Because of the preferential orienta

Because of the preferential orientation of CNTs, the percolation thresholds of the same composites measured in the direction perpendicular to the CNT alignment was only 0.05 wt% or equivalent to 0.034 vol%, which is more than one order of magnitude higher than that in the parallel direction. Upon application of an electric field, the CNTs align immediately and attempt lateral migration to form bundles even at a very low content (Fig. 2g). This results in the formation of extensive conducting networks to reach percolation in the direction of alignment while the similar conducting networks are not formed yet in the direction perpendicular to it at the same CNT content. Apart from CNT content, there are other factors, including applied voltage, polymerization time, applied temperature and viscosity of resin, which are also responsible for the anisotropy in percolation [25]. It should be noted that the transverse migration only starts once the networks in the parallel direction are developed. As the CNT content increases, the degree of interconnection in the transverse direction increases (see Fig. 6b), leading to a diminishing difference in the absolute conductivities and mechanical properties measured in the two directions.