Researchers in materials science and mechanical engineering at the University of Michigan (UM) have developed a new technique that can change plastic's molecular structure to help it cast off heat.
The new technique uses a process that engineers the structure of the material itself, considering the nature of plastics made of long chains of molecules that are tightly coiled and tangled like a bowl of spaghetti.
It used a chemical process to expand and straighten the molecule chains, giving heat energy a more direct route through the material. To accomplish this, the researchers first dissolved a typical polymer, or plastic in water, then added electrolytes to the solution to raise its pH, making it alkaline.
The individual links in the polymer chain, called monomers, take on a negative charge, which causes them to repel each other. As the monomers spread apart, they unfurl the chain's tight coils. Finally, water and polymer solution is sprayed onto plates using a common industrial process called spin casting, which reconstitutes it into a solid plastic film.
The uncoiled molecule chains within the plastic make it easier for heat to travel through it.
Researchers also found that the process has a secondary benefit: it stiffens the polymer chains and helps them pack together more tightly, making them even more thermally conductive.
"Polymer molecules conduct heat by vibrating, and a stiffer molecule chain can vibrate more easily," said Apoorv Shanker, a materials science and engineering graduate student who has been in the research team led by UM materials science and engineering professor Jinsang Kim. "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."
The process is a major departure from previous approaches, which have focused on adding metallic or ceramic fillers to plastics, a process that is expensive as well as can change the properties of the plastic in undesirable ways.
The work has important consequences because of the large number of polymer applications where temperature is important.
"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," said UM associate professor of mechanical engineering Kevin Pipe. "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."
The study has been published in the journal Science Advances.
