Process ‘problem’ probed

(NIST) and university researchers report a significant step toward solving the nanotube "problem", namely the challenge of overcoming processing obstacles so that the properties of the cylindrical structures can be exploited in new polymer composite materials of exceptional strength.

Described in the current issue of Physical Review Letters, their analysis reveals that, during mixing, carbon nanotubes suspended in viscous fluids can be encouraged to sort themselves by length. Achieving uniform sizes of nanotubes is one of several keys to producing affordable, high-quality polymer nanocomposites.

The team found that, under common processing conditions, shorter carbon nanotubes will flow toward the walls of mixing equipment, while the longer tubes tend to congregate in the interior.

Better understanding of factors that promote this self-sorting will point the way to process adjustments and devices that achieve desired arrangements of nanotubes during bulk manufacturing of polymer nanocomposites, says NIST's Erik Hobbie, leader of the collaboration, which included scientists from the University of Kentucky and Michigan Technical University.

Many times stronger than steel and possessing superlative thermal, optical and electronic properties, nanotubes have been called small-scale wonders, measuring a few nanometres in diameter and ranging greatly in length. Anticipated nanotube-based technologies range from hydrogen storage to transistors to space elevators. Nearest on the horizon are lightweight, high-strength carbon nanotube polymer structural composites.

With lasers, video microscopes and other optical monitoring equipment, the team tracked how nanotubes, both the single-wall and multiwall varieties, behave when suspended, at several different concentrations, in a polymer melt. They analysed suspensions ranging in viscosity from syrup-like to watery under different mixing conditions.

The results did not suggest a "magic bullet" for getting nanotubes to align uniformly in the same direction--also critical to reliable processing of high-quality nanocomposites. But the finding that, under "modest flow conditions," carbon nanotubes will sort by length could point the way to practical methods for bulk separation of nanotubes according to size.

Small-angle neutron scattering pattern provides an inverted representation of how carbon nanotubes flowing in a polymer melt sort themselves by length. Longer nanotubes, which scatter neutrons at lower angles, gather in purple regions, while medium-sized and short nanotubes are indicated by red and yellow, respectively. The dark blue circle in the centre of the image is the beam stop, which protects the sensitive detector from the transmitted beam of unscattered neutrons