“Unfortunately experience has shown that single-walled carbon nanotubes tend to clump together or form three-dimensional networks in water at concentrations where theories otherwise predict they will form a nematic liquid-crystal phase,” said Arjun Yodh of the University of Pennsylvania. “Our gels effectively increase the concentration of isolated single-walled carbon nanotubes without allowing them to bundle up or form networks.”
To make the nanotube gels, Yodh and colleagues coated single-walled carbon nanotubes with sodium dodecyl benzene sulphonate surfactant, and dispersed them at a low concentration in an aqueous solution of isopropyl acrylamide monomer. Then they treated the monomer so that it polymerized and cross-linked to form a gel. Heating the gel to 323 K caused the polymer to become hydrophobic and expel water, decreasing the gel’s volume by a factor of eight.
Birefringence measurements indicated that in gels containing a higher concentration of carbon nanotubes, the nanotubes aligned after this volume compression process. The sidewalls of these samples also buckled. The nanotubes tended to align parallel to the sidewalls and the alignment was larger near the edges of the gel. Adding water to the gels resulted in them swelling back to their original size and the nanotubes losing their alignment.
The researchers figure that the gel played two roles - it prevented the close contact between nanotubes that causes bundling, and it compressed the nanotubes to concentrations that favour their alignment and that aren’t achievable in aqueous suspensions.
“We expect the mechanical, electrical and perhaps thermal properties of the resulting composites to differ from their unaligned counterparts,” said Yodh’s colleague Mohammad Islam. “It might be possible to use a local influx of particular chemicals to cause mechanical deformations in the gel. Similarly, external fields could interact with the nanotubes, which in turn would interact and deform the background polymer network.”
The researchers reported their work in Physical Review Letters.