US physicists simplify the process of nanotube alignment.

US physicists simplify the process of nanotube alignment.

Single-wall carbon nanotubes (SWNTs) are remarkably strong and are famed for their ability to conduct heat and electricity. They work best when aligned parallel to one another to form a nematic liquid crystal phase, a set-up which is notoriously difficult to achieve. By trapping SWNTs in a gel, US physicists from the University of Pennsylvania, Philadelphia, claim to have aligned high concentrations of nanotubes.

In solutions with low SWNT concentrations, nanotubes are generally randomly oriented. Increasing the SWNT concentration encourages the nanotubes to line up but as ArjunYodh, the Pennsylvania team leader, explains, ’experience has shown that single-wall carbon nanotubes tend to clump together or form three dimensional networks in water at concentrations where theories otherwise predict that they will form this nematic liquid crystal phase’.

To get around this problem, the team dispersed SWNTs in an aqueous solution of a monomer which they polymerised to form a gel containing randomly oriented SWNTs. Increasing the temperature caused the gel’s polymer network to become hydrophobic, triggering the gel to collapse and expel water. Shrinking the gel aligned the SWNTs. ’Our gels effectively increase the concentration of isolated single-wall carbon nanotubes without allowing them to bundle up or form networks,’ explains Yodh.

Yodh told Chemistry World that the team is currently investigating how the gels respond to electric fields and to changes in the solvent. He suggests that such research may make the gels useful for ’sensing applications’. The researchers are also thinking of creating composite materials using the SWNT gels.

Emma Davies