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 angstrom, and their d-spacing values increased while the intensity of pi-pi 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 degrees C, respectively. These polymers exhibited excellent optical transparency (.10 < 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.