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4-Amino- N ′-(4-aminobenzoyl)benzohydrazide (AAPDA), a diamine monomer that contains two amide groups, was synthesised by amidation and reduction, after which it was polymerised with pyromellitic dianhydride (PMDA) to prepare AAPPI, a novel polyimide. AAPPI exhibited excellent barrier performance, with oxygen- and water-vapor-transmission rates (OTR and WVTR, respectively) of only 1.7 cm ^3 m ^−2 d ^−1 and 1.0 g m ^−2 d ^−1 , respectively. This polyimide (PI) also exhibits outstanding thermal properties, with a glass transition temperature (T _g ) of 423 °C, a 5% weight-loss temperature (T _d5% ) of 509 °C, and a coefficient of thermal expansion (CTE) of 2.58 ppm K ^−1 under nitrogen. The barrier performance of AAPPI was also compared to that of DABPI, a structurally similar PI. Molecular simulations, wide-angle x-ray diffractometry (WAXD), and positron annihilation lifetime spectroscopy (PALS) revealed that AAPPI forms many more interchain hydrogen bonds than DABPI due to its additional amide groups. Consequently, AAPPI has very tightly packed polymer chains, a high degree of crystallinity, a small free volume, and poor chain mobility. These factors generally inhibit the permeation of small molecules, which explains why AAPPI has better barrier properties than DABPI. This novel PI has broad applications for the packaging of flexible electronics.