Polyethylene multiwalled carbon nanotube composites Tony McNally a, * , Petra Po ¨tschke b , Peter Halley c , Michael Murphy c , Darren Martin c , Steven E.J. Bell d , Gerard P. Brennan e , Daniel Bein f , Patrick Lemoine g , John Paul Quinn g a School of Mechanical and Manufacturing Engineering, Queen’s University Belfast, Belfast BT9 5AH, UK b Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany c Division of Chemical Engineering, The University of Queensland, Brisbane, Qld 4072, Australia d School of Chemistry, Queen’s University Belfast, Belfast, UK e School of Biology and Biochemistry, Queen’s University Belfast, Belfast, UK f School of Electrical and Electronic Engineering, Queen’s University Belfast, Belfast, UK g Nanotechnology Research Institute, University of Ulster, Jordanstown BT37 0QB, UK Received 7 February 2005; received in revised form 14 June 2005; accepted 22 June 2005 Available online 21 July 2005 Abstract Polyethylene (PE) multiwalled carbon nanotubes (MWCNTs) with weight fractions ranging from 0.1 to 10 wt% were prepared by melt blending using a mini-twin screw extruder. The morphology and degree of dispersion of the MWCNTs in the PE matrix at different length scales was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). Both individual and agglomerations of MWCNTs were evident. An up-shift of 17 cm K1 for the G band and the evolution of a shoulder to this peak were obtained in the Raman spectra of the nanocomposites, probably due to compressive forces exerted on the MWCNTs by PE chains and indicating intercalation of PE into the MWCNT bundles. The electrical conductivity and linear viscoelastic behaviour of these nanocomposites were investigated. A percolation threshold of about 7.5 wt% was obtained and the electrical conductivity of PE was increased significantly, by 16 orders of magnitude, from 10 K20 to 10 K4 S/cm. The storage modulus (G 0 ) versus frequency curves approached a plateau above the percolation threshold with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behaviour. The ultimate tensile strength and elongation at break of the nanocomposites decreased with addition of MWCNTs. The diminution of mechanical properties of the nanocomposites, though concomitant with a significant increase in electrical conductivity, implies the mechanism for mechanical reinforcement for PE/MWCNT composites is filler- matrix interfacial interactions and not filler percolation. The temperature of crystallisation (T c ) and fraction of PE that was crystalline (F c ) were modified by incorporating MWCNTs. The thermal decomposition temperature of PE was enhanced by 20 K on addition of 10 wt% MWCNT. q 2005 Elsevier Ltd. All rights reserved. Keywords: Polyethylene; Multiwalled carbon nanotubes; Nanocomposites 1. Introduction The identification in 1991 of carbon nanotubes (CNTs) by Iijima [1] has stimulated intense research interest in the structure [2–8], properties [9–13] and applications [14–17] of these unique materials. The intrinsic superconductivity [9], field emission behaviour [10], potential as molecular quantum wires [11], ability to store hydrogen [12], unusually high thermal conductivity [13], use as sensors for gas detection [16] and the biocompatibility and potential for biomolecular recognition [17] of carbon nanotubes has been reported. However, it is the combination of exceptional conductivity (electrical and thermal), low density and mechanical properties [16] of CNTs that has resulted in their use in filled composites. Both theoretical and experimental studies have shown CNTs to have extremely high tensile moduli (O1 TPa for single walled carbon nanotubes, SWCNTs) and tensile strengths of the order of 500 GPa [18,19]. Carbon nanotubes are thermally stable up to 2400 8C in vacuo, have a thermal conductivity about Polymer 46 (2005) 8222–8232 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.06.094 * Corresponding author. Tel.: C44 28902 74712; fax: C44 28906 61729. E-mail address: t.mcnally@qub.ac.uk (T. McNally).