Constructing high-performance low-temperature curable PI materials by manipulating the side group effects of diamine

To date, low-temperature curable polyimide (PI) materials have been in great demand in the field of advanced packaging. However, their synthesis is still a challenge due to the distinct structural design contradiction between the low-temperature curing ability and other desired properties. In this work, we designed and synthesized two novel diamines with different side groups (phenyl and pyrimidine rings), named PhNH2 and SPMNH2, and applied them to construct high-performance PI. Inspired by our previously reported pyrimidine based diamine PMNH2 (X. Lv, S. Qiu, S. Huang, K. Wang and J. Li, Polymer, 2022, 261, 125418), we tried to explore the impact of nitrogen heterocycle position (in the side group or main chain) on low-temperature curing ability. Compared with the typical diamine complex with nitrogen heterocycles in the main skeleton, the well-designed pyrimidine side group endowed the PI films with a highly improved imidization degree (ID) and excellent thermal and mechanical properties at a low curing temperature (200 degrees C). It should be noted that the configurations and the side group effects of diamine could modulate the temperature of polymerization and the structure of polymers, which further influenced the final performance of PI films. The copolyimide films based on SPMNH2 exhibited a high elongation of 103.4%, Young's modulus of 3.20 GPa, tensile strength of 147 MPa, 5wt % decomposition temperature (T-d5%) of 524 degrees C, and a glass transition temperature (T-g) of 359 degrees C, which was much better than that for PI film copolymerization with PhNH2 or PMNH2 cured at 200 degrees C. Experimental results and theoretical calculations showed that the introduction of pyrimidine side groups allowed for a better catalytic effect in the reaction system, and stronger intermolecular force between the polymer chains. This is the first report about the side-group effects on low-temperature curable PI films, which should be enlightening for the design of novel and high-performance diamines applied for advanced packaging and other fields.