Structure of a poly(2,5-benzimidazole)/phosphoric acid complex

As-east films of poly(2,5-benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 degreesC, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Angstrom, b = 3.5 Angstrom, and c = 11.4 Angstrom, containing four monomer units of two chains. The absence of odd-order 001 reflections points to a 2(1) chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X-ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c-axis registry. (C) 2004 Wiley Periodicals, Inc.