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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.

To tolerate high processing temperature during the fabrication of low‐temperature polycrystalline silicon thin‐film transistors (LTPS–TFT) in flexible OLED devices, the polyimide (PI) films, which are used as substrate, should have ultra‐high glass transition temperature (Tg> 450°C) and ultra‐low coefficient of thermal expansion (CTE at 0–5 ppm K−1). In this paper, two novel heterocyclic monomers, namely, N,N'‐(xanthone‐2,7‐diyl)bis(4‐aminobenzamide) (p‐DAXBA) and N,N'‐(xanthone‐2,7‐diyl)bis(3‐aminobenzamide) (m‐DAXBA), which contain a xanthone moiety, are prepared and polycondensed with pyromellitic dianhydride (PMDA), respectively. PI films (PIa and PIb) with intrinsic high Tgand low CTE are designed from the perspective of rigid conjugate xanthone structure and hydrogen bonding interaction. It is found that the PIa films prepared by p‐DAXBA have better linear structure of molecular chains and show relatively higher Tgand lower CTE. The Tgof PIa‐40 is greater than 450°C, and CTE can reach as low as 2.7 ppm K−1, tensile strength of 179 MPa, modulus of 5.67 GPa, indicating potential application prospect as a flexible OLED substrate.

The utilization of 5-amino-2-(2′-hydroxy-4′-aminophenyl)-benzoxazole (HBOA) as third monomer was beneficial for the introduction of OH-type H-bonding into polyimide (PI) matrix through the copolymerization with 2,5-bis(4-aminophenyl)pyrimidine (PRM) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). And high-performance PI fibers were successfully fabricated via the dry-jet wet spinning process. The FTIR spectra confirmed that the strong interactions of intramolecular and intermolecular H-bonding existed simultaneously in PI fibers. Moreover, the glass transition temperatures (Tg) of all fibers were within 276–318 °C, and the temperatures at 5% weight loss (T5%) ranged from 574 °C to 604 °C, revealing the remarkable thermal stability of PI fibers. The two-dimensional wide-angle X-ray diffraction (2D WAXD) results illustrated a U-shaped change in the crystallinity (from 32.1% to 12.2% until 33.4%) of PI fibers with increasing HBOA content, which was also reflected in the density variation of fibers. The two-dimensional small-angle X-ray scattering (2D SAXS) patterns demonstrated that the micro-defects in all co-PI fibers profitably decreased in comparison with those in homo-PI fibers. Furthermore, the orientation factor of fibers presented an improvement from 0.843 to 0.899 after incorporating HBOA moiety. When the molar ratio of PRM/HBOA was 7/3, the co-PI fiber exhibited optimum mechanical properties with tensile strength and modulus as high as 4.15 GPa and 125.93 GPa, respectively. Interestingly, the variation in fiber orientation was in line with the same trend as that of mechanical properties of PI fibers, indicating orientation was also a crucial factor to improve the mechanical properties. The relationship between structure and mechanical property was also investigated.

Polymers with exceptional heat resistance are critically valuable in numerous domains, particularly as essential components of flexible organic light-emitting diodes. Among these, polyimides (PIs) demonstrate significant potential as substrate candidates for these next-generation flexible displays due to their robustness. However, traditional Edisonian approaches struggle to navigate the vast chemical space of PIs and also pose challenges of small data, which constrains the learnable chemical space for machine learning (ML). In this study, we propose a chemical-knowledge-based strategy to facilitate the design of PIs with high glass transition temperature (Tg) utilizing an atom-wise graph neural network and small data. Inspired by chemical intuition, our strategy leverages the available data on the same property (i.e., Tg) from other polymers, which is beneficial for expanding the chemical space used for ML. The trained ML model achieves an impressive performance in predicting Tgof polymers. We have also investigated the impact of the chemical space encompassed by the data sets on the performance of ML models. Through interpretability analysis, it has been demonstrated that our ML model has learned more accurate chemical knowledge. Utilizing the ML model, 89 PIs were rapidly discovered from over 106candidates, with experimental validation confirmed their exceptional heat resistance of the most promising PIs, which have been found to possess a Tgexceeding 405 °C and even 450 °C. These results, along with the trained ML model, have the potential to accelerate the discovery of polymer substrate materials for next-generation flexible display devices.