Cardo polyimides with high Tg and transparency derived from bisphenol fluorenes and 1,4-bis(4-fluorophthalimide)cyclohexanes via aromatic nucleophilic substitution

Two series of transparent cardo polyimides (PIs) derived from bisphenol fluorenes and 1,4-bis(4-fluorophthalimide)cyclohexanes (BFCH) with different ratios of trans- and cis-1,4-diaminocyclohexane were prepared via aromatic nucleophilic substitution. These PIs exhibited high glass transition temperatures (Tgs) of 271–352 °C, 5% weight loss temperatures (Td5%) of 400–433 °C in nitrogen, mechanical properties of tensile strengths of 62–107 MPa, tensile moduli of 3.0–4.4 GPa and elongations at break of 1.7–4.5% and great solubility in a variety of organic solvents. These PI films appeared excellent optical transparency, which displayed the cut-off wavelengths from 323 to 355 nm and high transmittance at 450 nm with more than 85 %. The steric effects of different substituent groups on bisphenol fluorene and different ratios of trans/cis configuration were also investigated.