An x‐ray and conformational study of Kapton H

Potential energy calculations were performed on the isolated chain of Kapton H (KH) in order to obtain information on the most probable conformations. A model with a COC angle of 130° and with the segments joining the ether oxygens forming a planar zigzag chain satisfactorily fits the x‐ray experimental data. Although the equatorial reflections are very few and diffuse, it seems likely that the space group is P21 and the unit cell parameters are a = 4.66 Å, b = 32.9 Å, c = 5.96 Å, β = 100°. The observed density is in agreement with two monomers in the unit cell. An intrachain vector distribution map was computed by rotating (angle θ) the pyromellitic acid dimide (DIPA) unit about the NC bond in the KH monomer. This map was compared with a one‐dimensional Patterson synthesis calculated along b, the fiber axis, to establish the value of θ. Van der Waals' energy calculations were subsequently accomplished in the crystal as a function of θ and of the rotation around the helical axis of the KH chains. The results allowed us to choose a reasonable macromolecular conformation and packing in the crystal. KH is mainly ordered along the fiber axis but shows little order in the packing of adjacent helixes. The macromolecules are held together by Van der Waals' and, probably, by charge‐transfer forces.