Structure and Gas Barrier Properties of Polyimide Containing a Rigid Planar Fluorene Moiety and an Amide Group: Insights from Molecular Simulations

[Image: see text] A novel diamine (FAPDA) bearing rigid planar fluorene and amide groups was successfully synthesized. Using such diamine and pyromellitic dianhydride (PMDA), a high-barrier polyimide (FAPPI) was obtained. FAPPI exhibits an outstanding gas barrier. Its water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) are as low as 0.51 g·m(–2)·day(–1) and 0.43 cm(3)·m(–2)·day(–1), respectively. Additionally, FAPPI shows excellent thermal stability with a coefficient of thermal expansion (CTE) of 5.8 ppm·K(–1) and a glass transition temperature (T(g)) of 416 °C. Molecular simulations, positron annihilation, and X-ray diffraction were utilized to gain insight on the microstructures for the enhanced barrier properties. Introducing fluorene moieties and amide groups improves the regularity and rigidity of molecular chains and increases interchain interaction of PI, resulting in low free volumes and decreased movement capacity of the chain. The low free volumes of FAPPI restrain the gas diffusivity and solubility. Meanwhile, the decreased chain movement reduces the diffusivity of gases. Consequently, barrier performances of FAPPI are improved. The polyimide possesses widespread application in the microelectronics packaging fields.