Synthesis and Mechanisms\nof Black Polyimide Films\nwith High Mechanical and Dielectric Strength: Effects of Multicomponent\nPolymerization and Doping

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