Thermo‐Reversible Cellulose Micro Phase‐Separation in Mixtures of Methyltributylphosphonium Acetate and γ‐Valerolactone or DMSO. Issue 7 (25th February 2022) - Holding, Ashley J.
Abstract: We have identified cellulose solvents, comprised of binary mixtures of molecular solvents and ionic liquids that rapidly dissolve cellulose to high concentration and show upper‐critical solution temperature (UCST)‐like thermodynamic behaviour ‐ upon cooling and micro phase‐separation to roughly spherical microparticle particle‐gel mixtures. This is a result of an entropy‐dominant process, controllable by changing temperature, with an overall exothermic regeneration step. However, the initial dissolution of cellulose in this system, from the majority cellulose I allomorph upon increasing temperature, is also exothermic. The mixtures essentially act as 'thermo‐switchable' gels. Upon initial dissolution and cooling, micro‐scaled spherical particles are formed, the formation onset and size of which are dependent on the presence of traces of water. Wide‐angle X‐ray scattering (WAXS) and 13 C cross‐polarisation magic‐angle spinning (CP‐MAS) NMR spectroscopy have identified that the cellulose micro phase‐separates with no remaining cellulose I allomorph and eventually forms a proportion of the cellulose II allomorph after water washing and drying. The rheological properties of these solutions demonstrate the possibility of a new type of cellulose processing, whereby morphology can be influenced by changing temperature. Abstract : Cellulose spherulites are formed during regeneration from the [P4441 ][OAc] ionic liquid and DMSO or GVL co‐solvent electrolyte mixtures, after dissolution at increased temperature and cooling of the mixtures. The thermodynamics of the initial gelation, upon cooling, support a UCST‐type transition, with additional spherulite formation occuring during the gelation process. Cellulose solvation in [P4441 ][OAc] electrolytes was also found to be highly dependent on co‐solvent polarity.