Synergistic regulation of the thermal–optical properties of polyimides via amide bonds and trifluoromethyl groups: from molecular design to flexible CNT TFT application

Abstract

With the rapid advancement of flexible display technology, stricter requirements are imposed on the high-temperature resistance and optical performance of flexible display substrates. Highly transparent, high-temperature-resistant polyimide (PI) is one of the most promising substrates, yet enhancing heat resistance via single molecular structure design often compromises transparency, making it difficult to balance the two properties. To balance the thermal stability and optical transparency of polyimide (PI), three diamine monomers (3F–P, 3F–M, and 3F–O) containing amide bonds and trifluoromethyl groups (–CF₃) were synthesized through molecular design, followed by the preparation of PI-1 to PI-6. Amide-derived intermolecular hydrogen bonds restricted molecular chain mobility and enhanced rigidity to improve heat resistance, while the electron-withdrawing property of –CF₃ inhibited electron transfer, thereby blocking the charge transfer complex (CTC) effect to boost transparency. Systematic characterization showed that the PI materials had a glass transition temperature (T_g) of 294.8–332.6 °C due to amide bonds. The meta-substituted PI exhibited optimal performance (T_g = 317.9 °C, transmittance = 88.73%, and b* = 2.16). Transfer characteristic curve tests on carbon nanotube thin film transistors (CNT TFTs) fabricated with PI-2 as the substrate confirmed the potential of PI-2 as a flexible electronic substrate. This work provides crucial insights and data for PI flexible substrate optimization and its flexible electronic applications.