Abstract
Solutions of two types of cellulose in the ionic liquid 1-butyl-3-methyl-imidazolium acetate (BmimAc) have been analyzed using rheology and fast-field cycling nuclear magnetic resonance (NMR) spectroscopy, in order to analyze the macroscopic (bulk) and microscopic environments, respectively. The degree of polymerization (DP) was observed to have a significant effect on both the overlap (c*) and entanglement (ce) concentrations and the intrinsic viscosity ([η]). For microcrystalline cellulose (MCC)/BmimAc solutions, [η] = 116 mL g-1, which is comparable to that of MCC/1-ethyl-3-methyl-imidazolium acetate (EmimAc) solutions, while [η] = 350 mL g-1 for the commercial cellulose (higher DP). Self-diffusion coefficients (D) obtained via the model-independent approach were found to decrease with cellulose concentration and increase with temperature, which can in part be explained by the changes in viscosity; however, ion interactions on a local level are also important. Both Stokes-Einstein and Stokes-Einstein-Debye analyses were carried out to directly compare rheological and relaxometry analyses. It was found that polymer entanglements affect the microscopic environment to a much lesser extent than for the macroscopic environment. Finally, the temperature dependencies of η, D, and relaxation time (T1) could be well described by Arrhenius relationships, and thus, activation energies (Ea) for flow, diffusion, and relaxation were determined. We demonstrate that temperature and cellulose concentration have different effects on short- and long-range interactions.
Highlights
Ionic liquids (ILs) are made up of a cationic and anionic species and can be defined as low-melting point salts; Walden defined ILs as salts with a melting point below 100 °C.1,2 ILs are non-volatile and potentially recyclable and can be designed for specific processes by altering the cation and anion structures.[3]
The flow curves for A-cell in butyl-3-methyl-imidazolium acetate (BmimAc) (0−4 wt %) were analyzed in order to compare the solvent properties of BmimAc to those of ethyl-3-methyl-imidazolium acetate (EmimAc) and other acetate-based ILs
It should be noted that for all of the cellulose−BmimAc solutions, no positive inflexion was observed in the data toward the higher shear rates, and it was assumed that η(∞) was approximately the value of the pure solvent when fitting the data
Summary
Ionic liquids (ILs) are made up of a cationic and anionic species and can be defined as low-melting point salts; Walden defined ILs as salts with a melting point below 100 °C.1,2 ILs are non-volatile and potentially recyclable and can be designed for specific processes by altering the cation and anion structures.[3]. ILs have had an enormous impact in the area of cellulose processing, owing in part to their ability to break the strong network of hydrogen bonds that forms between polymer chains.[5] Mild conditions, relatively easy recovery of cellulose, and potential recyclability of the solvent make ILs superior to many other reported cellulose solvents, such as N-methylmorpholine N-oxide, which often require high energy inputs and are thermally unstable.[6,7] high viscosity[8] and cost of ILs have led to practical issues with their scale-up and have restricted their use in industrial cellulose processing.[9] Dissolving small amounts of cellulose in an IL can result in a large viscosity increase, which hinders further solubilization[10] and complicates IL−cellulose handling on a large scale Their ability to dissolve cellulose decreases as the amount of water within the system increases,[11] since water causes cellulose to coagulate.[12,13] Almost all ILs are hygroscopic, and as a result, small-scale cellulose dissolution is generally carried out within a sealed system or a glovebox, in order to reduce water vapor uptake from the atmosphere. Dissolution may alternatively require vacuum distillation throughout the process in order to remove water.[14]
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