Acyloxy-Containing Fluorene- and Benzimidazole-Based Polyimides with Improved Solubility, Transparency, and Dielectric Properties by Postmodification

Polyimide (PI) films with a low dielectric constant (Dk) and dissipation factor (Df) at high frequency remain an intriguing study for the ever-increasing demand for 5G high-frequency transmission. In this study, an effective molecular design strategy for preparing low-dielectric PI films with excellent overall performance is presented by simultaneously introducing an acyloxy-containing fluorene-based Cardo structure and rigid benzimidazole unit with an H-bonding site. Based on this concept, a series of different-length acyloxy-containing fluorene- and benzimidazole-based PIs were easily prepared by a moderate postmodification of a hydroxyl-containing PI (PI–OH-NH). Because of the presence of rigid fluorene- and benzimidazole-based structures, all modified PI films still possess decent thermal stability, mechanical properties, and coefficient of thermal expansion (CTE) values. Introducing the acyloxy side groups effectively reduces the packing density of molecular chains and increases the fractional free volume, resulting in simultaneously improving solubility, transparency, and dielectric properties of the modified PI films. Testing results show that the modified PIs keep good solubility in high-polar solvents and display improved solubility in low-polar solvents. In addition, the transmittances at 500 nm (T500) of the modified PI films obviously increase to 84.0%–86.4% from 69.6% of the PI–OH-NH film. The Dk and Df values at 1 MHz of the modified PI films reduce to 2.57–3.42 and 0.0105–0.0210 from 3.58 and 0.0282 of PI–OH-NH film, respectively. Similarly, the Dk and Df values at 10 GHz of the modified PI films also reduce to 2.68–2.91 and 0.0131–0.0227 from 3.20 and 0.0354 of PI–OH-NH film, respectively. The excellent integrative properties make these modified PI films favorable candidates for interlayer insulation dielectric materials and flexible circuit substrates in the microelectronics and optoelectronic engineering industries.