Rheological investigation on the anisotropic phase of cellulose-MMNO/H2O solution system

The cellulose solutions in N-methyl-morpholine-N-oxide (MMNO)/H2O were prepared. The cellulose concentration was varied from 5 to 25 wt.%. The mole ratio of water to MMNO was fixed at 0.8 and propyl gallate (similar to 1 wt.%) was added as an antioxidant. To observe the anisotropic phase formation, polarized light microscopy, DSC, dynamic rotational rheometry and capillary rheometry experiments were performed. Polarized light micrographs revealed anisotropy for the cellulose solutions of 20 wt.% and higher concentrations. The anisotropy become clearer when sheared. It was difficult to detect the isotropic transition temperature and concentration from DSC and dynamic viscosity. In capillary rheometry measurements, however, both viscosity drop at the anisotropic to isotropic transition concentration and flow activation energy change at the transition temperature were observed. The phase transition temperature and concentration were 95 degrees C-100 degrees C and about 23 wt.%, respectively. Time-temperature superposition principle was applied successfully to the solutions below 20 wt.% concentration but failed for the solutions above 20 wt.% concentration. The largest deviation from superposition was found at 23 wt.% solution which was the critical concentration. (C) 1999 Elsevier Science Ltd. All rights reserved.