Electrical Conduction and Rheological Behaviour of Composites of Poly(Ε-Caprolactone) and MWCNTs

Two mechanisms of conduction were identified from temperature dependent (120 K - 340 K) DC electrical resistivity measurements of composites of poly(ε-caprolactone) (PCL) and multi-walled carbon nanotubes (MWCNTs). Activation of variable range hopping (VRH) occurred at lower temperatures than that for temperature fluctuation induced tunneling (TFIT). Experimental data was in good agreement with the VRH model in contrast to the TFIT model, where broadening of tunnel junctions and increasing electrical resistivity at T > Tg is a consequence of a large difference in the coefficients of thermal expansion of PCL and MWCNTs. A numerical model was developed to explain this behavior accounting for a thermal expansion effect by supposing the large increase in electrical resistivity corresponds to the larger relative deformation due to thermal expansion associated with disintegration of the conductive MWCNT network. MWCNTs had a significant nucleating effect on PCL resulting in increased PCL crystallinity and an electrically insulating layer between MWCNTs. The onset of rheological percolation at ~0.18 vol% MWCNTs was clearly evident as storage modulus, G' and complex viscosity, h* increased by several orders of magnitude. From Cole-Cole and Van Gurp Palmen plots, and extraction of crossover points (Gc) from overlaying plots of G' and G" as a function of frequency, the onset of rheological percolation at 0.18 vol% MWCNTs was confirmed, a similar MWCNT loading to that determined for electrical percolation