Aqueous dispersions surfactant-SWCNTs

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... 

Direct contact of the dispersed SWCNTs and the conductive polymer is favorable, because this presumably decreases the non-contact resistivity between the CNTs by the formation of conductive bridges between adjacent CNTs in the CNT network. The high affinity of conjugated polymers for CNTs through it-it electronic interactions ensures a close conductive polymer-CNT contact. Applying these conductive polymers to a water-based system requires the conductive polymer to have a surfactant-like nature. Water-soluble polythiophenes have been used to disperse SWCNTs in water, but no quantitative information with regard to the level of SWCNT individualization was provided. Polymeric surfactants like poly(styrene sulfonate] (PSS] have been reported to effectively disperse CNTs in water. PSS is also used in the preparation of an aqueous dispersion (latex] of the conductive polymer poly(3,4-... 

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... 

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. ... 

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. 

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