A Simple Regeneration Process Using a CO₂-Switchable-Polarity Solvent for Cellulose Hydrogels

Cellulose is one of the main components of plant cell walls, abundant on earth, and can be acquired at a low cost. Furthermore, there has been increasing interest in its use in environmentally friendly, carbon-neutral, sustainable materials. It is expected that the applications of cellulose will expand with the development of a simple processing method. In this study, we dissolved cellulose in aqueous N-butyl-N-methylpyrrolidinium hydroxide solution ([C₄mpyr][OH]/H₂O) and investigated the cellulose regeneration process based on changes in solubility upon application of CO₂ gas. We investigated the effect of transformation of the anion chemical structure on cellulose solubility by flowing CO₂ gas into [C₄mpyr][OH]/H₂O and conducted pH, FT-IR, and ¹³C NMR measurements. We observed that the changes in anion structure allowed for the modulation of cellulose solubility in [C₄mpyr][OH]/H₂O, thus establishing a simple and safe cellulose regeneration process. This regeneration process was also applied to enable the production of cellulose hydrogels. The hydrogel formed using this method was revealed to have higher mechanical strength than an analogous hydrogel produced using the same dissolution solvent with the addition of a cross-linker. The ability to produce cellulose-based hydrogels of different mechanical properties is expected to expand the possible applications.