Colorless Copolyimide Films With High‐ T g and Low CTE From a Twisted Benzimidazole Diamine and HPMDA / CBDA Blends for Flexible Display Substrates

A series of colorless copolyimide (Co‐PI) films was prepared by one‐step polycondensation of a twisted benzimidazole diamine (1‐(4‐amino‐2‐methylphenyl)‐2‐methyl‐1H‐benzo[d]imidazol‐5‐amine) with HPMDA and CBDA at varying molar ratios. The rigid cyclobutane ring of CBDA significantly influenced the thermal property and dimensional stability of the resulting copolymers. All films exhibited excellent thermal stability with 5% weight loss temperatures ( T d5% ) in the range of 464°C–480°C. The glass transition temperature ( T g ) increased from 448°C to 485°C with rising CBDA content due to restricted chain mobility, while the coefficient of thermal expansion (CTE) decreased markedly, reaching as low as 38 ppm/K. The copolymer with 50 mol% CBDA offered a balanced performance (CTE = 39 ppm/K) while maintaining high optical transparency. Mechanical properties were moderate (Young's modulus 2.4–3.3 GPa, tensile strength 79–87 MPa, elongation 4.1%–7.2%). Optically, all films showed high transmittance at 400 nm (79%–84%) and 450 nm (85%–87%) with cutoff wavelengths of 299–302 nm, and colorimetric analysis confirmed near‐colorless appearances ( L * > 95, YI < 4). The copolymers dissolved in common polar aprotic solvents, enabling direct solution processing without post‐imidization. This study confirms that copolymerizing a twisted benzimidazole diamine with HPMDA and CBDA is an effective strategy for producing high‐performance colorless transparent polyimide films.