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 ( T d5 ) between 552 and 563 °C, and the glass transition temperatures ( T g ) 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 T g 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 ( T 450 = 86.1%), thereby meeting the performance requirements for flexible electronic devices.