ABSTRACTThis work concerns solutions of the rigid polymers: poly(p-phenylene benzobisthiazole)-PBZT, and poly(p-phenylene benzobisoxazole)-PBO, in polyphosphoric acid (PPA), and formation of a crystal solvate phase by cocrystallization of polymer and solvent. Several crystal-solvate phases have been observed in these solutions, depending on temperature and the extent of exposure to moisture. The structure of these phases is studied by X-ray diffraction. Transitions between the solvated phases involve changes in the lateral packing as well as the axial registry of parallel chains. All PBOPPA crystal solvates observed so far exhibit three-dimensional order, whereas such order appears only in the PBZT-PPA phase obtained at low moisture levels. These differences are interpreted in terms of the state of protonation of the polymer chains, whereby the highly ordered structures reflect unique associations of the protonated polycation and PPA anions. Observations by electron microscopy indicate that the transition from the highly ordered crystal solvate phase to the less ordered phase involves formation of thin microfibrils, less than 100 Å in width. These microfibrils may be the precursor of the microfibrillar morphology observed in fibers formed from solutions of the rigid polymers.
