Enhanced thermal conductivity in immiscible polyimide blend composites with needle-shaped ZnO particles

Novel blend composite films were prepared by mixing two kinds of immiscible polyimides (PIs), one containing sulfur (SD) and one containing fluorine (TF), with needle-shaped zinc oxide (n-ZnO) particles. Optical microscopy and cross-sectional SEM images revealed that vertical double percolation (VDP) structures were successfully formed in the PI blend films by spontaneous macro-phase separation. In the VDP structures, each phase is separately aligned along the out-of-plane direction, and the n-ZnO particles were selectively incorporated in the TF-rich phase. These blend composite films exhibited significantly higher thermal conductivity (TC) in the out-of-plane direction than blend composite films containing pyramidal ZnO (p-ZnO) particles. The wide-angle X-ray diffraction (WAXD) results revealed that the n-ZnO particles were more randomly oriented in the blend films than those in homopolymer films, in which the particles were preferentially aligned parallel to the film. The n-ZnO particles confined in the VDP structures and partially oriented along the out-of-plane direction contributed to the enhancement of the out-of-plane TC.