Heat‐resistant polyimides with low CTE and water absorption through hydrogen bonding interactions

A novel diamine, 1H,1′H-(2,2′-bibenzimidazole)-5,5′-diamine (DPABZ), containing bisbenzimidazole unit was successfully synthesized, and used to prepare a series of copolyimides BPDA:(ODAm/DPABZn) by polycondensation with 4,4-diaminodiphenyl ether (ODA) and 4,4-biphthalic anhydride (BPDA). For comparison, a series of copolyimides BPDA:(ODAm/PABZn) based on another benzimidazole diamine 5-amino-2-(4-aminobenzene)-benzimidazole (PABZ) was also prepared. As a result, with the increase of PABZ or DPABZ content, the heat resistance (Tg and Td) and mechanical properties (σ and E) of the resulting polyimide (PI) films increased, while the coefficient of thermal expansion (CTE) decreased. Overall, the DPABZ-based PIs showed higher Tg values and much lower CTE values than PABZ. As the content of PABZ increased, the water absorption of PABZ-based PIs increased obviously, but no significant change in DPABZ-based PIs. The intramolecular hydrogen bonding in DPABZ-based PIs caused by the diamine DPABZ was believed to be the reason for the aforementioned differences. The BPDA: DPABZ film with low-water adsorption of 2.1%, high-Tg value of 436°C and low-CTE value of 5.4 ppm/°C could be a promising new generation of flexible display substrates.