Phase Transition and Rheological Behaviors of Concentrated Cellulose/Ionic Liquid Solutions

Abstract

The phase transition and rheological behaviors of concentrated solutions of microcrystalline cellulose (MCC) in an ionic liquid of 1-allyl-3-methylimidazolium chloride (AMIMCl) have been investigated. Polarized optical microscopy (POM) measurements indicate that the two critical cellulose concentrations for the appearance of biphase and fully anisotropic phase for MCC/AMIMCl solutions are 9 and 16 wt%, respectively. POM and differential scanning calorimetry (DSC) measurements coherently indicate that the clearing temperature, T(c) increases with increasing cellulose concentration. Oscillatory shear measurements show that the crossover frequency first moves to lower values and then moves back to higher values with increasing cellulose concentration, which indicates that most cellulose chains are aligned or oriented to reduce chain entanglements when the cellulose concentration is above 14 wt%. From the steady shear measurements, it is surprising to find that the viscosity versus shear rate curves exhibit four flow regions including two plateaus and two shear-thinning regions when the cellulose concentration exceeds 9 wt%. The influences of cellulose concentration and temperature on the first normal stress differences (N(1)) are analyzed according to the Larson theory. The peak of N(1) always appears at the intermediate part of the first shear-thinning region, and the following minimum of N(1) appears at the onset of the second shear-thinning region. The viscosity versus shear rate curves only exhibit two flow regions when temperature is above the respective T(c); meanwhile, negative N(1) values disappear and N(1) increases monotonically. The above results suggest that melting of the liquid crystal domains at high temperature results in the disappearance of the second plateau for the viscosity versus shear rate curves.