Mechanically Robust and Colorless Polyimide Films Cross-Linked with a Cycloaliphatic Amide Linkage

A method for cross-linking a conventional colorless and transparent polyimide (PI) with a diamine was explored to enhance its mechanical properties. The PI was synthesized using two fluorinated monomers─4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 2,2-bis(trifluoromethyl)benzidine─through chemical imidization. Cross-linked polymer films with smooth surfaces and excellent appearances were achieved without phase separation or optical defects by adding a small amount of cycloaliphatic diamine to the PI varnishes beforehand. Conversely, using a linear aliphatic diamine as an additive resulted in difficulty controlling the cross-linking reaction due to rapid gelation, while aromatic diamine did not participate in the cross-linking reaction due to its low nucleophilicity. PI films cross-linked with 1,4-bis(aminomethyl)cyclohexane (CHAM) exhibited significantly improved mechanical properties compared to their un-cross-linked counterparts, achieving an elastic modulus of 7.2 GPa, a tensile strength of 113 MPa, and a Shore D hardness of 97.5 at 5 wt % CHAM. Thermal decomposition, which leads to yellowing, was minimal even at high temperatures during film drying, maintaining colorless transparency. These findings will be useful in developing polymer films for cover windows and transparent substrates for flexible and foldable displays.