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Carbon Nanotube Dispersions

The major roadblock to achieving progress in using Single Walled Carbon Nanotubes (SWNTs) to produce high performance materials has been the difficulty of arraying SWNTs into ordered macroscopic samples. Recent major advances indicate that macroscopic objects of SWNTs can be produced by processing SWNTs in the liquid state. By using a variety of means such as stabilization by surfactants, or by dissolution in superacids, pristine SWNTs have been dispersed at sufficient concentrations to enable the extrusion of continuous, well-aligned, macroscopic fibres. Similarly, films of SWNTs have been produced by coating and spraying processes. A key determinant of the properties of liquid-spun fibres and films is the extent of alignment of SWNTs in the dispersion induced by the processing. This aspect is related to the rheological properties of SWNT dispersions, particularly the coupling between rheological properties and the flow-induced evolution of the microstructure. Because extrusion and coating flows are extension-dominated, understanding the extensional rheology of SWNT dispersions is fundamental to controlling the liquid state processing of SWNTs.

Fig 1. Postulated Phase Diagram for SWNTs in acids (image kind courtesy of Matteo Pasquali).

The present study is aimed at characterizing the extensional behaviour of SWNT dispersions using the filament stretching rheometer. Currently, published measurements of the extensional behaviour of rodlike polymer solutions are scanty, except for a recent publication by Sridhar’s group on the uniaxial extensional flow of non-Brownian fibre suspensions. Efforts so far on characterizing the shear rheology of SWNT dispersions have focused on the relationship between the intrinsic shear viscosity and mean carbon nanotube length. Pure SWNTs prepared by the High-Pressure Carbon Monoxide (HiPCO) process have been shown to have a length in the range of 500 to 700 nm, which has been corroborated independently by Atomic Force microscopy experiments. In the present work, various concentrations of SWNTs have been dispersed in a range of surfactant solutions consisting of pluronic F68 in water, or mixtures of pluronic F68 and polyethylene glycol (PEG). The viscosity of the solvent was tailored to achieve an extensional flow of the uniformly dispersed SWNT dispersion. At the dispersion concentrations that could be achieved, however, reliable experiments could only be performed at extension rates at which the high strain rate asymptotic extensional viscosity, which is independent of the imposed strain rate and where Brownian forces are likely to be negligible, was measured. We show that identical extensional viscosities are measured, independent of the particular solvent used, provided the dispersion concentration and the viscosity ratio of solution to solvent is the same. An estimate of the mean length of the SWNTs obtained by fitting the steady state extensional viscosity to Batchelor’s theory for the extensional viscosity of rods is shown to lead to values that are in excellent agreement with previous measurements through shear rheology and atomic force microscopy.

Collaborators

1. T. Sridhar, Monash University
2. M. Pasquali, Rice University

Funding

1. Engineering Faculty Grants Scheme, Monash University
2. Australian Research Council, Discovery Grants Scheme

Conference Presentations and Publications on this topic

1. D. A. Nguyen, B. Dan, N. Parra-Vasquez, M. Pasquali, J. R. Prakash, T. Sridhar, Extensional Rheology of Single Walled Carbon Nanotubes in Liquids, Fifteenth International Congress on Rheology, Monterey, USA, 3 Aug. - 8 Aug., 2008.