Polyimide synthesis and characterizations: DFT-assisted computational studies on structural units

Abstract

A diamine was obtained from commercial pyridin-4-amine through a two-step synthesis (dinitro to diamine). The final combination of diamine, disulfonic acid, and pyromellitic dianhydride resulted in poly(amic acid)_x (PAA_x), and the original sulfonated polyimide_x (SPI_x) was produced by polycondensation. Various characterizations of dinitro, diamine, and SPI_x were performed, with each material analyzed individually. Quantum chemical calculations of SPI_x, Density Functional Theory (DFT) studies, and innovative analyses derived from DFT output data were carefully examined. An important point is that the molecular weight of SPI_x (M_n 28,300 g.mol⁻¹) was calculated using end-group signals obtained by proton nuclear magnetic resonance (¹H NMR) spectra (-NH at 2.21 ppm and -NH₂ at 1.15 ppm). Similarly, to gain prior knowledge about the conductivity or insulation properties of the material, the λ_max value (430 nm) and band gap (Δ) (2.88 eV) in the ultraviolet–visible (UV–vis) spectrum were calculated and compared with the band gap (Δ) energy values (1.784 eV, 0.988 eV, and 1.719 eV) provided by the theoretical E_HOMO-E_LUMO calculations. Thermal thermograms suggest that the final polymer can exhibit high thermal resistance and perform well as an engineering material. The morphological properties of the polymer were clearly presented through cross-sectional scanning electron microscopy (SEM) observations, providing qualitative insights into the product’s behavior under environmental stress. All these characterizations and theoretical calculations offer predictions for the future application areas of this newly synthesized polymer. Theoretical data can be used to comment on the hydration properties, proton transfer, electrical properties, and optical properties of the polymer. Our study is actually aimed at this.

Graphical abstract