Preparation of highly thermally conductive, flexible and transparent AlOOH/polyimide composite film with high mechanical strength and low coefficient of thermal expansion

With the rapid development of next-generation optoelectronic devices, there is urgent demand for transparent and highly thermal conductive polymer film materials exhibiting superior performance. However, achieving a balance among these diverse properties remains a significant challenge. In this study, we propose a solution to this dilemma by introducing AlOOH nanowires into colorless polyimide (CPI) films. AlOOH/CPI composite films were prepared using a blade coating strategy. Remarkably, the in-plane arrangement of AlOOH nanowires within the CPI matrix facilitates the formation of thermal conductivity pathways. The resulting optimized composite film demonstrates a substantially higher in-plane thermal conductivity (3.568 W·m-1·K-1) compared to pure CPI (0.626 W·m-1·K-1), representing a more than fivefold increase. And the through-plane thermal conductivity showed a marginal enhancement. Moreover, due to the smaller length of the AlOOH nanowires (100 nm) in comparison to the wavelength of visible light, 10 wt% AlOOH/CPI composite film maintains a high transparency of 86.7%, which is only 2.7% lower than that of the pure CPI film. Besides, the formation of hydrogen bonds between AlOOH nanowires and the CPI matrix enhances the mechanical properties and dimensional stability of the AlOOH/CPI composite film. The Young's modulus (3.1 GPa) increases to 148% of the original, while the thermal expansion coefficient (33.7 ppm·°C-1) decreases to 73% of the original value. Furthermore, AlOOH/CPI composite films exhibit excellent flexibility, chemical stability, flame retardancy, and electrical insulation property. These remarkable properties render AlOOH/CPI composite films highly promising for application in advanced optoelectronic devices. This study offers valuable insights into the preparation of CPI-based composite films with excellent comprehensive properties.