Molecular design strategy for high-performance colorless polyimides: Tuning heterocyclic structure and substituent polarity

Colorless polyimide (CPI) films have emerged as essential materials for flexible optoelectronics, serving as substrates or protective layers in foldable displays and wearable devices. Achieving a balanced combination of high thermal stability, mechanical durability, and optical transparency, however, remains a persistent challenge. In the present work, three diamine monomers containing different heterocyclic rings and side groups were synthesized, and a series of CPI films were prepared from the three heterocyclic diamines and four commercial dianhydrides (6FDA, BPDA, BTDA and BPADA) via thermal imidization. The effects of heterocyclic structure, substituent type, and dianhydride type on the film properties were systematically investigated. All films exhibited amorphous structures and excellent thermal stability, with T g ranging from 220 to 315 °C and T 5% between 498 and 595 °C. In this study, the Pym-2FBAP-based films exhibited the best overall performance among all the films investigated. Showing the highest T g , superior mechanical performance with tensile strengths of 103–133 MPa and elongations at break of 8%–48%, and outstanding optical transparency with a maximum transmittance of 87.6% at 550 nm, low yellowing and haze values. These advantages were attributed to the symmetric pyrimidine ring and strong electron-withdrawing fluorine substituents, which promoted dense chain packing and suppressed charge-transfer complex formation, as further supported by DFT calculations that revealed E g in the range of 3.11–3.44 eV. Flexibility tests confirmed excellent bending durability, as the films withstood 100,000 cycles at a 3 mm radius without visible damage. The combination of these properties positions the Pym-2FBAP-based CPI films as promising candidates for flexible electronic applications.