Synthesis and characterization of pyromellitic dianhydride based sulfonated polyimide: Survey of structure properties with DFT and QTAIM

Pyromellitic dianhydride (PMDA) and 4,4′-diamino-2,2′-stilbenedisulfonic acid (DSD) were polymerized by polycondensation in 1:1 Sulfonated Polyimide (SPI) m-Cresol. The SPI was characterized by spectroscopic analyses (Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (1H-NMR), carbon nuclear magnetic resonance (13C-NMR), ultraviolet–visible spectroscopy (UV–Vis), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)/ differential thermal analysis (DTA), solubility tests and elemental analysis (EA) techniques. Furrhermore, quantum chemical calculations of the synthesized SPI were analyzed by density-functional theory (DFT) with Becke-3-Parameter-Lee–Yang–Parr (B3LYP)/6–311-G basis set to understand electrical and optic features of SPI. Natural Bond Orbital (NBO) analysis and molecular electrostatic potential (MEP) surfaces were calculated. Quantum theory analysis of atoms in molecules (QTAIM) was made using the AIMAll program to find H-bonding, proton interaction, proton transfer and water uptake script. The second degradation stage was observed at a temperature higher than 450 °C in TGA/DTA curve. The variation of the heat flow in reverse heat flow was based on the Tg of SPI occurring around 353.11 °C.