New polymer films conduct heat instead of trapping it

New polymer films conduct heat instead of trapping it
By mixing polymer powder in solution to generate a film that they then stretched, MIT researchers have changed polyethylene’s microstructure, from spaghetti-like clumps of molecular chains (left), to straighter strands (right), allowing heat to conduct through the polymer, better than most metals. Image courtesy of the researchers

Polymers are usually the go-to material for thermal insulation. Think of a silicone oven mitt, or a Styrofoam coffee cup, both manufactured from polymer materials that are excellent at trapping heat.

Now MIT engineers have flipped the picture of the standard polymer insulator, by fabricating thin polymer films that conduct heat — an ability normally associated with metals. In experiments, they found the films, which are thinner than plastic wrap, conduct heat better than many metals, including steel and ceramic.

The team’s results, published in the journal Nature Communications, may spur the development of polymer insulators as lightweight, flexible, and corrosion-resistant alternatives to traditional metal heat conductors, for applications ranging from heat dissipating materials in laptops and cellphones, to cooling elements in cars and refrigerators.

“We think this result is a step to stimulate the field,” says Gang Chen, the Carl Richard Soderberg Professor of Power Engineering at MIT, and a senior co-author on the paper. “Our bigger vision is, these properties of polymers can create new applications and perhaps new industries, and may replace metals as heat exchangers.”

Chen’s co-authors include lead author Yanfei Xu, along with Daniel Kraemer, Bai Song, Jiawei Zhou, James Loomis, Jianjian Wang, Migda Li, Hadi Ghasemi, Xiaopeng Huang, and Xiaobo Li from MIT, and Zhang Jiang of Argonne National Laboratory.

In 2010, the team reported success in fabricating thin fibers of polyethylene that were 300 times more thermally conductive than normal polyethylene, and about as conductive as most metals. Their results, published in Nature Nanotechnology, drew the attention of various industries, including manufacturers of heat exchangers, computer core processors, and even race cars.

It soon became clear that, in order for polymer conductors to work for any of these applications, the materials would have to be scaled up from ultrathin fibers (a single fiber measured one-hundredth of the diameter of a human hair) to more manageable films.

“At that time we said, rather than a single fiber, we can try to make a sheet,” Chen says. “It turns out it was a very arduous process.”

The researchers not only had to come up with a way to fabricate heat-conducting sheets of polymer, but they also had to custom-build an apparatus to test the material’s heat conduction, as well as develop computer codes to analyze images of the material’s microscopic structures.

In the end, the team was able to fabricate thin films of conducting polymer, starting with a commercial polyethylene powder. Normally, the microscopic structure of polyethylene and most polymers resemble a spaghetti-like tangle of molecular chains. Heat has a difficult time flowing through this jumbled mess, which explains a polymer’s intrinsic insulating properties.

Read more: New polymer films conduct heat instead of trapping it

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