Unique Gelation Behavior of Cellulose in NaOH/Urea Aqueous Solution

A transparent cellulose solution was prepared by mixing 7 wt % NaOH with 12 wt % urea aqueous solution which was precooled to below −10 °C and which was able to rapidly dissolve cellulose at ambient temperature. The rheological properties and behavior of the gel-formed cellulose solution were investigated by using dynamic viscoelastic measurement. The effects of temperature, time, cellulose molecular weight, and concentrations on both the shear storage modulus (G') and the loss modulus (G' ') were analyzed. The cellulose solution having a viscosity-average molecular weight (Mη) of 11.4 × 104 had its sol−gel transition temperature decreased from 60.3 to 30.5 °C with an increase of its concentration from 3 to 5 wt %. The gelation temperature of a 4 wt % cellulose solution dropped from 59.4 to 30.5 °C as the Mη value was increased from 4.5 × 104 to 11.4 × 104. Interestingly, at either higher temperature (above 30 °C), or lower temperature (below −3 °C), or for longer gelation time, gels could form in the cellulose solutions. However, the cellulose solution remains a liquid state for a long time at the temperature range from 0 to 5 °C. For the first time, we revealed an irreversible gelation in the cellulose solution system. The gel having been formed did not dissolve even when cooled to the temperature of −10 °C, at which it was dissolved previously. Therefore, this indicates that either heating or cooling treatment could not break such stable gels. A high apparent activation energy (Ea) of the cellulose solution below 0 °C was obtained and was used to explain the gel formation under the cooling process.