Dissolution and Metastable Solution of Cellulose in NaOH/Thiourea at 8 °C for Construction of Nanofibers

To develop a facile approach for the dissolution of cellulose, a novel solvent (9.3 wt % NaOH/7.4 wt % thiourea aqueous solution) was used, for the first time, to dissolve cellulose within 5 min at 8 °C. The results of NMR and Raman spectra demonstrated that stable thiourea···OH^- complexes were formed through strong hydrogen bonds in NaOH/thiourea at room temperature. Moreover, the strength of the hydrogen bonds in thiourea···OH^- complexes was much higher than that in urea···OH^- complexes, and the number of thiourea···OH^- complexes increased significantly in 9.3 wt % NaOH/7.4 wt % thiourea compared to that in 9.5 wt % NaOH/4.5 wt % thiourea, which dissolved cellulose at -5 °C, leading to the dissolution of cellulose at a relatively high temperature (8 °C). The cellulose that dissolved at such a high temperature was metastable. The results of dynamic light scattering and transmission electron microscope experiments confirmed that the extended cellulose chains and their aggregates coexisted in the dilute cellulose solution. Interestingly, stiff cellulose chains could be self-assembled in parallel to form nanofibers in the metastable cellulose solution, from which cellulose microspheres consisting of nanofibers could be easily produced by inducing heating. This work not only proposed a novel method for the dissolution of cellulose in aqueous system at temperatures over 0 °C but also opened up a new pathway for the construction of nanofibrous cellulose materials.