Dispersion of CNTs in polymer matrix

Well-dispersed and long-term stable carbon nanotubes/polyol dispersions can also be prepared by a mechanochemical approach with the aid of a dispersing agent (Tang and Xu, 1999). Good dispersion of CNTs in polymer matrix can be achieved by means of high-power dispersion, compatibilizer, polymer-assisted blending, and surfactants (Cochet et al., 2001). 

Although the approach of covalent functionalization of CNT surface is an effective means to obtain a homogeneous dispersion of CNTs in polymer matrix and a strong interfacial interaction with the polymer, it inevitably destroys the intrinsic properties of CNTs such as the unique ji-electron system of pristine CNTs is affected due to formation of covalent bonds and shortening of length of CNTs during chemical treatments (70). 

Barrau and co-workers have found that amphiphilic palmitic add could be favorable for an efSdent dispersion of CNTs in an epoxy matrix. The hydrophobic part of palmitic acid was absorbed onto the surface of CNTs, whereas the hydrophilic head group induced electrostatic repulsions between CNTs, effeaively preventing their aggregation. The cosolvent has also been found to affect the dispersion of CNTs in polymer matrix. Very recently, Camponeschi a reported the use of trifluoroacetic acid as a cosolvent for the dispersion of MWNTs in a conjugated polymer poly (3-hexylthiophene) and PMMA via a solution process. SEM, optical microscopy, and light transmittance studies indicated that a better dispersion of CNTs in polymer matrices was obtained by using trifluoroacetic acid. Many other polymer composites such as polyurethane/CNT, PS/CNT, epoxy/ CNT, poly(vinyl alcohol)/CNT, " P(MMA-co-EMA)/ CNT, polyacrylonitrile/CNT, and polyethylene/CNT have also been fabricated by this method. ... 

Because of the unique combination of mechanical, electrical, and thermal properties, the CNTs have been excellent candidates to substitute or complement the conventional nanofillers in the fabrication of multifunctional PNs. The first PNs using CNTs as the nanoadditive was reported in 1994.20 By far, the CNTs have been the second most investigated nanoadditives to reduce the flammability of the polymers through nanocomposite technology. A difficulty of the application of the CNTs in polymers is the dispersion of CNTs in the matrix polymer, and the high cost of the CNTs is another problem. 

The main factor that controls the performance of the composites is the state of dispersion of CNTs in the matrix. Carbon nanotubes can easily form bundles and this aggregation decreases their aspect ratio thus reducing their efficiency as fillers. So, all processing methods used to prepare polymer nanotube composites aim to improve dispersion of CNTs in order to fully exploit the potential of these materials. 

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. 

As emphasized in the preceding, the dispersion of CNTs in polymer matrices is a critical issue in the preparation of CNT/polymer composites. Better reinforcing effects of CNTs in polymer composites will be achieved if they do not form aggregates and as such, they must be well dispersed in polymer matrixes. 

Generally, realizing effective reinforcement of CNTs in polymer needs two factors (21) (1) good dispersion in matrix, and (2) strong interface strength between CNTs and polymer. Dispersion is probably a more fundamental issue. Nanotubes must be uniformly dispersed as isolated nanotubes and individually coated with polymer. This results in a more uniform stress distribution and minimizes the presence of stress-concentrated centers. Furthermore, strong interface between CNTs and polymer leads to efficient stress transfer... 

Generally, the thermal stability of the composites was improved by the addition of the CNTs, which may be attributed to the following combined effects (1) the uniformly dispersed carbon nanotubes presumably provided thermo-oxidative stability to the polymers in the vicinity of the tube surfaces (2) the enhanced thermal conductivity of the composite can facilitate heat transport and thus increase its thermal stability (81) (3) it is possible that the formation of compact chars of CNTs and polymer matrix during the thermal degradation is beneficial to the improvement of thermal stability of the composites (82). 

Among the applications discussed in this chapter, the most prominent in recent years is CNT-reinforced polymer nanocomposites. The use of CNTs in polymers can provide superior mechanical properties (60). For instance, the addition of 1% CNTs might increase the stiffness of polymers by 10% and increase their resistance to fracture however, improvements in the properties of CNT-reinforced polymers largely depend on the dispersion of CNTs within the polymer matrix and the polymer-CNT interfacial properties. The following section highlights several studies regarding the processing of PLA-CNT nanocomposites. 

CNTs can be further functionalized through noncovalent or covalent interactions for better dispersibility and stability in polymer matrix. The noncovalent functionalization of CNTs normally involves van der Waals, n-n, CH-n, or electrostatic interactions between polymer molecules and CNT surfaces. The advantage of noncovalent functionalization is that it does not destroy the conjugated system of the CNT sidewall, and therefore it does not affect their final properties (Spitalsky et al., 2010). It should be noted that, the graphitic sidewalls of CNTs provide the possibility for tt-stacking interactions with conjugated and aromatic... 

---