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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Multi-wall Carbon Nanotube - Polypropylene Composites and Tapes

Author: DENG, Hua

Year: 2008

Supervisor(s): Ton Peijs

In the current study, the potential of multi-wall carbon nanotubes (MWNTs) in polypropylene (PP) matrices has been investigated. Research has been focused on the mechanical and electrical properties of isotropic composites and oriented tapes. PP/MWNT composites were prepared through melt compounding and hot-pressing method. These isotropic composite films were then drawn in the solid state into highly oriented tapes where MWNTs and polymer chains are aligned. Mechanical property studies showed that a high reinforcing efficiency can be obtained at relative low draw ratios (<10), providing good alignment, low filler content (0.25 wt.%), and good dispersion. In order to fabricate conductive polymer tapes, larger amounts of MWNTs are needed to form a conductive percolating network (up to 5.3 wt.%) in the composites. Next to MWNTs, carbon black is used as a conductive filler to draw a comparison between carbon nanofillers of different aspect ratio. It was found that solid state drawing destroys the conductive network, leading to an increase in resistivity with draw ratio. In order to counteract the decrease in conductivity in drawn tapes and with this the increase in percolation threshold, a new concept is introduced to create multi-functional oriented polymer nanocomposite tapes (or fibres) that combine high stiffness and strength with good electrical properties at a low percolation threshold. The concept is based on a bicomponent construction
consisting of a highly oriented polymer core together with a skin based on a conductive polymer composite with a lower melting temperature than the core, enabling thermal annealing of the skin to improve conductivity through a kinetic reagglomeration process while retaining the properties of the core and hence that of the tape or fibre. The annealing is found to recover the conductive network in the skin layers which are destroyed by solid state drawing, leading to exceptionally low percolation thresholds for highly drawn fibres or tape. The low percolation threshold is the result of interesting nanotube morphologies, which involved oriented MWNT bundles that form lateral connections between bundles through ‘hairy’ nanotube bundles. In-line electrical measurement show a time and temperature dependence of the network formation during annealing. X-ray and mechanical property studies confirmed that polymer orientation is retained in the core while the orientation is lost in skin layers. A new concept of producing high strength conductive polymer tape/fibre is achieved.

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