IONIC LIQUIDS GO BUCKY
Carbon nanotube-ionic liquid gels yield potentially useful new materials
Carbon nanotubes and room-temperature ionic liquids can be blended to form gels that may be used to make novel electronic devices, coating materials, and antistatic materials, according to researchers in Japan.
University of Tokyo chemistry professor Takuzo Aida, researcher Takanori Fukushima, and coworkers prepare the "bucky gel" materials by grinding suspensions of high-purity single-walled carbon nanotubes in imidazolium cation-based ionic liquids in an agate mortar [Science, 300, 2072 (2003)]. The researchers then use a centrifuge to separate excess ionic liquid from the desired black, viscous gel.
"We believe this discovery provides an important milestone for the fabrication of carbon-nanotube-based materials and may significantly contribute to their device applications," Aida tells C&EN. "Since bucky gels of ionic liquids are soft materials, they can be readily processed into bucky cables, bucky films, bucky sheets, and even bucky inks."
Single-walled carbon nanotubes normally exist as 3-D networks of heavily entangled bundles.
"We used transmission electron microscopy to show that the carbon nanotubes in the bucky gels are considerably untangled to give much finer bundles that physically cross-link due to the cation- interactions between the imidazolium ions of the ionic liquids and the carbon nanotube surfaces," Aida explains. "Since ionic liquids are nonvolatile, the bucky gels are thermally stable and do not shrivel, even under vacuum."
The team also shows that lowering the temperature of the gels results in long-range ordering of the ionic liquid ions and the consequent formation of crystalline materials.
||POLYMERIZABLE Imidazolium ionic liquid is used to fabricate electroconductive "bucky plastic."
© SCIENCE 2003
"This phenomenon is reminiscent of biological mineralization, during which local clustering of inorganic salts on the surface of an organic scaffold triggers controlled nucleation and growth of inorganic crystals," the authors observe.
In a separate experiment, the Japanese researchers used a bucky gel formed with a polymerizable ionic liquid to prepare a highly electroconductive plastic material reinforced with single-walled carbon nanotubes. They first prepared a black bucky gel by grinding a suspension of the nanotubes and a polymerization initiator, 2,2'-azobisisobutyronitrile, in 1-(4-acryloyloxybutyl)-3-methylimidazolium hexafluorophosphate (structure shown) for one hour. A homogeneous black polymer composite containing the nanotubes formed after heating the gel for 10 hours.
Aida says that by incorporating just 4% by weight of the carbon nanotubes, the mechanical properties of the polymer, such as dynamic hardness, are enhanced by around 400% because of the strong interfacial interaction between the nanotubes and the ionic liquid polymer.
The research was carried out under the Aida Exploratory Research for Advanced Technology Nanospace Program, which is supported by Japan Science & Technology Corp.