In common with the discovery in 2002 that a camera flash could ignite single-walled carbon nanotubes, the researchers stumbled across the effect when trying to photograph their samples.

"I was very surprised," said Richard Kaner of the University of California, Los Angeles. "My graduate student, Jiaxing Huang, decided to take some pictures of his polyaniline nanofibres one evening when he heard a distinct popping sound and smelled burning plastic."

"Jiaxin recalled a paper that we had discussed during a group meeting reporting that carbon nanotubes burned up in response to a camera flash," continued Kaner. "By adjusting the distance of the camera flash to his material he was able to produce smooth films with no burning, making this new discovery potentially useful."

The team found that exposure to a camera flash caused the polyaniline fibres to chemically cross-link. According to the scientists, although the energy from the camera flash isn't enough to melt bulk polymer, it's likely to cause local hot spots, particularly around the chromophores on the polyaniline chains.

The researchers say these chromophores absorb visible light due to photothermal conversion of polyaniline, which may cause more exothermic crosslinking reactions between polymer chains. The heat released by the photothermal conversion and crosslinking reactions could then accumulate within the nanofibres, as polyaniline has a low thermal conductivity.

Unlike carbon nanotubes, the nanofibres did not burn unless the flash was less than 0.5 cm away. The scientists propose that melting of the polymer nanofibres may drain the heat away benignly, avoiding combustion.

Exposing a random network of conducting polyaniline nanofibres to a camera flash caused the nanofibres on the surface to form a continuous film. The fibres on the underside of the film remained unaffected.

As the cross-linking process also changes the solubility of the fibres, the technique could form a relatively quick and easy way of making asymmetric films for use in separation membranes, chemical sensors and actuators.

And by using a photomask, the researchers were able to weld certain areas of the film selectively. This imprinted a pattern of smooth cross-linked areas. Kaner and colleagues say that since many properties of polyaniline (such as conductivity, surface area, optical absorption, permeability and thermal stability) change after cross-linking, this technique could be especially useful for organic micro- or nano-devices.

The team also showed that they could flash-weld polyaniline nanofibres to one-micron diameter spheres of polystyrene.

"We can envision welding other materials together as well," said Kaner. "One way to do this is to take two blocks of a conventional polymer and insert polyaniline nanofibres between them, then induce the cross-linking reaction to produce enough heat to weld the polymer blocks together. We can weld polyaniline to itself or to another polymer or potentially use it to join conventional polymers together."

The researchers reported their results in Nature Materials.