School of Engineering and Materials Science
Research Student Awards
PhD Thesis: The Potential of Carbon Nanotubes in Polymer Composites
Author: CISELLI, Paola
Supervisor(s): Ton Peijs
In the present study, the real potential of CNTs as reinforcing fibres in polymer composites has been explored, with particular emphasis to oriented systems like tapes and fibres. In addition, the use of carbon nanotubes as conducting filler has been investigated, with special attention to the application as sensor materials. Ultra-high-molecular-weight polyethylene (UHMW-PE)/MWNTs composites have been prepared by a novel approach. The method involves the use of a mixture of solvents during the gelation process. The electrical properties of the as-prepared films and drawn tapes have been investigated. Above percolation, the conductivity was gradually decreased during the stretching process, which is responsible for a change in distribution and alignment of MWNTs. However, it is interesting to note that the conductivity at a draw ratio of 30 can still reach 10-4 S/m, probably because of the high aspect ratio of the MWNTs. This level is two orders of magnitude higher than the minimum required to provide electrostatic discharge, making these fibres interesting from a mechanical and electrical point of view. Although the prospect of creating the ultimate conductive unidirectional nano-composite fibre by combining high strength PE fibre with carbon nanotube is very intriguing, a simple composite model showed that at least 5 wt% SWNTs, homogeneously dispersed, fully aligned and with perfect matrix interaction, are needed to increase the strength of standard UHMW-PE fibre by 32%. Below this critical concentration, the contribution of the SWNTs to the composite fibre strength is only 35 GPa, which is fairly low compared to their theoretical strength of 100-150 GPa. The highly apolar character of a polyolefin combined with the inert nature of carbon nanotube make the task of obtaining homogeneous nano-composites at such high loading a real challenge and defeats to some extent the idea of nanocomposites where property improvement is sought at low loadings. In conclusion, carbon nanotubes can be used as fillers in UHMW-PE in order to create conductive films or fibres. However, the idea of using carbon nanotubes, especially MWNTs, as reinforcing nanofibres for UHMW-PE should be carefully reconsidered, since the ultimate mechanical properties might be compromised by the addition of carbon nanotubes to the perfect structure of the pure polymer fibre. A second system has been investigated, poly(vinyl alcohol) (PVA)/SWNTs nanocomposites. They were prepared by a mild processing route, involving the use of an ionic surfactant, sodium dodecyl sulfate (SDS) in aqueous solutions. In order to evaluate the influence of this surfactant on the mechanical properties of the nano-composites, this system was compared with a surfactant-free system based on the use of an organic solvent, dimethyl sulfoxide (DMSO). In contrast to the results found for UHMW-PE, significant enhancements of mechanical properties have been found for both PVA based systems at low nanotube loading. Micromechanical analysis showed that the nanotubes contribution to the nanocomposite strength was as high as 68 GPa for the aqueous SDS based system and 75 GPa for the DMSO system. These values are very high when compared to most other data reported in literature and starts to exploit the theoretical strength of the CNTs. The systems were very similar in terms of dispersion, stress transfer and mechanical behaviour. However, X-ray studies demonstrated that the two systems were very different in terms of the effect that nanotubes had on the polymer morphology. SWNTs were found to change the pre-orientation of the crystals in the cast films in the aqueous system, while they had no effect in the DMSO system. This result is very important for the assessment of the true reinforcing efficiency of the nanotubes. While the improvement of the mechanical properties in the aqueous system can be ascribed to some extent to the change in initial morphology of the polymer, the enhanced mechanical properties in the DMSO system can be fully attributed to the true reinforcement of nanotubes within the polymer matrix. This result highlights that it is possible to exploit the great potential of carbon nanotubes in terms of mechanical reinforcement. However, it also shows that it is of paramount importance to assess the effect of nanotubes on polymer morphology in order to establish whether true reinforcement is present. Finally, the effect of incorporating MWNTs in an elastomer, ethylene-propylene-dienemonomer (EPDM) has been investigated. The main focus of the study was on the electrical behaviour of the nanocomposites, in view of possible sensor applications. The percolation threshold has been determined and was consistent with the predictions of Munson-McGee for a system of conductive fillers of aspect ratio between 40 and 130. A linear relation has been found between conductivity and strain up to 10% strain. This could be used for applications such as sensor materials. Cyclic experiments were conducted to establish whether the linear relation was reversible, which is an important requirement for sensor materials. These measurements showed that the change in conductivity presents a reversible part and an irreversible one. A similar trend was previously reported for short carbon fibre filled epoxy and was attributed to damage (the irreversible part) and piezoresistivity (the reversible part). An analogous conclusion can be drawn in this case, where the reversible part could be due to piezoresistivity and the irreversible part to damage. Although this behaviour is detrimental for the use as sensors, the experiments also showed that the material stabilises during the measurement, when the change in normalised conductivity becomes more repeatable. Although further optimisation is needed, these initial results showed that CNTs have great potential for the creation of conductive sensor fibres that can be used in future smart textile applications.
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