Plastics are excellent insulators, meaning they efficiently trap heat—a quality that is an advantage in something such as a coffee cup sleeve. But this insulating property is less desirable in products such as plastic casings for laptops and mobile phones, which can overheat in part because the coverings 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 to dissipate heat rather than insulating it. The new polymers, which are lightweight and flexible, conduct 10 times as much heat as most commercially used polymers.
“Traditional polymers are both electrically and thermally insulating,” says Yanfei Xu, a postdoc in MIT’s Department of Mechanical Engineering. “The discovery and development of electrically conductive polymers has led to novel electronic applications such as flexible displays and wearable biosensors.
“Our polymer thermally conducts and removes heat much more efficiently,” Xu continues. “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.”
Most polymers look like this under magnification: a tangled web of polymer strands. The tangled structure inhibits heat dissipation. A new method straightens out the polymers and lets heat move easily in all directions through the polymer. (Image: Chelsea Turner/MIT)
Polymers are made from long chains of monomers, or molecular units, linked end to end. These chains are often tangled in a spaghetti-like ball. Heat has a hard time moving through this disorderly mess and tends to get trapped within the polymeric snarls and knots.
And yet, 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, meaning they do not conduct electricity, and can therefore 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 one led by Gang Chen, head of MIT’s Mechanical Engineering Dept. Back in 2010, Chen and his team invented a method to create “ultradrawn nanofibers” from a standard polyethylene sample. The technique stretched disordered polymers into ultrathin, ordered chains. Chen found that the resulting chains let heat move easily through the material, and that the polymer conducted 300 times as much heat as ordinary plastics.
But the insulator-turned-conductor could only dissipate heat in one direction, along the length of each polymer chain. Heat couldn’t travel between polymer chains, due to weak Van der Waals forces, a phenomenon that essentially attracts two or more molecules close to each other. In an attempt to engineer polymers with high thermal conductivity, the team wanted to combine intramolecular and intermolecular forces. The team ultimately produced a heat-conducting polymer known as polythiophene, a conjugated polymer commonly used in many electronic devices.
The researchers developed a new way to create a polymer conductor using oxidative chemical vapor deposition (oCVD). In this process, two vapors are injected into a chamber and onto a substrate, where they interact and form a film. In this case, an oxidant vapor and a vapor of monomers were sent on silicon or glass substrates in the chamber.
The team measured each sample’s thermal conductivity using time-domain thermal reflectance, a technique in which a laser is shot onto the material to heat up its surface. The drop in its surface temperature is then monitored by measuring the material’s reflectance as the heat spreads into it.
On average, the polymer samples conducted heat at about 2W/meter per degree kelvin, about 10 times faster than what conventional polymers can achieve.
The polymer samples appeared nearly isotropic, or uniform, so the team theorized that the material’s properties, such as its thermal conductivity, should also be nearly uniform. Following this reasoning, the team predicted 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 looking for ways the material can be used in electronics and other products, such as casings for batteries, and films for printed circuit boards.