Color-Tunable to Intrinsically Black: A Side-Chain Oxidation Strategy for Polyimide Films

Intrinsically black polyimides hold great promise for applications in optoelectronics and flexible electronics. However, conventional preparation methods face challenges such as complex monomer synthesis, poor process compatibility, or compromised material performance. To address these challenges, this study developed a strategy that integrates “molecular design” with “in situ oxidation,”. In this study, four oxidizable diamine monomers with distinct side-chain structures were designed and synthesized: 2,4-diaminodiphenylamine (aniline side chain), 2,4-diamino-4′-naphthyl diphenylamine (naphthylamine side chain), 2,4-diamino-4′-biphenyl diphenylamine (4-aminodiphenyl side chain), and 2,4-diamino-4′-diphenylamine diphenylamine (4-aminodiphenylamine side chain). These monomers were copolymerized with ODPA to prepare the corresponding polyimide films. By introducing an oxidizing agent during the high-temperature film-forming process, in situ oxidation of side-chain aromatic amines is achieved, revealing how side-chain structure affects film properties. The results indicate that effective conjugation length and planarity of the side chains are key factors: the 4-aminodiphenylamine side chain yielded films exhibiting the best near-black appearance ( L * = 10.56) and the broadest visible light absorption ( λ max = 624 nm, T 300–700 = 1.2%) after oxidation. All films retained good thermal and mechanical properties ( T 5% > 324°C, tensile strength > 89 MPa). Graphical Four oxidizable diamine side chains control polyimide color via conjugation length and planarity. The 4-aminodiphenylamine unit gives a near-black film ( L * = 10.56) with broad visible absorption ( λ max = 624 nm, T ₃₀₀–₇₀₀ = 1.2%) after in situ oxidation, while retaining good thermal ( T ₅% > 324°C) and mechanical (> 89 MPa) properties.