Investigation of multi-walled carbon nanotube-reinforced high-density polyethylene/carbon black nanocomposites using electrical, DSC and positron lifetime spectroscopy techniques

Background: The positive temperature coefficient (PTC) effect on material properties has attracted much attention in recent years due to the prospects of many applications like temperature sensors, thermistors, self-regulating heaters, etc. It has been suggested that incorporation ofmulti-walled carbon nanotubes (MWNTs) into carbon black (CB)-filled polymers could improve the electrical properties of composites due to high conductivity and network structure and significantly reduce the required CB loading. Results: We observed no change in melting temperature and crystalline transition temperature on addition of MWNTs. However, the heat of fusion decreases as the amount of conducting carboxylated MWNT (c-MWNT) filler increases and the resistivity of the composite decreases. The free volume shows an increase up to 1.5 wt‰ of c-MWNT content and then decreases. Conclusion: Well-developed crystals could not be formed due to restricted chain mobility as filler content increases. This results in minimum intermolecular interactions, and thus a decreased heat of fusion. A composite with c-MWNT content of 0.5 wt% showed the highest PTC and higher resistivity at 150 °C possibly due to the formation of flocculated structures at elevated temperature. For filler content greater than 1.5 wt‰, the decrease in free volume may be due to restricted chain mobility.