Surface functionalization of CNTs

The most common method for the production of carbon nanotubes is hydrocarbon-based chemical vapor deposition (CVD) [97] and adaptations of the CVD process [98, 99], where the nanotubes are formed by the dissolution of elemental carbon into metal nanoclusters followed by precipitation into nanotubes [100]. The CVD method is used to produce multiwalled carbon nanotubes (MWCNTs) [101] and double-walled carbon nanotubes (DWCNTs) [102] as well as SWCNTs [103], The biomedical applications of CNTs have been made possible through surface functionalization of CNTs, which has led to drug and vaccine delivery applications [104,105],... 

TEM revealed that the surface functionalization of CNT with phenol or 1-octadecanol improved their dispersion in a NR matrix. 

Table 25.1 Modifiers used for surface functionalization of CNTs...

It was found that the above-mentioned dopants attached to CNTs change the electronic properties of CNTs and can therefore be used for surface functionalization of CNTs (Wang et al. 2007 Zhang and Zhang 2009). The analysis of operating characteristics of CNT-based gas sensors with functionalized... 

Moreover, surface functionalization of CNTs influenced the final properties of the composites. Thus, they are focused on enhancing the CNTs/polyester master batches without styrene through various kinds of functional groups to obtain the desired mechanical properties and microstructure of composites (Seyhana et al. 2009 Hilmi et al. 2010 Esteban et al. 2013 Ziyan et al. 2014). 

Surface properties of CNTs induce chemical interactions between CNTs and polypropylene, which in turn improve the mechanical behaviour of the composites (Girei et al. 2012 Pascual et al. 2012 Kim et al. 2013a, b, c). With that in mind, the interface between CNTs and polypropylene was simulated using contact elements. It is recorded that the length of CNTs significantly affects the reinforcement phenomenon of the polypropylene composites (Sulong et al. 2013 Wu et al. 2013a, b, c). Indeed, to increase the surface properties of CNT-filled polypropylene composites, some studies focused on the surface functionalization of CNTs. For... 

Surface functionalization of CNTs can be applied improve the compatibility of structure. A variety of methods based on covalent attachment and non-cova-lent interactions have been developed to achieve effective dispersions of CNTs. For obtaining the effective dispersions of CNTs, it can be utilized the covalent attachment and non-covalent interactions. 

A completely different strategy for the surface functionalization of CNTs with nitrogen-containing groups is the treatment of CNTs under an atomic-nitrogen flow obtained by molecular nitrogen dissociation in microwave plasma. X-ray photoelectron spectroscopy of the nanotube surface demonstrated the presence of amides, oxides and, mainly, amine and nitrile groups [63]. 

An alternative approach to functionaUzed CNTs is direct surface functionalization of CNTs with reactive groups. For example, Theato and coworkers used partly oxidized multiwall CNTs to install PFP esters on the surface, which could be used for further surface functionaUzation or for use in the reactive LbL assembly of multilayer polymer films. It is worth noting that the obtained thin films were transparent and conductive, making them suitable candidates for solution-processed transparent electrodes [209]. 

The surface properties of CNTs are paramount for their hybridization with other components. The formation of large bundles due to van der Waals interactions between hydrophobic CNT walls further limits the accessibility of individual tubes. Functionalization of CNTs can enhance their dispersion in aqueous solvent mixtures and provide a means for tailoring the interfacial interactions in hybrid and composite materials. Functionalization techniques can be divided in covalent and non-covalent routes, which will be described in greater detail in Chapter 3. 

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]. 

Pristine CNTs are chemically inert and metal nanoparticles cannot be attached [111]. Hence, research is focused on the functionalization of CNTs in order to incorporate oxygen groups on their surface that will increase their hydrophilicity and improve the catalyst support interaction (see Chapter 3) [111]. These experimental methods include impregnation [113,114], ultrasound [115], acid treatment (such as H2S04) [116— 119], polyol processing [120,121], ion-exchange [122,123] and electrochemical deposition [120,124,125]. Acid-functionalized CNTs provide better dispersion and distribution of the catalysts nanoparticles [117-120],... 

Nanotubes are functionalised to improve their solubility in water or to attach to their surface biologically active substances such as peptides and drugs. The ability to attach biological substances has raised an interest in using nanotubes as carriers for delivery of drugs and vaccines. A number of researchers performed functionalization of CNT with physiologically active molecules and macro-objects. These results are summarized in Table 2.3 [30 0]. 

Figure 3. Functionalization of CNTs (a) non-covalent interacions with polymers and biomolecules (b) covalent surface chemical modification (end-functionalization and side-wall functionalization).

A number of studies on CNT-polymer composites have focused on improving the dispersion and load transfer efficiency in other words the compatibility between the CNTs and polymer matrix through covalent chemical functionalization of CNT surface (12,40). Many of the studies reported above have used acid-functionalized CNTs to fabricate MWCNT-PMMA composites with improved mechanical properties using different processing methods (24,25,27,62). Yang et. al (68) modified the acid functionalized CNTs with octadecylam-ine (ODA) to obtain ODA-functionalized CNTs. These CNTs were reinforced in a copolymer P(MMA-co-EMA) to form composites with improved dispersion and mechanical properties. 

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). 

The use of surfactants and the chemical functionalization of CNT surfaces have also been investigated in efforts to improve CNT dispersion as well as enhance the CNT-polymer interfacial bond (10,27-29). However, although chemical functionalization can lead... 

The electrochemical response of CNTs is dominated by double-layer charging with only a small contribution of faradaic pseudocapacitance due to surface oxides (Frackowiak and Beguin, 2000 Kavan et al., 2001), so that faradaic processes are essentially absent. In contact with aqueous electrolytes, however, weak peaks may eventually appear. These peaks are presumably due to faradaic pseudocapacitance associated with oxygen-containing surface functionalities on CNTs and/or... 

As described above, ILs for functionalization of CNTs are generally low molecular organic salts. Recently, Chen et al. [146] have employed PIL-functionalized MWCNTs to serve as the templates for fabrication of metal NP hybrids. In this method, the IL of 3-ethyl-l-vinylimidazolium tetrafluoroborate (EVimBF ) was in situ polymerized to form a poly(EVunBF ) shell on the surface of MWCNTs (Fig. 15.11). This not only makes MWCNTs uniform dispersion in water but also serves as the medium to stabilize and grow metal NPs. It is noted that metal NPs (e.g., Pt, PtRu) are uniformly dispersed on the supported surface of poly(EVimBF )/... 

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