Rheology and phase behavior of the system PBZT-phosphoric acid-water

Abstract

Solutions of poly-p-phenylenebisbenzthiazole (PBZT) in phosphorous acid were examined in a rheogoniometer for shear response and relaxation as a function of concentration and temperature based on knowledge of the phase diagram of the system. It was found that the various regions of the phase diagram (i.e., the isotropic, the nematic, and the biphasic regions) and the various kinds of crystal solvates were reflected by their rheological behavior. This shows that rheological properties can be used to map, at least, the broad features of phase diagrams; conversely, the knowledge of the phase diagram is a prerequisite for the appreciation of rheological behavior. At the same time, the stress involved in the rheological tests themselves can modify, to a lesser extent, the positions of the phase dividing lines compared to those in the static system, which at appropriate points in the phase space can lead to stress-induced phase transitions. The principal rheological effect, here used diagnostically, is in itself novel; it is a spontaneous stress buildup in the stationary state of the rheometer after the stresses generated by the preceding shearing have decayed following stoppage of the rotor. The effect itself will be followed up in a separate study.