Colorless polyimide copolymers derived from isomeric biphenyltetracarboxylic dianhydrides and 2,2′-bis(trifluoromethyl)benzidine

Colorless polyimide (CPI) copolymers were synthesized by one-step or two-step methods, using 2,2′-bis(trifluoromethyl)benzidine (TFMB), 2,2′,3,3′-biphenyltetracarboxylic dianhydride (3,3′-BPDA), and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (4,4′-BPDA) as the monomers. These polymers exhibited a d-spacing value of 5.7–6.7 Å, and their d-spacing values increased while the intensity of π-π stacking interactions decreased with the increase of 3,3′-BPDA/TFMB contents. The solubility of the copolymers were gradually enhanced as the increase of 3,3′-BPDA/TFMB contents. Their coefficients of thermal expansion and glass transition temperatures increased with 3,3′-BPDA/TFMB contents, ranging from 15 to 68 ppm K−1 and 300 to 359 °C, respectively. These polymers exhibited excellent optical transparency (λ0 < 390 nm, T450 > 83.2%, and b* < 4). CPI-4 and CPI-5 with 40 mol% and 20 mol% of 3,3′-BPDA/TFMB contents exhibited well-balanced solution processability and physical properties. The CPI copolymers can be potentially used in optical and optoelectronic fields owing to their desirable overall properties.