Thermal Gelation of Cellulose in a NaOH/Thiourea Aqueous Solution

Utilizing a novel solvent of cellulose, 6 wt % NaOH/5 wt % thiourea aqueous solution, for the first time, we prepared the thermally induced cellulose gel. We investigated the thermal gelation of cellulose solutions with rheometry and the structure of the gel with ¹³C NMR, wide-angle X-ray diffraction, environmental scanning electron microscopy, and atomic force microscopy. The cellulose solutions revealed an increase in both the storage modulus (G′) and the loss modulus (G″) with an increase in the temperature during gelation. The temperature at the turning point, where G′ overrides G″ because of the onset of gelation, decreased from 38.6 to 20.1°C with an increase of cellulose concentration from 4 to 6 wt %. Given enough time, G′ of all solutions can exceed G″ at a certain temperature slightly lower than the gelation temperature, indicating that the occurrence of the gelation is also a function of time. Each of the assigned peaks of NMR of the cellulose gel is similar to that of the cellulose solution, suggesting that the gelation resulted from a physical cross-linking. The gels were composed of relatively stable network units with an average diameter of about 47 nm. At either a higher temperature (at 60°C for 30 s) or a longer gelation time (at 30°C for 157 s), the gel in the 5 wt % cellulose solution could form. A schematic gelation process was proposed to illustrate the sol-gel transition: the random self-association of the cellulose chains having the exposed hydroxyl in the aqueous solution promotes the physical cross-linking networks.