Novel polymer can keep cool as the heat rises

Polymers are used to develop various materials, including plastics, nylons and rubbers. In their most basic form, polymers are made up of many of identical molecules joined together like a chain. Engineering these molecules to join together in specific ways can allow researchers to control the characteristics of the resulting polymer.

Using this method, an international team led by Sheng Shen, an associate professor of mechanical engineering at Carnegie Mellon University, created a polymer thermal regulator that can quickly transform from a conductor to an insulator, and back again. When it's a conductor, heat transfers quickly; when it's an insulator, heat transfer much more slowly. By switching between the two states, the thermal regulator can control its own temperature, as well as the temperature of its surroundings, such as a refrigerator or computer.

In order to switch between high and low thermal conductivity, the very structure of the polymer has to change. This transformation is activated solely with heat. The polymer starts "with a highly-ordered crystalline structure," Shen said. "But once you increase the temperature of the polymer fiber, to around 340K, then the molecular structure changes and becomes hexagonal." Shen and his team report their work in a paper in Science Advances.

The transformation occurs because the heat targets the polymer’s molecular bonds. "The bonding of the molecules becomes pretty weak," Shen explained. "So the segments can rotate." And once the segments rotate, the structure becomes disordered, greatly reducing its thermal conductivity. This type of transition is known as a solid-solid transition: although the polymer reaches temperatures close to its melting point, it remains a solid throughout the process.

When studying the polymer's transformation, Shen focused on how its conductivity changed. He also gathered data on other phase transitions so he could compare the ratios. "When you look at all the materials we have on Earth, the conductivity change is, at most, a factor of four," Shen said. "Here, we've already discovered a new material that can have a conductivity change of around 10."

Additionally, the structural change can happen quickly, within a range of 5K. It's also reversible, which allows it to be turned on and off like a switch.

The polymer can handle much higher temperatures than other thermal regulators, remaining stable up to 560K. It's hard to break down, so it can survive many transitions. And since it's heat-based, it doesn't require as many moving parts as typical cooling methods, making it much more efficient.

While this research has been explored theoretically in the past, Shen's work is the first time it's been shown experimentally. He believes that the polymer will have many real-world applications. "This control of heat flow at the nanoscale opens up new possibilities. Such as developing switchable thermal devices, solid-state refrigeration, waste heat scavenging, thermal circuits and computing."

This work builds on previous research in Shen's lab, where his team developed a polymer nanofiber that was strong, lightweight, thermally conductive, electrically insulating and biocompatible – all at less than 100nm wide.

This story is adapted from material from Carnegie Mellon University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.