Jinsang Kim group developed heat-conducting plastics

New plastic engineered to let heat escape 10 times faster
Jinsang Kim group developed heat-conducting plastics

Gun-Ho Kim, a research fellow in Jinsang Kim's research group, holds a sample of a polymer.

The spaghetti-like internal structure of most plastics makes it hard for them to cast away heat, but a University of Michigan research team has made a plastic blend that does so 10 times better than its conventional counterparts.


Plastics are inexpensive, lightweight and flexible, but because they tend not to pass thermal energy, they can’t be used more widely in technologies like computers, smartphones, cars or airplanes – places that could benefit from their properties but where heat control is important. The new U-M work could lead to lighter, versatile, metal-replacement materials that make possible more powerful electronics or more efficient vehicles.


“People have spent a lot of time designing polymers that conduct electricity for organic LEDs or solar cells, but no one has looked at how to engineer the thermal properties and we need polymers that conduct heat a lot better than the ones we have today.” said Kevin Pipe, an associate professor of mechanical engineering and corresponding author of a paper on the work published in the current issue of Nature Materials.


“A polymer is a long linear molecule made of smaller repeating units so-called monomer, like a long pearl necklace made of pearl beads as a monomer.” said Jinsang Kim, an associate professor of materials science and engineering and another corresponding author of the paper.


In order to efficiently pass heat through a material, constituent atoms and molecules must be strongly bound each other to provide continuous pathway for the heat to conduct. Otherwise substances stay hot when this energy can’t escape. The newly developed plastic mixture results from one of the first attempts to engineer the isotropic flow of heat in what the researchers call an amorphous polymer film.


“The polymer chains in a plastic are like spaghetti,” Pipe said. “They’re long and they don’t bind to one another well. When heat is transferred by vibrational waves in there, the spaghetti moves around a lot, but the heat has a hard time propagating out.”


Pipe and Kim research groups devised a way to link long polymer chains of polyacrylic acid (PAA) with short strands of polyacryloyl piperidine (PAP) like long spaghetti noodles that bind themselves better through molten cheese. The hydrogen bonds that form between them are 10 to 100 stronger than the so-called van der Waals forces that loosely hold the long strands in most other plastics.


“We improved those connections so the heat energy can find continuous pathways out of the material,” Kim said. “There’s still a long way to go, but this is a very important step we made to understand how to engineer plastics in this way. Ten times better is still a lot lower heat conductivity than metals, but we’ve opened the door to continue improving.”


To arrive at these results, the researchers synthesized PAP with a controlled length and blended with three different plastics having chemical structures that would enable hydrogen bonding with PAP, respectively. Then they tested how each conducted heat depending on the chemical structures.


“Some samples were found to conduct heat exceptionally fast, and we were somewhat surprised by the values,” said Gun-Ho Kim, first author of the paper and a postdoctoral fellow in mechanical engineering and materials science and engineering. “Later, other independent evidences supported the finding, from which we learned many important material design principles required to enhance heat transfer through amorphous polymers.” Two other first authors are Dongwook Lee and Apoorv Shanker, graduate students in Macromolecular Science and Engineering.


The other common approaches to channeling heat in plastics involve adding metal or ceramic particles to the mix. But those have drawbacks of affecting other properties of plastics. They increase the materials’ weight and cost, make them more opaque, and can affect how the materials conduct electricity and reflect light.


The paper is titled “High thermal conductivity in amorphous polymer blends by engineered interchain interactions.” The research was funded by the U.S. Department of Energy, Office of Basic Energy Sciences as part of the Center for Solar and Thermal Energy Conversion in Complex Materials, an Energy Frontier Research Center. Gun-Ho Kim has also received a fellowship from the U-M Energy Institute.


Based on the news article by Nicole Casal Moore http://ns.umich.edu/new/multimedia/slideshows/22539-heat-conducting-plastic-developed-at-u-michigan


For more information: Abstract of “High thermal conductivity in amorphous polymer blends by engineered interchain interactions:” http://www.nature.com/nmat/journal/vaop/ncurrent/pdf/nmat4141.pdf


Jinsang Kim: http://www.mse.engin.umich.edu/people/jinsang