Weak interactions and their impact on cellulose dissolution in an alkali/urea aqueous system

Abstract

Weak interactions, though sometimes easily ignored, play an important role in macromolecule dissolution. In this work, the characterization of weak interactions between urea and cellulose in a LiOH/urea aqueous solution was accomplished and confirmed in situ, for the first time, using PFG-SE NMR, FT-IR and solvatochromic methods, etc. The NMR results indicated the binding of urea with cellulose in the solution, demonstrating the existence of the weak interactions between them. Subsequently, the solvatochromic methods revealed that urea hardly affected the hydrogen bond donor (HBD acidity) and hydrogen bond acceptor (HBA basicity) properties of the solvent, but was related to its dipolarity and polarizability, indicating that dispersion forces existed therein, but not likely hydrogen bonding, which was also supported by the FT-IR. Furthermore, the impact of weak interactions between urea and cellulose was demonstrated to facilitate the dissolving process. The fine dispersion and good stability of cellulose in the solution were maintained by mitigating the effect of the hydrophobic portions from all the dilute, semi-dilute and concentrated regimes, supported by the results of dynamic light scattering (DLS), rheology, NMR, etc. Therefore, the transmittance and mechanical properties of the regenerated cellulose materials prepared from the cellulose solution in the alkali/urea aqueous system were enhanced, compared with those in the alkali only system. This work provided significant and new experimental insights into the non-covalent weak interactions between urea and macromolecules from the viewpoints of polymer physics and physical chemistry, which could never be ignored and underestimated. The indispensable weak interactions in the system are also important for the green conversion of natural biomass into new materials via physical processes.