Controllable low dielectric porous polyimide films templated by silica microspheres: microstructure, formation mechanism, and properties.

In the paper, controllable low dielectric macroporous polyimide (PI) films were prepared using silica microspheres as template, based on the solvent-evaporation-assisted in situ self-assembly of polyamic acid (PAA, precursor of PI) and silica microspheres in N,N-Dimethylformamide (DMF) solution. The interior microstructure of the macroporous PI films and its relationship with mechanical and dielectric properties were investigated. It was found that the macroporous PI films had triple-layer porous structures: ordered porous surface, dense layer in the middle, and interconnected porous structure at the bottom. The thickness of the dense layer in the middle changed with increasing weight content of silica. When SiO(2) wt% was above 40%, the dense layer nearly disappeared. The interconnected porous structure at the bottom was quite homogenous, and the pore size could be controlled by the diameter of silica microspheres. Based on the experimental results, possible formation mechanism of the macroporous PI films was addressed. The mechanical properties of the macroporous PI films were degraded due to the introduction of air void. It was found that the degradation of mechanical properties of macroporous PI films with porosity of 37% is more obvious than that of PI films with porosity below 31%, which could be attributed to the triple-layer microstructure of the films. The air void introduction could effectively reduce the dielectric constant from 4.11 of PI dense films to 2.57 of porous PI films with porosity of 37%. The dielectric constant was only closely related to porosity and had no obvious connection with pore size.