Dispersions surfactant-SWCNT

The separated metallic S WCNTs were also found to enhance transparent conductive performance in composite films with conductive polymers, particularly the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOTPSS) blend as it is optically transparent in the visible spectral region (Figure 16.20). In such composites, the conductive polymer blend served the function of dispersion agents, so no surfactants were necessary in the film fabrication. In the work by Wang et al, suspensions of nanotubes (enriched metallic or nonseparated S WCNTs) in DMSO were mixed with aqueous PEDOTPSS in various compositions, and the resulting mixtures were sprayed outo au optically transparent substrate. The sheet resistance results demonstrated that the composite films with enriched metallic SWCNTs were consistently and substantially better in performance than those with nonseparated SWCNTs (and both better than films with neat PEDOTPSS). Aqueous PEDOTPSS is not as effective as commonly used surfactants in the dispersion of SWCNTs, which... 

For Method 1, PEDOT PSS was used as a surfactant for the dispersion of SWCNTs in an aqueous environment [similar system to that reported previously]. This dispersion was subsequently blended with a PS latex stabilized with SDS. This case is illustrated in Figure 6.8 [A]. 

Shin et al. (2008) reported a systematic study on the dispersion of SWCNTs in aqueous solution by comparing three different types of neutral, cationic, and anionic surfactants. All three surfactants could facilitate the good SWCNT dispersion at the optimum concentration, which is a little higher than their critical micelle concentration. The SWCNTs were aggregated and formed bundles in all cases at very high concentrations. As a result, it is not benehcial to add more surfactant above the optimum concentration. 

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-... 

Surfactants, such as sodium dodecyl sulfate (SDS) and Triton X-100, and particularly ILs, have been lately used as binders, since the latter improve the conductivity of the paste material and, depending on the molecular structure of the IL, can also provide catalytic properties or selectivity toward certain compounds. SWCNTs were dispersed in a 1 wt% SDS aqueous solution and sonicated... 

DNA is often modeled as a hydrophilic cylinder with a constant charge density of 5.8 e /nm. Recent experiments have shown that DNA-dispersed SWCNTs have similar electrostatic properties, with a charge density on the order of 5.8 to 6.2 e /nm. Thus a similar theoretical framework can be applied to both. In the case of surfactant-dispersed nanotubes less information is available, but presumably the micelle enclosing the SWCNT can possess an even higher charge due to the large number of surfactant molecules (of which SDS is commonly used) surrounding the nanotube. It should be noted that surfactant systems are complicated by the presence of empty surfactant micelles and by interaction of the surfactant with certain surfaces. 

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. 

This "excluded volume" concept was further illustrated by Winey and her coworkers who prepared SWCNT/PS nanocomposites by homogeneously coating SWCNTs (exfoliated in aqueous solutions without using surfactant) on the surface of softened flakes or pellets of PS, maintained above the glass transition of the polymer. After processing of the coated PS particles by compression molding, it was shown that SWCNTs were predominantly present in the interfacial volume between the pellets and formed a continuous three-dimensional cellular network. The nanocomposites obtained had conductivity values of the order of 10 S/m for 1 wt% of SWCNTs and a percolation threshold of about 0.2-0.3 wt%, i.e., half of the value of one of the reference samples for which SWCNTs of the same batch were homogeneously dispersed into the same PS matrix by an alternative method. ... 

The mechanism of MWCNT de-entanglement is expected to be very similar to the one described for SWCNTs, although the initial state is different. Note that if the sonication treatment is too aggressive and/ or lasts too long, it can lead to local damage of the CNT walls, if not to CNT shortening. Local damage deteriorates both electrical and mechanical properties. At the end of the sonication process, a dispersion of mainly individual CNTs with adsorbed surfactant molecules on their surface is obtained. 

With the simple and efficient UV-Vis tool to monitor the sonication-driven debundling of SWCNTs and MWCNTs, the optimum debundling conditions can be determined to produce stable surfactant-CNT dispersions that can be further stored and employed to make conductive nanocomposites. Finding trends in the general debundling behavior of several types of CNTs, as well as selecting the right power of sonication and the optimum CNT and surfactant concentrations, are dealt with in this section. 

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... 

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