Oriented liquid-crystalline solutions of the rigid heteroaromatic polymers poly (p-phenylene benzobisthiazole) and poly(p-phenylene benzobisoxazole) in polyphosphoric acid were transformed into crystalline phases by absorption of moisture. At moderate moisture levels crystal-solvate phases are formed by co-crystallization of the polymer and its solvent, whereas at high water content the crystalline polymer phase is formed by deprotonation of the polymer. Two crystal-solvate forms have been identified, as the moisture content increases up to about 10% (w/w), denoted form I and form II. Transitions between the different phases, as a function of temperature and time, were studied by X-ray diffraction using the high-intensity synchrotron source. The intensity of the crystalline reflections from the form I state decrease gradually with increasing temperature up to final melting at about 70°C. The orientation of the crystalline phase is maintained up to its melting. Recrystallization is observed upon quenching, showing the reversibility of this transition. The behaviour of the form II phase differs markedly. As the temperature is increased, a gradual crystal-crystal transition occurs in the temperature range of 250-300°C from the form II state to the crystalline polymer state. This transformation is irreversible. A novel recrystallization phenomenon is observed when the liquid-crystalline phase is heated to about 250°C. Crystalline reflections appear at spacings close to those of the crystalline polymer state. These reflections disappear upon cooling. These transformations are discussed in terms of the state of protonation of the polymer in its solution in polyphosphoric acid. Form I is considered a complex of the protonated polymer and the acid anions, which dissolves at moderate temperatures. Recrystallization at elevated temperature is suggested to result from deprotonation of the polymer. The state of protonation of the polymer in the form II state is still not understood.
Посилання на статтю:
Phase transitions in solutions of rigid polymers studied by synchrotron radiation / Y. Cohen and S. Buchner and H. G. Zachmann // Polymer. – 1992. – Vol 33. – P. 3811-3817.