Nanotube dispersion state

Despite the results presented above, near-field microscopy has not been extensively used to characterize polymer/nanotube composites However, it can be noticed that AFM is a useful technique to locally probe the mechanical properties of the composites (at the polymer-nanotube interface for example). One possible reason for the small amount of studies by AFM and STM could be that observing the surface only does not permit to obtain much information on the nanotube dispersion state. For that kind of characterization, transmission electron microscopy is a key technique owing to the small nanotube diameter. 

Obtaining a homogeneous nanotube dispersion state in a polymer matrix is still a major issue. From a general point of view, the dispersion state is influenced from the following parameters (53) ... 

As a consequence, the nanotube dispersion state has to be characterised at several pertinent scales, including that covered by TEM. 

In some studies, a statistical description of the nanotube dispersion state was obtained from TEM images. For example, Uchida et al. (56) measured the diameter distribution of SWNT bundles in poly(acrylonitrile), with and without a purification treatment involving sonication in methanol. The different bundle diameter distributions (especially the mean diameter) could explain the different composite tensile moduli. Fornes et al. (57) also determined the diameter distribution of SWNTs bundles in a polymer matrix (namely polycarbonate). To improve the contrast in the bright-field TEM images and better measure the bundle diameter, they dissolved the polymer in chloroform and studied the remaining SWNT network. 

TEM remains certainly the most powerful technique to get bulk information, but due to the low sample thickness required for observation, in most cases, CNT are cut and it is almost impossible to observe them surrounded by their neighbours and so, to analyse their mutual interactions. However, in situ spectroscopy leads to more and more precise data on CNT - matrix interface, which one of the key-point of macroscopic behaviour. It can be noticed that SEM can be performed in transmission, leading to images similar to what can be obtained by TEM. However, in the magnification range covered by both techniques, SEM provides images of thicker samples with a higher contrast, which should provide reliable results on the nanotube dispersion state. However, TEM remains unavoidable to locally characterise the nanotube-matrix interface and the nanotube-nanotube contacts. 

Regardless of the nature of these nanotubes, their dispersion state in the host polymer is crucial and its improvement is a challenge to achieve the best lire performance of the corresponding nanocomposites. For example, Kashiwagi et al.9 have shown that in PMMA well-dispersed SWNTs led to a strong decrease in PHHR in cone calorimeter tests, while poorly dispersed SWNTs did not modify HRR in comparison with pristine PMMA. 

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. 

Song, YS, Youn, JR. 2005. Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites. Carbon Ah 1378-1385. 

A closely related method to the in situ polymerisation processing of composites is based on epoxy resins thermosets. In this approach, CNTs can be dispersed in a liquid epoxy precursor and then the mixtures can be cured by the addition of hardener, such as triethylene tetramine (TETA), and the application of temperature or pressure. In most cases, the epoxy monomer exists in liquid state, facilitating nanotube dispersion. Curing is then carried out to... 

Figure 5.3. Illustration about different states of nanotube dispersion (a) uniform coverage probability indicated by Qp 1, (b) separation indicated by Qp <1, and (c) cluster formation indicated by Qp > 1...

Next to the interfacial properties between the nanotubes and the polymer as well as the mixing conditions, the structure of primary agglomerates can have a significant influence on the state of nanotube dispersion and distribution. As an example, four different kinds of raw MWCNT materials are considered in the following NanocyF NC 7000... 

Li, J. Ma, P.C. Chow, W.S. To, C.K. Tang, B.Z. Kim, J.K. (2007b). Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes. Advanced Functional Materials, 17, 3207-3215. 

S. Bal, "Influence of dispersion states of carbon nanotubes on mechanical and electrical properties of epoxy nanocomposites," Journal of Scientific and Industrial Research, vol. 66, pp. 752-756, 2007. 

Closely related to the ID dispersion relations for the carbon nanotubes is the ID density of states shown in Fig. 20 for (a) a semiconducting (10,0) zigzag carbon nanotube, and (b) a metallic (9,0) zigzag carbon nanotube. The results show that the metallic nanotubes have a small, but non-vanishing 1D density of states, whereas for a 2D graphene sheet (dashed curve) the density of states... 

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