Producing high performance fibres from long, flexible objects like polymer chains or carbon nanotubes (CNTs) is challenging because of their tendency to tangle and stick to each other in a random arrangement. CNT fibres can be 'spun' directly from a continuous floating catalyst chemical vapour deposition (FCCVD) method, a process first demonstrated here in the Department nearly 20 years ago, but the strengths and conductivities of the unprocessed fibres are much lower than the component CNTs.

However, by adding surface charge to the CNTs by using a strong (chlorosulfonic) acid, the fibres can be stretched and aligned, before being treated with a solvent (chloroform) to remove any excess acid and straighten the remaining tubes. We call this the "double-drawing" process. The resulting fibres have a dramatically improved combination of mechanical and electrical/thermal properties, which makes them suitable for applications like heat shields or giving protection from lightning strikes.

Figure caption: A 2D schematic of the optimization of "effective bundles" and the effective length of tubes within a bundle after Double-Drawing process. (a) in the disordered CNT network, tubes (lines) aggregate as bundles (marked as ① to ④). Within the schematic cell under vertical stretching, only shortest bundle connecting the vertical surfaces takes load, i.e., being "effective" (coloured in red). Tubes outside the loaded bundle remain idle despite portions being oriented parallel to the load (circled with blue dotted circle). For tubes that link to multiple bundles, only the length section of tubes attached to an effective bundle take load (solid blue arrows), while the external length section remains idle (hollow blue arrows). (b) With the Double-Drawing process, the crumpled CNTs are straightened as the shortest pathway and more tubes are compressed into a large bundle ([②③④]), extending the effective length section.