Synthesis and Characterization of High Glass Transition Temperature Colorless Polyimides Containing Hydrogen Bonding Carbazole Diamine for Optoelectronic Devices

Maintaining ultrahigh heat resistance, a low thermal expansion coefficient (CTE), and adequate colorless transparency concurrently poses a significant challenge for colorless polyimides (CPIs), especially as substrate materials for flexible optoelectronic devices. In this work, we designed and synthesized a hydrogen-bonding carbazole tetraphenyl aromatic diamine, 2,7-bis[2-trifluoromethyl-4-aminophenyl]-9H-carbazole (2,7-CPFDA). The corresponding polyimide (PI) films were synthesized via the copolymerization of 2,7-CPFDA and 2,2′-bis(trifluoromethyl)benzidine (TFDB) with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) at varying molar ratios. All copolymer PI films presented high heat resistance with the 5% weight loss temperatures (Td5) between 552 and 563 °C, and the glass transition temperatures (Tg) ranged from 354 to 380 °C. As the content of 2,7-CPFDA increased, the CTE decreased from 17.6 to 10.4 ppm K–1, while the tensile modulus (E) rose from 5.7 to 6.7 GPa, and the elongation at break (ε) improved from 5.4% to 28%. When BPDA was substituted with 9,10-diphenyl-9,10-bis(trifluoromethyl)-9,10-dihydroanthracene-2,3,6,7-tetraacid dianhydride (6FDPDA) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), the CPI films exhibited overall favorable properties. Notably, C–PI-7 exhibited a high Tg of 456 °C, excellent mechanical properties (E = 6.7 GPa, ε = 11.9%), low CTE (8.7 ppm K–1), and high transmittance at 450 nm (T450 = 86.1%), thereby meeting the performance requirements for flexible electronic devices.