Aug 3 2017
Advanced plastics could result in the emergence of cheaper, lighter, more energy-efficient product components, including those used in LEDs, vehicles and computers — if only they were better at dispersing heat.
A sample of heat - conducting polymer is tested for thickness in U-M’s Lurie Nanofabrication Facility. (Image credit: Joseph Xu, Michigan Engineering)
A new method that can alter plastic's molecular structure to help it disperse heat is a promising step in that direction.
A team of University of Michigan Researchers in Materials Science and Mechanical Engineering have developed this inexpensive and scalable method. Details of their findings can be found in Science Advances.
The concept can possibly be adapted to a range of other plastics. In initial tests, it created a polymer nearly as thermally conductive as glass — still far less so than ceramics or metals, but six times better at dispelling heat than the same polymer without the treatment.
Plastics are replacing metals and ceramics in many places, but they're such poor heat conductors that nobody even considers them for applications that require heat to be dissipated efficiently. We're working to change that by applying thermal engineering to plastics in a way that hasn't been done before.
Jinsang Kim, U-M Materials Science and Engineering Professor
The process is a key departure from former approaches, which have concentrated on incorporating ceramic or metallic fillers to plastics. This has met with partial success; a large quantity of fillers must be added, which is costly and can alter the properties of the plastic in adverse ways. Instead, the new method uses a process that engineers the material’s structure itself.
Plastics are composed of long chains of molecules that are firmly coiled and tangled like a bowl of spaghetti. As heat moves through the material, it must pass along and between these chains—a difficult, roundabout journey that obstructs its progress.
The team also comprising of U-M Associate Professor of Mechanical Engineering Kevin Pipe, Mechanical Engineering Graduate Researcher Chen Li and Materials Science and Engineering Graduate Student Apoorv Shanker used a chemical process to enlarge and straighten the molecule chains. This offered heat energy a more direct route through the material. To achieve this, they began with a common polymer, or plastic. They first dissolved the polymer in water, and then included electrolytes to the solution to increase its pH, making it alkaline.
The separate links in the polymer chain — called monomers — take on a negative charge, which makes them repel each other. As they spread apart, they unfold the chain's tight coils. Finally, the polymer and water solution is sprayed onto plates using a regular industrial process called spin casting, which reconstitutes it into a solid plastic film.
The unfolded molecule chains within the plastic make it easier for heat to pass through it. The team also discovered that the process has a secondary advantage — it hardens the polymer chains and helps them pack together more firmly, making them a lot more thermally conductive.
Polymer molecules conduct heat by vibrating, and a stiffer molecule chain can vibrate more easily. Think of a tightly stretched guitar string compared to a loosely coiled piece of twine. The guitar string will vibrate when plucked, the twine won't. Polymer molecule chains behave in a similar way.
Apoorv Shanker, Materials Science and Engineering Graduate Student
Pipe says that the study can have significant consequences because of numerous polymer applications in which temperature is crucial.
Researchers have long studied ways to modify the molecular structure of polymers to engineer their mechanical, optical or electronic properties, but very few studies have examined molecular design approaches to engineer their thermal properties. While heat flow in materials is often a complex process, even small improvements in the thermal conductivities of polymers can have a large technological impact.
Kevin Pipe, U-M Associate Professor of Mechanical Engineering
The team is currently aiming to make composites that integrate the new method with several other heat dissipating strategies to further raise thermal conductivity. They are also involved in applying the concept to other types of polymers outside of what they used in this research. A commercial product is probably several years away.
"We're looking at using organic solvents to apply this technique to non- water soluble polymers," Li said. "But we believe that the concept of using electrolytes to thermally engineer polymers is a versatile idea that will apply across many other materials."
The research is titled "High thermal conductivity in electrostatically engineered amorphous polymers."