Scientists have been studying carbon nanotubes for the last 20 years because these materials have many remarkable properties that include extremely high tensile strength and high electrical conductivity. Now, Ming Xu of AIST in Tsukuba and colleagues have discovered yet another exceptional property in these tubes – viscoelasticity over a wide temperature range.

Viscoelastic materials behave like thick liquids (for example, honey) but are also reversibly elastic, like rubber bands. One example of such a material is polymer foam – widely used in earplugs that adapt themselves to the shape of your ear yet recover their original form after they are removed. Viscoelasticity is seen in a variety of materials, including amorphous and semicrystalline polymers, some biomaterials, crystals and even some metallic alloys.

Random networks

The new rubber is made from a random network of interconnected single-, double- and triple-walled carbon nanotubes and has the same viscoelasticity as that of the most thermally resistant silicone rubber at room temperature. However, silicone rubber only retains its viscoelasticity between –55 °C and 300 °C. The new material remains flexible over a much higher temperature range, can recover its shape after being repeatedly deformed and shows excellent fatigue resistance.

Xu's team began by depositing metal catalysts on a silicon substrate. These catalysts act as seeds for growing the nanotubes from a carbon source, such as ethylene. A drop of water (100–200 ppm) added to the mix greatly increases carbon nanotube growth and produces long tubes.

The carbon nanotubes normally just grow upwards using such a technique, but by pre-treating the catalyst, the researchers succeeded in lowering the density of the tubes to create an entangled network of long tubes as growth progresses – similar to vines in a jungle, says Xu. "Importantly, an individual carbon nanotube cannot stand on its own, so as one tube grows from the substrate, it touches another tube for support. This results in a network of tubes that contact each other via Van der Waals forces."

Zipping and unzipping

According to the team, the network is highly stable over a broad temperature range thanks to the energy dissipated as the individual nanotubes zip and unzip at the points of contact. The carbon nanotubes themselves are also very heat resistant – between 2000 °C and 3000 °C – so an even broader temperature range might be possible for this rubber.

As for potential applications, Xu says that they are not yet sure since the material made is totally new and unique with hitherto unseen properties. "We are currently searching for applications that could benefit from such temperature invariant properties," Xu said.

Yury Gogotsi of Drexel University says that the entangled nanotube material "is a kind of versatile rubber that could be used in cold interstellar space or inside a high-temperature vacuum furnace". With further developments, such a material may find use not only in space vehicles but also in more down-to-earth applications, such as wrinkle-free textiles or viscoelastic shoe insoles that reduce mechanical shocks, he adds.

The Japan team, which includes researchers from the National Science and Technology Agency in Kawaguchi, would now like to tailor the viscoelastic properties in the nanotube networks to create softer, stronger or more elastic materials. "Such an approach would also help us meet the demands of target applications," says Xu.

The work was reported in Science.