Covalent functionalization sidewall

From an atomic configuration point of view, a nanotube can be divided into two parts that are generated by curvatures the end caps and sidewall. The end caps are close to the hemispherical fullerene and are curved in 2D, and the sidewall contains less-distorted carbon atoms and is curved in ID (Polizu et al., 2006). Owing to their specific curvatures, the chemical reactivity at the sidewall is significantly lower than that at the end caps The sidewall is thought to be inert and highly reactive agents are required for the covalent functionalization of CNT sidewalls (Wei et al., 2007). 

Thermal decomposition of peroxides generates a variety of radical groups which can covalently react with CNTs. This strategy has been employed in particular by Peng and co-workers who functionalized sidewalls with benzoyl, lauroyl and carboxyalkyl derivatives [38]. 

The use of CNTs in composites for optical, mechanical, electronic, biological and medical applications, etc., requires the chemical modification of their surface in order to meet specific requirements depending on the application [140]. While searching for how to perform the covalent functionalization of CNTs, it was found that the tips of CNTs were more reactive than their sidewalls [142,143]. 

It has also been demonstrated that CNT sidewalls can be covalently fluorinated [148 150], or they can be derivatized with certain highly reactive chemicals such as dichlorocarbene [142], In this context, Chen et al. applied derivatization chemistry with thionychloride and octadecylamine in order to obtain organic soluble SWCNTs and later they performed a reaction with dichlorocarbene that led to the covalent functionalization of the nanotube walls. 

Hou et al. (43) reported the poly(vinyl alcohol) nanocomposites using single walled (SWNT), few walled (FWNT) and multi walled (MWNT) nanotubes. The nanotubes were covalently functionalized to generate acid functionalities on the sidewalls. The incorporation... 

For example, Dillon et al. [154] reported a covalent attachment of branched polyethyleneimine (PEI) to the sidewalls of SWNTs through the use of fluorinated single-wall CNTs as precursors. The structural integrity of the original purified SWNT is maintained upon covalent functionalization with PEI. Solid-state NMR shows the presence of carboxylate substituents due to carbamate formation as a consequence of the reversible CO2 absorption to the primary amine substituents of the PEI. Desorption of CO2 is accomplished by heating under argon at 75 °C, while the dependence of the quantity of CO2 absorbed on temperature and the molecular weight of the PEI is also observed. 

Compared to covalent functionalization, the main advantage of non-cova-lent functionalization is the preservation of the nanotube s structure by preventing disruption of the intrinsic sp structure and conjugation while significantly improving their solubility. Zhao et al. reported extensive research on non-covalent functionalization, including (i) aromatic small molecule based non-covalent functionalization, (ii) biomacromolecule based non-covalent functionalization and (iii) polymer based non-covalent functionalization. Aromatic molecules were employed to interact with the sidewalls... 

Figure 6.1. Strategies for covalent functionalization of CNTs. Scheme A direct sidewall functionalization. Scheme B defect functionalization Reproduced from [26] with permission from Elsevier...

Carbon nanotubes (CNTs) are unique one-dimensional (1-D) nanomaterials composed entirely of sp hybridized carbon atoms. Unlike other 1-D nanomaterials, every atom in a CNT is located on the surface, which gives rise to unique properties desirable for many applications. In order to utilize this nanomaterial in most applications, CNTs must be chemically functionalized. Covalent functionalization of CNTs represents a vibrant field of research. Often in covalent modification, the sidewalls or the end groups are subject to functionalization (Figure 1) the primary problem with this approach, however, is that the physical properties of the nanotube are impaired. As this chapter does not cover this topic, interested readers are referred to high-quality review articles. In order to chemically functionalize CNTs while preserving their physical properties, supramolecniar chemistry of CNTs needs to be developed. 

The tendency for as-prepared SWNTs to aggregate in aqueous environments through van der Waals interactions has hampered their utility in biological applications. While it is possible to covalently functionalize the sidewalls of the nanotube, such alterations have negative effects on CNT properties. Moreover, while surfactants could be utilized to disperse SWNTs in aqueous environments, there exists a risk that the surfactant will denature biological molecules. To overcome these limitations, O Connell et al. employed a surfactant coupled with the water-soluble polymer polyvinyl pyrrolidone (PVP, Figure 12a,i,ii) to wrap around SWNTs, which enabled individual, pristine SWNTs to be dispersed at the gram per liter concentrations in aqueous environments. ... 

Overall, covalent functionalization of CNTs has diverse mechanical and electrical attributes caused by the intervention of the attached moieties and the modification of the structural p-network (Wang et al. 2010). This structural alteration occurred at the termini of the tubes and/or at the sidewalls. Moreover, the direct sidewall functionalization associated with rehybrization of one or more sp C ate of C network into a sp configuration and concurrent loss of conjugation (Lee et al. 2013). 

The fluorination of CNTs becomes prevalent for early investigation of the covalent functionalization due to the fact that CNTs sidewalls are expected to be inert (Karousis et al. 2010 Li et al. 2012). The fluorinated CNTs have C-F bonds that are easily broken than those in alkyl fluorides, and therefore providing substitution sites... 

CNTs lend themselves to a range of chemical modifications. Both covalent and non-covalent functionalizations are possible at intact CNT sidewalls, at defect sites on sidewalls or at the tip of the nanotubes. The most common modification is the formation of carboxyl residues [39, 40]. The non-covalent functionalization of CNTs can be carried out by coating CNTs with amphiphilic surfactant molecules or polymers (poly-ethylene-glycol). 

The huge advantage related to physical functionalization is that it does not destroy the conjugated systems of the CNTs sidewalls, and therefore it does not affect the final structural properties of the material. Non-covalent functionalization is an alternative method for tuning the interfacial properties of the CNTs. 

Covalent functionalization of CNTs is based on the formation of a covalent bond between functional entities and the carbon backbones of CNTs, conducted at the termini of the CNTs or at their sidewalls. It could also be divided into direct covalent sidewall functionalization and indirect covalent functionalization (defect functionalization). Direct covalent sidewall functionalization is associated with a change of hybridization from sp to sp and a simultaneous loss of conjugation. The latter takes advantage of chemical transformations of the already present defect sites, which can be the open... 

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