Facile synthesis of acyloxy-containing fluorene-based Cardo polyimides with high optical transparency, fluorescence and low dielectric constant

A series of acyloxy-containing fluorene-based Cardo polyimides (PIs) were facilely synthesized by the post-polymerization modification based on a simple esterification of hydroxyl-containing PIs. Incorporation of acyloxy groups may reduce the packing density of PI chains and weaken both inter- and intra-chain charge transfer complexes (CTC) formation. Accordingly, the modified PIs exhibit the improved solubility in low-boiling-point solvents and optical transparence. The optical transmittance at 400 nm (T400) of all modified PI films is significantly increased to 80.1–83.5% from 41.6–54.5% of the precursor PI films. Meanwhile, the modified PI films exhibit a bright fluorescence. The modified PIs still display the superior heat resistance with a glass transition temperature (Tg) of 301.1–390.9 °C as well as admirable mechanical properties with a tensile strength (TS) of 72.1–81.3 MPa. In addition, the dielectric constant (k) value at 1 MHz of the modified PI films is decreased to 2.58–2.90 from 3.95–4.48 of the precursor PI films, which is attributed to the enlarged free volume and reduced water absorption afforded by the incorporation of acyloxy groups. This work provides an efficient post-polymerization modification approach to simultaneously affording the multifunctional PI films with high Tg, high T400 and low k, probably as the potential candidates for the practical application in display and microelectronic industries.