Extensional rheology of cellulose-ionic liquid solutions

Various cellulose-ionic liquid solutions were measured with a rotational plate-andplate rheometer and a Capillary Break-up Extensional Rheometer (CaBER). Comparison of the shear and extensional rheological properties showed good agreement in some cases. The CaBER, however, seems to be more sensitive to small variations in solute composition. INTRODUCTION Since almost a decade now, nonderivatizing dissolution of ligno-cellulosic material in so called ionic liquids (ILs) is explored extensively. The term ionic liquids refers to compounds that solely consist of ions and exhibit a melting point below 100 °C. These special salts usually comprise inorganic anions and bulky, organic cations like 1-alkyl-3methylimidazolium, 1-alkylpyridinium, 1alkyl-1-methylpyrrolidinium, tetraalkylammonium, guanidinium or phosphonium ions. Reduced interaction between the ions due to their size and charge delocalization and an entropically favoured liquid state lead to the exceptionally low melting points. In 2002, Rogers et al. re-discovered the remarkable capability of ILs to dissolve cellulose directly, i.e. without derivatization of the hydroxyl groups, in high concentrations. In the meantime, several IL-systems have been reported to be suitable cellulose solvents. Especially 1-ethyl-3methylimidazolium acetate ([emim] OAc, Fig. 1) seems to be a promising solvent due to its low viscosity, low toxicity, and good cellulose dissolution properties and was thus studied by many research groups. Nonderivatizing cellulose-dissolution and subsequent regeneration via dry-jet wet spinning provide access to environmentally benign and economically viable biomass processing. Similar to polymer-melt spinning, the cellulose-ionic liquid solution (dope) is spun via an air-gap into a coagulation bath filled with an anti-solvent, where the cellulose is regenerated in the form of value added products like fibers or films. To produce stable filaments in the air gap, however, detailed knowledge of the dope’s rheological properties is required. Unfortunately, only few reports on the rheological properties of cellulose-IL solution have been published. Most of them mainly deal with shear rheological investigations of dilute solutions of microcrystalline cellulose (MCC). Figure 1. 1-Ethyl-3-methylimidazolium acetate [emim] OAc. Extensional rheology of cellulose-ionic liquid solutions Michael Hummel, Anne Michud, and Herbert Sixta Department of Forest Products Technology, Aalto University, Espoo, Finland Herein we present two rheological studies on cellulose with high degree of polymerization (DP) in [emim] OAc. The first one deals with in influence of the concentration on the rheological properties, whereas in the second the molecular weight distribution (MWD) of the solute was varied. In both cases shear rheological data is compared to elongational-rheological properties assessed by means of a Capillary Break-up Extensional Rheometer (CaBER).