STRESS-RELAXATION BEHAVIOR OF LIQUID-CRYSTALLINE SOLUTIONS OF ETHYL CELLULOSES WITH DIFFERENT MOLECULAR-WEIGHT IN M-CRESOL

The stress relaxation behaviour of liquid crystal‐forming ethyl celllulose (EC) solutions in m‐cresol was determined by means of a cone‐plate type viscometer at 30°C. The effect of molecular weight (MW) on the behaviour was also determined. The relaxation behaviour could be fitted with the following equation: (Formula Presented.) where σi and σf are steady‐state shear stresses at shear rate \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm i}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm f}$\end{document}, σ(t) is time‐ dependent stress, A1 and A2 are constants, τ1 and τ2 are relaxation times, t is time, and tc is a characteristic time. When log σ* was plotted against time, one straight line was obtained for isotropic solutions, whereas anisotropic solutions yielded two straight lines. This suggests that the liquid crystalline solutions have two separate relaxation processes: Process 1 has a relatively short relaxation time, and process 2 has a long one. The parameters τ1, τ2, and A2 were greatly dependent on polymer concentration, combination of \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm i}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$\dot \gamma _{\rm f}$\end{document}, and MW, whereas A1 was independent thereof and was close to unity. The process 1 was supposed to be valid for individual molecules, and process 2 for liquid crystalline domains or randomly aggregated or entangled molecules. © 1990 Hüthig & Wepf Verlag, Basel