Associating polymer features of native cellulose in ionic liquid solutions

Cellulose is the most abundant bio-based polymer natural resource on Earth today. Certain ionic liquids (IL) have been discovered to be the best solvents to dissolve cellulose at the molecular level. Despite the promising role that natural polymers may undertake in the near future in replacing synthetic polymers, still many challenges exist. Here we address the associating polymer features exhibited by native cellulose in solutions with varying IL, in both linear and nonlinear viscoelastic regimes. We observed three aspects attributed to associations between cellulose chains: i) For a given cellulose sample at any particular concentration, the viscoelasticity is very different in the three ionic liquids we study. ii) The concentration dependence of entanglement plateau width is considerably stronger than that of polymer solutions with no associations. iii) The shear stress growth coefficient in steady-shear flow overcomes the complex viscosity, violating the empirical Cox-Merz relationship. We also used the Sticky-Reptation molecular model to estimate the association lifetime and the average number of associating sites from the observed data.