Phase equilibria in solutions of cellulose derivatives and the rheological properties of solutions in various phase states

Abstract

Phase equilibria in solutions of hydroxypropyl cellulose in such solvents as water, propylene glycol, poly(ethylene glycols) of various molecular masses, triethyl citrate, and DMSO are analyzed. The phasetransition lines are constructed with the use of microinterference and polarization microscopy methods. In a certain temperature-concentration range, all solutions undergo a sequence of transitions typical for solutions of stiff-chain crystallizable polymers: isotropic and LC states separated by the two-phase corridor and, in some cases, the crystalline state. Of special interest are systems in which the superposition of amorphous (with the LCST or UCST), LC, and crystalline equilibria is realized. The features of the phase states of solutions are compared with the physicochemical characteristics of solvents and the intensity of their interaction with hydroxypropyl cellulose macromolecules. Solutions occurring in various phase states are tested by rheological methods. These solutions demonstrate the typical rheological behavior of anisotropic solutions: the presence of yield stress and the extremum concentration dependence of viscosity with a maximum, when the LC phase appears, and a minimum, when it transforms into the 100% phase. In the case of single-phase solutions, viscosity increases with concentration, while for two-phase solutions, viscosity decreases with an increase in the fraction of the LC phase. The rheological data are found to be sensitive to phase transitions observed in solutions.