(165b) Novel Nanocomposites Made from Polymer and as-Received, Unmodified Graphite: Effects of Graphite Level and Dispersion on Mechanical and Electrical Properties, Crystallization, and Thermal Stability

Many hundreds of experimental studies have been published related to polymer-carbon nanotube nanocomposites or hybrids. In contrast, only a relatively small number of experimental studies have been published related to polymer-graphite nanocomposites or hybrids, with almost all of those focused on chemically modified or thermally pre-treated graphite, e.g., graphite oxide, expanded graphite, or thermally exfoliated graphite oxide. This is because effective dispersion or exfoliation of as-received, unmodified graphite is practically impossible with conventional melt processing. Even with pre-treatment, nanocomposite production by conventional processing is challenging due to thermodynamic and/or kinetic limitations, sometimes leading to limited property enhancement. However, relative to carbon nanotubes and modifed graphite, there are significant economic and environmental advantages associated with using the natural, as-received, unmodified graphite as a nanofiller.

To overcome the challenges associated with using graphite as a nanofiller, we have employed solid-state shear pulverization (SSSP) to produce polymer-graphite nanocomposites that are not subject to the thermodynamic/kinetic limitations associated with conventional processes. We have demonstrated that the continuous, scalable SSSP process can result in polypropylene (PP) nanocomposites containing well-dispersed unmodified, as-received graphite at levels up to 2.5 - 3.0 wt% graphite. When PP/graphite (2.5 wt%) nanocomposites are made by SSSP, the resulting materials exhibit a 100% increase in Young's modulus and a ~60% increase in yield strength in comparison with neat PP. Even at extremely low levels of graphite (0.3 wt%), the resulting PP/graphite nanocomposites made by SSSP can exhibit extroardinary enhancements in mechanical properties relative to neat PP, including a 50% increase in Young's modulus and 45% increase in yield strength with little to no reduction in impact strength or elongation at break. However, we have found that increasing the graphite to levels well in excess of 2.5 - 3.0 wt% can lead to poor exfoliation of the graphite via SSSP, with only improvements in Young's modulus and yield strength being no better than with 0.3 wt% graphite and with substantial embrittlement of the nanocomposite.

Measurements of the shear storage moduli of our resulting PP/graphite nanocomposites reveal a solid-like rheological response at low frequency with graphite levels of 2.5 wt% or greater and a liquid-like rheological response with graphite levels of 0.3 - 1.0 wt%. These results indicate the presence of a network-like percolated structure at graphite levels of 2.5 wt% graphite and greater, which is accordance with our measurements of electrical conductivity which indicate a percolation threshold between 1.0 wt% and 2.5 wt% graphite.

Besides mechanical properties and electrical conductivity, we will also describe the effects of graphite level and dispersion on the crystallization behavior and thermal degradation of PP.