10/29/2010
Polypropylene-graphite nanocomposites made by solid-state shear pulverization: Effects of significantly exfoliated, unmodified graphite content on physical, mechanical and electrical properties 
(5525-5531) Polymer 51 #23 (2010)  Wakabayashi et al of Northwestern University found substantial graphite exfoliation at 0.3–2.7 wt% graphite  with less exfoliation at higher graphite concentrations. 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 processing.  (RDC 12/12/2010)