SWCNT batches

To further probe the SWCNTs purity, their intrinsic conductivities were measured by preparing buckypapers from the respective dispersions. Conductivities of buckypapers prepared from HiPCO and Carbolex SWCNTs were approximately 1 x 10 and 1 x 10 S m, respectively. The significant difference in intrinsic conductivities indicates that there is certainly a difference in the electronic properties of the SWCNTs. The lower conductivity of the Carbolex SWCNTs could be attributed to more wall defects, a larger fraction of semi-conducting SWCNTs or a carbonaceous coating on the SWCNT wall. To determine the main cause of these lower conductivities would require further investigation. For the application intended here, this difference is of utmost importance. One further prerequisite is that the aspect ratio of the two SWCNT batches is not vastly different. 

Fig. 8. The effect of potassium intercalation on the temperature dependence of the resistivity of a bulk SWCNT sample. Curve a, pristine material from a different batch than in Fig. 6 curve b, after doping with potassium curve c, after heating in the cryostat vacuum to 580 K overnight curve d, after 3 days at 580 K [35].

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. ... 

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. ... 

Firstly it was found that, contrary to treatments based on nitric acid, this mild HCl-based purification did not seem to significantly oxidize the surface of the CNT walls, as shown by the comparison of the IR spectra of purified and non-purified CNTs. Secondly, the results of TGA measurements clearly pointed out that the efficiency of the purification was batch-dependent in terms of relative percentage of iron particle removal. The HIPCO SWCNTs of a batch containing a... 

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).

These two experimental observations point out that the purified CNTs are more bundled at the end of the exfoliation than the as-produced CNTs. This was confirmed by SEM images of the purified HiPCO SWCNTs of the high quality batch, which showed that these... 

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... 

Please note that detailed analysis of UV-Vis-NIR spectra features of dispersions of surfactant-purified SWCNT constitute a useful tool to assess the degree of purity of a given batch of CNTs, providing the ability for optimization and/or comparison of production or purification methods. ... 

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. 

Ultimately, the quality of the CNT will play the most influential role on the conductive nature of the final composite. A substantial increase in ultimate conductivity [beyond the percolation threshold] with a decrease in melt-viscosity was reported for systems in which the inherent SWCNT conductivity was much lower than those used in the composites presented here. The higher inherent conductivity of the CNTs used in the present study could explain the absence of a large increase in ultimate conductivity with decreased melt viscosity. Differences in ultimate conductivities have been shown to vary widely across different CNT batches, thus making comparisons between different systems inaccurate. ... 

Results presented on the influence of the matrix viscosity on the percolation threshold, collected using one batch of SWCNT and one batch of MWCNTs, not only agree with the report on the decrease in percolation threshold with an increase in the processing temperature, but also agree with results reported for carbon blackfilled polymers. The influence that the matrix viscosity has on the formation of equilibrated CNT network structures cannot be overlooked. It has been shown that a reduced matrix viscosity, achieved by higher processing temperatures or introducing low molar mass material, leads to similar reductions in the percolation threshold. 

A different batch of SWCNTs, namely Carbolex SWCNTs, were used in the preparation of SWCNT/PS/PEDOT PSS composites.Carbolex SWCNTs are produced by arc-discharge technology. This technique is used to produce SWCNTs on a large scale but with the disadvantage... 

SWCNTs. This implies that the SWCNT surface chemistry (degree of graphitization] could differ greatly for the two batches, although this counters the observation of the TGA results. It should, however, not be forgotten that a featureless absorption spectra could also be linked to a lesser degree of individualization of the SWCNTs. 

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