Synthesis and characterization of novel optically transparent and organosoluble polyimides based on diamines containing cyclohexane moiety

Two new aliphatic diamine monomers, one containing cyclohexane moiety and the other containing both cyclohexane and trifluoromethyl moieties, were synthesized. Two series of semi-aromatic polyimides (PIs) derived from the two diamines were prepared by one-step solution polycondensation using m-cresol as solvent. These PIs had inherent viscosities between 0.32 and 1.20 dL/g and could be processed to colorless, flexible and tough films. Compared with the PIs without trifluoromethyl group, the PIs with trifluoromethyl group showed better solubility in many solvents such as DMAc, NMP and m-cresol. It was also found that all the PI films had high optical transparency in the visible region with the cut-off wavelength of 291–379 nm. However, compared with the non-fluorinated PI 5 series, the PI 4 series showed lighter color. Furthermore, thermal analysis showed that the synthesized PIs had 5 % weight lost temperature (T _5%) in N₂ of 372–425 °C, and the glass transition temperature (T _g) of 221–270 °C. Moreover, all the PIs showed good mechanical properties with the tensile strengths of 80–114 MPa, tensile moduli of 2.7–3.5 GPa, and elongation at breaks of 5.1–26.5 %.