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

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 (T 450 ) 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 (T g ) 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.