Improved mechanical, thermal properties and ideal dielectric properties of polyimide composite films by incorporation of boron nitride nanosheets and aramid nanofibers

In this work, a high-performance composite film polyimide (PI)/boron nitride nanosheets (BNNS) + aramid nanofibers (ANFs) was prepared by in-situ polymerization. Modified two-dimensional (2D) BNNS were prepared by urea-assisted ball milling to exfoliate and -NH2 functionalize hexagonal boron nitride(h-BN) in one step. One-dimensional (1D) ANFs prepared by deprotonation method and 2D BNNS were simultaneously incorporated into PI and used as effective reinforcement nanofillers. This novel PI/BNNS + ANFs composite film has excellent thermal stability and mechanical properties. Compared with pure PI, the glass transition temperature (T-g) increased by approximately 18 degrees C, and the coefficient of thermal expansion (CTE) decreased by 11.45%. Remarkably, the tensile strength and elongation at break of PI/BNNS + ANFs-1 wt% composite film increased by 18.6% and 133.4%, respectively. The improvement of thermal stability and mechanical properties were ascribed to the uniform dispersion of BNNS + ANFs and the interfacial interaction between BNNS + ANFs and PI matrix. Furthermore, the PI/BNNS + ANFs composite films show good dielectric properties and optical transparency. These excellent comprehensive properties of the PI/BNNS + ANFs composite films make it a good candidate for the flexible substrate.