Structural Characterization of an Ordered Aromatic Polyimide: Pyromellitic Dianhydride-Oxydianiline

The three-dimensional (3-D) ordered structure of semiflexible pyromellitic dianhydride oxydianiline (PMDA-ODA) is calculated using the CHARMM molecular modeling program. Polymorphism in the ordered chain conformation is calculated with chain periodicities n = 2, 3, and 4, where n is the number of monomers within the period. A planar zigzag chain cross-section is observed in the n = 2 and 4 structures while a triangular cross-section is observed in the n = 3 structure. The optimum ordered chains are packed to form a 3-D crystal by considering relative chain translations parallel and perpendicular to the chain axis by adjusting the number of basis chains, m in the unit cell. Although the n = 3 and 4 periodicities have not been reported, we calculate an approximate 3-D crystal structure factor. Corresponding structure factors along and perpendicular to the chain are calculated and compared with wide-angle X-ray scattering experiments, with good agreement. The lowest energy 3-D structure consists of an n = 2 conformation, with single chain periodicity along the interplanar packing direction and double chain periodicity along the edge-on stacking direction.