Nanoengineering Pure Polymers with High Thermal Conductivity

Several researchers are pursuing a better understanding of thermal conductivity in polymers. Significant progress has been made; but the interplay of structure and thermal transport is still not well understood in polymers. Polymers with high thermal conductivity typically have a high degree of crystallinity. Such polymers are usually stretched to align their chains, which is a process that is known to enhance thermal conductivity. But it is difficult to achieve enhanced thermal conductivity with current approaches while retaining the tough, amorphous structure of bulk polymers because aligned, long chains have a thermodynamic preference to fold into crystals.  Using electropolymerization in nanoscale templates, in contrast, we found that the thermal conductivity of amorphous polythiophene nanofibers can be over 20 times the bulk value. This enhancement results from significant molecular chain re-orientation in the direction of heat conduction and from high thermal conductivity in individual polythiophene chains. Surface interactions during the templated electropolymerization align polymer chains and prevent them from folding into crystals, demonstrating a unique approach to tailoring macromolecular structure and thermal transport properties. Using vertical arrays of the nanofibers, we demonstrate effective heat transfer at critical contacts in high-power, high-temperature electronics, and in a new class of thermally-conductive flexible substrates. In addition to these results, I will discuss the unique thermal metrology used to extract the thermal conductivity of large area polymer nanofiber arrays.