Synthesis and characterization of low retardation colorless polyimides containing <i>m</i> -phenylenediamine with different pendant groups

In this study, 4-biphenyl-3,5-diaminobenzoate (E- m-PDA) was synthesized by three steps using 3,5-dinitrobenzoic acid as a starting material. Two series of colorless polyimides (CPIs) PI-(1a-1d) and PI-(2a-2d) were prepared by traditional two-step polycondensation of m-phenylenediamine ( m-PDA) based diamines with different pendant groups (-H, CF 3 , COOH and biphenyl ester) with aromatic dianhydrides BPADA and 6FDA, respectively. All of the prepared CPIs exhibited high thermal stability with 5% weight loss at temperatures ranging from 428 to 539°C in nitrogen, and glass transition temperatures ( T g s) ranging from 228 to 332°C. These PI films displayed a UV-vis absorption cut-off wavelength at 338–376 nm and transparency of 76%–89% at a wavelength of 450 nm. Both series of colorless PI films also showed low birefringence values of 0.0021–0.0100. In addition, the optical retardations of these CPIs were extremely low (48–220 nm), indicating low optical anisotropy of these CPI films. Colorless PI film PI-2d prepared based on E- m-PDA and 6FDA had good comprehensive properties of low birefringence ([Formula: see text] n of 0.0064), high T g (319°C) and excellent transparency ( T 450 of 88%).