Figure 6. Scanning electron micrographs of the epoxy matrix composite materials reinforced with multi-walled carbon nanotubes.
The effect of carbon nanotube weight fraction on the volume resistivity of the epoxy matrix composite materials is illustrated in Figure 7 which are reinforced with multi-walled carbon nanotubes. For the highly dispersed structure and the partially agglomerated structure, the electrical percolation threshold occurs at a very low concentration. The percolation threshold for conductive particles embedded in an insulating polymer matrix is sensitive to the structure of the reinforcement, and the decrease in electrical resistivity with an increase in reinforcement content is attributed to the probability of reinforcement contact. For carbon nanotube composites that have large carbon nanotube aspect ratios, extremely low percolation thresholds can be achieved. The low percolation threshold of the polymer matrix nanocomposites indicates that the relatively large aspect ratios of the carbon nanotubes are likely maintained during processing and the multi-walled carbon nanotubes form a percolating network throughout the polymer matrix. Electrical properties are sensitive to local statistical perturbations in the microstructure that create a conducting path for transport. For the partially agglomerated structure the mean values of electrical resistivity are higher than for the more highly dispersed structure, particularly at lower fractions of multi-walled carbon nanotubes. Because of local agglomeration of carbon nanotubes, the statistical fraction of carbon nanotubes participating in conductive percolation is lower, resulting in higher volume resistivity. A homogeneous dispersion of the carbon nanotubes in a polymer matrix results in an improved stiffness, strength, and conductivity of the final polymer nano-composites. The process provides carbon nanotubes reinforced polymers having a percolation threshold at significantly lower loading of the carbon nanotubes compared to other mixing techniques. With percolation threshold is meant the amount of carbon nanotube needed to retrieve a significant increase in the electrical conductivity of the polymer nano-composites. However, there is a continuous need to improve the conductivity of polymers or to lower the percolation threshold of polymer nano-composites without being confronted with the above disadvantages.