High glass transition temperature and ultra-low thermal expansion coefficient polyimide films containing rigid pyridine and bisbenzoxazole units

Polyimide (PI) films with extremely high heat-resisting and dimensional stability are ideal substrate materials for flexible organic light-emitting diodes. In this study, three diamines containing rigid bisbenzoxazole structures, 2,2 '-p-phenylenebis(5-aminobenzoxazole) (phDBOA), 2,2 '-m-pyridylenebis(5-aminobenzoxazole), and 2,2 '-p-pyridylenebis(5-aminobenzoxazole) (p-PDBOA), were polymerized with 3,3 ',4,4 '-biphenyl tetracarboxylic dianhydride and pyromellitic dianhydride (PMDA) separately with traditional two-step approach to prepare a set of PI films. Introduction of bisbenzoxazole improves stiffness of molecular chain, whereas that of pyridine increases in-plane orientation and molecular chain tend to be densely stacked. Among prepared films, PI-6 (PMDA/p-PDBOA) shows the most excellent mechanical properties, with an ultra-high glass transition temperature of >450 degrees C, an ultra-low thermal expansion coefficient of <5 ppm K-1 (50-450 degrees C), a tensile strength of 259 MPa, and a modulus of 8.3 GPa. Given these excellent properties, PI-6 could be applied in flexible display.