Formation of core–shell structures and viscous fingering in cellulose beads regenerated from [DBNH][OAc]/DMSO

Superbase Ionic Liquids (SBILs) are efficient direct-dissolution solvents for cellulose and have found applications such as manufacturing of man-made textile fibers. In this study cellulose beads were prepared from microcrystalline cellulose dissolved in a mixture of SBIL 1,5-diazabicyclo[4.3.0]non-5-enium acetate with dimethyl sulfoxide, [DBNH][OAc]/DMSO, by drop-wise regeneration using water as an antisolvent. This resulted in cellulose regeneration by spinodal decomposition phase separation. The cross-sections of freeze-dried beads were thoroughly investigated using SEM, revealing a complex internal bead structure. Special attention was paid to structures resulting from the inwards moving regeneration front, where the solvent and antisolvent interdiffuse in opposite directions. The phase boundary at the regeneration front showed evidence of Saffman–Taylor instability, i.e., viscous fingering. Altering the diffusion environment surrounding the bead during regeneration resulted in nested layers of cores and shells. The number and placement of the core–shell separations was regulated by the number of transfers between two antisolvent baths and the duration of alternating periods of fast and slow interdiffusion of water and [DBNH][OAc]/DMSO through the bead perimeter.