Polymers with defective fillers increase the heat transfer of plastics, studies reveal

In an exploration of the next generation of materials for modern devices, Amherst has made discoveries to design what is better at radiating the heat – a team of researchers led by Amherst at the University of Massachusetts.

This study, published in Science Advances, discovered that experimentally and theoretically, polymers (commonly referred to as plastics) are made with thermally conductive fillers 160% better than those with full fillers. This counterintuitive finding challenges long-standing assumptions that flaw the performance of materials. Instead, it refers to a promising new strategy for engineering polymer composites with ultra-high thermal conductivity.

The study was led by UMass Amherst with collaborators from Massachusetts Institute of Technology, North Carolina State University, Stanford University, Oak Ridge National Laboratory, Argonne National Laboratory and Rice University.

Polymer revolutionized modern devices with unparalleled lightness, electrical insulation, flexibility and ease of processing. Polymers are embedded in every corner of a high-tech environment, from high-speed microchips and LEDs to smartphones and soft robotics.

However, common polymers are thermal insulators with low thermal conductivity, which can lead to overheating problems. Their inherent insulation properties trap heat, SAP performance, and create dangerous hot spots that accelerate wear, increasing the risk of catastrophic failures and fires.

For years, scientists have been trying to improve the thermal conductivity of polymers by incorporating thermally conductive fillers such as metals, ceramics and carbon-based materials. The logic is simple. Blending thermally conductive filler should improve overall performance.

However, in reality, it’s not as simple as this. Consider a polymer mixed with diamonds.

Given the exceptional thermal conductivity of diamonds of about 2,000 watts per Kelvin (W M-1 K-1), a polymer made up of 40% diamond filler could theoretically achieve a conductivity of about 800 W M-1 K-1. However, the challenges of filler aggregation, defects, high contact resistance between the polymer and filler, and low thermal conductivity of polymer matrixes that impair heat transfer lead to lack of practical results.

“Understanding the heat transport mechanisms of polymeric materials has been a long-standing challenge due to complex polymer structures, ubiquitous defects and obstacles,” says Yanfei Xu, assistant professor of mechanical and industrial engineering at UMass Amherst and corresponding author.

For their research, the team, which aimed to understand the heat transport of polymeric materials and lay the foundation for controlling heat transfer across non-uniform interfaces, created two polymer composites, polyvinyl alcohol (PVA).

As expected, the full filler of its own was more thermally conductive than the incomplete filler.

“The perfect filler (graphite) has a high thermal conductivity of about 292.55 W M-1 K-1 in itself.

But surprisingly, when these fillers were added to the polymer, the polymers made with defect-containing graphite oxide fillers were 160% better than those with perfect graphite fillers.

The team used a combination of experiments and models in characteristic transport measurements, neutron scattering, quantum mechanical modeling, and molecular dynamics simulations to study how it affects heat transport in polymer composites.

They found that defective fillers promote more efficient heat transfer, as uneven surfaces do not allow the polymer chain to clog tightly as well as perfectly smooth fillers. This unexpected effect, known as the reinforced vibrational bond between the polymer and the defective filler at the polymer/filler interface, increases thermal conductivity, reduces resistance, and makes the material more efficient for heat transfer.

“We’ve seen a lot of experience in the field of mechanical aerospace engineering,” said Jun Liu, an associate professor at North Carolina State University. “It’s true, flaws can sometimes lead to better results.”

Xu believes that experimentally and theoretically these results lay the basis for engineering new polymeric materials with ultra-high thermal conductivity. These advancements present new opportunities for devices, from high-performance microchips to next-generation soft robots.