Influence of the polymer structure and nanotube concentration on the conductivity and rheological properties of polyethylene/CNT composites

A series of multi-walled carbon nanotube /polyethylene (MWNT/PE) composites with several concentrations (0.5, 1, 2.5, 5, 7 wt%) of chemical vapour deposition (CVD)-grown carbon nanotubes (CNTs) have been investigated. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) have been chosen as matrix. The nanocomposites were prepared by melt mixing; a good dispersion in the matrix and a good CNT–polymer interfacial adhesion have been verified by scanning electron microscopy (SEM). In Raman measurements the characteristic bands of the MWNTs are shifted to slightly higher wavenumbers when increasing the CNT content in the nanocomposite, indicating an effective interaction between MWNTs and polymer matrix. Melt rheological investigations in dynamic mode indicate the dispersion degree and the percolation state of the CNTs within the polymer matrix. The rheological percolation threshold of the nanocomposites is between 1 and 2.5 wt%. For HDPE/CNT as well as for LDPE/CNT composites, we found a six orders of magnitude increase in electrical conductivity from 1 to 2.5 wt%, that is the same percolation threshold as determined by rheology. Below percolation threshold we found reproducible diodelike behaviour with different conductivity in forward and reverse bias direction for HDPE sample.