Liquid phase purification involves refluxing in an acid such as nitric acid at an elevated temperature.In general,carbon nanotubes produced by the arc dis- charge process requires more extensive purification than the other two processes. Carbon nanotubes are often formed as long,entangled bundles.The "cut- ting"process is used to shorten their lengths,disentangle them,open up the ends,and provide active sites for functionalization.The cutting process can be either mechanical (e.g.,by ball-milling)or chemical (e.g.,by treating them in a 3:1 mixture of concentrated sulfuric acid and nitric acid). Carbon nanotubes are available in a variety of forms.One of these forms is called the bucky paper,which is a thin film of randomly oriented SWNTs.It is made by filtering SWNTs dispersed in an aqueous or organic solution and then peeling off the nanotube film from filter paper.Carbon nanotube fibers and yarns containing aligned SWNTs have also been produced [19]. 8.3.3 FUNCTIONALIZATION OF CARBON NANOTUBES Carbon nanotubes are functionalized for a variety of purposes.Among them are(1)improve their dispersion in the polymer matrix,(2)create better bonding with the polymer matrix,and(3)increase their solubility in solvents.Functio- nalization has also been used for joining of nanotubes to form a network structure. Functionalization can occur either at the defect sites on the nanotube wall or at the nanotube ends.The functional groups are covalently bonded to the nanotubes using either oxidation,fluorination,amidation,or other chemical reactions.Functionalization can also be achieved using noncovalent inter- actions,for example,by wrapping the nanotubes with polymer molecules or adsorption of polymer molecules in the nanotubes.The covalent functionaliza- tion is generally considered to provide better load transfer between the nano- tubes and the surrounding polymer matrix,and therefore,improved mechanical properties.On the other hand,noncovalent functionalization may be preferred if it is required that the electronic characteristics of carbon nano- tubes remain unchanged. The covalent bonds can be produced in two different ways:(1)by direct attachment of functional groups to the nanotubes and (2)by a two-step functionalization process in which the nanotubes are chemically treated first to attach simple chemical groups(e.g.,-COOH and-OH)at the defect sites or at their ends(Figure 8.7),which are later substituted with more active organic groups.Carbon nanotubes have also been functionalized using silane-coupling agents [20].Silane-coupling agents are described in Chapter 2. A variety of chemical and electrochemical functionalization processes have been developed [15].One of these processes is a two-step functionalization process in which acidic groups,such as the carboxylic (-COOH)groups or the hydroxyl(-OH)groups,are first attached by refluxing carbon nanotubes in concentrated HNO3 or a mixture of H2SO4 and HNO3.One problem in acid 2007 by Taylor Francis Group.LLC.Liquid phase purification involves refluxing in an acid such as nitric acid at an elevated temperature. In general, carbon nanotubes produced by the arc dis￾charge processrequires more extensive purification than the othertwo processes. Carbon nanotubes are often formed as long, entangled bundles. The ‘‘cut￾ting’’ process is used to shorten their lengths, disentangle them, open up the ends, and provide active sites for functionalization. The cutting process can be either mechanical (e.g., by ball-milling) or chemical (e.g., by treating them in a 3:1 mixture of concentrated sulfuric acid and nitric acid). Carbon nanotubes are available in a variety of forms. One of these forms is called the bucky paper, which is a thin film of randomly oriented SWNTs. It is made by filtering SWNTs dispersed in an aqueous or organic solution and then peeling off the nanotube film from filter paper. Carbon nanotube fibers and yarns containing aligned SWNTs have also been produced [19]. 8.3.3 FUNCTIONALIZATION OF CARBON NANOTUBES Carbon nanotubes are functionalized for a variety of purposes. Among them are (1) improve their dispersion in the polymer matrix, (2) create better bonding with the polymer matrix, and (3) increase their solubility in solvents. Functio￾nalization has also been used for joining of nanotubes to form a network structure. Functionalization can occur either at the defect sites on the nanotube wall or at the nanotube ends. The functional groups are covalently bonded to the nanotubes using either oxidation, fluorination, amidation, or other chemical reactions. Functionalization can also be achieved using noncovalent inter￾actions, for example, by wrapping the nanotubes with polymer molecules or adsorption of polymer molecules in the nanotubes. The covalent functionaliza￾tion is generally considered to provide better load transfer between the nano￾tubes and the surrounding polymer matrix, and therefore, improved mechanical properties. On the other hand, noncovalent functionalization may be preferred if it is required that the electronic characteristics of carbon nano￾tubes remain unchanged. The covalent bonds can be produced in two different ways: (1) by direct attachment of functional groups to the nanotubes and (2) by a two-step functionalization process in which the nanotubes are chemically treated first to attach simple chemical groups (e.g., –COOH and –OH) at the defect sites or at their ends (Figure 8.7), which are later substituted with more active organic groups. Carbon nanotubes have also been functionalized using silane-coupling agents [20]. Silane-coupling agents are described in Chapter 2. A variety of chemical and electrochemical functionalization processes have been developed [15]. One of these processes is a two-step functionalization process in which acidic groups, such as the carboxylic (–COOH) groups or the hydroxyl (–OH) groups, are first attached by refluxing carbon nanotubes in concentrated HNO3 or a mixture of H2SO4 and HNO3. One problem in acid
2007 by Taylor & Francis Group, LLC