Micelle-Encapsulated Carbon Nanotubes
Single-walled carbon nanotubes (SWNTs) has been a great interest due to their unique properties and those have motivated researchers to investigate their use in variety of application such as electrostatic discharge, structural reinforcement, and thermal dissipation. However, SWNT has main weakness that hinders the development of its application which is poor solubility and processibility. Many processing techniques and modification on the surface of nanotubes, either covalently or noncovalently, have been developed.
Taton's group from University of Minnesota developed a way to encapsulate carbon nanotubes using crosslinked, amphiphilic copolymer micelles.
The strategy to encapsulate SWNTs within amphiphilic copolymer shells by dissolving the copolymer, in this case is poly(styrene)-block-poly(acrylic acid) (PS-b-PAA), in dimethylformamide (DMF) and SWNTs were suspended in the same solution. Dissolving PS-b-PAA in DMF does not cause micellisation as both hydrophobic PS and hydrophilic PAA are solvated well by DMF.
The formation of micelles was induced by the addition of water to the suspension. Under the micellisation, PS blocks point inward toward the nanotubes forming hydrophobic core while PAA blocks point outward to give hydrophilic shell. This self-assembly shows how PS-b-PAA could stabilise SWNT effectively. After the micellisation, the micellar shells were crosslinked permanently using diamine linker and carboimide activator EDC. Furhtermore, SWNTs remanined solublised during the cross-linking process. The encapsulated SWNTs (e-SWNTs) can be purified from excess reagents using several cycles of centrifugation and e-SWNTs can be easily redispersed in water, both hydrophobic and hydrophilic solvents, and also polymer solution; SWNTs and PS-b-PAA cannot be redisperesed. Furthermore, e-SWNTs can also be casted from combined solution of e-SWNTs and polymers yielding bulk e-SWNTs composite gels, rubbers and solids in which the SWNTs were well dispersed.
The degree of shell cross-linking can be varied by adjusting diamine linker and interestingly complete cross-linking is not necessary to stabilise e-SWNTs; the minimum 25% of acid group converted to amide is required. Besides that, high yield of isolated e-SWNTs was observed at high H2O:DMF ratio and it might be due to stabilisation of PS block core at high water content. Furthermore, SWNTs cannot be stabilised by simply mixing empty, preformed, or precross-linked PS-b-PAA. This shows that the necessity of PS-b-PAA to be assembled in the presence of SWNTs.
The e-SWNTs were characterised using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The characterisation of unpurified e-SWNTs using tapping mode AFM showed a mixture of spherical, empty micelles, and straight cylindrical e-SWNTs. The cylindrical objects were straighter than typical wormlike micelles and this indicates the presence of straight SWNT within the micelle's core. Furthermore, TEM images showed that the micelles contained only single SWNTs rather than SWNTs bundles.
This method of encapsulating SWNTs using block copolymer surfactant provides a novel way to enhance SWNTs solubility and processibility. Furthermore, e-SWNTs are also compatible with a wide variety of solvent and polymer matrices, e-SWNTs can be used as alternative starting material to pure SWNTs for production and investigation of nanotube composite materials.
Referemce
Y. Kang and T. A. Taton, J. Am. Chem. Soc., 2003, 125 5650-5651.
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| Scanning tunnelling microscopy image of nanotubes. (T. W. Odom, J.-L. Huang, and C. M. Lieber, J. Phys.: Condens. Matter, 2002, 14, R145-R167). |
The strategy to encapsulate SWNTs within amphiphilic copolymer shells by dissolving the copolymer, in this case is poly(styrene)-block-poly(acrylic acid) (PS-b-PAA), in dimethylformamide (DMF) and SWNTs were suspended in the same solution. Dissolving PS-b-PAA in DMF does not cause micellisation as both hydrophobic PS and hydrophilic PAA are solvated well by DMF.
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| The general strategy of SWNTs encapsulation. |
The degree of shell cross-linking can be varied by adjusting diamine linker and interestingly complete cross-linking is not necessary to stabilise e-SWNTs; the minimum 25% of acid group converted to amide is required. Besides that, high yield of isolated e-SWNTs was observed at high H2O:DMF ratio and it might be due to stabilisation of PS block core at high water content. Furthermore, SWNTs cannot be stabilised by simply mixing empty, preformed, or precross-linked PS-b-PAA. This shows that the necessity of PS-b-PAA to be assembled in the presence of SWNTs.
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| AFM images of (a) unpurified e-SWNT material, (b) a bundled pair of e-SWNTs, (c) purified e-SWNTs; (d) and (e) TEM images of purified e-SWNTs. |
This method of encapsulating SWNTs using block copolymer surfactant provides a novel way to enhance SWNTs solubility and processibility. Furthermore, e-SWNTs are also compatible with a wide variety of solvent and polymer matrices, e-SWNTs can be used as alternative starting material to pure SWNTs for production and investigation of nanotube composite materials.
Referemce
Y. Kang and T. A. Taton, J. Am. Chem. Soc., 2003, 125 5650-5651.
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