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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: Conductive TPU - CNT Composites for Strain Sensing

Author: ZHANG, Rui

Year: 2009

Supervisor(s): Ton Peijs, Mark Baxendale

This thesis aims to fabricate thermoplastic polyurethane/carbon nanotube (TPU/CNT) conductive polymer composite fibres and evaluate their strain sensing abilities. First, TPU/CNT composite films were produced by casting its solution. The conductive networks of the films were studied with percolation theory (pc ~0.35 wt.%), and the conduction mechanism was identified as fluctuation induced tunnelling (FIT). An exponential relationship between conductivity and strain was found, which was attributed to an increased tunnelling resistance originated from a strain induced inter-particle separation. A conductive yarn was then produced by coating a Spandex yarn with the aforementioned solution, showing similar strain sensing behaviour. The equivalent CNT loading in the conductive yarn can be as low as 0.015 wt.%. In order to explore the possibilities of industrial applications of these materials, efforts were then made to produce TPU/CNT conductive fibres with a melt-spinning process. A melt-spinning grade TPU was used as matrix and the dynamic percolation process in this system was studied with special attention. This process was described as a time-temperature dependent process controlled by the polymer viscosity. Based on this understanding, the melt-spinning parameters were optimised and highly conductive fibres were spun with CNT loading as low as 1 wt.%. It was also demonstrated that their conductivity can be successfully predicted. The strain sensing behaviours of the coated yarn and melt spun fibre were also studied, demonstrating the possibilities using these materials as flexible strain sensors in applications such as smart textiles.