Unique CO2-switched cellulose solution properties in the CO2/DBU/DMSO solvent system and the preparation of regenerated materials

Abstract

The reaction of cellulose with CO₂ in the presence of various organic superbases resulted in a novel CO₂ derivative dissolution strategy for cellulose, presenting a special solvent system for cellulose dissolution processing and derivatization. The properties of the cellulose solution are important for the preparation of particular regenerated materials via dissolution processing. Herein, properties of the cellulose solution in a CO₂/1.8-diazabicyclo[5.4.0]undec-7-ene/dimethyl sulfoxide (CO₂/DBU/DMSO) solvent system were first systematically studied, and it was found that the apparent viscosities of cellulose solutions were highly dependent on the cellulose type, concentration and temperature. The overlap concentrations (c*) were determined to be 2 and 0.6 wt% for the microcrystalline cellulose (MCC) and wood pulp solutions, respectively. The Cox–Merz rule was valid for cellulose solutions in the CO₂/DBU/DMSO solvent system. Furthermore, taking advantage of the reversible chemistry of CO₂ in this solvent system, a reversible CO₂-controlled sol–gel transition of the newly achieved cellulose solution in CO₂/DBU/DMSO was identified by dynamic rheology of measuring the temperature dependence of storage modulus (G′) and loss modulus (G′′). Furthermore, the interesting CO₂-controlled sol–gel transition was developed via integrated thermal-induced CO₂ release and anti-solvent extraction for the preparation of regenerated cellulosic gel materials and then cellulose films with a tensile strength of 44.3 MPa. Interestingly, it was found that the regenerated cellulose through this approach turned out to be a cellulose IV_I crystalline structure.