High thermal conductivity in noncrystalline enriched polyethylene fibers.

Polyethylene represents an exceptional thermal conductor in theory: The perfectly extended chain is predicted to conduct heat extremely well. However, its practical scalable forms fall far below this limit because noncrystalline structures disrupt heat transport across multiple length scales. Here, we identify that a partially ordered, noncrystalline transitional phase in highly aligned polyethylene is not a negative by-product of processing but a key contributor to heat conduction. Guided by this insight, we develop a gel-state intermittent slow stretching method that directs noncrystalline evolution during polyethylene fiber formation. This approach enables chain relaxation and structural reorganization in regions typically regarded as amorphous and interfacial, promoting their conversion into the transitional phase and their seamless integration with crystalline domains. The resulting structure extends the continuity of ordered shish segments within the period structure, increasing the distance over which heat can travel quasi-ballistically. As a result, polyethylene fibers with 26.79% noncrystalline content achieve thermal conductivities up to 70.61 watts per meter per kelvin, representing 1.45 to 2.60 times of leading commercial polyethylene fibers. These findings establish control of noncrystalline structure as an essential route to unlocking high thermal conductivity in polymers and open a pathway toward lightweight, fully organic thermal conductors.