The relationship of electrical conductivity and percolation in composites of multiwall carbon nanotubes and thermoplastic polyurethane

Abstract

Abstract The relationship of electrical conductivity and percolation has been characterised in detail for multiwall carbon nanotubes (MWCNT) in thermoplastic polyurethane (TPU). Weight fractions of MWCNT of up to 5 wt% were studied. Ultrasonic dispersion of the MWCNT during synthesis has been shown to significantly improve homogeneity and percolation of the composite material as evidenced by electron microscopy. Extending ultrasonication time improves the homogeneity of the composite material. The percolation threshold for homogenous MWCNT/TPU material has been determined as 0.95 wt%. Observing surface charging effects with electron microscopy is suggested as a method to probe mesoscopic uniformity. Material uniformity also relates to the magnitude of the apparent conductivity exponent, when conductivity data are fitted with the percolation law. The integration of MWCNT in the polyurethane matrix has been depth-profiled with electron microscopy by varying the energy of the primary electrons. This has shown that many MWCNT curl up into horseshoe shapes. The observation challenges the common assumptions that MWCNT retain their high aspect ratio in the composite material and that they can be modelled as straight rods.