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Solution Rheology of Dry Native Cellulose in Ionic Liquids : Weakly Associating Polymers ?

Ralph H. Colby, Materials Science and Engineering, Penn State University, University Park, PA 16802 USA

We report solution rheology of six dry native cellulose samples of different molecular weight in three different ionic liquids ; 1-ethyl-3-methylimidazolium acetate [EMIm]Ac, 1-butyl-3-methyl imidazolium chloride [BMIm]Cl1 and 1-ethyl-3-methylimidazolium methylphosphonate [EMIm][P(OCH3)(H)O2].2 These solutions do not crystallize, making it possible to measure the glass transition using oscillatory shear rheology. Trace amounts of water impart a yield stress to these solutions so the experience 20 min at 80 C in the rheometer after loading to remove water.3 Concentration dependences of specific viscosity and relaxation time make each solution appear to be a ‘normal’ polymer solution, with dilute, semidilute unentangled and entangled regimes of concentration. However, the same cellulose sample in different solvents at the same concentration has a different width of the rubbery plateau and there is a strange failure of the Cox-Merz rule (shear viscosity larger than linear complex viscosity in the shear-thinning region) for cellulose solutions in ionic liquids. Both of those observations suggest that native cellulose is not a simple flexible polymer in solution and that it instead has some inter-chain hydrogen bonds between cellulose chains. Urea is known to compete for hydrogen bonds and adding urea to these solutions lowers the viscosity, most dramatically for the solutions with the widest rubbery plateau. An additional complication with these solutions is that cellulose seems to adsorb to the air interface to create a viscoelastic film and the consequences of that adsorption on literature for the intrinsic viscosity of cellulose in [EMIm]Ac will be discussed.


1. X. Chen, Y. Zhang, H. Wang, S.-W. Wang, S. Liang and R. H. Colby, Solution Rheology of Cellulose in 1-butyl-3-methyl Imidazolium Chloride, J. Rheol. 55, 485 (2011).
2. X. Chen, S. Liang, S.-W. Wang, and R. H. Colby, Linear Viscoelastic Response and Steady Shear Viscosity of Native Cellulose in 1-ethyl-3- methylimidazolium methylphosphonate, J. Rheol. 62, 81 (2018).
3. B. Nazari, N. W. Utomo, and R. H. Colby, The Effect of Water on Rheology of Native Cellulose/Ionic Liquids Solutions, Biomacromolecules 18, 2849 (2017).