PHASE-SEPARATION OF RIGID POLYMERS IN POOR SOLVENTS .1. (HYDROXYPROPYL)CELLULOSE IN WATER

Aqueous solutions of (hydroxypropyl) cellulose, which can be either is otropic (dilute regime) or cholesteric (concentrated regime) at room t emperature, exhibit reversible phase separation upon heating above sim ilar to 37 degrees C. The biphasic system does not gel and includes a concentrated phase having the appearance of an emulsion of poorly orga nized microaggregates, and a dilute solution from which the concentrat ed phase cannot be completely separated. Ellipticity data suggest the onset of cholesteric order at temperatures slightly below those at whi ch isotropic solutions undergo phase separation. This hint of the form ation of the mesophase is detected at concentrations unusually low (si milar to 0.4% v/v). The conjugation of the two phases was evidenced by determination of the individual volumes and concentrations. The pseud o phase diagram includes boundaries for the high-temperature biphasic region which are not readily interpreted in terms of a heat-induced cr ystallization process. It is suggested that the primary event controll ing phase separation is the driving force for the formation of a mesop hase in a solvent which becomes poorer upon increasing temperature ('' inverted'' wide region of the theoretical diagram). The development of well-organized liquid crystalline domains, and their possible evoluti on toward a crystalline state, may be greatly slowed down by kinetic e ffects.