Carboxyl-functionalized MWNT

Analogous results have been recently obtained by other research groups. The effect of -COOH- and phenol-functionalized CNTs on mechanical, dynamic mechanical and thermal properties of polypropylene (PP) nanocomposites were considered, and the results confirmed in both cases that the percentage crystallinity was found to increase on phenol and carboxylic functionalization of MWNTs. 

Figures 6.16(a)-(c) show the frequency dependence of AC conductivity of solution cast PVDF-nanocomposites filled with pristine, carboxylic- and ester-functionalized MWNTs. An ethanol solvent was used for dispersing nanotubes. At low filler contents, the conductivi-...

Figure 6.16. Frequency dependence of AC conductivity of (a) pristine, (b) carboxyl- and (c) ester-functionalized MWNT/PVDF composites with different MWNT contents. Reproduced from [90] with permission from Elsevier...

The best fits of the experimental permittivity values to Eq. 6.3 give = 3.8 vol% for all composites, and s = 1.05, 1.13 and 1.23 respectively for the pristine, carboxyl- and ester-functionalized MWNT/PVDF composites (inset figures). 

Bandarian, Shojaei, and Rashidi (2011) found out that the iucorporatiou of reactive modified MWNTs, particularly the ones with hydroxyl and carboxyl functional groups, helped in improving the sound absorption properties of flexible PU foams by inducing the formation of microcells of less than 5 pm in the open-cell walls of... 

Nanotube functionalization is performed in other examples listed in Table 11.1. In 1999, Shaffer et al. used carboxylated MWNTs. By contrast to studies described in the followings, this allows to the achievement of homogeneous composites with a large nanotube fraction of about 50 wt%. However, the improvement of the Young s modulus is still weak, particularly if we consider the large amount of CNTs included in the matrix. Liu et al. (47) reported an improvement of mechanical properties with SWNTs which are covalently functionalized by OH groups, compared to the same neat nanotubes simply dispersed with SDS. 

The reaction is conducted the same way as with MWNT with the sole difference of employing toluene instead of DMF as a solvent. This is because the nanotubes present as impurity in the sample are insoluble in toluene, whereas in DMF, they would be dispersed too, and participate in the reaction with the azomethine yiides. In toluene, on the other hand, only fullerenes and onions are sufficiently dispersed. The obtained samples feature interesting nonlinear optics (NLO) properties. In the meantime, one has also succeeded in generating on the onions surface carboxyl groups subsequently to be functionalized by reaction with amines (Figure 4.40). 

Functionalized multi-walled carbon nanotubes (MWNTs) were prepared by acid treatment followed by reaction with 3-aminopropyltriethoxysilane. Reaction of silane with oxidized nanotubes was confirmed by Fourier transform infrared (FTIR) spectroscopy and energy dispersive X-ray (EDX) analysis to confirm silicon on the surface of the MWNTs. Raman spectroscopy of the acid-treated MWNTs confirmed formation of surface defects due to carboxyl... 

The nanocomposites obtained firom phenolic resin and carboxylated MWNTs showed an improvement of the thermal stability than the neat phenolic resin. The highest thermal stability was obtained in the case of the nanocomposites obtained by in situ polymerization, due to the quality of dispersion of the functionalized MWCNTs [94]. An enhancement of the thermal stability was also obtained in the case of nanocomposites containing boron phenolic resin and MWCNTs modified with nitric acid, 4,4 -diaminodiphenyl methane and boric acid. This effect was ascribed to better interfacial interactions between modified MWCNTs and the resin matrix [95]. 

The grafting-from approach is based on the mitial immobilization of initiators onto the nanotube surface, followed by in situ polymerization with the formation of the polymer molecules bound to the nanotube. The benefit of this technique is that polymer-functionalized nanotubes with high grafting density can be prepared. However, this method requires a strict control of the amoimts of initiator and substrate [53, 54]. The grafting- from technique is also used for the preparation of styryl-grafted nanotubes [55]. In this work, the carboxylic acid groups on the surface of oxidized MWNTs have been reacted with 4-vinyl-benzyl chlorides via an esterification reaction followed by the polymerization to produce polystyrene (PS)-grafted CNTs [34,36-38]. 

Recently, Kevlar-functionalized nanotubes were prepared by heating Kevlar with MWNTs in the presence of sulfuric and nitric acids under reflux. This resulted in the partial oxidation and functionalization of CNTs with carboxylic acid groups, which formed hydrogen bonds with amido-groups, as well as terminal carboxylic acid and amino groups in Kevlar. This process produced Kevlar-coated CNTs, which have been utilized for the fabrication of MWNT-polyvinylchloride (PVC) composites. Plasma polymerization treatment enables the coating of CNTs with a very thin (3 nm) polymer layer. Polymer composites based on these coated nanotubes enhanced interfacial bonding in a PS polymer matrix [22, 91]. 

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