Conformational disorder in the pseudohexagonal form of atactic polyacrylonitrile

Accurate wide angle X-ray diffraction measurements for a highly oriented polyacrylonitrile (PAN) fiber suggest the presence, in the pseudohexagonal form of PAN, of sequences with an average periodicity close to 2.4 Angstrom. This is in qualitative agreement with the mean periodicity anticipated by the knowledge of lateral spacing of the pseudohexagonal cell and of the experimental density value. A conformational analysis has allowed location of minimum energy extended conformations for stereoregular and stereoirregular model chains of PAN. The Fourier transforms of these minimum energy extended model chains have been calculated and compared to the experimental X-ray diffraction data. This comparison indicates that the paracrystalline pseudohexagonal phase of PAN is formed by elongated chain stretches, with atactic configuration. The backbone dihedral angles of these atactic stretches in the minimum energy conformation assume values close to 180 degrees, corresponding to r diads, but may present also large deviations from 180 degrees, corresponding to m dads. Where these deviations occur, the length of the projection along the chain axis of the vector connecting the centers of consecutive monomeric units is shorter than 2.5 Angstrom and close to 2.3 Angstrom, giving a mean chain periodicity in a good agreement with the experimental value.