Turning plastic insulator into heat conductor

2 min read

Plastics are excellent insulators — a quality that can be an advantage in something like a coffee cup sleeve. But it is less desirable in products such as plastic casings for laptops and mobile phones, which can overheat, as they can trap the heat that the devices produce.

Now a team of engineers at MIT has developed a polymer thermal conductor — a plastic material that works as a heat conductor, dissipating heat rather than insulating it. These lightweight, flexible polymers are claimed to conduct 10 times as much heat as most commercially used polymers.

“Traditional polymers are both electrically and thermally insulating. The discovery and development of electrically conductive polymers has led to novel electronic applications such as flexible displays and wearable biosensors,” explained Yanfei Xu, a postdoc in Professor Gang Chen’s lab at MIT’s Department of Mechanical Engineering. “Our polymer can thermally conduct and remove heat much more efficiently. We believe polymers could be made into next-generation heat conductors for advanced thermal management applications, such as a self-cooling alternative to existing electronics casings.”

At the microscopic level, polymers are made from long chains of monomers linked end to end. These chains are often tangled in a spaghetti-like ball. Heat carriers have a hard time moving through this disorderly mess and tend to get trapped.

Still, researchers have attempted to turn these natural thermal insulators into conductors. For electronics, polymers would offer a unique combination of properties, as they are lightweight, flexible, and chemically inert. Polymers are also electrically insulating and can therefore be used to prevent devices such as laptops and mobile phones from short-circuiting in their users’ hands.

Several groups have engineered polymer conductors in recent years, including Prof Chen’s group which, in 2010, invented a method to create ‘ultradrawn nanofibers’ from a standard sample of polyethylene. The technique stretched the polymers into ultrathin, ordered chains. Chen found that the resulting chains enabled heat to move along and through the material, and that the polymer conducted 300 times as much heat compared with ordinary plastics.

But the insulator-turned-conductor could only dissipate heat along the length of each polymer chain, not between the chains, due to weak Van der Waals forces.

In an attempt to engineer polymers with high thermal conductivity, Xu simultaneously engineered intramolecular and intermolecular forces in the hope that heat could be transported along and between the polymer chains.

The team ultimately produced a heat-conducting polymer known as polythiophene, a type of conjugated polymer that is commonly used in many electronic devices.

Xu, and members of Prof Chen’s lab teamed up with Karen Gleason, Professor of chemical engineering, and her lab members to develop a new way to engineer a polymer conductor using oxidative chemical vapor deposition (oCVD), whereby two vapours are directed into a chamber and onto a substrate, where they interact and form a film. “Our reaction was able to create rigid chains of polymers, rather than the twisted, spaghetti-like strands in normal polymers.” Xu says.

In their studies, the oxidant flowed into a chamber, along with a vapour of monomers — individual molecular units that, when oxidised, form into the polymer chains at relatively large-scale samples of 2cm2.

“Because this sample is used so ubiquitously, as in solar cells, organic field-effect transistors, and organic light-emitting diodes, if this material can be made to be thermally conductive, it can dissipate heat in all organic electronics,” said Xu.

The team measured each sample’s thermal conductivity using time-domain thermal reflectance — a technique where a laser is shot onto the material to heat up its surface and then monitor the drop in its surface temperature by measuring the material’s reflectance as the heat spreads into the material.

On average, the polymer samples conducted heat at about 2 watts per metre per kelvin — about 10 times faster than conventional polymers. A team at Argonne National Laboratory found that polymer samples appeared nearly isotropic, or uniform. This means that the material should conduct heat equally well in all directions, increasing its heat-dissipating potential.

Going forward, the team will continue exploring the fundamental physics behind polymer conductivity, as well as ways to enable the material to be used in electronics and other products, such as casings for batteries, and films for printed circuit boards.

Xu concluded: “If we can understand how thermal transport [works] in these disordered structures, maybe we can also push for higher thermal conductivity. Then we can help to resolve this widespread overheating problem, and provide better thermal management.”