Green Covalent Functionalization Of Multiwalled Carbon Nanotubes And Surface Supramolecular Chemistry

Carbon nanotubes (CNTs) consist of one or more graphene sheets seamlessly wrapped into cylinder-shaped nanotubes. Their pseudo one-dimensional nanostructures enable them to possess unusually optical, electrical, magnetical, thermal and mechanical properties. CNTs have already shown great potential applications in both functional and structural materials. However, nano-sized CNTs are intrinsically insoluble in solvents, easy of aggregation and difficult of processing and manipulation due to their large aspect ratio and attractive van der Waals force. Therefore, realistic applications of CNTs are being encumbered. Organic covalent modification has been proved to be one of the most effective methods to overcome these limitations. At present, most of functionalization methods have been limited to the use of strong acids, thionyl chloride and volatilizable solvents, which all cause environmental pollution, chemical corrosion to equipments and harm to human health. Meanwhile, such methods have required multistep processingfor a long time, thus affording low functionalization efficacy.In the present dissertation, several green methods have been proposed to chemically functionalize multiwalled carbon nanotubes (MWNTs) with organic molecules and polymers based on the research background mentioned above. First, versatile functional groups were covalently attached to the surface of MWNTs by one-step free radical addition of water-soluble azo initiators. The coverage density of functional groups is rationally controlled either by adjusting the feed ratio of azo initiators to MWNTs or by utilizing the starting chemicals of multifunctional groups. The functionalized-MWNTs have much better dispersibility and stability than pristine MWNTs in polar solvents. The attached carboxylic groups were then used as scaffolds to chelate Ag+ions affording MWNT/Ag nanohybrids. Secondly, MWNTs have been successfully functionalized by free radical addition of4,4’-azobis(4-cyanopentanol) in aqueous media to generate the terminal-hydroxyl-rnodified MWNTs (MWNT-OH), followed by surface-initiated in situ ring-opening polymerization of ε-caprolactone in1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) to obtain poly(ε-caprolactone)-grafted MWNTs (MWNT-g-PCL). The amount of grafted-PCL synthesized in BmimBF4is clearly higher than that obtained in1,2-dichlorobenzene under comparable conditions. Thirdly, functionalization of MWNTs with biodegradable supramolecular polypseudorotaxanes has been successfully performed by utilizing surface-initiated ring-opening polymerization of e-caprolactone to yield PCL-grafted MWNTs (MWNT-g-PCL), followed by forming inclusion complexes between grafted-PCL chains and a-cyclodextrins (a-CDs) to give a-CD-NTPCL hybrids. There are significant differences in the morphology and solubility of MWNTs before and after introduction of a-CD. Some protuberances are clearly observed for a-CD-NTPCL as compared with MWNT-g-PCL Furthermore, the host-guest stoichiometry (monomeric unit of CL/a-CD molar ratio) for a-CD-NTPCL is much higher than that of polypseudorotaxanes consisted of linear PCL and alpha-CDs. This observation can be explained by a combination of several reasons including the steric hindrance of grafted-PCL, the competitive exclusion between adjacent PCL chains toward a-CD, and the addition order of a-CD as well as the host-guest feed ratio.The proposed methodology here uses water and room temperature ionic liquids as reaction media and promises a green chemical process for functionalizing CNTs. Moreover, surface supramolecular chemistry would open up a new opportunity for manipulation and processing of CNTs. Such novel supramolecular hybrids are expected to provide an entry to extend the applications of CNTs to medicine and biology fields through embedding the functional polymers and heterogeneous components.