Synthesis and Mechanisms of Black Polyimide Films with High Mechanical and Dielectric Strength: Effects of Multicomponent Polymerization and Doping

Black polyimide (PI) film is a promising polymer material that has been widely applied in the electronic circuit industry and aerospace. However, the addition of black additives shows a trade-off effect on the mechanical and electrical properties of PI films. Herein, we propose a novel preparation method to eliminate the above trade-off effect. It combines pyromellitic dianhydride (PMDA) and biphenyl dianhydride (BPDA) as monomers in polycondensation. The flame retardants are doped to minimize the addition of black additives. In this way, after optimization of the preparation conditions, the obtained films are observed to possess outstanding mechanical (tensile strength of 165.2 MPa, elongation at break of 22.1%, elastic modulus of 4587.4 MPa, and puncture strength of 521.3 N/mm), electrical (dielectric strength of 160.0 kV/mm) and optical properties (transmittance of 0.20%). A mechanistic study by microscopic tests and theoretical calculations shows that, with thermal decomposition, the flame retardants could form a dispersed carbon deposit. The dispersed carbon deposit and carbon additive synergistically reduce the light transmittance. With this method, the addition of black additive decreases from over 5 to 0.5 wt %. PMDA and BPDA regulate the rigidity of the polymer chains and the intermolecular interactions, thereby improving the mechanical and dielectric strength. This method has also been scaled up for industrial production without sacrificing the stability and physicochemical properties of the film.