Ultrahigh thermal-stability polyimides with low CTE and required flexibility by formation of hydrogen bonds between poly(amic acid)s

[Display omitted] •The diphenylpyridine and diphenylpyrimidine moiety were introduced into PI chains.•The ultralow CTE (< 5 ppm K−1), high Tg (> 450 °C) and good flexibility were achieved.•Effect of chain rigidity, chain packing and in-plane orientation on CTE was studied.•The impact of hydrogen bonds formed at the PAAs stage on CTE was discussed. The flexibility of organic light-emitting diode (OLED) displays highly depends on the properties of the flexible substrates. In this paper, a series of aromatic polyimides have been fabricated via the copolycondensation of pyromellitic dianhydride (PMDA), the two different rigid heterocyclic diamines, 2,5-bis(4-aminophenyl)pyrimidine (PRM) or 2,5-bis(4-aminophenyl)pyridine (PRD), and another flexible diamine, 4,4′-oxydianiline (ODA). The performance of the polyimide films could be systematically tailored by means of adjusting the main-chain rigidity, as well as the close packing and orientation of polymer chains by the formation of the intermolecular hydrogen bonds between poly(amic acid)s. The optimal results (PIb-4, PIb-5, PIc-2) showed that the polyimides were endowed with ultra-high glass transition temperature (Tg) exceeding 450 °C, low coefficient of thermal expansion (CTE) at 0–5 ppm K−1 and excellent thermal stability (Td5% = 570–590 °C). Meanwhile, all of them exhibited sufficient flexibility, the elongation at break at of 40–60%, extremely high tensile strength of 250–380 MPa and modulus of 4.1–6.1 GPa. Hence, the polyimide films should be the promising candidates for application as the polymer substrates for flexible OLED displays.