Synthesis and Properties of Colorless Transparent Polyimides with Low CTE and High Tensile Strength

To develop the colorless transparent fluorinated polyimide (PI) with low coefficient of thermal expansion (CTE) and high tensile strength, a series of ternary copolymerized fluorinated PIs were synthesized by 4,4-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2'-bis(trifluoromethyl) benzidine (TFMB), and the third monomer 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). The physical properties of the PI films were effectively regulated and optimized by adjusting the ratio of the rigid BPDA and flexible 6FDA components. By increasing the BPDA content, the thermal stability, dimensional stability, and mechanical properties of the polymers were enhanced. Among them, the PI film which molar ratio of 6FDA and BPDA was 5 : 5 exhibited an excellent comprehensive performance with a λ0 of 372 nm, Tmax nearly 91%, the tensile strength beyond 108 MPa and possessed high glass-transition temperature Tg which was 315.8°С. Furthermore, the CTE of the PI film was calculated about 21 ppm/K in the range of 25–200°С, indicating great dimensional stability.