The crystal structure of a rigid-rod aromatic polyimide: application of a molecular simulation technique to calculate the structure

Abstract A molecular simulation technique, combined with X-ray diffraction measurements, was applied to the determination of the crystal structure of poly(4,4′-diphenylene pyromellitimide), a high-modulus aromatic polyimide. Simulation of the crystal structure was carried out with the programs POLYGRAPH and CERIUS (Molecular Simulations Inc.) using the Dreiding force field approach. Initial models optimized by molecular mechanics were set up in order to minimize the energy of an isolated chain and then located in the crystalline lattice by using the unit cell parameters obtained from the X-ray diffraction measurements. These structures were then optimized by the anneal-dynamics method by fixing the unit cell. The computed diffraction patterns calculated for the simulated structure agree well with the X-ray diffraction patterns obtained experimentally. The two benzene rings of the biphenyl group are coplanar and rotated by 65° from the pyromellitimide group. The extended polymer chains run along the direction of the c -axis, with the pyromellitimide plane being inclined at 58° to the (010) plane. The structure has the space group Pbam, with unit cell parameters of a =8.45, b =5.71 and c ( fibre axis )=16.78 A . Two adjacent pyromellitimide groups are aligned side-by-side at the same height along the c -axis and form a zigzag plane along the a - and b -axes.