Nanotube-Matrix Interaction

Nanocarbon structures such as fullerenes, carbon nanotubes and graphene, are characterized by their weak interphase interaction with host matrices (polymer, ceramic, metals) when fabricating composites [99,100]. In addition to their characteristic high surface area and high chemical inertness, this fact turns these carbon nanostructures into materials that are very difficult to disperse in a given matrix. However, uniform dispersion and improved nanotube/matrix interactions are necessary to increase the mechanical, physical and chemical properties as well as biocompatibility of the composites [101,102]. 

Chapter 1 Carbon Nanotube-Reinforced Polymers 4.3.2 Nanotube-Matrix Interaction... 

The nanotubes were first oxidized in nitric acid before dispersion as the acidic groups on the sidewalls of the nanotubes can interact with the carbonate groups in the polycarbonate chains. To achieve nanocomposites, the oxidized nanotubes were dispersed in THF and were added to a separate solution of polycarbonate in THF. The suspension was then precipitated in methanol and the precipitated nanocomposite material was recovered by filtration. From the scanning electron microscopy investigation of the fracture surface of nanotubes, the authors observed a uniform distribution of the nanotubes in the polycarbonate matrix as shown in Figure 2.3 (19). 

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. 

Thus, the above reports demonstrated that both non-covalent and covalent functionalisation of nanotubes can serve as effective tools for improving both the nanotube dispersion and nanotube-matrix interfacial interaction, enabling the fabrication of reinforced polymer composites. 

As mentioned above, Raman spectroscopy has been used to evaluate the state of dispersion of CNTs in polymer-based composites. The shift to higher wavenumbers of the Raman bands has been taken as an indication of fewer inter-tube interactions. A further upshifting of Raman bands occurs when the tubes are disentangled and embedded in the polymer medium, as was reported for the SBR matrix. Said upward shifts are also considered as the indication of intimate nanotube/polymer interaction, as will be discussed in Section 2.5.2. 

The diflerences in the composite manufaeturing methods, i.e. powder-(MWCNTs and ball milled SWCNTs) or liquid- (ehemieally treated SWCNTs) based approach, cannot account for the diflferenees in the properties, sinee both methods were used for the SWCNTs composites and resulted in similar thermal behavior. They concluded that in this case, there must be a very large interface resistance to the heat flow associated with poor phonon eoupling between the stiff nanotubes and the (relatively) soft polymer matrix. Also, it is possible that the phonon vibrations in the SWCNTs are dampened by the matrix interaction, while in the MWCNTs the phonons ean be earned in the inner walls without hindrance. 

Another aim of this research was to achieve a good bonding between the nanotubes and the matrix. A load transfer between them can only be achieved with a sufficiently good bonding, which should result in an improvement of the fracture mechanical properties. TEM provides a possibility of obtaining useful qualitative information concerning the nanotube-epoxy matrix interaction. 

A different approach can be used to induce nanopartide self-assembly on surfaces or matrixes both by electrostatic interactions and chemical bonding between a functionalized nanoparticle and a surface. This is a vast area of research in which many types of substrates like Si, Si02, Ti02, A1203, MgO, carbon nanotubes, and so... 

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