Fluorine-free transparent polyimides with highly heat-resistance derived from 3,3′,4,4′-diphenylsulfone tetracarboxylic acid anhydride and methyl group substituted diamines

As is known to all, it is an effective strategy to achieve transparent aromatic polyimides by introduction of fluorine-containing monomer. However, due to the prohibition of perfluoroalkyl and polyfluoroalkyl substances (PFAS), fluorine-free polyimides have attracted more and more attention. In order to investigate the structure-properties relationship of fluorine-free transparent polyimides, a new diamine monomer, 2′,3,3'',5′-tetramethyl-[1,1′:4′,1''-terphenyl]-4,4''-diamine (3TMTD) containing methyl group and terphenyl structure, was successfully synthesized, and a series of highly heat-resistant polyimides were synthesized by 3,3′,4,4′-diphenylsulfone tetracarboxylic acid anhydride (DSDA) with 2,2′-bis(methyl)-4,4′-benzidine (2,2′-DMBZ), 3,3′-bis(methyl)-4,4′-benzidine (3,3′-DMBZ), 3TMTD, 9,9-bis(4-aminophenyl)fluorene (FDA) and 9,9-bis(3-methyl-4-aminophenyl)fluorene (MFDA). And the polyimide containing fluorine element derived from DSDA and 2,2′-bis(trifluoromethyl)-4,4′-benzidine (TFDB) was also prepared for comparison. The effects of different methyl substitution positions and numbers, as well as fluorine-containing diamine and fluorine-free diamines, on the properties were well studied. The resulting polyimides exhibited high T g s of 373–426 °C, transmittances at 450 nm of 66–87 %, and tensile moduli of 2.8–4.0 GPa. Among them, PI-3 derived from DSDA and 3,3′-DMBZ showed the highest T g of 426 °C, while PI-4 derived from DSDA and 3TMTD dispalyed the best comprehensive performance with T g of 405 °C, transmittance of 80 % at 450 nm, tensile strength of 167.6 MPa and tensile modulus of 4.0 GPa.