffraction indicates that SSSP employing harsh pulverization conditions yields substantial graphite exfoliation at 0.3-2.7 wt% graphite content with less exfoliation being achieved at higher graphite content. With increasing graphite content, thermal degradation temperature and non-isothermal onset crystallization temperature increase substantially (by as much as 35 and 23 °C relative to neat polypropylene) while isothermal crystallization half-time decreases dramatically. In contrast, Young’s modulus and tensile yield strength exhibit maxima (∼100% and ∼60% increases, respectively, relative to neat polypropylene) at 2.7 wt% graphite content, with all nanocomposites retaining high elongation at break values except at the highest filler loading. Electrical conductivity measurements indicate percolation of graphite at 2.7 wt% and higher graphite content, consistent with rheology measurements showing the presence of a solid-like response of melt-state shear storage modulus as a function of frequency. Significant tunability of graphite exfoliation and property enhancements is demonstrated as a function of SSSP processing.