Ultrahigh Thermal Conductivity in a 12‐nm Polymer Crystal Limited by Boundary Scattering and Surface Amorphization

Polymer lamellar crystals with highly ordered crystalline structures are ideal systems for understanding and engineering thermally conductive polymers. However, their nanometer-scale thickness, hard-to-eliminate defects, and limited lateral dimensions have impeded experimental characterization, leaving key thermal transport mechanisms unresolved. Here, we address this knowledge gap by devising a multilayer single-crystal stack for a non-contact measurement technique, combined with advanced theoretical calculations. The measured cross-plane thermal conductivity is 4 W m^-1 K^-1 for a 12-nm-thick polyethylene lamellar single crystal, representing the highest value observed for dielectric materials in this thickness range. Theoretical analyses indicate that this value is nevertheless limited by the combined effects from boundary scattering and surface amorphization, offering critical insights for molecular design and understanding of nanoscale heat transfer in ultrathin soft materials.