Welding done in a flash

Chemists have found that an ordinary camera flash causes the instantaneous welding together of nanofibres made of polyaniline, a synthetic polymer that can be made in conducting or insulating form.

UCLA Chemists have found that an ordinary camera flash causes the instantaneous welding together of nanofibres made of polyaniline, a synthetic polymer that can be made in conducting or insulating form. The discovery, which the chemists call “flash welding,” is published in the November issue of the journal Nature Materials.

Numerous applications could potentially result from this research in such areas as chemical sensors, separation membranes and nano devices.

“We used an ordinary 35-millimeter camera, but you could also use a laser, or any other high-intensity light source,” said Richard B. Kaner, UCLA professor of inorganic chemistry and materials science and engineering, and a member of the California NanoSystems Institute at UCLA.

“I was very surprised,” Kaner said. “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. Jiaxing recalled a paper that we had discussed during a group meeting reporting that carbon nanotubes burned up in response to a camera flash. 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 camera flash induces a chemical reaction; it starts a chain reaction in which the tiny nanofibres interact and cross-link, producing heat, which leads to more spontaneous cross-linking across the entire surface of the nanofibres, welding them together, Kaner said. Unlike carbon nanotubes, which burn up, this material is thermally absorbent and can dissipate the heat well enough so that it does not burn.

“We can envision welding other materials together as well,” Kaner added. “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.”

Because only the part exposed to light welds together, chemists can create patterns by covering sections that they do not want welded, thus controlling what parts weld together.

Kaner’s research team searched for whether any conventional techniques have this same welding property. They found a recent commercial process called laser welding, now used in the electronics industry, in which a laser beam is used to weld together conventional polymers. “The trouble with laser welding,” Kaner said, “is that lasers generally have a small cross-section and consume a lot of power. Our research has the potential of revolutionising this process.”

Nanofibres have high surface areas and important properties, from sensing to flash welding. “This shows why nano is important,” Kaner said. “Here’s a good example of where nano materials possess a property that conventional materials do not have.”

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