Influence of hydrostatic pressure during gelation on physicochemical properties of cellulose hydrogels prepared from ionic liquid/DMSO solution

Various studies have been conducted on the effect of gelation temperature on the physicochemical properties of hydrogels; however, the effect of hydrostatic pressure on these properties is yet to be investigated. In this study, cellulose hydrogels were prepared by dissolving cellulose in 60 wt% 1-butyl-3-methylimidazolium acetate/dimethyl sulfoxide (DMSO) and gelating the resultant solution by displacing it with water at hydrostatic pressures ranging from 0.1 to 250 MPa. The relationship between the hydrostatic pressure during gelation and physicochemical properties of the prepared cellulose hydrogels (shrinkage rate, water content, mechanical strength, and crystallinity) was thoroughly investigated. The results revealed that with the increase in the hydrostatic pressure, the water content of the cellulose hydrogels decreased and the cellulose hydrogels became denser. In contrast, the crystallinity (structure and orientation within the cellulose bundles that form the backbone of the cellulose hydrogels) and compressive modulus did not exhibit hydrostatic pressure dependence. Furthermore, by conducting molecular dynamics simulations of the cellulose/ionic liquid/water system under ambient and hydrostatic pressure (150 MPa and 250 MPa, respectively), the cellulose–ionic liquid, cellulose–DMSO, and cellulose–water radial distribution functions and coordination numbers were investigated to elucidate the effect of the hydrostatic pressure during gelation on the gel physicochemical properties from a molecular theoretical perspective. It was found that with increasing pressure, the water and DMSO coordinated around the cellulose both increased while the ionic liquid decreased. Thus, it was concluded that this decrease in the coordination number of ionic liquids to cellulose at higher pressure promotes aggregation between the cellulose bundles that form the backbone of cellulose hydrogels. This results in a decrease in the water content and an increase in the mechanical strength of the cellulose hydrogel. These results provide deep insight into the dependence of gel physicochemical properties on hydrostatic pressure during gelation and a platform to expand these applications using a convenient and environmentally friendly process.