Imidazolium-based ionic liquids are low vapour pressure, powerful cellulose solvents, allowing the dissolving of cellulose up to 25 wt.%; they are thus very promising solvents for making fibres, films and aerogels. We have studied cellulose solutions in the ionic liquid 1-butyl-3-methylimidazolium chloride (BMIMCl). This system has been investigated using NMR spectroscopy, diffusion, relaxometry and rheology. Our set of cellulose concentrations (0, 1, 2, 5, 8 and 10 % w/w) were examined over a range of temperatures (30 °C to 100 °C). The change in 1H chemical shift, measured at 400 MHz, revealed that the resonances belonging to the imidazolium ring had the strongest dependences on cellulose concentration, with them moving upfield on the addition of cellulose. This is consistent with the usual explanation for the dissolution of cellulose by this IL, in which the Cl anion preferentially leaves the cation to break the inter- and intra- hydrogen bonds of the cellulose chains. NMR relaxometry was carried out both at high field (400 MHz) and low field (20 MHz). At high field the relaxation times measured showed a weak dependence on cellulose concentration, indicating that fast local reorientations of the protons (timescale ~ 1 / 400 MHz) were not affected by the dissolution of cellulose. Conversely, the low field relaxation times were significantly altered on the addition of the carbohydrate. The dependences of the spin-lattice and spin-spin relaxations from these low field experiments were found to be well-described by the classic Bloembergen-Purcell-Pound theory. From this analysis a rotational correlation time for the cation as a function of temperature and cellulose concentration was determined. Rotational correlation times from the low field NMR relaxometry tauc were combined with the NMR diffusion D and viscosity data showing that the Stokes-Einstein-Debye formulae correctly relate these parameters, see the Figure below. From this analysis a microviscosity for the ions was determined.

NMR spectroscopy, relaxometry, diffusion, and rheological studies of cellulose in the ionic liquid 1-butyl-3-methylimidazolium chloride

249th National Meeting (2015)

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