Nanotube agglomerates

Carbon nanotubes are hollow carbon cylinders with hemispherical endcaps of less than 1 nm to a few nanometres in diameter and several microns in length. The aspect ratios are of the order of 1000 and more. The elementary nanotubes agglomerate in bundles or ropes that are difficult to disperse. 

The gap between the predictions and experimental results arises from imperfect dispersion of carbon nanotubes and poor load transfer from the matrix to the nanotubes. Even modest nanotube agglomeration impacts the diameter and length distributions of the nanofillers and overall is likely to decrease the aspect ratio. In addition, nanotube agglomeration reduces the modulus of the nanofillers relative to that of isolated nanotubes because there are only weak dispersive forces between the nanotubes. Schadler et al. (71) and Ajayan et al. (72) concluded from Raman spectra that slippage occurs between the shells of MWNTs and within SWNT ropes and may limit stress transfer in nanotube/polymer composites. Thus, good dispersion of CNTs and strong interfacial interactions between CNTs and PU chains contribute to the dramatic improvement of the mechanical properties of the... 

The percolation takes place if the critical volume fraction of secondary nanotube agglomerates Vy ggg is reached. According to classical percolation theory, the conductivity increase can be described with power law behavior (Eq. 5.9) with cr the plateau value of conductivity and the critical exponent (see also Appendix). 

So, the breakdown of the carbon nanotubes agglomerates is caused by the collapse of microbubbles in the interstices of the aggregates and by collisions between aggregates at high speeds and by liquid microjets that reach the surface of aggregates and bundles of carbon nanotubes. 

Wick P, Manser P, Limbach LK, ttlaff-Weglikowska U, Krumeich F, Roth S, Stark WJ, Bruinink A (2007) The degree and kind of agglomeration affect carbon nanotube cytotoxicity. Toxicol. Lett. 168 121-131. 

Figure 3. Electron microscopy images of the graphite (a) and nickel (b) explosion products in the toluene a) agglomeration of nanotubes, b) separated multy-walled nanotube.

Wang Y., Wei F., Luo G., Yu H., Gu G. The large-scale production of carbon nanotubes in a nano-agglomerate fluidizcd-bed reactor. Chemical Physics Letters, 2002,364, 568-572. 

Undoubtedly, understanding the reinforcement of carbon nanotubes based nanocomposites requires the characterization of several parameters of the nanotubes like diameters, lengths (and their distribution), structures (SWNT, MWNT). Moreover, after processing, the three-dimensional distribution of nanotubes within the polymer matrix, the presence of agglomerates, the interfacial properties have also to be precisely characterized. However, CNT based... 

As was detailed in this section, TEM can bring numerous pieces of information regarding the polymer/nanotube composite microstructure. However, it has to be recalled that nanofillers such as nanotubes easily agglomerates and their dispersion state has to be characterised from the micron to the nanometre scale. This is one reason, among others, why Scanning Electron Microscopy is another widely used to characterise polymer/nanotube composites. 

As-synthesized MWCNT and SWCNT exist as bundles or ropes and tend to agglomerate due to strong van der Waals forces (13) (Figure 7.1). Unless the CNTs are separated in to individual tubes and dispersed in the polymer matrix, the interactions of the nanotubes with the polymer will be weak. The mechanical failure of such composites will occur due to slippage of the tubes in the bundle that are not bonded to the matrix. In addition, the aggregates or bundles reduce the aspect ratio of the reinforcement which affects electrical properties as well (15). Because of these factors the first step will be to open up these bundles to separate individual tubes by using different techniques to increase the volume of interface between the CNT and the matrix (40). 

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