Synthesis and properties of fluorine-free polyimides with ultrahigh Tg and excellent transparency based on the synergistic regulation of fluorenyl and methyl groups

It is a new challenge to develop fluorine-free transparent polyimides due to the prohibition of perfluoroalkyl and polyfluoroalkyl substances (PFAS). Hence, a series of fluorine-free polyimides were synthesized by 9,9-bis(3,4-dicarboxyphenyl) fluorene dianhydride (BPAF) with 3,3′-bis(methyl)-4,4′-benzidine (2,2′-DMBZ), 3,3′-bis(methyl)-4,4′-benzidine (3,3′-DMBZ), 1,4-diaminobenzene ( p -PDA), 1,3-diaminobenzene ( m -PDA), 2,6-toluenediamine ( m -MPDA) and 2,4,6-trimethyl-1,3-phenylenediamine ( m -TMPDA) through one-step and two-step methods. Then the corresponding polyimide films were obtained by solution casting. All polyimides exhibited high glass transition temperatures ( T g s) of 404-523 °C, coefficients of thermal expansion (CTEs) of 39-65 ppm/K, optical transmittances at 400 nm ( T 400 s) of 36-82 %, tensile strengths of 104.6-156.3 MPa, and tensile moduli of 2.6-4.3 GPa. The prepared fluorine-free polyimides had better performances than typical fluorine-containing polyimide derived from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl)-4,4′-benzidine (TFDB). In particular, PI-6 exhibited comprehensive properties with T g of 523 °C, T 400 of 82%, and CTE of 42 ppm/K.