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The Effect of Water on Rheology of Native Cellulose/Ionic Liquids Solutions Behzad Nazari, Nyalaliska Utomo, and Ralph H. Colby Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b00789 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017
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The Effect of Water on Rheology of Native Cellulose/Ionic Liquids Solutions Behzad Nazari, Nyalaliska Utomo, Ralph H. Colby* Materials Science and Engineering and the Materials Research Institute, Penn State University, University Park, PA 16802, United States Abstract Cellulose coagulates upon adding water to its solutions in ionic liquids. Although cellulose remains in solution with much higher water contents, here we report the effect of 0-3 wt.% water on solution rheology of cellulose in 1-butyl-3-methylimidazolium chloride (BMImCl) and 1-ethyl-3methylimidazolium acetate (EMImAc). Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), and polarized light microscopy were also used to study water absorbance to the solutions. Tiny amounts of water (0.25 wt.%) can significantly affect the rheological properties of the solutions, imparting a yield stress, while dry solutions appear to be ordinary viscoelastic liquids. The yield stress grows linearly with water content and saturates at a level that increases with the square of cellulose content. Annealing the solutions containing small amounts of water at 80°C for 20 minutes transforms the samples to the fully dissolved “dry” state.
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Introduction Cellulose, the most abundant renewable polymer on Earth, consists of chains that aggregate via intermolecular hydrogen bonds to form a crystalline structure1, 2. Cellulose is the major structural material of plants; i.e. wood is largely cellulose, and cotton is almost pure cellulose3, 4. This natural polymer is utilized in a large number of applications including paper, textile, construction, polymer, and food industries5. Although some processes involving cellulose do not require the break-down of its crystalline nature, dissolution of cellulose is quite often of essential importance for its processing6. This is attributable to the melting point of cellulose being higher than its decomposition temperature, making solutions play an inevitable role in cellulose processing3. Only a short list of solvent systems for native cellulose have been suggested, for instance, dimethylacetamide/lithium chloride (DMAc/LiCl), dimethylformamide/dinitrogen tetroxide (DMF/N2O4), dimethyl sulfoxide/tetra-n-butylammonium fluoride (DMSO/TBAF)7. However, most of these solvents were found to be contaminating or corrosive and required special equipment with large power consumption3, 7. Ionic liquids (ILs) are salts which melt below 100 °C because of their structural irregularity8. Since 2002, certain ILs, with anions that compete for hydrogen bonds, have been reported to fully dissolve native cellulose (up to 20 wt.%) without pretreatments or derivatizing effects8, 9. These ILs have negligible vapor pressure, in addition to ease of recycling, their choice as a solvent could prevent their release into the environment, offering an advantage over most traditional solvents6, 10. It is well accepted that rheology plays an important role in many processing operations involving polymer solutions such as fiber spinning, film casting, etc8, 11,12. Rheology of cellulose/IL solutions has been studied with cellulose contents ranging from dilute to entangled regimes6, 8, 11-17, mostly focusing on 1-butyl-3-methylimidazolium chloride (BMImCl)8, 13, 16, 17
and 1-ethyl-3-methylimidazolium acetate (EMImAc)6, 11, 15, 17. For both BMImCl and
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EMImAc, via oscillatory and steady shear tests for different cellulose concentrations, and based on the concentration dependences of the specific viscosity, cellulose solutions were separated into dilute, semidilute unentangled, and entangled regimes, similar to solutions of many flexible neutral polymers in θ solvents. The two transition concentrations separating these regimes were reported to be the overlap concentration c* = 0.5 wt.% and the entanglement concentration ce = 2.0 wt.% cellulose with Mw = 120-140 kg/mol8, 14. Dissolved cellulose can be regenerated from the solution by adding a non-solvent (e.g. water) that is miscible with the IL5. This sol-gel transition of the cellulose solution seems to be of essential importance in many applications for cellulose/ILs such as wet spinning, electro-spinning, composites, etc5, 18, 19. For instance, coagulation baths can be utilized as a way to extract IL from the solutions due to the fast gel formation by immersing the solution into a container of non-solvent which will lead to the precipitation of cellulose fibers5, 18, 19. At low contents of the coagulation agent, a three-dimensional gel consisting of cellulose, IL, and non-solvent (e.g. water) forms5, 18, 19
, whose structure is not yet clear. Based on turbidity, optical microcopy, and X-ray diffraction
measurements, it seems non-crystalline cellulose aggregates are the underlying structures that form the gel upon adding moderate amounts of non-solvent (e.g.
