SciTech

Polyethylene nanofiber displays extraordinary thermal properties

Diagram of molecular shapes of polymers in their normal amorphous state, polyethylene nanofiber in its linear state, and polyethylene nanofiber as a thermal insulator. The dots indicate regions with rotational disorder that slow heat transfer. (credit: Kian Nassre/) Diagram of molecular shapes of polymers in their normal amorphous state, polyethylene nanofiber in its linear state, and polyethylene nanofiber as a thermal insulator. The dots indicate regions with rotational disorder that slow heat transfer. (credit: Kian Nassre/)

Materials research has seen tremendous evolution over the past century, and continues to evolve today. One of the more recent discoveries in the field is a polymer that can rapidly switch between acting as a thermal conductor and insulator.

This development arose through research conducted by Carnegie Mellon University associate professor of mechanical engineering Sheng Shen, in his work investigating mechanically fabricated polyethylene nanofibers. These nanofibers are produced by stretching ordinary polyethylene, the plastic used in plastic bags.

“Generally in bulk polyethylene, we have the polymer chains tangled. But when we get the nanofiber by stretching it, it becomes highly aligned, and that makes it crystalline”, said Xiao Luo, a master’s student working under Shen, in an interview with The Tartan. “What’s interesting is that at low temperatures, it is very thermally conductive. But at a certain temperature, the polymer chains become twisted in subregions. Such disorder indicates a huge decrease in thermal conductivity”

Polymers are molecular chains that typically tangle into formations resembling a pile of wet spaghetti. Organic polymers have a spine of carbon atoms with different R groups branching off the side. The more complex the R group, the less likely that the polymers will form straight lines.

Polyethylene has one of the simplest R groups: lone hydrogen atoms. At room temperature, the polyethylene nanofiber conducts heat roughly 100 times better than bulk polyethylene. When the nanofiber reaches roughly 430 Kelvin (314 ºF), rotational disorder occurs without disrupting the long-range order, essentially creating a firewall that slows down the transmission of heat. As a result, above 430 Kelvin, the polymer maintains its unusual stiffness but rapidly switches from being a thermal conductor to a thermal insulator.

This property was not entirely surprising to the team, as it has been hypothesized for some time. “The crystallization of polyethylene by stretching it was discovered as early as the 1980s. But at that time, the technique was limited to a microscale. Even the 10-micrometer nanofiber, they found, had high crystallinity,” Luo said, “This piece of work was inspired by a molecular dynamics simulation… We proved that this kind of material behavior exists in nature.”

As of now, the fabrication of the nanofiber is likely too inefficient to be jumping ahead towards commercial uses, but the properties may someday have astounding uses. Luo hopes to improve the fabrication process to make the nanofiber longer, beyond 1 millimeter. Since polyethylene is relatively inexpensive, we may see it used as a sensor in all kinds of microelectronic devices.