Hot-pressing induced alignment of boron nitride in polyurethane for composite films with thermal conductivity over 50 Wm−1 K−1

Abstract

Flexible insulating composites with superior thermal conductivity (usually below 5 W m−1 K−1 for the previously-reported composites) are highly desirable for further miniaturization of high-power electronics and portable devices. Herein hexagonal boron nitride (hBN)/thermoplastic polyurethane (TPU) composites were readily fabricated by solution ball-milling of hBN and TPU in N,N′-dimethylformamide followed by precipitation in ethanol. The coagulated solids were then processed by hot-pressing to produce TPU composite films with well-dispersed hBN platelets. The amount of hBN in the TPU matrix was loaded as high as 95 wt% while retaining mechanical flexibility. The in-plane thermal conductivity variations' trend with hBN loadings exhibited a sudden jump happened with the loading level over 40 wt% in the polymer matrix. Intriguingly, the TPU composite film with 95 wt% hBN exhibits an in-plane thermal conductivity of up to 50.3 W m−1 K−1 (a 264-fold increase compared to pure TPU), due to the hot-compression induced alignment of hBN platelets and strong interactions in/with the TPU matrix. The corresponding out-of-plane thermal conductivity is also as high as 6.9 W m−1 K−1. The hBN/TPU composite films were further employed as heat spreaders for light-emitting diodes, resulting in a significant decrease by about 40 °C in the set-point and the saturation temperatures. Such superior thermal transport performance has rarely been reported in the polymer composites. The proposed method provides a facile and novel strategy for the scalable fabrication of high-performance polymer composites containing two dimensional layered materials.