HiPCO CNT dispersions

Figure 3.4 Evolution of the UV-Vis spectra of aqueous SDS-HiPCO CNT dispersions plotted as a function of sonication time. The first measurement [spectrum (aj, 0 Joule) was performed before the beginning of the sonication. (b) A sample sonicated for 10 min at 20 W. (c) A sample sonicated for 40 min, which was imaged by cryo-TEM (see Figure 3.2a). The fourth spectrum (d) corresponds to a 130 min-sonicated CNT solution, which was also studied with cryo-TEM (see Figure 3.2b).

Figure 3.8 shows the evolution of the value of the absorbance at 272 nm for several HiPCO CNT dispersions with different SDS concentrations. Vigolo et al. have already observed that if the initial amount of surfactant is too low to stabilize all the CNTs that can be potentially exfoliated, only a fraction of these CNTs is finally effectively exfoliated. The results of Grossiord et al. are in line with this observation the value of the UV-Vis absorbance is directly proportional to the concentration of absorbing species in solution, l.e., of individual CNTs that have been peeled off from the CNT bundles. The plateau value of the UV-Vis absorbance becomes higher with increasing SDS concentrations. As soon as there are enough surfactant molecules present in the system to stabilize the free dispersion of all the CNTs that can potentially be exfoliated, the plateau value of the UV-Vis absorbance reaches a maximum, and its value does not increase further upon further increase of the SDS concentration. 

Figure 3.8 Evolution of the absorbance at 272 nm for aqueous 0.1 wt% HiPCO CNT dispersions, containing different SDS concentrations. All samples were diluted 150 times to ensure that all the absorbance values at 272 nm remain lower than 1, which implies a reduction of the error introduced by...

Figure 3.9 UV-Vis absorbance at 272 nm, at the end of ultrasonication-induced debundling of HiPCO CNT dispersions containing different CNT concentrations as a function of the ratio of the SDS concentration and the SWCNT concentration.

Figure 3.13 Evolution of the value of the absorbance of the peak located at 272 nm for an aqueous 0.1 wt% HiPCO CNT dispersion, containing 1 wt% of SDS. The SWCNTs for which the exfoliation behavior is monitored here come from a lower quality batch, and are exfoliated either before ( ) or after purification (O).

Figure 4.6.6 shows scanning electron micrographs of HiPco coating surfaces (a) with and (b) without the blocky VA/AA copolymer dispersing agent. More defined CNT fibers are seen for the coatings with the VA/AA material, which indicates better CNT dispersion. 

The same Carbolex and HiPCO SWCNT dispersions as those previously studied in Section 3.1.2 were examined with UV-Vis spectroscopy. Samples were taken regularly during the sonicating process, diluted and UV-Vis spectra were recorded. Since desorption processes are typically quite slow, it was assumed that the amount of SDS molecules adsorbed on the CNT walls was not significantly influenced by dilution, and UV-Vis spectra were typically immediately recorded after dilution. " Please note that the dilution factor — in other words, the CNT concentration after dilution [i.e., 6.7 x 10 wt% for the standard SWCNT dispersions) — was chosen in such a way that all the UV-Vis absorbance values remained below 1 so that the error inherently present in the measurement itself is reduced. At this dilution, the contribution of scattering can be ignored. " ... 

The UV-Vis spectra recorded for aqueous HiPCO SWCNT dispersions, obtained after different energy-inputs and sonication times, are given in Figure 3.4. The corresponding spectra for Carbolex SWCNTs show a similar development, but exhibit one maximum instead of two around 250-300 nm. This difference indicates that the UV-Vis spectra obtained are specific for the CNT type studied. This is not surprising since the Carbolex and HiPCO CNTs studied... 

It appears that all the CNT dispersions [of SDS-HiPCO SWCNTs, SDS-Carbolex SWCNTs, and SDS-MWCNTs (thin MWCNTs of the batches MWA P041206 and 060213 provided by Nanocyl SA]] presented in this chapter are stable for at least several months, if not years, since the value of the UV-Vis absorbance of these dispersions remained virtually constant during this period. ... 

The nature and the quantity of impurities removed are strongly connected to the surfactant type, as well as to the CNT type and batch (notably their density and the homogeneity of the batch). In order to Illustrate this last point, SDS-Carbolex SWCNT and SDS-HiPCO SWCNT dispersions were exfoliated and subsequently centrifuged at... 

Basic studies on diazonium-CNT chemistry led to two very efficient techniques for SWCNT derivatization solvent-free functionalization [176] and functionalization of individual (unbundled) nanotubes [175], With the solvent-free functionalization (Scheme 1.26), heavily functionalized and soluble material is obtained and the nanotubes disperse in polymer more efficiently than pristine SWCNTs [176], With the second method, aryldiazonium salts react efficiently with the individual (unbundled) HiPCO produced and sodium dodecyl sulfate (SDS)-coated SWCNTs in water. The resulting functionalized tubes (one addend in nine tube carbons) remained unbundled throughout their entire lengths and were incapable of reroping. [175],... 

Another coating property that was investigated for the VA/AA/HiPco system was its capability of thermoelectric effect, in which the coating was heated when DC electricity was applied across it (Caneba and Axland, 2004). Finally, the same type of coating was found to function as a high emissivity material, due to its capability of dispersing the CNT in the polymer coating. 

Surfactants — either anionic surfactants such as sodium dodecylsulfate [SDS], or sodium dodecyl benzene sulfate [SDBS], or polysaccharide [Gum Arabic GA] — were first used to disperse, and exfoliate as-produced SWCNTs in water by ultrasonication, and to stabilize the resulting aqueous CNT suspension, see Figure 2.12. The SWCNTs were synthesized by either the AD method [about 30 % of impurities], or by the HiPCO process [having a catalyst particle content of about 5 wt%]. Please note that not only short surfactant molecules, but also polymeric surfactants such as polystyrene sulfonate, or even conductive polymers having a surfactant nature, can also be successfully used to disperse CNTs in water. 

Figure 3.5 shows the absorbance at different wavelengths as a function of the energy supplied to the aqueous SDS-HiPCO dispersion. Since the power of sonication is kept constant throughout the experiments, it is equivalent to plot the absorbance at a certain wavelength either versus the time of sonication or versus the energy supplied to the SDS-CNT mixture. 

Figure 3.15 UV-Vis spectra of SDS-HiPCO SWCNT aqueous dispersions before and after ultracentrifugation. Notice the broadening of the absorption features due to the presence of small CNT bundles in the non-centrifuged sample.

Raman spectroscopy is often used to assess the quality of CNTs. -A typical Raman spectrum of a CNT mat or dispersion shows a few characteristic bands. These include the "graphene-like or G-band observed between 1,500 and 1,600 cm , the "disorder-induced or D-band observed at 1,300 cm , and the radial breathing mode [RBM] observed between 100 and 500 cm . A comparison between the G-and D-bands from two samples has been reported to indicate their relative purity.Raman spectra taken for SDS-stabilized dispersions of HiPCO and Carbolex SWCNTs are given in Figure 6.13. 

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