Colorless polyimide (PI) films with outstanding mechanical and thermal properties are highly demanded in flexible electronics. Herein three 4,4′-diaminodiphenyl ether (ODA)-based diamines with different substituents (–CH3, –CF3) and substitution positions (ortho and meta positions) were synthesized. After reacting with a combination of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in a typical sequential reaction manner, four highly transparent copolyimide films with excellent mechanical strength, high heat-resistance and superior fracture toughness were fabricated. A copolyimide film made using m-3F-ODA exhibits a fairly high transmittance at 400 nm of up to 86.8%, tensile strength as high as 102 MPa, tensile modulus of 2.76 GPa and glass transition temperature above 308 °C. The desirable solubility enables the prepared PIs to be readily cast from solution into robust films. All these encouraging results make these PI films potential substrates in place of ultrathin glass in flexible displays.
