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

Abstract

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.

Glycerin/NaOH Aqueous Solution as a Green Solvent System for Dissolution of Cellulose

“…Peaks located at 104.1 ppm, 79.5 ppm, 74.3 ppm and 61 ppm were ascribed to the carbon atom of C1, C4, C2, C6 as inserts in Figure 3. Twin peak located at 75.7 ppm was ascribed to the carbon atom of C3,5 [26]. Compared with the reported cellulose I obtained from the NaOH/urea solvent system, a higher magnetic field shifting from 79.2 ppm [27] to 79.5 ppm implies the destruction of intra-molecule hydrogen bonding, which is similar with that reported dissolution of wood pulp in LiCl/DMAc [28].…”

Optically Transparent Multiscale Composite Films for Flexible and Wearable Electronics

“…[84,93] Creating nanopapers in bottom-up fashion, on the other hand, obviates the need for a high-energy apparatus by taking advantage of supramolecular self-assembly of NFs from a solution. Several solvent systems (e.g., aqueous alkali/urea, [94,95] halide/ organic solvent, [96] ionic liquids, [97] etc.) have been proven to yield a homogenous solution of cellulose and/or chitin, despite their insolubility in most organic solvents; detailed information can be found in several review articles.…”

Molecular‐Scale Design of Cellulose‐Based Functional Materials for Flexible Electronic Devices

“…[47] Compared with the above solvents, novel green solvent systems, such as ionic liquids, alkali/urea, deep-eutectic solvents, and dimethylacetamide (DMAc)/LiCl, are characterized by environmental friendliness, high thermal stability and solubility, and recyclability, so they show great potential for cellulose dissolution and constructing functional materials. [24][25][26][27][48][49][50][51] Due to the strong interactions between the anion/cation of the solvent and the hydroxyl group of cellulose, [52][53][54][55][56][57] the original hydrogen-bonding networks of the cellulose can be easily destroyed through purely physical means. [58][59][60][61][62][63][64] Then, the cellulose can be rapidly and completely dissolved into single molecular chains (Figure 1b,c), which allows the design of materials based on the molecular-scale concept.…”

“…Green solvents that are environmentally friendly and easy to use, such as ionic liquids and alkali/urea, can gently dissolve cellulose into single molecular chains. [ 24–27 ] Then, combined with designable and controllable supramolecular self‐assembly and molecular hybridization, it is convenient to develop cellulose‐based materials with unique structures, such as dynamic topological networks, and optimal properties, such as stretchability, ionic conductivity, and self‐healing. [ 4 ] Meanwhile, nanomaterials such as conductive polymer sheets, metal oxide nanoparticles, 2D covalent layers, and responsive polymer molecules, can be well‐integrated with cellulose macromolecules during self‐assembly processes to create functional materials with conductive, magnetic, electrochemical, and stimuli responsiveness.…”