Chain dynamics and glass transition of dry native cellulose solutions in ionic liquids

Dry native cellulose solutions in 1-butyl-3-methylimidazolium methylphosphonate (EMImMPO₃H), 1-butyl-3-methylimidazolium acetate (EMImAc), and 1-butyl-3-methylimidazolium chloride (BMImCl) ionic liquids (IL) were investigated using subambient linear viscoelastic oscillatory shear. Glass transition temperatures (T_g) of solutions with various cellulose concentrations up to 8.0 wt% were observed as the peaks of loss tangent tan(δ) and loss modulus G'' in descending temperature sweeps at 1 rad s^-1. Cellulose/IL solutions showed a minimum in T_g at ∼2.0 wt% cellulose content before increasing with cellulose concentration, suggesting a perturbation of the strongly structured IL solvents by the cellulose chains. Isothermal frequency sweeps in the vicinity of T_g were used to construct time-temperature-superposition master curves. The angular frequency shift factor a_T as a function of temperature indicates Arrhenius behavior within a 9 K range near T_g, allowing calculation of fragility, which was found to be constant up to 8.0 wt% cellulose concentration. This result implied that increasing cellulose concentration initially decreases T_g due to disrupted ionic regularity of ILs, but does not seem to change their fragility.