CRIS -Creative Research Institution- Hokkaido University

【Division of Innovative Research】Bunshi FUGETSU

Bunshi FUGETSU Nanotechnology & Materials
Creation and characterization of hybrid-materials with carbon nanotubes as the key-elements
Bunshi FUGETSUProfessor

Many potential applications implied by their remarkable electrical, mechanical properties and the unique morphologies have been proposed for the single-walled carbon nanotubes (SWCNTs). Their self-assembling characteristics, i.e., they self-assemble to form crystalline bundles (ropes), on the other hand, have been the important barriers for their chemical and/or physical manipulation, and thus their use in practical applications. Surfactant-based isolation, i.e., the use of surfactants, such as sodium dodecyl sulfate (SDS) to isolate the individual (unbundled) SWCNTs from the bundled ones, is the cornerstone for preparation of the unbundled SWCNTs. Although other methods, such as the so-called non-covalent and/or the covalent P side-wall functionlization and the polymer-molecular wrapping find also applications. The surfactant-based method, in fact, is rather time-consuming, for a number of reasons. First, a mechanical disassembling procedure (such as the high-shear mixing and/or sufficient ultra-sonication) must be used throughout to produce the primarily individual SWCNTs. In other words, the entire surfactant power is not high enough to disassemble the SWCNT bundles. Second, fine (smaller) bundles and the individual tubes are both contained in the homogeneous dispersion. A centrifugation machine of ultra-high performance was required for isolating the individual tubes from the fine bundles. Third, the procedures for mechanical disassembly, for surfactant dispersion, and for the centrifugation isolation must be done repetitively, till all the bundles were being converted into the individual tubules. We have established a simple yet powerful method with which large quantities of unbundled SWCNTs can be obtained within a single step. Dipole/dipole electrostatic interactions were used as the driving forces to disassemble the SWCNT bundles. When mixed with zwitterionic surfactants the pristine SWCNTs formed highly viscous gels after being ground. The gels were highly dispersible in aqueous solution. Typical atomic force microscopy (AFM) and the high-resolution transmission electron microscopy (HR-TEM) images of aqueous SWCNTs samples are shown in Figure 1.

 When mixed with SWCNT bundles, the zwitterionic surfactants form the self-assembled diads and/or quartets because of the electrostatic attraction. These constituent blocks then attach on to the SWCNT bundles and aggregate with one another to form large amphiphiles, which may be called "self-assembled zwitterionic monolayer (SAZM)". The positive/negative charged headgroups orientated toward the terminal-end, and most of them form anti-parallel doublets. The formation of the anti-parallel doublets is a general phenomenon, driven largely by the electrostatic forces to diminish the repulsions between the same charges of the headgroups of the zwitterionic surfactants. The self-assembled zwitterionic surfactant monolayer should have high propensities to interact with the others because of the strong dipole/dipole electrostatic interactions. The dipole/dipole electrostatic interactions occurred readily, typically as the SAZM/SWCNT -bundles being well ground. The SWCNT bundles were disassembled into their constituent elements as the dipole/dipole interactions taking place (Figure 2). After the bundles are being disassembled, a cylindrical SAZM have further created with the unbundled SWCNTs being acting as kernels.

The unbundled SWCNTs obtained using this method can be used directly for developing numbers of SWCNT-composites. Preparation of SWCNT-embedded materials by adding the unbundled SWCNTs in to desirable matrix is under investigation by our group.