Electrical conductivity of PUR/MWCNT nanocomposites in the molten state, during crystallization and in the solid state

The electrical conductivity of a nanocomposite constituted of multiwalled carbon nanotubes (MWCNT) dispersed in a semicrystalline polyurethane matrix, is investigated during cooling from the melt to the solid state. The same percolation threshold, ϕc = 0.85 wt.%, is obtained in the molten and in the solid state, although the exponent t of the percolation equation is significantly higher in the solid state. A remarkable increase of the conductivity during crystallization is observed for nanocomposites of MWCNT content above ϕc, but for contents below ϕc the conductivity decreases. Combined conductivity and PVT results, lead us to discard the hypothesis of an increase of the density of the conductive network (associated with volume shrinkage) as being the cause of the conductivity enhancement during crystallization. Instead, the analysis of the parameters of the percolation equation for the molten and the solid state, suggests a transition from a less effective conductive network to a more performing three dimensional network.