Tuning the heat resistance properties of polyimides by intermolecular interaction strengthening for flexible substrate application

Intrinsic polyimides (PIs) with high glass transition temperature (T-g), excellent thermal stability, and favorable mechanical properties have been in great demand in recent years with the rapid development of flexible displays. In this work, an interchain-interaction strengthener has been introduced to obtain PIs with improved thermal resistance and stability. Benzimidazole-based diamines imparting rigidity, conjugation, crosslinking, and especially hydrogen bonding have been incorporated into benzoxazole-polyimide. Frequency shifts in the IR bands spectra of C=O and N-H bonds were indicative of strong hydrogen bonding in benzimidazole-incorporating PIs, resulting in an increase in T-g from 333 degrees C to 442 degrees C. Furthermore, because of more extensive hydrogen bonding, PIs containing bis-benzimidazole units exhibited much higher crystallinity and T-g than those containing mono-benzimidazole units. The 5% weight loss temperature (T-d(5)) was above 550 degrees C because of the rigidity of the conjugated structure and the tightly packed state of the polymer chains. T-g and T-d(5) of PIs annealed in air were higher than those of samples annealed in vacuo, whereas the opposite trend was seen in crystallinity and mechanical properties. Finally, we delineated the effects of intermolecular interaction strengthener and annealing atmosphere on the properties of PIs, which may be helpful for the future design of flexible PI substrates.