The electronic, mechanical and optical properties of carbon nanotubes make them highly attractive for use in a variety of technological applications. To make most effective use of their properties, however, it is necessary to align the nanotubes uniformly — a process made difficult by the fact that carbon nanotubes have low solubility and hence are reluctant to disperse well in solution. Kunihiko Okano, Ikuyo Noguchi and Takashi Yamashita from the Tokyo University of Science1 have now developed a preparation method that solves both problems.

One approach to improving the solubility of carbon nanotubes is to affix them with solubilizing groups. As covalent modification results in structural changes that can be detrimental to their physical properties, however, most existing approaches rely on what is known as ‘supramolecular functionalization’ — the binding of solubilizing groups to the nanotubes by a combination of many weak intermolecular forces.

Yamashita and his colleagues adopted a supramolecular approach, but with a twist. “We thought that we should be able to use a surfactant effect to disperse the nanotubes,” explains Yamashita. “By adding a polymer with both aromatic and polar ionic groups, the nanotubes could be dissolved in water.” The polymer backbone contains multiple aromatic groups, which are attracted to the aromatic framework of the carbon nanotubes, as well as multiple sulfonic acid groups, which help to solubilize the nanotube–polymer conjugate in water.

Fig. 1: Crossed-polarization microscopy images showing the uniformity of nanotube alignment in liquid crystal. (Left) Pure liquid crystal. (Center, Right) Liquid crystal with 1.0 and 10 mg/mL of carbon nanotubes. A more even appearance indicates higher uniformity of molecular alignment.From Ref. 1. Reproduced with permission. © 2010 ACS

To align the nanotubes, the researchers chose a polymer with liquid crystalline properties. In the liquid crystalline state, the molecules could move around like in a liquid, but with a limited range of motion, causing the molecules to align with one another. In a dilute solution in water, the nanotube–polymer conjugates tended to self-align (Fig. 1), and when the solution was cast onto a surface and the water evaporated, the alignment of the nanotubes was locked in place.

“Our work will allow further practical applications of carbon nanotubes, such as in the production of printable or flexible electronics,” says Yamashita. “The liquid crystal properties also present the opportunity to prepare devices that can be switched under heating or light irradiation, and we plan to look into this in our future research.”