(629b) Polymer-Graphite and Polymer-Carbon Nanotube Naoncomposites Processed by Solid-State Shear Pulverization: Major Enhancements in Mechanical Properties, Thermal Stability, Crystallizability, and Electrical Conductivity

A continuous, industrially scalable process method called solid-state shear pulverization (SSSP) is shown to be effective in producing well-dispersed polypropylene (PP)/ carbon nanotube and PP/ as-received graphite nanocomposites that cannot be obtained by conventional melt processing alone. With SSSP, the polymer and nanofiller mixture experience high shear and compressive forces exceeding those normally found in conventional melt processing. The SSSP processing leads to energy storage in the material which is relieved by the creation of new surfaces via fragmentation and/or delamination of the material. Repeated fusion and fragmentation steps during SSSP results in high levels of nanofiller exfoliation/dispersion. In some cases, the excellent exfoliation/disperson of nanofiller accompanies processing by SSSP only; in other cases excellent exfoliation/dispersion is achieved by a two-step process of SSSP followed by melt mixing.

Major improvements in Young's modulus (> 100% increase) and yield strength (65% increase) are obtained in the nanocomposites made by SSSP, with the maximum increases being observed in samples containing the highest concentrations of nanofiller that are well-dispersed. In contrast, electrical conductivity is correlated to dispersion of carbon-based nanofiller in a different manner, with conductivity increasing with increasing nanofiller content even when an increase in nanofiller content results in less effective dispersion or exfoliation. Values of electrical conductivity on the order of 0.2 S/m have been achieved at 8.4 wt% graphite content after SSSP processing; this conductivity can be eliminated by use of melt processing after SSSP when the melt processing leads to high levels of nanofiller orientiation and a loss of the percolated state of the nanofiller. Thermal degradation temperatures are strongly correlated to dispersion of carbon-based nanofiller, with degradation temperatures (@ 1 wt% loss in nitrogen) in nanocomposites containing 1 wt% nanofiller increasing by as much as ~40 K relative to neat PP. The carbon-based nanofillers also lead to major increases in PP crystallization rate.