Rationale for the dispersion method

CNTs were naturally entangled and efforts were required to debundle and stabilize them into homogeneous suspensions prior to further treatments. As both the chemical functionalisation of the CNTs and very harsh sonication were initially excluded to avoid damaging the CNTs, only pure solvents or surfactant/solvent solutions were investigated as dispersion media.

Previous studies [234, 235, 238, 248, 337] showed that, although efficient in dispersing CNTs [338, 339], surfactant molecules used to disperse CNTs tended to stick very well onto CNT surface thus making their removal and cleaning complicated. Tests were performed on CNTs dispersed in propan-2-ol with 0.1 and 0.5 % of Trimeric Nonylphenol Polyoxyethylene- ether (TNP) and it was shown that even after 3 back-washings the contact angle with deionised water was still lower by 10-15 degrees (i.e. more hydrophilic) than that of the CNTs dispersed into pure propan-2-ol. Since the surface energy of the membrane was critical and needed to be absolutely reproducible, the use of surfactant was also avoided.

For these reasons a method involving low CNT concentrations, sonication and temperature was initially adopted.

3.3.1 Dispersion of the CCI CNTs

A study undertaken by Dr. Dong Wook Chae, former post-doctoral fellow at the CSIRO MSE was focused on a new dispersion mechanism to produce long term stability CCI suspension. This work has not published yet but should be submitted very soon. To facilitate the current work, CCI CNTs pre-dispersed by this method were used as a starting material.

The process involved the dispersion of the CCI CNTs under high shearing stress in a viscous media. The CNTs were blended with a poly(saccharide) based solution and mixed to be mechanically dispersed. After mixing, the solution was first diluted with hot deionised water and stirred. The CNTs were filtered and collected through a cellulose filter media and rinsed with 60 ºC deionised water to remove the remaining sugar. Several of

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these rinsing steps were necessary until all the poly(saccharide) was removed and clear permeate obtained by filtration across the CNT cake. Once cleaned the CNTs were easily redispersed into alcohol based solutions.

This dispersion method yielded highly dispersed and stable solutions that could last for several months. However, the high shear forces involved heavily damaged and shortened the CNTs. Unfortunately no other dispersion method was found to provide sufficient energy to break apart the CCI CNTs bundles without damaging the CNT walls and thus creating defects in their structures as shown in Figure 3-5. Amongst the defects formed during this process, collapsed walls, presence of amorphous carbon and CNT cleavage were commonly found. For this reason CCI CNTs were not used extensively for BP production, but only used for comparisons and in mixtures with the CVD CNTs (Chapter 4).

Figure 3-5 TEM of CCI CNTs after high shearing dispersion

3.3.2 Dispersion of CVD CNTs

3.3.2.1 Effect of different solvents

As shown in Appendix 1, CNTs do behave very differently depending on the polarity, volatility and surface tension of the solvent. Exposed to the right kind of solvent CNT forests can for example form fractal patterns of very precise geometries. The choice of a suitable solvent for the CNT dispersion will rely on its affinity with the CNTs and on the stability of the CNT/solvent interactions.

Several solvents were first tested and mixed with small amounts of CNTs. Figure 3-6 shows CNT-solvent solutions composed of ~ 10 ml of

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solvent and a ~0.05 mg of CVD CNTs. The solvents used are listed in Figure 3-6. The CNTs were dispersed by first shaking by hand the flasks and followed by two sonication sessions at 30ºC for 5 min. As shown in Figure 3-6 the degree of dispersability varied greatly depending on the solvent used. The size and shape of bundles are clearly different and vary from a low density ‘cloud’ of CNT for propan-2-ol to small and dense bundles in deionised water.

The stability of the CNT bundles was also tested in the different solvents after mild sonication (150 W for 10 min at 20 ºC). No major change in the visual classification was found even after several cycles. Other parameters such as the solvent price, toxicity or volatility were considered. Propan-2-ol was found to be a cheap, flammable but reasonably safe and non toxic solvent. It was also available in reasonable quantities in the laboratory which was critical to disperse large quantities of CNTs.

For these reasons pure propan-2-ol was preferred and will be the only solvent used in the following work.

Figure 3-6 Dispersion of CVD CNTs into different solvents in a stable state

3.3.2.2 Dispersion protocol

Amongst the most efficient and convenient techniques, sonication in solution has been shown to produce reproducible suspensions. However, the sonication method and intensity have to be carefully chosen to avoid damaging the CNT walls and shortening them [340]. During sonication of CNTs in solution, the liquid jet streams resulting from ultrasonic cavitation, overcome the bonding forces between the nanotubes, and separate the tubes [248, 341, 342].

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The work in the next section was undertaken in order to find an appropriate method to qualitatively assess the CNTs dispersion state. A study to characterize the CNT bundle size distribution would have been interesting but was not pursued as it did not directly fall within the scope of this PhD. Figure 3-7 shows an example of CVD CNT dispersed by horn sonication.

The intensity used (given in the figure) is critical and the debundling effectiveness clearly improves with increasing energy. Acceptable dispersion states were obtained for the CVD CNTs by horn sonication after only 15 to 30 s of treatment. The bundles were clearly broken and solutions were this way stable for a few minutes at room temperature.

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Figure 3-7 Example of the horn sonication efficiency at various powers. Nominal power was 150 W = 100%. The time is given in seconds of sonication. The solutions were initially at room temperature.

However as shown in Figure 3-8, horn sonication did lead to damaged CNTs. The TEM micrographs show how CNTs walls were broken and defects induced into the CNT structure. This was not desired as any defects in the CNT structure, as shown in the next chapter was a possible weak point in the structure. It could induce both mechanical defects in the BPs and wetting of the surface and pores. For this reason a bath sonicator was preferred (Figure 3-9) as it induced lower degree of damage to the CNTs. However, as the horn sonicator efficiency was higher, it was used to pre-disperse some highly entangled batches which will be specified when used in this work.

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Figure 3-8 TEM of CVD CNTs after horn sonication

Figure 3-9 Example of solution sonicated in the bath sonicator. The bath is composed of deionised water

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