Molecular Modeling of Pure and Phosphoric Acid-Doped Poly(2,5-benzimidazole)

Abstract

Various oligomers and infinite chains of pure and phosphoric acid-doped poly(2,5-benzimidazole) (ABPBI) were studied by a quantum-chemical method within the framework of the cluster approximation with the functional ωB97XD and basis 6-31G**, taking into account periodic boundary conditions with the functional PBE and the basis of the projector-augmented plane waves PAW, and also by the molecular dynamics method MD-VASP. It was found that random orientations of ABPBI units and the presence of free rotation around a single bond connecting two monomeric segments of ABPBI lead to a minimal possibility of p–p interactions. ABPBI has a form wavy chain i.e. it has local curvatures and with energies of ∼0.1 eV can rotate around single bonds. In the case of doping with phosphoric acid, an acid-base reaction can occur in some monomer segments of ABPBI between the imidazole moieties in the polymer chain and phosphoric acid to form dihydrogen phosphate ions and protonated imidazolium cations ABPBIH⁺. When the temperature increases from 0 to 600 K, in addition to protons, acid molecules, acid residues, formed OH-fragments, and released intrastructural water also actively participate in the transport process. However, the structure of the fragments depends on the speed and uniformity of heating.