THE USE OF SUPERCRITICAL CARBON DIOXIDE TO PRODUCE POLYCARBONATE/NANOTUBE COMPOSITES WITH IMPROVED ELECTRICAL CONDUCTIVITY

摘要

Supercritical carbon dioxide (scCO_2) treatment of carbon nanotubes (CNT) has led to improved properties of thermoplastic nanocomposites with minimal damage to the CNT aspect ratio. In this work, the supercritical carbon dioxide treatment of CNT was optimized to enhance the bundle separation by using fundamental thermodynamic principles. Furthermore, the scCO_2 method was applied to the previously developed scCO_2 aided melt blending method to form polycarbonate/ nanotube composites with different degrees of nanotube separation. Bulk density measurements and transmission electron microscopy (TEM) were conducted to investigate the effect of different processing conditions on the separation of nanotube bundles and ensure that the nanotubes destruction was minimized. Conductivity was measured to examine the effect of the optimized scCO_2 processing on the final composite properties. It was found that an optimal expansion was obtained at soak temperatures and pressures corresponding to a CO_2 fluid density of ～730 kg/m~3. This resulted in an expansion factor of ～12 fold by volume compared to pristine nanotubes. The composite prepared with 3 wt % nanotubes at the optimal degree of nanotube separation showed a surface conductivity of 5.71 × 10~(-5) Siemens (S), compared to 9.16 × 10~(-6) S without scCO_2 treatment. It was found that beyond a critical degree of nanotube separation (expansion factor of ～5 fold), conductivity failed to improve appreciably. Improved mixing techniques may result in continuing improvement in conductivity when combined with the scCO_2 treatment.