Rheological transitions and in-situ IR characterizations of cellulose/LiCl•DMAc solution as a function of temperature

Cellulose was dissolved in LiCl/DMAc solvent system at various concentrations. Using rheological measurements, we observed that the cellulose solution becomes gelled upon heating and jelly-like during cooling. Interestingly, while phase separation occurs during heating (from room temperature to higher) along with a sol-gel transition, and this transition is irreversible, the cooling (from room temperature to lower) induced solution property transition from more viscous to more elastic is completely reversible. The structural changes of cellulose solution during heating and cooling cycles are in situ monitored by infrared spectroscopy, and we found that the temperature of the solution affects the interactions among DMAc, lithium cations, chloride anions, and the hydroxyl groups in cellulose chains. While heating induces phase separation and gelation of cellulose solutions, cooling simply turns the solution into a reversible jelly status. Also, when cellulose concentration increases, the gelation temperature caused by heating becomes significantly lower. Possible mechanisms of the rheological transitions of cellulose/LiCl center dot DMAc solutions during either heating or cooling are proposed in this study.