Wrapping SWNTs with Polymers

Not only limited to the PFO polymer, another fluorene-based polymer poly(9,9-dioctylfluorene-a/t-benzothiadiazole) (F8BT) is also able to discrimi-nately wrap SWNTs with larger diameters such as a (15,4) tube. It has been suggested that a stable exciplex between poly[(9,9-dioctyl-fluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) and SWNTs exists, in which the energy level matching between SWNTs and fluorene-based polymers is responsible for the chirality selectivity. 

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Since the discovery of SWNTs, they have been expected to become the building blocks of the next generation of functional nanomaterials. However, their strong cohesive property and poor solubility have restricted the use of SWNTs for fundamental and applied research fields. One method to overcome these problems is to make the SWNTs more soluble by wrapping them with polymers [31]. At the same time, the fabrication of high-performance carbon nanotube (CNT)-based composites is driven by the ability to create anisotropy at the molecular level to obtain appropriate functions. 

As with fullerenes, carbon nanotubes are also hydrophobic and must be made soluble for suspension in aqueous media. Nanotubes are commonly functionalized to make them water soluble although they can also be non-covalently wrapped with polymers, polysaccharides, surfactants, and DNA to aid in solubilization (Casey et al., 2005 Kam et al., 2005 Sinani et al., 2005 Torti et al., 2007). Functionalization usually begins by formation of carboxylic acid groups on the exterior of the nanotubes by oxidative treatments such as sonication in acids, followed by secondary chemical reactions to attach functional molecules to the carboxyl groups. For example, polyethylene glycol has been attached to SWNT to aid in solubility (Zhao et al., 2005). DNA has also been added onto SWNT for efficient delivery into cells (Kam et al., 2005). 

As discussed in Section 10.2.3, PVP and DNA have been used to wrap and water-solublize SWNTs. For specific actuator, electrical and electro-optic applications, SWNTs have been wrapped by piezoelectric polyvinylidene fluoride and trifluor-oethylene copolymer [50] or with conjugated polymers [51, 52]. The conjugated polymer used to form a composite with MWNTs and an electron-transport layer in light-emitting diodes is poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV) [53]. Wrapping coupled with electron doping has been achieved with polyethylene imine to form p-n junction devices ([40], see footnote 1). 

Unfortunately, fluorination and other sidewall functionalization methods can perturb the electronic nature of the SWNT. An approach by Smalley,and Stoddart and Heath, to increasing the solubility without disturbing the electronic nature of the SWNTs was to wrap polymers around the SWNTs but leave individual tubes electronic properties unaffected. Stoddart and Heath found that the SWNT ropes were not separated into individually wrapped tubes the entire rope was wrapped. Smalley found that individual tubes were wrapped with polymer the wrapped tubes did not exhibit the roping behavior. While Smalley was able to demonstrate removal of the polymer from the tubes, it is not clear, however, how easily the SWNTs can be manipulated and subsequently used in electronic circuits. In any case, the placement of SWNTs into controlled configurations has been by a top-down methodology, for the most part. Significant advances will be needed to take advantage of controlled placement at dimensions that exploit a molecule s small size. 

Figure 4.1 Molecular dynamic simulation snapshots of the wrapping of a SWNT (10,10) by different polymers (cross-sectional view). Reproduced with permission from Foroutan and Nasrabadi." °...

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. ... 

SWNTs were successfully dispersed by wrapping with other polymers including polystyrene sulfonate (PSS, Figure 12a,iii), poly(l-vinylpyrrolidone-co-vinyl acetate), poly(l-vinyl pyrrolidone-coacrylic acid), poly(l-vinyl pyrrolidone-co-dimethylaminoethyl methacrylate), polyvinyl sulfate, poly(sodium styrenesulfonic acid-co-maleic acid), dextran (Figure 12a,iv), dextran sulfate, and bovine serum albumin. Other polymers failed to disperse SWNTs, including poly(methyl methaciylate-co-ethyl acrylate), poly(vinyl alcohol), poly(ethylene glycol), and poly(allyl amine). 

Porphyrin-methacryUc acid polymer 48, prepared by polymerization of methyl methacrylate and transesterification as shown in Scheme 3, was also reported to form a stable complex with SWNTs in DMF through polymer wrapping [131]. The complex was found to create a long-lived charge-separated state upon photoexcitation. 

Although the supramolecules of SWNTs wrapped with porphyrin-containing polymers mentioned above were prepared by mixing SWNTs and the polymer in solution, a very stable complex of porphyrin-pyrene copolymer 49/SWNT was also synthesized by copolymerization of porphyrin and pyrene in the presence of soluble SWNTs as shown in Scheme 4 [132]. 

Blau and co-workers prepared a CNT-polymer hybrid by suspended SWNTs in organic solvents poly (p-phenylenevinylene-co-2,5-dioctyloxy-m-phenylenevinylene) to wrap the copolymer around the CNTs. The electrical properties of these hybrids were improved relative to those of the individual components. A noncovalent method has been used to functionalize SWNTs by encapsulating SWNTs within cross-linked and amphiphilic poly(styrene)-blocfe-poly(actylic add) copolymer micelles (Figure 19). This encapsulation significantly enhanced the dispersion of SWNTs in a wide variety of polar and nonpolar solvents and polymer matrices because the copolymer shell was permanently fixed. Thus, encapsulated SWNTs may be stabilized with respect to typical polymer processing and recovery from the polymer matrix. 

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