Turning polymer insulators into heat conductors

Polymer films conduct heat better than ceramics and many metals.

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A continuous increase in power density and miniaturizing electronics makes heat dissipation one of the most complex technological challenges1. The worldwide semiconductor industry has acknowledged that Moore's law is nearing its end2. A major issue is that vast amounts of waste heat generated during device operation leads to overheating problems3. A need for new materials to manage overheating arises. Polymers have been used for electronics packaging, thanks to their unparalleled properties: light weight, electrical insulation and easy processability4. But common polymers, which are generally regarded as thermal insulators, are undesirable for efficient heat dissipation5. Disorders and defects in polymers act as scattering sites for heat carrier transport6 and result in low thermal conductivity on the order of 0.1 W m-1 K-1. Turning thermally insulating polymers into thermal conductors will provide new opportunities for better thermal management applications.

In April 2019, we reported polyethylene films with a metal-like thermal conductivity of ~62 W m-1 K-1, over two orders of magnitude greater than that of typical polymers and exceeding that of many metals and ceramics7. Conventional approaches of focusing on crystalline phases in polymers at times marginally enhance the thermal conductivity of polymers. In contrast to these conventional approaches, we emphasized the need to engineer the spaghetti-like amorphous phases in polymers that acted as bottlenecks for efficient heat transfer. We identified high thermal conductivity (~16 W m-1 K-1) in the amorphous region was critical to achieving overall high thermal conductivity ~ 62 W m-1 K-1. In order to create efficient thermal transport pathways in polymers, we turned entangled and coiled chains into disentangled and aligned chains. In particular, we started from commercial semi-crystalline polyethylene powders and dissolved the powders in decalin, allowing the initially entangled chains to disentangle. This greatly reduced the entanglements for the subsequent processing. Afterward, the hot solution was extruded through a custom-built Couette-flow system, which imparted a shear force on the polymer chains and led to further disentanglements. Finally, these extruded films were further mechanically pressed and drawn inside a heated enclosure using a continuous and scalable roll-to-roll system. Structural studies and thermal modeling revealed that the drawn film consisted of nanofibers with crystalline and amorphous regions. These studies also revealed that the amorphous region had a remarkably high thermal conductivity, over ~16 W m-1 K-1, which was essential for achieving overall high thermal conductivity ~62 W m-1 K-1. For more details, please check out the paper “Nanostructured polymer films with metal-like thermal conductivity” on Nature Communications. https://www.nature.com/articles/s41467-019-09697-7

We foresee that further improvement of the thermal conductivity of the amorphous phase will play a key role in developing the next generation of diamond-like polymers with even higher thermal conductivity. For more details, please check out our blog  (behind the paper).

After the paper, we are striving to develop heat conducting polymers with diamond-like thermal conductivity. From the fundamental perspective, developing polymers with high thermal conductivity, especially semicrystalline and amorphous polymers, requires a deep understanding of the thermal transport mechanisms in polymers (Figure 1). We are exploring the relationships between thermal transport properties in polymers and chain structures at multiscale levels ranging from the atomic level and the nanoscale level to the microscale level and the macroscale level. In particular, we are investigating how thermal conductivity is influenced by the polymer chain morphology, chain topology and chemical compositions. In addition to top-down approaches for developing heat conducting polymers by disentangling commercial polymers at the atomic level, we are developing bottom-up approaches for synthesizing new polymers at the monomer level. These new polymers are expected to have effective thermal transport pathways and high thermal conductivity. From the application perspective, we note that polymers with simultaneously high thermal conductivity and high melting temperatures are desirable for thermal management applications such as polymeric heat exchangers. Polymers with high thermal conductivity and low elastic modulus are ideal for thermal management applications such as soft actuators. We believe that the heat conducting polymers with their unparalleled combination of characteristics (light weight, electrical insulation, chemical stability, easy processability etc.) hold promise for many existing and unforeseen applications.

Figure 1. Developing heat conducting polymers and understanding the thermal transport mechanisms. (a) Investigating the relationships between thermal transport properties in polymers and chain morphologies. (b) Demonstrating heat conducting polymers for thermal management application. (c) Exploring how thermal transport properties in polymers are influenced by chain topology. (d) Exploring how thermal transport properties in polymers are influenced by chemical compositions.

This blog was written by Yanfei Xu with comments from Yu Guo and Yanfei Xu on our future perspectives and research areas of heat conducting polymers at the University of Massachusetts Amherst.

Reference

  1. Moore, A. L. & Shi, L. Emerging challenges and materials for thermal management of electronics. Materials Today 17, 163–174 (2014).
  2. Waldrop, M. M. The chips are down for Moore’s law. Nature News 530, 144–147 (2016).
  3. Ball, P. Computer engineering: feeling the heat. Nature 492, 174–176.
  4. Peplow, M. The plastics revolution: how chemists are pushing polymers to new limits. Nature News 536, 266–268 (2016).
  5. Sperling, L. H. Introduction to physical polymer science. (John Wiley & Sons, Inc., Lehigh University Bethlehem, Pennsylvania 2006).
  6. Chen, G. Nanoscale energy transport and conversion, a parallel treatment of electrons, molecules, phonons, and photons. (Oxford University Press, 2005).
  7. Xu, Y., Kraemer, D., Song, B., Jiang, Z., Zhou, J., Loomis, J., Wang, J., Li, M., Ghasemi, H., Huang, X., Li, X. & Chen, G. Nanostructured polymer films with metal-like thermal conductivity. Nature Communications 10, 1771 (2019).

Yanfei Xu

Assistant Professor, University of Massachusetts Amherst

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