The evolution of macromolecular packing and sudden crystallization in rigid-rod polyimide via effect of multiple H-bonding on charge transfer (CT) interactions

The evolution of macromolecular packing of rigid-rod aromatic PI with high Tg containing benzimidazole units in the main chain was studied. The active H-bonging at high temperature (300 ∼ 420 °C) affected the CT interactions and then dominated the macromolecular packing. The PI fibers and films were investigated by tensile testing, DMA, FTIR, PCMW2D, WAXD, SAXS and UV/vis. The results of FTIR indicate multiple H-bonding interactions were formed in PAA and after full thermal imidization one kind of H-bonding between the –NH– of benzimidazole units and CO of imide rings still existed, acting as physical cross-linking points and driving PI into the displacement packing with amorphous domain and low tensile strength based on the data from WAXD structural analysis and tensile test. Only when the annealing temperature increased to 400 °C, this type of H-bonding became weakened and transferred to another type of H-bonding, resulting in sudden crystallization and high tensile strength. Simultaneously, the cut-off wavelength in UV/vis transmission decreased from 447 nm to 431 nm, indicating that the CT interactions were also weakened.