Molecular Packing and Crystalline Order in Polybenzobisoxazole and Polybenzobisthiazole Fibers

Abstract

The structures of poly(p-phenylenebenzobisthiazole) (PBZT) and poly(p-phenylenebenzobisoxazole) (PBO) fibers have been investigated by fiber diffraction techniques. d-spacings were obtained from equatorial and meridional scans recorded on a four-circle diffractometer. Intensity data were derived from x-ray rotation patterns taken on Weissenberg and vacuum cylindrical cameras. Unit cells were found to be monoclinic and non-primitive, each containing two chains per cell of dimensions a = 11.79(2), b = 3.539(5), c = 12.514(9) A, γ = 94.0(2)o for PBZT; and = 11.20(1), b = 3.540(2), c = 12.050(3) Å, γ = 101.3(1) for PBO. The fiber axes correspond to c. The conformational torsion angle between the bisthiazole and phenylene units and the orientation of chains within the unit cells were obtained from a ‘linked-atom least-squares’ (LALS)refinement procedure. A packing model is proposed for each polymer in which two independent molecular chains are displaced longitudinally by discrete rather than random increments. Primitive unit cells (Z = 1), besides requiring perfect axial registry of molecular chains, suffer from the occurrence of short intermolecular contacts and are rejected from further consideration.

Modeling the effect of crosslinking in methyl-pendant poly(p-phenylene benzobisthiazole)

“…An interesting feature of the PBZT unit cell is the noncoplanarity between the phenyl ring and the heterocycle, 30 attributed to chain-packing considerations. The same noncoplanarity is also observed in 2,6-diphenylbenzo[1,2-d:4,5-dЈ]bisthiazole, 22 a model compound of PBZT.…”

A Novel Random Preferential Orientation of the Crystal A‐axis Along the Radial Direction Confirmed on the Poly‐<i>p</i>‐phenylenebenzobisoxazole (PBO) Fiber Made with a Water

“…We propose a hypothesis that in the course of detachment of the phosphoric acid from the PBO molecule and its outflow to the outside, the acid molecule can rotate the phenylene ring in the PBO molecule and align the phenylene ring of PBO molecule to the direction of the flow (Figure 9(1)), because there is the chemical force of solvation between PBO and polyphosphoric acid. It should be noted that the direction of the phenylene ring normal to the molecular axis is almost parallel to the direction of the a-axis of the PBO crystal 18 , suggesting the reason of presence of the radial pattern of the a-axis in the fiber structure is due to this outflow of the solvent by coagulation.…”

“…Therefore, the fiber shows relatively small crystallite size, especially along the a-and b-axes. Here, the diffraction spots are indexed after Fratini et al 18 in this article.…”

High-Performance Fibers

“…The PBO fiber is crystalline, with monoclinic unit cells [83]. The crystallinity of PBO improves with heat treatment, which is also accompanied by a significant improvement in fiber modulus [84].…”