Transparent polyamide-imides containing fluorene groups with high glass transition temperature and good dimensional stability

With the rapid development of the flexible display industry, the requirements for the transparent polyimide flexible substrate are becoming more and more stringent. In this work, two diamines, 9,9-bis(4-(4′-aminobenzoylamino)phenyl)fluorene (FDAADA) and N,N′ -(9,9′-spirobifluorene-2,2′-diyl)bis(4-aminobenzamide) (SBFADA), were synthesized. Cyclopentanone bis-spironorbornane tetracarboxylicdianhydride (CpODA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) were utilized as dianhydride monomers to prepare a series of transparent polyamide-imides (PAIs). All PAIs exhibited high glass transition temperatures ( T g s) of 423–468 ℃, coefficients of thermal expansion (CTEs) of 36.1–64.2 ppm/K, optical transmittances of 15–85 % at 400 nm, tensile strengths of 89.8–147.4 MPa, and tensile moduli of 2.9–4.3 GPa. In particular, PAI-1 ( T g =454 ℃, T 400 =85 %, CTE=36.1 ppm/K) and PAI-4 ( T g =468 ℃, T 400 =66 %, CTE=50.0 ppm/K) exhibited good optical properties, excellent thermal properties and low CTEs, which demonstrated the potential application in flexible substrate materials.