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

Abstract

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°C, an ultra‐low thermal expansion coefficient of <5 ppm K−1 (50–450°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.