The researchers investigated a range of nanotube diameters (5–8, 10–20, 20–30 and 50–70 nm) and lengths (10–20, 1–2 µm). In addition, the group coated some of its nanotube samples with an ultra-thin PMMA layer (2–3 nm) to make the material easier to disperse.

"By coating the nanotubes with a low surface energy polymer, we thought that we could limit the amount of clustering and exploit the MWCNTs remarkable surface area to weight ratio to the maximum," Nikhil Koratkar told nanotechweb.org.

The larger surface area to volume ratio of carbon nanotubes means that they should outperform larger micro-sized fibres in use today as crack-bridging agents, but only if the MWCNTs can be well-dispersed throughout the epoxy host.

Bridging the gap
Fatigue testing in accordance with ASTM standard E647-05 revealed that the MWCNTs with the highest aspect ratios produced the best results.

"We measured up to a 10-fold lowering in the crack growth rates with the addition of less than 0.5% weight of nanotube additives," said Koratkar.

By using PMMA-coated MWCNTs, the team was able to take this result a step further and showed that a 20-fold reduction in crack growth rate is possible with only 0.25% weight of filler.

AFM images revealed that the PMMA-coated nanotubes are dispersed much more uniformly in the solvent before mixing with epoxy than the untreated material.

The researchers presented their work in Nanotechnology.