The crystal structure of poly(2,6-naphthalenebenzobisthiazole)

The crystal structure of poly(2,6-naphthalenebenzobisthiazole) (Naph-2,6-PBT) was studied using X-ray and molecular modeling methods. The X-ray pattern of the annealed Naph-2,6-PBT fiber showed several Bragg reflections as well as streaks along the layer lines indicating that the registry between adjacent chains exists in the crystal with a great deal of axial disorder. Disordered structure in the crystal was fitted into the triclinic unit cell with the unit cell parameters of a=6.78 Å, b=3.46 Å, c=14.61 Å, α=88.0°, β=114.7°, and γ=94.8° with P I ¯ space group. The calculated density, 1.68 g/cm3 was comparable with the observed density, 1.56 g/cm3. The Δc/c (staggering ratio) representing the registry between the adjacent chains in the ac plane was −0.19, which is in good agreement with the energy calculation although another local energy minimum was found at Δc/c=0.31. The disordered structure in Naph-2,6-PBT was probably due to the discrete axial shift between Δc/c=0.31 and −0.19 in the ac plane. The LALS refinement showed that the naphthalene group was rotated by 9 (±3)° from the ac plane on the projected structure along the chain axis with a torsion angle between the naphthalene and benzobisthiazole rings of 23°.