  Thermally conductive polymers are of fundamental interest and can also be
exploited in thermal management applications. Recent studies have shown
stretched polymers can achieve high thermal conductivity. However, the
transport mechanisms of heat in thermally conductive polymers have yet to be
elucidated. Here we report a method for scalable fabrication of polyethylene
films with a high thermal conductivity of 62 W/m-K. The achieved thermal
conductivity is over two orders-of-magnitude greater than that of typical
polymers (~0.1 W/m-K), and exceeds those of many metals and ceramics used as
traditional heat conductors. Careful structural studies are carried out and
reveal that the film consists of nanofibers with crystalline and amorphous
regions. Contrary to conventional wisdom, we reveal the importance of the
amorphous morphology in achieving such high thermal conductivity, rather than
simply from enhancements in the degree of crystallinity and crystallite
alignment. The amorphous phase reaches a remarkably high thermal conductivity
of ~16 W/m-K. Even still, we identify that the presence of this amorphous phase
is the dominant factor as the film thermal conductivity is still much lower
than the predicted values for bulk single-crystal polyethylene (237 K/m-K).
This work lays the foundation for the rational design and synthesis of
thermally conductive polymers, and opens up new opportunities for advanced heat
management, particularly when flexible, lightweight, chemically inert and
electrically insulating thermal conductors are desired.
