Effects of sonochemical modification of carbon nanotubes on electrical and electromagnetic shielding properties of epoxy composites

Electrical and electromagnetic interference shielding properties of epoxy/multi-walled carbon nanotube (MWCNTs) composites are studied for low nanotube contents from 0.03 to 0.3 wt.% and different processing conditions. In situ chemical modification of carbon nanotube surfaces was achieved by ultrasonic irradiation of carbon nanotubes either in polyethylene polyamine or in liquid epoxy resin, resulting in amine-grafted or epoxy-grafted composites, respectively. Raman, TGA, and SEM analyses indicate a successful grafting of polymer chains to the nanotube surfaces. The amine-grafted MWCNT/epoxy composites show the presence of a thick polymer layer that has formed an extra phase on the carbon nanotube walls, which affects significantly the electrical conductivity and radio frequency response properties. Hence, the amine-grafted composites demonstrate a slightly higher percolation threshold (pc = 0.08 wt.%) and, at the same time, a lower absolute values of dc-conductivity as compared to that of the epoxy-grafted composites, with pc = 0.05 wt.%. In the radio frequency range, almost 3 orders of magnitude rise of the values of ac-conductivity was observed for small contents of the amine-grafted MWCNTs in epoxy. The epoxy-grafted composites could be proposed for producing effective antistatic and electrostatic dissipation coatings. Around and above the percolation threshold, the amine-grafting is critical in the radio frequency range for producing coatings with large dielectric losses. The absolute values of microwave attenuation were found to be independent of the surface modification procedure, but strongly dependent on the concentration of MWCNTs and the thickness of the composite layer.