Colorless polyimides from 2,2′,3,3′-biphenyltetracarboxylic dianhydride and fluorinated diamines

In this work, 2,2′,3,3′-biphenyltetracarboxylic dianhydride (3,3′-BPDA) was exploited to produce colorless polyimides (CPIs), and their properties were systematically investigated. High-temperature polycondensation of 3,3′-BPDA with fluorinated diamines enabled the synthesis of CPIs with high molecular weights. The resulting CPIs displayed deficient chain packing owing to the steric hindrance and suppressed formation of charge transfer complexes (CTC), as confirmed by the interchain distances (5.9–6.7 Å). These CPIs were highly soluble in common organic solvents due to the enlarged interchain distances. Depending on the chain rigidity, the glass transition temperatures of the CPIs ranged from 287 to 414 °C. More importantly, these CPIs exhibited outstanding optical properties, with a cut-off wavelength of 355–370 nm, transmittance at 450 nm of 79.6–89.2%, and yellow index of 1.4–13.4.