Preparation and Properties of Fluorine‐Free and Organo‐Soluble Polyesterimides and the Films With Good Optical Transparency and Thermal Stability

ABSTRACT Fluorine‐free, fully aromatic polyimide (PI) films, characterized by the excellent optical transparency and high‐temperature endurance, have been successfully synthesized through either homopolymerization or copolymerization of an ester‐containing diamine, 2‐(4‐aminobenzoate)‐5‐aminobiphenyl (ABABP) and two distinct dianhydrides: 2,2‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) and 9,9‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]fluorene dianhydride (BPFPA). Owing to the good solubility of the developed PI‐I to PI‐V resins in polar aprotic solvents, particularly in N,N‐dimethylacetamide (DMAc), N‐methyl‐2‐pyrrolidone (NMP) and N,N‐dimethylformamide (DMF), the synthesis process was performed through a two‐stage chemical imidization technique. The polymer solutions of PI‐I to PI‐V in DMAc were fabricated into films that showed exceptional optical clarity, featured with the ultraviolet cutoff wavelength (λ) below 375 nm, the light transmission at 450 nm (T450) surpassed 80%, the b* values (CIE indices) below 4.5, and the turbidity percentage (haze values) under 0.5%. Despite the fact that a higher molar concentration of BPFPA components in the dianhydride segments led to a decline in the optical characteristics of the films, the thermal resistance was concurrently enhanced. The resulting copolymerized PI films exhibited a glass transition temperature (Tg) over 264.9°C, along with the coefficients of linear thermal expansion (CTE) values of 56.7 ~ 65.9 ppm/K in temperature range of 50 ~ 200°C.