Exfoliated CNTs

Surfactants are therefore effective solubilizers that can exfoliate CNTs by physical adsorption, which occurs at interfaces, allowing self-assembling into supramolecular structures [50]. 

The last main route to produce CNT/polymer composites with well-dispersed CNTs is based on the use of a third component, assisting the optimum incorporation of exfoliated CNTs into the polymer matrix, preferably without altering the intrinsic properties of the CNTs. In most of the cases reported in literature, this third component is a surfactant, but it can also be a conductive polymer, as exemplified in the next paragraph. 

Z.5.3.4 Use of surfactant spraying of exfoliated CNTs on polymer powder... 

Use of surfactant mixing of exfoliated CNTs with pre-made polymer latex... 

The resulting aqueous, surfactant-stabilized CNT suspension was then centrifuged to remove catalyst particles and large, non-exfoliated CNT bundles, which were possibly still present in the aqueous dispersion. After centrifugation, the solution separated into a solid precipitate and an ink-like supernatant, which mostly contains exfoliated CNTs or very small CNT bundles consisting mostly of two or three tubes. This supernatant was then mixed with latex particles [see Figure 2.13],... 

It is worth mentioning that the described concept used to monitor the debundling of CNTs, i.e., taking the absorbance value of the UV-Vis spectrum of a CNT dispersion as a direct measure for the concentration of individual exfoliated CNTs, can also be applied... 

Consequently, stopping the sonication when the maximum amount of exfoliable CNTs has been unbundled [e.g., at 100,000 J for HiPCO SWCNTs] should prevent further cutting of the SWCNTs, though SWCNT surface damage may not be particularly minimized. 

The surfaces of the potentially exfoliable CNTs are presumably completely covered with surfactant molecules. If that is the case, then one would expect a linear relation between the plateau of the UV absorbance and the SDS concentration till all CNTs are exfoliated, after which the absorbance should remain constant. Since SDS is inactive in the UV-Vis region, CNT dispersions having a different excess of surfactant should all exhibit the same absorbance, as soon as the maximum debundling is reached. This seems to be roughly the case, as shown in Figure 3.9. It is clear that the minimum SDS concentration necessary to stabilize all the CNTs that can potentially be exfoliated is about 0.17 wt% [or 5.88 mol SDS/m ater) for 0-1 wt% of CNTs. Consequently, the wt% ratio surfactant over CNT concentration at full coverage is about 1.7. 

For the production of nanocomposites with the latex-based concept, it is preferable to be able to work at (relatively] high concentrations of exfoliated CNTs for reproducibility purposes (in order, for example, to be able to prepare a full series of nanocomposites of different CNT loadings by diluting the same surfactant-CNT dispersion], as well as for commercial reasons. 

At the end of the debundling process, aqueous surfactant-CNT dispersions not only contain dispersed and mostly exfoliated CNTs, covered by surfactant molecules and detectable by UV-Vis spectroscopy. Also present are carboneous impurities such as graphene nanoparticles or amorphous carbon, catalyst particles, and possibly some bundles of CNTs that were not exfoliated. 

Centrifugation leads to the preferential removal of the densest constituents of the multi-component systems constituted by aqueous surfactant-CNT dispersions. The nature and the quantity of species removed are strongly related to the CNT type and batch [as already illustrated in the previous paragraph] and to the surfactant types. Regarding this last point, it has been foreseen and/or demonstrated many times that all surfactants do not necessary have the same ability to exfoliate CNTs [see refs. 13, 14, 189, among others]. In particular. Tan and Resasco developed a method based on UV-Vis spectroscopy for quantifying the [individual CNT]/[CNT bundle] ratio. This method leads to the ranking of the relative ability of surfactants to exfoliate SWCNTs. 

One synthesis approach that does not rely on CNT formation from the gas phase is molten salt synthesis. The reactor consists of a vertically oriented quartz tube that contains two graphite electrodes (i.e. anode is also the crucible) and is filled with ionic salts (e.g. LiCl or LiBr). An external furnace keeps the temperature at around 600 °C, which leads to the melting of the salt. Upon applying an electric field the ions penetrate and exfoliate the graphite cathode, producing graphene-type sheets that wrap up into CNTs on the cathode surface. Subsequently, the reactor is allowed to cool down, washed with water, and nanocarbon materials are extracted with toluene [83]. This process typically yields 20-30 % MWCNTs of low purity. 

One final example worth mentioning is the reductive alkylation/arylation with lithium and alkyl/aryl halides in liquid ammonia. This is a two-step process in which negatively charged nanotubes are formed via electron transfer from the metal. This step is relatively easy and fast due to the CNTs electron sink properties, and it enables exfoliation of the tubes through electrostatic repulsion in the second stage, the alkyl/aryl halides react with the charged tubes to form a radical anion which can dissociate into the alkyl radical and the halide anion, with the former species undergoing addition to the CNT sidewalls [42]. 

The majority of studies have used surfactants that wrap around nanocarbons via van der Waals interactions [37]. For instance, surfactants such as sodium dodecylsulfate (SDS) are commonly used to disperse CNTs in aqueous solutions [38,39] while other surfactants, such as Pluorinc-123, are used to mechanically exfoliate graphene from graphite flakes (Fig. 5.4(a)) [40,41]. The polar head group of the surfactant can be used to further hybridize the nanocarbon via a range of covalent or noncovalent interactions [42]. For example, nanoparticles of Pt [43,44] and Pd [45] have been decorated onto SDS-wrapped MWCNTs. Similarly, Whitsitt et al. evaluated various surfactants for their ability to facilitate the deposition of Si02 NPs onto SWCNTs [46,47]. As an exam-... 

There are two types of surface modifications for CNTs noncovalent interactions (Figure 6.54) and covalent sidewall (Figure 6.55) or defect-sitet (Figure 6.56) functionalization.Both methods, as well as physical techniques such as sonica-tion, are successful in separating individual SWNTs from bundles - a process known as exfoliation. In general, it is most desirable to incorporate isolated SWNTs in a composite rather than bundles, since the latter features poor intertube interactions resulting in a lower overall strength - especially at low CNT concentrations. 

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