Solution Rheology Of Cellulose In Ionic Liquids

Ionic liquids, as a class of designable green solvent, have great application potential in preparation of cellulose fibers. In this dissertation, solution rheology of cellulose in ionic liquids is reported using oscillatory and steady shear for cellulose concentrations spanning the dilute, semidilute unentangled, and entangled regimes. Based on the rheological behaviors, the interaction between ionic liquids-cellulose, as well as the dynamics of cellulose in such ionic liquids were investigated. The solubility and the solution behavior of cellulose in two ionic liquids with different structures was also studied.The concentration dependence of specific viscosity, relaxation time andη0/τof cellulose/[BMIM]Cl were firstly investigated. On the basis of the concentration dependence, the overlap concentration c* of our cellulose in [BMIM]Cl is 0.5 wt% and the entanglement concentration ce is a factor of 4 larger (2 wt%). Therefore, the cellulose solutions in [BMIM]C1 can be separated into three regimes:the dilute regime, semidilute unentangled regime, and entangled regime. In dilute regime,ηsp andη/τscales as c andτis independent of c. For semidilute unentangled solutions (between 0.5 and 2 wt%), the specific viscosity, relaxation time and terminal modulus exhibit concentration dependencesηsp-c2,τ-c, G-c, respectively while for entangled solutions (between 2 and 10 wt%) we findηsp-c14/3,τ-c2.3, G-c7/3, consistent with scaling predictions for neutral polymers in aθsolvent. The fact that ce/c*= 4 also suggests that 25℃is in the 0 regime for this cellulose, since good solvents usually have ce/c*≈10. However, the entanglement concentration of cellulose in [EMIM][P(OCH3)(H)O2] is only 0.8 wt%, and the c* has not been found in the concentration range from 0.1 wt% to 10 wt%, so the cellulose/[EMIM][P(OCH3)(H)O2] solutions we studied can be only separated into two regimes:semidilute unentangled and entangled solutions. For semidilute unentangled solutions (between 0.1 and 0.8 wt%), the specific viscosity, relaxation time and terminal modulus exhibit concentration dependencesηsp-c1.3,τ-c, G-c, respectively while for entangled solutions (between 0.8 and 10 wt%) we findηsp-c3.9,τ-c1.6, G-c2.3, consistent with scaling predictions for neutral polymers in a good solvent.Time-temperature superposition (TTS) was applied to get master curves, which can be used to characterize the full linear viscoelastic response of each cellulose solution. TTS worked very nicely for the linear viscoelastic oscillatory shear response of cellulose/[BMIM]Cl and cellulose/[EMIM][P(OCH3)(H)O2] solutions, this demonstrates the high stability of all those cellulose solutions in such wide range of temperature. Rouse model and Zimm model were applied to describe the dynamics of cellulose in ionic liquids through the reduced loss modulus and storage modulus. It was found that the transition of the Rouse-like behavior to Zimm-like behavior took place with decreasing cellulose concentration of the semidilute unentangled cellulose/[BMIM]Cl solutions. This transition indicates that dominate hydrodynamic interactions also exist in the semidilute cellulose/[BMIM]Cl solutions due to the strong interactions between ions-cellulose molecules, although the concentration is higher than overlap concentration. As expected, the linear viscoelastic response of dilute cellulose/[BMIM]Cl solution was described completely by the Zimm model (υ=0.5, for 0 solvent). It is thereby concluded that [BMIM]Cl should be 0 solvent for cellulose. On contrary to cellulose/[BMIM]Cl solutions. However, all the semidilute cellulose/[EMIM][P(OCH3)(H)O2] solutions exhibit Rouse-like behavior even the mono-chain Rouse model can not describe their linear viscoelastic response well, so the interactions between cellulose chains are dominant in semidilute cellulose/[EMIM][P(OCH3)(H)O2] solutions. And the hydrodynamic interactions are screened by the strong inter-macromolecular interactions. Cellulose in [EMIM][P(OCH3)(H)O2] has lower ce than cellulose/[BMIM]Cl, and the extent of entanglement of cellulose in cellulose in [EMIM][P(OCH3)(H)O2] is about 2-3 orders of magnitude higher than that extent of entangled cellulose/[BMIM]Cl solutions. This means that the molecule size of cellulose in [EMIM][P(OCH3)(H)O2] is much larger than that in [BMIM]Cl, espeically while c>c*, the strong interactions between cellulose chains would determine the rheological behavior of the solution. But the unusual deviations from the Cox-Merz rule indicate that cellulose/ionic liquid solutions are not simple neutral flexible polymers.