NSTI Nanotech 2009

Potential of Nanoscale Carbon Fibre Reinforcements for High Performance Semicrystalline Polymers

G. Broza, K. Schulte
Technical University Hamburg, DE

Keywords: carbon nanotubes, semicrystalline polymers, nanocomposites

Abstract:

Nanoscale carbon fibres CNFs, supplied by Applied Sciences PyrografTM III Carbon Nanofibres, USA, are a new class of material with a potential as reinforcement in polymer composites.Typically, they are hundreds of microns long and only 100 to 400 nm in diameter. They are much smaller than conventional carbon fibres. The CNFs are produced by a catalysed pyrolytic reaction involving hydrocarbon gases and metallic catalyst particles. The partly show a bamboo structures and are curved and graphitic with d-spacing of 3.4 A. The combination of the CNFs with known polymer matrix systems is a novel step in composites technology. The fundamental understanding necessary on the atomic scale will further enhance the exchange of materials for high performance fibre-reinforced semi-crystalline polymers. Ultra-thin films of polymers poly(1-butene), polypropylene and polyethylene including nanoscale carbon fibres are prepared according to the method of Petermann and Gohil: polymers with CNF are dissolved in xylene (0.5 %). Some droplets of this solution are then deposited on the smooth surface of a glass slide where the solution itself disperses uniformly. When heating the sample to a temperature only a few degrees below the melting temperature of the polymer, the solvent evaporates. From the resulting melt a highly oriented ultra thin film is drawn by a motor driven cylinder. The dispersion process developed in this project leads to well-separated carbon nanofibres in an uniaxially oriented semi-crystalline poly(1-butene), polypropylene and polyethylene film. It is the purpose of this paper to investigate whether the lattice orientation and spacing of the carbon nanofibres influence the uniaxially oriented polymer matrix. Additionally, the influence of the chemical nature of the polymeric matrix and their lattice structure on the dispersion of carbon nanofibres is investigated by many different drawing temperatures during melt-spinning of the uniaxially oriented semi-crystalline matrix. The crystallisation behaviour and morphology of oriented polymer composites with CNFs has been investigated by transmision electron microscopy and by light microscopy. In the interface between polymer and CNF surface crystals act as nucleation centres for the high density polymer matrix. After crystallisation from the melt and independent of air-cooling a transcrystalline layer was found with lamellar crystal grown the perpendicular to the CNF fibre axis. The comparison of the results observed by transmision electron microscopy yield to new insights on the interface morphology on uniaxially oriented semi-crystalline polymer composites and the architecture of the transcrystalline layer. The transcrystalline layers only reflect the morphology of the matrix crystallised under certain conditions and without interaction with the nanoscale carbon fiber. The surface crystals of the fibres act as nucleation centres for the matrix, and may result from uniaxial oriented crystallisation of polymer. The mechanical properties of the nanocomposites are currently investigated. The tests include the characterisation of the tensile, compressive and toughness properties. Furthermore, the electrical properties of these films are investigated by AC impedance spectroscopy. Due to the high aspect ratio of the tube-like filler the filler weight fraction necessary to form a three-dimensional conductive path throughout the matrix should be reduced compared to dispersing a particulate filler such as carbon black.
 
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