Enhancing thermal conductivity of polyimide composite film by electrostatic self-assembly and two-step synergism of AlO microspheres and BN nanosheets.

To improve the perfection of a three-dimensional thermally conductive network in polyimide (PI) composite film and with respect to the economy and simplicity of processing, a strategy of the two-step synergism of AlO microspheres and hexagonal boron nitride (BN) nanosheets was proposed. First, BN nanosheet-coated AlO microspheres (AlO@BN) were prepared by electrostatic self-assembly method for the first step of the synergism. Then, the AlO@BN&BN/PI composite film containing AlO@BN and BN was fabricated by a two-step method for the second step of the synergism, and was systematically characterized. With an optimized mass ratio of 2 : 1 of AlO@BN to BN, the thermal conductivity of the 35 wt% AlO@BN&BN/PI composite film reached 3.35 W m K, and was increased by 1664% compared to that of pure PI. The synergism of the AlO and BN was the most significant in the AlO@BN&BN/PI composite film with the thermal conductivity, which was 36.6%, 23% and 22% higher than that of the AlO/PI, BN/PI and AlO@BN/PI composite films, respectively. The enhancement mechanism of heat conduction was clearly demonstrated. The BN coated on the surface of AlO mainly played a bridging role between the AlO and the BN network, which improved the perfection of the thermally conductive network. The AlO@BN segregated the PI matrix to construct the BN network with the typical segregated structure in the composite film, resulting in an efficient thermally conductive network. This work provided a novel strategy for the preparation of conductive polymer composites.